Search

The application of deep‐learning‐based seismic phase pickers has surged in recent years. However, the efficacy of these models when applied to monitoring volcano seismicity has yet to be fully evaluated. Here, we first compile a data set of seismic waveforms from various volcanoes globally. We then show that the performances of two widely used deep‐learning pickers deteriorate systematically as the earthquakes' frequency content decreases. Therefore, the performances are especially poor for long‐period earthquakes often associated with fluid/magma movement. Subsequently, we train new models which perform significantly better, including when tested on two data sets where no training data were used: volcanic earthquakes along the Cascadia subduction zone and tectonic low‐frequency earthquakes along the Nankai Trough. Our model/workflow can be applied to improve monitoring of volcano seismicity globally while our compiled data set can be used to benchmark future methods for characterizing volcano seismicity, especially long‐period earthquakes which are difficult to monitor. Plain Language Summary: Earthquake activity at volcanic regions is often monitored to indicate volcanic activity. Identifying the time when the energy radiated from an earthquake source arrives at a seismometer is essential for locating the earthquake, which can be difficult for volcanic earthquakes because of high noise levels, high event rates, and obscured onsets. Previous studies have demonstrated that deep learning can excel in picking the arrival times of regular earthquakes. However, it is unclear how sensitive these detectors are to earthquakes in volcanic regions. Here, we first compile a data set of earthquakes from various volcanoes globally. We then show that existing deep‐learning‐based detectors can miss a large fraction of these earthquakes, especially those without an abrupt change in signal amplitude. We then provide two new models which can better detect volcanic earthquakes than existing models. Our model/workflow can be applied to improve monitoring of volcanic earthquakes globally. Key Points: We compile a data set of seismic waveforms from various volcanic regions globallyWe show that existing deep‐learning phase pickers' performances deteriorate with decreasing earthquake frequency contentOur retrained models perform better and are more generalizable for monitoring volcano seismicity, especially long‐period earthquakes [ABSTRACT FROM AUTHOR]

Differential rotation is one of the basic characteristics of the Sun, and it plays an important role in generating the magnetic fields and its activities. We investigated rotation rate using chromospheric features such as plages, enhanced network (EN), active network (AN), and quiet network (QN) separately (for the first time). The digitized Ca-K images from Kodaikanal Observatory for 1907–1996 are used to study rotation over 0°–80° latitudes at an interval of 10°. We find that plages and all types of networks exhibit the differential rotation of the chromosphere. Furthermore, the rotation rate shows a decreasing pattern as one move from the equator to the higher polar latitudes for all the features used in the study. At the equator the rotation rate (rotation period) is obtained to be ∼13.98° day−1 (25.74 days), ∼13.91° day−1 (25.88 days), ∼13.99° day−1 (25.74 days), and ∼14.11° day−1 (25.51 days) for plage, EN, AN, and QN areas, respectively. By analyzing how the area of chromospheric features varies over time, we can effectively map the Sun's rotation rate at all latitudes, including the polar regions. Interestingly, both plages and small-scale networks exhibit a similar differential rotation rate. This suggests these features likely rooted at the same layer below the visible surface of the Sun. Therefore, the long-term Ca-K data is very useful for studying the solar rotation rate at all latitudes including the polar regions. [ABSTRACT FROM AUTHOR]

Recent photometric observations of massive stars have identified a low-frequency power excess which appears as stochastic low-frequency variability in light-curve observations. We present the oscillation properties of high-resolution hydrodynamic simulations of a |$25\,\,{\rm{M}_\odot }$| star performed with the PPMstar code. The model star has a convective core mass of |$\approx 12\,\,{\rm{M}_\odot }$| and approximately half of the envelope simulated. From this simulation, we extract light curves from several directions, average them over each hemisphere, and process them as if they were real photometric observations. We show how core convection excites waves with a similar frequency as the convective time-scale in addition to significant power across a forest of low and high angular degree l modes. We find that the coherence of these modes is relatively low as a result of their stochastic excitation by core convection, with lifetimes of the order of 10s of days. Thanks to the still significant power at higher l and this relatively low coherence, we find that integrating over a hemisphere produces a power spectrum that still contains measurable power up to the Brunt–Väisälä frequency. These power spectra extracted from the stable envelope are qualitatively similar to observations, with the same order of magnitude yet lower characteristic frequency. This work further shows the potential of long-duration, high-resolution hydrodynamic simulations for connecting asteroseismic observations to the structure and dynamics of core convection and the convective boundary. [ABSTRACT FROM AUTHOR]

The Periadriatic Fault system (PAF) ranks among the largest post-collisional structures of the European Alps. Recent geodetic data suggest that a fraction of the Adria–Europe convergence is still being accommodated in the Eastern Alps. However, the historical seismicity records along the easternmost segment of the PAF are ambiguous, and instrumental records indicate that seismotectonic deformation is mostly concentrated in the adjacent Southern Alps and Dinarides. Both electron spin resonance (ESR) and optically stimulated luminescence (OSL) dating methods can be used to date coseismic slip (with a combined range covering a few decades to a couple of million years) in slowly deforming fault zones, such as the PAF. Since the saturation doses of the quartz ESR signals are larger than quartz and feldspar OSL, ESR enables establishing a maximum age of the last resetting event of the system, while OSL allows constraining their minimum age when the signal is in saturation. We collected fault gouge samples from three localities along the easternmost segment of the PAF. For ESR, we measured the signals from the Al center in quartz comparing the results from the single aliquot additive dose (SAAD) and single aliquot regenerative (SAR) dose protocols. For OSL, we recorded the infrared stimulated luminescence signal at 50°C (IR50) and post-infrared infrared-stimulated luminescence signal at 225 °C (pIRIR225). Our dating results indicate that the studied segment of the PAF system accommodated seismotectonic deformation during the Quaternary, with a maximum age for the last resetting event of the system ranging from 1075 ± 48 to 552 ± 26 ka (ESR SAR) and minimum ages in the range from 196 ± 12 to 281 ± 16 ka (saturation of pIRIR225). The obtained ages suggest that the studied segment of the PAF could be considered at least as a potentially active fault. [ABSTRACT FROM AUTHOR]

We calculated stress drops for 2875 small intraslab earthquakes at intermediate depths beneath Tohoku, Japan. We applied an S-coda-wave spectral ratio method to almost 900,000 event pairs. Detailed velocity values for the oceanic crust (OC) were adopted from previous observational studies. The median stress drops in the OC are about half those in the oceanic mantle (OM). The median stress drop for earthquakes in the OC decreases from depths of 70 to 120 km and increases from 120 to 170 km. Our preferred interpretation is that the rigidity in the OC decreases and then increases with depth due to combined effects of the dehydration associated with the eclogite formation and the increasing temperature with depth. These depth variations are consistent with results of a similar study beneath Hokkaido. The median stress drops in the oceanic plate beneath Tohoku are generally smaller than those beneath Hokkaido. Previous studies imaging the seismic structure at shallow depths and b-value analyses of intraslab earthquakes indicate that the near-trench region of the oceanic plate off Tohoku is more hydrated than that off Hokkaido. Taken together, these results suggest that differences in the degree of hydration of the oceanic plate in the near-trench regions could produce the different behaviors of stress drops of intermediate-depth earthquakes observed in Tohoku and Hokkaido. [ABSTRACT FROM AUTHOR]

Partially ionized plasmas (PIP) constitute an essential ingredient of our plasma universe. Historically, the physical effects associated with partial ionization were considered in astrophysical topics such as the interstellar medium, molecular clouds, accretion disks and, later on, in solar physics. PIP can be found in layers of the Sun's atmosphere as well as in solar structures embedded within it. As a consequence, the dynamical behaviour of these layers and structures is influenced by the different physical effects introduced by partial ionization. Here, rather than considering an exhaustive discussion of partially ionized effects in the different layers and structures of the solar atmosphere, we focus on solar prominences. The reason is that they represent a paradigmatic case of a partially ionized solar plasma, confined and insulated by the magnetic field, constituting an ideal environment to study the effects induced by partial ionization. We present the current knowledge about the effects of partial ionization in the global stability, mass cycle and dynamics of solar prominences. We revise the identified observational signatures of partial ionization in prominences. We conclude with prospects for PIP research in prominences, proposing the path for advancing in the prominence modelling and theory and using new and upcoming instrumentation. This article is part of the theme issue 'Partially ionized plasma of the solar atmosphere: recent advances and future pathways'. [ABSTRACT FROM AUTHOR]

The properties of rockfalls, such as the volume and geometry of the detached rock mass as well as the number and diameter of rock fragments, greatly affect their propagated behavior, and different strategies are required to mitigate rockfall hazards. Landquakes generated by rockfalls are regarded as good proxies for those properties. To explore the free‐fall impact dynamics of rockfalls, a series of free‐fall experiments on granular masses were designed and conducted with the characteristics of the generated landquakes versus initial variables being analyzed. The results show that the particle diameter is a dominant factor affecting landquakes, with the most significant effect on the mean frequency. The impact area is another key factor, with which both the maximum seismic amplitude and radiated seismic energy have strong positive correlations, while the mean frequency of landquakes shows a weak negative correlation. The maximum seismic amplitude appears to have a poor correlation with the numbers of layers of the free‐falling granular masses, and it was proven that the maximum seismic amplitude is determined by only the lower part of the granular masses. Quantitative relationships between the number of particles and seismic signal characteristics indicate that disintegration needs to be accounted for when landquakes are used to investigate the characteristics of a rockfall. Moreover, the frequency of landquakes decreases with a decrease in the vertical velocity and increase in the horizontal velocity during granular mass collapse. Plain Language Summary: The movement of a rockfall exerts forces on the ground over which it travels and generates outward‐propagating seismic signals. These signals are considered valuable sources of rockfall information. However, the interpretation of the generated seismic signals may be challenged by the various effects of the properties of rockfall masses. Here, we conducted a series of controlled laboratory experiments on the impacts of free‐falling granular masses on a flat plate with several different experimental conditions and seismic measurements. The power laws established allow us to further characterize the different effects of rockfall properties, including the rockfall volume, initial disintegration, and impact area, on the resulting seismic signatures. An important result is the dependency of the seismic signal frequency on the granular mass properties, which can be used to improve the inferences of rockfall characteristics from seismic signals. Key Points: Particle size is a dominant factor affecting landquakes, with the most significant effect on the mean frequencyThe maximum seismic amplitude is mainly governed by the particle size and impact area rather than the numbers of layers of falling massesLinks between rockfall dynamics and landquakes are studied, revealing a positive trend between the vertical velocity and seismic frequency [ABSTRACT FROM AUTHOR]

We propose a new approach capable of measuring local seismic anisotropy from 6C (three‐component translation and three‐component rotation) amplitude observations of ambient seismic noise data. Our recent theory demonstrates that the amplitude ratio of 6C cross‐correlation functions (CCFs) enables retrieving the local phase velocity. This differs from conventional velocity extraction methods based on the travel time. Its local sensitivity kernel beneath the 6C seismometer allows us to study anisotropy from azimuth‐dependent CCFs, avoiding path effects. Such point measurements have great potential in planetary exploration, ocean bottom observations, or volcanology. We apply this approach to a small seismic array at Pin˜ $\widetilde{n}$on Flat Observatory (PFO) in southern California, array‐deriving retrieves rotational ground motions from microseismic noise data. The stress‐induced anisotropy is well resolved and compatible with other tomography results, providing constraints on the origin of depth‐dependent seismic anisotropy. Plain Language Summary: In contrast to the well‐known translational displacement of seismic waves, rotation measures the angle of rotation of a point in a medium as it deforms. Traditionally, the travel time of cross‐correlation functions of translational displacement of ambient seismic noise and its corresponding propagation distance is used to calculate the velocity of the subsurface medium. However, our recent theory shows that the amplitude ratio of translational displacement and rotation allows us to reveal the local velocity of the medium beneath the receiver from ambient seismic noise data and to study anisotropy from earthquake events. This paper points out for the first time that the amplitude observations of 6C cross‐correlation functions of ambient seismic noise provide a new approach to measure the azimuth‐dependent velocity in anisotropic media. Key Points: We show a new approach enabling the extraction of local phase velocity from 6C cross‐correlation functions of ambient noise dataAzimuth‐dependent 6C cross‐correlation functions allow us to study local seismic anisotropy and its depth dependenceLocal anisotropy at Pin˜ $\widetilde{n}$on Flat Observatory is compatible with compression stress, providing constraints on stress‐induced anisotropy [ABSTRACT FROM AUTHOR]

Volcanic tremor is a semi‐continuous seismic and/or acoustic signal that occurs at time scales ranging from seconds to years, with variable amplitudes and spectral features. Tremor sources have often been related to fluid movement and degassing processes, and are recognized as a potential geophysical precursor and co‐eruptive geophysical signal. Eruption forecasting and monitoring efforts need a fast, robust method to automatically detect, characterize, and catalog volcanic tremor. Here we develop VOlcano Infrasound and Seismic Spectrogram Network (VOISS‐Net), a pair of convolutional neural networks (one for seismic, one for acoustic) that can detect tremor in near real‐time and classify it according to its spectral signature. Specifically, we construct an extensive data set of labeled seismic and low‐frequency acoustic (infrasound) spectrograms from the 2021–2022 eruption of Pavlof Volcano, Alaska, and use it to train VOISS‐Net to differentiate between different tremor types, explosions, earthquakes and noise. We use VOISS‐Net to classify continuous data from past Pavlof Volcano eruptions (2007, 2013, 2014, 2016, and 2021–2022). VOISS‐Net achieves an 81.2% and 90.0% accuracy on the seismic and infrasound test sets respectively, and successfully characterizes tremor sequences for each eruption. By comparing the derived seismoacoustic timelines of each eruption with the corresponding eruption chronologies compiled by the Alaska Volcano Observatory, our model identifies changes in tremor regimes that coincide with observed volcanic activity. VOISS‐Net can aid tremor‐related monitoring and research by making consistent tremor catalogs more accessible. Plain Language Summary: Volcanic tremor is a persistent vibration of the ground, atmosphere, or both that can occur before and during volcanic eruptions. Despite its importance in volcano monitoring and eruption forecasting, volcano observatories do not have a reliable way of automatically detecting and identifying tremor due to the variable intensities and frequencies at which it occurs. In order to accomplish this, we develop and test a pair of machine learning models that classify spectrograms (i.e., images representing a signal's frequency content over time) from seismic and low‐frequency acoustic data. The models are trained on manually labeled images derived from the recent 2021–2022 eruption of Pavlof Volcano, Alaska, which demonstrated substantial signal diversity (e.g., different tremor types, earthquakes, explosions and noise). Our models achieve 81.2% and 90.0% accuracy on the seismic and low‐frequency acoustic test sets respectively, and perform well when applied to data recorded from past Pavlof Volcano eruptions. In addition, transitions in tremor sequences identified from our analysis generally coincide with shifts in eruptive patterns from Pavlof Volcano. Our tools can help volcano observatories systematically monitor tremor, and advance tremor research by making catalogs of their occurrences more consistent and accessible. Key Points: We develop a pair of convolutional neural networks that detect and classify volcano seismic and acoustic signalsWe apply our models to Pavlof Volcano eruptions (2007, 2013, 2014, 2016, and 2021–2022) and derive volcano seismoacoustic timelinesThe seismoacoustic timelines reveal shifts in unrest regimes linked to explosions and effusive activity [ABSTRACT FROM AUTHOR]

A moderate size earthquake (ML 5.9) occurred near Mechuka in Siang Valley on 23rd April 2019 at 20:15:50.00 UTC in the rupture zone of the great Assam earthquake of 15th August 1950. This shallow-focused earthquake occurred close to NE trending Main Central Thrust has a thrust fault mechanism striking parallel to this major tectonics of the Himalaya. Including this event, a dataset of 46 local, micro and low magnitude earthquakes (1.5 ≤ ML ≤ 5.9) of the Siang Valley of Arunachal Pradesh, NE India, are evaluated for the characteristics of the earthquake source, seismicity, stress drop and tectonic structures. The earthquake events were recorded by a close digital network of eight broadband seismic stations installed in December 2018 in the Siang Valley. The stress drop (Δσ) obtained for the Mechuka earthquake in the Siang Valley is 114.71 bars, the seismic moment (Mo) of 6.86E + 16 Nm and the source radius (r) of 1392.41 m. The seismic moments obtained from the microearthquakes vary between 2.44E + 11 and 6.86E + 16 Nm, and the source radius varies from 172.90 to 1392.41 m based on Brune's circular source model 1970. The corner frequencies (fc) of the events range from 1.03 to 8.04 Hz. Seismically intense clustering of microearthquakes has a low-stress drop below 10 bar except for the Mechuka earthquake, which varies from 0.05 to 114.72. This high-stress drop of moderate earthquake compared to its low values of lower magnitude earthquakes indicates that the earthquake of 23rd April 2019 is associated with high-strength material accumulating high strain during the earthquake-building process. Moment magnitude of whole data set ranges from 1.52 to 5.16. The scaling relationship between Mo and fc is obtained as 9.089E + 16fc–9.52, and for Mo and Δσ are modelled to be related as Mo = 8.96E + 10Δσ2.86. Evaluated low-stress drop for the smaller size of earthquakes shows brittleness of the upper crustal region. Research highlights: Modelling of earthquake source parameters to obtain scaling relations for Siang Valley, Arunachal Pradesh. The estimated Mo, fc, r, ∆σ and Mw are 2.44 × 1011 to 6.86 × 1016 Nm, 1.03–8.04 Hz, 172.90–1392.41 m, 0.05–114.71 bar and 1.52–5.16, respectively. Scaling relation between Mo and fc, Mo and Δσ and Mw and fc is Mo = 9.089E + 16fc−9.52, Mo = 8.96E + 10 Δσ2.86 and Log fc = –0.39Mw + 2.39, respectively. [ABSTRACT FROM AUTHOR]

Tsunami inundation maps are crucial for understanding the impact of tsunamis and planning mitigation measures. Our research focuses on creating a database of stochastic tsunami scenarios along the Chilean subduction zone and probabilistic inundation maps for 11 coastal cities. We divided the Chile-Perú subduction zone into four seismic segments based on historical seismicity. Stochastic rupture scenarios, ranging from 8.0 to 9.6 magnitudes, were generated using the Karhunen-Loeve expansion. The Stochastic Reduced Order Model (SROM) helped select representative tsunami scenarios for each segment and magnitude bin. We then used the NEOWAVE model to simulate these scenarios to an inundation level, creating probabilistic tsunami maps for various return periods. Our findings reveal that local geography significantly influences tsunami inundation, with some areas facing high inundation risks while others experience minimal impacts. As a result, a uniform planning and design criterion across the entire country is not advisable; site-specific studies are necessary. These probabilistic scenarios can provide tailored solutions for different Chilean coastal cities, enhancing their resilience. Additionally, this research marks the first comprehensive probabilistic tsunami hazard analysis for the Chilean coast, considering multiple seismic sources, marking a crucial step toward full tsunami risk assessment for coastal communities. [ABSTRACT FROM AUTHOR]

Lake Tennyson's basin and sedimentary record were assessed for multiple branches of paleoenvironmental research. A geophysical investigation of the lake depositional setting was supported by physical and chemical analysis of multiple sediment cores. The basin reaches 60 m depth and several subaerial landforms identified were likely deposited from seismic activity. Deformed sediments and turbidites in gravity cores are attributed to one or more large (pre)historic earthquakes, and episodic grain size changes in sediment cores are related to either seismic and/or climatic/meteorological processes. A hummocky slump deposit located on the western lake bed margin is linked to widespread sediment deformation that was likely caused by movement of the Awatere Fault. Radiocarbon dating constrains the most recent major deformation event between the late thirteenth century CE and mid-twentieth Century. Folded turbidites in the stratigraphy suggest multiple sedimentary deformation episodes occurred during the last millennium. Micro-XRF elemental signatures suggest proxies of past water column mixing (windiness proxy) can be reconstructed via identification of changing redox conditions. Records from Lake Tennyson show potential for several palaeoenvironmental research applications, and combining them with basin morphostratigraphy and land-based palaeoclimate and palaeoseismic evidence can enhance understanding of environmental change in the northern South Island. [ABSTRACT FROM AUTHOR]

Induced earthquakes tend to be shallow, while tectonic events often occur in deeper parts of the Earth. A well-estimated hypocenter with uncertainties may help to evaluate whether an event is of an induced or tectonic origin. In this study, we focus on the development of a hypocenter method that helps to better define the source location of an earthquake and reduce the spatial error of the measurement. The hypocenter and the uncertainty is obtained by using the P- and S-wave phase time difference for a station and the P-wave traveltime differences between pairs of stations simultaneously in the hypocenter analysis. The uncertainty inherent to an imperfect reference velocity model, modelling, instrumental inaccuracy and phase time picking is propagated into the spacial hypocenter solution. A refined hypocenter methodology is successfully tested in a synthetic experiment with shallow (∼ 5 km), intermediate (∼ 10 km) and deep source points (∼ 15 km). The synthetic experiment indeed shows that it is possible to separate earthquakes by their depth solution, hence offering an indication that the event is either induced or tectonic. Case studies are presented of estimations of hypocenters and error ellipses for (1) induced seismicity at sites for gas storage in salt domes, geothermal production and gas extraction as well as (2) tectonic events. [ABSTRACT FROM AUTHOR]

The earthquake swarm in Palghar, western India, is a natural laboratory to understand long-duration seismogenesis. However, it requires an accurate estimation of the key parameters, such as earthquake locations and the velocity model of the region. Two separate studies performed by two organisations in India, the CSIR-National Geophysical Research Institute (NGRI) and the National Center for Seismology (NCS) reported significantly differing results from the first 8–12 months of earthquake monitoring in Palghar. CSIR-NGRI reported the depth of earthquakes in the range of 4–15 km, whereas NCS reported a much shallower depth down to 7 km only. We investigate the reason for this depth discrepancy by analysing datasets from the networks of both these organisations. We find that no available velocity models are able to reconcile the depth difference between the two datasets. Selection criteria generally adopted to determine a good velocity model, such as travel-time residuals and location errors, also failed to identify a suitable velocity model. Several synthetic tests helped us to identify that the network geometry and source-station separation are contributing factors to the observed depth difference. However, the main reason for the discrepancy was found to be inconsistent velocity models used for location at both networks. The main objective of our work is to estimate a velocity model which minimises the depth discrepancy between both datasets. In this regard, we synthetically generated hundreds of models with different VP and VP/VS ratios, followed by a grid search of VP and VP/VS ratios, which harmonises the location from the two datasets. The best model consists of a VP lying between 5.55–5.85 km/s and a VP/VS ratio between 1.77–1.81. For this velocity model, there is a greater overlap of the hypocentral clouds from both networks, and about 80% of the earthquakes of the CSIR-NGRI network were located within depths ≤ 7 km. We find that location errors vary seasonally, which is also related to the VP/VS of the crust. Since earlier studies have also found a strong correspondence between rainfall and seismicity in this region, we infer that the high VP/VS ratio might have been caused by the rainfall saturation of cracks in the Palghar seismicity zone. [ABSTRACT FROM AUTHOR]

Declustering of earthquake catalogs, that is determining dependent and independent events in an earthquake sequence, is a common feature of many seismological studies. While many different declustering algorithms exist, each has different performance and sensitivity characteristics. Here, we conduct a comparative analysis of the five most commonly used declustering algorithms: Garnder and Knopoff (1974), Uhrhammer (1986), Reasenberg (J Geophys Res: Solid Earth 90(B7):5479–5495, 1985), Zhuang et al. (J Am Stat Assoc 97(458):369–380, 2002), and Zaliapin et al. (Phys Rev Lett 101(1):4–7, 2008) in four different tectonic settings. Overall, we find that the Zaliapin et al. (Phys Rev Lett 101(1):4–7, 2008) algorithm effectively removes aftershock sequences, while simultaneously retaining the most information (i.e. the most events) in the output catalog and only slightly modifying statistical characteristics (i.e. the Gutenberg Richter b-value). Both Gardner and Knopoff (1974) and Zhuang et al. (J Am Stat Assoc 97(458):369–380, 2002) also effectively remove aftershock sequences, though they remove significantly more events than the other algorithms. Uhrhammer (1986) also effectively removes aftershock sequences and removes fewer events than Gardner and Knopoff (1974) or Zhuang et al. (J Am Stat Assoc 97(458):369–380, 2002), except when large magnitude events are present. By contrast, Reasenberg (J Geophys Res: Solid Earth 90(B7):5479–5495, 1985) only effectively removed aftershocks in one of the test regions. [ABSTRACT FROM AUTHOR]

The Alborz Mountains are among the areas exhibiting high tectonic and seismic activity in northern Iran. Studying key parameters of tectonic structures, including quantitative analysis and observational studies, in such active regions is essential to identify potential active faults and assess the consequent seismic hazards. This study focuses on seismicity and seismotectonics by analyzing seismic parameters, including b-value, mean seismic activity rate, earthquake recurrence time, seismic moment, and fractal dimension derived from micro and teleseismic data. The b-values vary between 0.6 and 1.1 in the tectonically active parts of the study area, corresponding with the reverse/thrust and strike-slip active faults. Large earthquakes might be prone to occur at 10–25 km depth because both catalogues show low b-values (b < 1.0) concentrations at this depth range. The high fractal dimension (> 1.5), high seismic activity rate, large seismic moment parameters, and its continuously increasing trend. Short recurrence periods (20–50 years) of M 6.5 events also emphasize the high seismic activity and high seismic hazard. On the other hand, the prevalence of low b-values is notably observed in areas encompassing densely populated cities such as Rasht, Lahijan, Amol, Babol, Sari, Behshahr, Gorgan, and the megacity of Tehran. Furthermore, we have identified asperities where the Gorgan Plain, the Khazar, and the Alamutrud Fault Zones are located. These findings emphasize the seismic hazard potential in the identified areas and urban centers within the study area. Therefore, particular attention should be directed towards areas exhibiting low b-values when assessing and mitigating seismic hazards. It underscores the necessity for additional focus on seismic hazard assessment and implementation of mitigation strategies in the Alborz region. [ABSTRACT FROM AUTHOR]

We studied the earthquake detection capacity of DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis) operating in the Nankai Trough, a target region monitored for future megathrust earthquakes. The focus of this paper was to evaluate the impact on this capacity from the malfunction of parts of the network. For this purpose, the completeness magnitude, above which all earthquakes are considered to be detected by a seismic network, was used. Then, a catalog that includes events observed by DONET was used. We found spatiotemporal variability of completeness magnitude, ranging from values below 1 in one of the areas where stations are densely deployed to values above 2 at the periphery and outside of the DONET area. We conducted a simulation computation for cases of malfunction of densely distributed stations. The results showed that completeness estimates in the area near the malfunctioning stations were about 1 magnitude larger. This implies that malfunction repair and/or replacement with new stations would be desirable because they pronouncedly affect earthquake monitoring. We then demonstrated an example of how to use the information of completeness magnitude as prior knowledge to compute the b value of the Gutenberg-Richter distribution. The result indicates the b value as a proxy that can help to image stress heterogeneity when there is a magnitude-6 class slow slip event on the Nankai Trough plate boundary. [ABSTRACT FROM AUTHOR]

The precise determination of earthquake location is the fundamental basis in seismological community, and is crucial for analyzing seismic activity and performing seismic tomography. First arrivals are generally used to practically determine earthquake locations. However, first-arrival traveltimes are not sensitive to focal depths. Moreover, they cannot accurately constrain focal depths. To improve the accuracy, researchers have analyzed the depth phases of earthquake locations. The traveltimes of depth phases are sensitive to focal depths, and the joint inversion of depth phases and direct phases can be implemented to potentially obtain accurate earthquake locations. Generally, researchers can determine earthquake locations in layered models. Because layered models can only represent the first-order feature of subsurface structures, the advantages of joint inversion are not fully explored if layered models are used. To resolve the issue of current joint inversions, we use the traveltimes of three seismic phases to determine earthquake locations in heterogeneous models. The three seismic phases used in this study are the first P-, sPg- and PmP-waves. We calculate the traveltimes of the three seismic phases by solving an eikonal equation with an upwind difference scheme and use the traveltimes to determine earthquake locations. To verify the accuracy of the earthquake location method by the inversion of three seismic phases, we take the 2021 MS6.4 Yangbi, Yunnan earthquake as an example and locate this earthquake using synthetic and real seismic data. Numerical tests demonstrate that the eikonal equation-based earthquake location method, which involves the inversion of multiple phase arrivals, can effectively improve earthquake location accuracy. [ABSTRACT FROM AUTHOR]

Fast Radio Bursts (FRBs) are energetic phenomena that have significant implications for understanding fundamental physics and the Universe. Recent observations of FRB 121102, FRB 20220912A, and FRB 20201124A by the Five-hundred-meter Aperture Spherical Telescope showed high-burst rates and distinctive energy distribution and temporal properties. In this study, we examine these observations to investigate the scale invariance of the waiting times between bursts for intervals longer than approximately 1 s. Our analysis revealed a unified scaling law for these longer intervals, which is similar to the behaviour of solar flares. This discovery inspires us to suggest a dual analogy of the FRB scenario across the entire time intervals: with earthquake dynamics at subsecond scales and with solar flare dynamics beyond the one-second threshold. This threshold potentially aligns with the dynamic time-scale of neutron star crusts, offering insight of the occurrence of FRBs into the internal processes of neutron stars. [ABSTRACT FROM AUTHOR]

Sparse representation and inversion have been widely used in the acquisition and processing of geophysical data. In particular, the low-rank representation of seismic signals shows that they can be determined by a few elementary modes with predominantly large singular values. We review global and local low-rank representation for seismic reflectivity models and then apply it to least-squares migration (LSM) in acoustic and viscoacoustic media. In the global singular value decomposition (SVD), the elementary modes determined by singular vectors represent horizontal and vertical stratigraphic segments sorted from low to high wavenumbers, and the corresponding singular values reflect the contribution of these basic modes to form a broadband reflectivity model. In contrast, local SVD for grouped patch matrices can capture nonlocal similarity and thus accurately represent the reflectivity model with fewer ranks than the global SVD method. Taking advantage of this favorable sparsity, we introduce a local low-rank regularization into LSM to estimate subsurface reflectivity models. A two-step algorithm is developed to solve this low-rank constrained inverse problem: the first step is for least-squares data fitting and the second is for weighted nuclear-norm minimization. Numerical experiments for synthetic and field data demonstrate that the low-rank constraint outperforms conventional shaping and total-variation regularizations, and can produce high-quality reflectivity images for complicated structures and low signal-to-noise data. [ABSTRACT FROM AUTHOR]

The presence of coherent noise in seismic data leads to errors and uncertainties, and as such it is paramount to suppress noise as early and efficiently as possible. Self‐supervised denoising circumvents the common requirement of deep learning procedures of having noisy‐clean training pairs. However, self‐supervised coherent noise suppression methods require extensive knowledge of the noise statistics. We propose the use of explainable artificial intelligence approaches to 'see inside the black box' that is the denoising network and use the gained knowledge to replace the need for any prior knowledge of the noise itself. This is achieved in practice by leveraging bias‐free networks and the direct linear link between input and output provided by the associated Jacobian matrix; we show that a simple averaging of the Jacobian contributions over a number of randomly selected input pixels provides an indication of the most effective mask to suppress noise present in the data. The proposed method, therefore, becomes a fully automated denoising procedure requiring no clean training labels or prior knowledge. Realistic synthetic examples with noise signals of varying complexities, ranging from simple time‐correlated noise to complex pseudo‐rig noise propagating at the velocity of the ocean, are used to validate the proposed approach. Its automated nature is highlighted further by an application to two field data sets. Without any substantial pre‐processing or any knowledge of the acquisition environment, the automatically identified blind masks are shown to perform well in suppressing both trace‐wise noise in common shot gathers from the Volve marine data set and coloured noise in post‐stack seismic images from a land seismic survey. [ABSTRACT FROM AUTHOR]

Improving the resolution of seismic migration images plays an important role for geophysical interpreters to characterize underground reservoirs. However, the classical image domain least‐squares migration method based on the local‐stationary assumption cannot obtain a satisfactory high‐resolution seismic image due to the significant spatial variant characteristics of the point spread function. To mitigate this problem, we proposed a high‐resolution point spread function deconvolution method and applied it to two‐dimensional cases. Nevertheless, extending the two‐dimensional method to three‐dimensional problems directly would fail due to the intrinsic complexity in three‐dimensional cases. In this study, we resolve the differences encountered in the point spread function deconvolution method for two‐ and three‐dimensional cases and provide specific strategies for achieving high‐resolution imaging with low computational cost when extending the point spread function deconvolution method to three‐dimensional cases. The main schemes include (1) incorporating the analytical expression of the point spread function to guide the generation of three‐dimensional point spread function distributions, (2) extending the point spread function filter calculation method from a two‐dimensional square to a three‐dimensional rectangular prism and (3) interpolating to obtain more compact point spread functions for reducing migration artefacts. Results from the three‐dimensional synthetic Overthrust model and field data set demonstrate that our techniques could effectively enhance the spatial resolution of the migration images with reduced migration artefacts. With these specific strategies, the space‐variant point spread function deconvolution algorithm shows superior performance on three‐dimensional cases at a much lower computational cost compared with the classical least‐squares migration method and the local‐stationary deblurring method. Synthetic tests and real data applications confirm that the space‐variant point spread function deconvolution method has distinct advantages over both two‐ and three‐dimensional problems and can be widely adopted in practice. [ABSTRACT FROM AUTHOR]

The Mongolian Plateau in Central Asia is an intracontinental tectonic system far from active plate boundaries. Despite its distance from these boundaries, the plateau is characterized by intense crustal deformation accompanied by voluminous Cenozoic volcanism and active modern seismicity. However, the intraplate deformation mechanism has long been debated owing to the scarcity of observations and contradictions between different results. In recent years, growing geophysical studies have been conducted on the Mongolian Plateau, providing constraints on its lithospheric structure and dynamics. Here, we review the geophysical research on the Mongolian Plateau over the last decade, including seismological, geodetic, gravity, magnetotelluric, and geodynamic aspects. This review aims to (a) describe crustal and mantle structures based on multiscale seismic images; (b) describe deformation patterns based on seismic anisotropy, focal mechanisms, and global positioning system (GPS) observations; and (c) discuss the mechanisms behind intraplate deformation, volcanism, and seismic activity across the Mongolian Plateau. Seismic images show that the crustal structure of the plateau has significant east-west differences. Several blocks in the western Mongolian Plateau have thick crusts, including the Altai Mountains, Hovsgol Rift, and Hangay Dome. The lithospheric deformation across the Mongolian Plateau has strong lateral variation, with NE-SW shortening in the Altai Mountains and W-E or NW-SE shear deformation in the Hangay Dome region and the eastern part. The varied deformation may result from the superposition of multiple mechanisms, including far-field stress in the Altai Mountains, mantle upwelling, and mantle flow in the Hangay Dome region. However, it is difficult to identify the geodynamics of the formation of the entire Mongolian Plateau because the deformation is too complicated, and the present models are not sufficient and are always partial. Overall, this review encompasses recent advances in seismic observations of the Mongolian Plateau, illuminates the heterogeneities in the crust and mantle structure and deformation of the plateau, and discusses the mechanisms behind the deformation, magmatism, and seismicity. [ABSTRACT FROM AUTHOR]

The pseudo-differential operator with symbol | k |α has been widely used in seismic modelling and imaging when involving attenuation, anisotropy and one-way wave equation, which is usually calculated using the pseudo-spectral method. For large-scale problems, applying high-dimensional Fourier transforms to solve the wave equation that includes pseudo-differential operators is much more expensive than finite-difference approaches, and it is not suitable for parallel computing with domain decomposition. To mitigate this difficulty, we present a truncated space-domain convolution method to efficiently compute the pseudo-differential operator |$\sqrt{-\nabla ^2}$|⁠ , and then apply it to viscoacoustic reverse-time migration. Although |$\sqrt{-\nabla ^2}$| is theoretically non-local in the space domain, we take the limited frequency band of seismic data into account, and constrain the approximated convolution stencil to a finite length. The convolution coefficients are computed by solving a least-squares inverse problem in the wavenumber domain. In addition, we exploit the symmetry of the resulting convolution stencil and develop a fast spatial convolution algorithm. The applications of the proposed method in Q -compensated reverse-time migration demonstrate that it is a good alternative to the pseudo-spectral method for computing the pseudo-differential operator |$\sqrt{-\nabla ^2}$|⁠ , with almost the same accuracy but much higher efficiency. [ABSTRACT FROM AUTHOR]

Seismic traveltime anomalies of waves that traverse the uppermost 100–200 km of the outer core have been interpreted as evidence of reduced seismic velocities (relative to radial reference models) just below the core–mantle boundary (CMB). These studies typically investigate differential traveltimes of SmKS waves, which propagate as P waves through the shallowest outer core and reflect from the underside of the CMB m times. The use of SmKS and S(m-1)KS differential traveltimes for core imaging are often assumed to suppress contributions from earthquake location errors and unknown and unmodelled seismic velocity heterogeneity in the mantle. The goal of this study is to understand the extent to which differential SmKS traveltimes are, in fact, affected by anomalous mantle structure, potentially including both velocity heterogeneity and anisotropy. Velocity variations affect not only a wave's traveltime, but also the path of a wave, which can be observed in deviations of the wave's incoming direction. Since radial velocity variations in the outer core will only minimally affect the wave path, in contrast to other potential effects, measuring the incoming direction of SmKS waves provides an additional diagnostic as to the origin of traveltime anomalies. Here we use arrays of seismometers to measure traveltime and direction anomalies of SmKS waves that sample the uppermost outer core. We form subarrays of EarthScope's regional Transportable Array stations, thus measuring local variations in traveltime and direction. We observe systematic lateral variations in both traveltime and incoming wave direction, which cannot be explained by changes to the radial seismic velocity profile of the outer core. Moreover, we find a correlation between incoming wave direction and traveltime anomaly, suggesting that observed traveltime anomalies may be caused, at least in part, by changes to the wave path and not solely by perturbations in outer core velocity. Modelling of 1-D ray and 3-D wave propagation in global 3-D tomographic models of mantle velocity anomalies match the trend of the observed traveltime anomalies. Overall, we demonstrate that observed SmKS traveltime anomalies may have a significant contribution from 3-D mantle structure, and not solely from outer core structure. [ABSTRACT FROM AUTHOR]

The northern Chile Triple Junction (CTJ) is characterized by the ongoing subduction of the Nazca plate beneath the South American plate. The geological structures within the subduction zone undergo complex changes, resulting in significant tectonic activities and intense seismicity along the western margin of South America. Based on the Gravity Recovery and Climate Experiment (GRACE) data and earthquake catalogues, this study selects the northern CTJ area (25° S–40° S, 75° W–65° W) as the research object, adopts the mathematical methods of independent component analysis (ICA) and principal component analysis (PCA) to separate the earthquake-related signals within the GRACE data, and fits the changes of seismic b -values through the frequency–magnitude relationship. The characteristics of gravity changes before and after seismic events, the seismic activity parameter b -values, and the relationship between the gravity signals and b -values are discussed. The results show that mathematical methods can effectively extract seismic-related gravity components from the GRACE data. ICA, compared to PCA, provides better results in capturing the temporal variations associated with b -value time-series, which exhibit good consistency in long-term trend changes. The average change of b -values in the study area is 0.66 ± 0.003, fluctuating over time. Generally, prior to larger seismic events, b -values tend to decrease. Along the western margin of South America, b -values are low; this aligns with the active tectonic activities between subducting plates. Additionally, a certain correlation between b -values and gravity changes is observed, but due to the influence of tectonic activities, the correspondence between b -values and gravity anomalies may not be consistent across different areas. The b -value is highly consistent with the strain rate model. Low b -values correspond to high strain rates along the western edge of South America, which is in line with the tectonic characteristics of frequent seismic activity in this area. A gradual concentration of gravity anomalies before major earthquakes is observed, accompanied by the gradual accumulation of smaller seismic events. Meanwhile, several months before the two major earthquakes, the spatial distribution of gravity appears to be similar to the coseismic signals, but the nature of its generation remains to be explored. These methods and results not only add to the applications of GRACE in seismic studies but also raise questions for further exploration. [ABSTRACT FROM AUTHOR]

The seismic b -value in Gutenberg–Richter law is an important parameter in earthquake science research and earthquake risk assessment. People have tried to use b -values for short-term earthquake forecasts, and this requires the premise of estimating reliable b -values for real-time seismic catalogues. However, estimating b -values for real-time catalogues, which are usually of poor qualities, is usually faced with many difficulties and problems. In this study, through a series of numerical tests, we investigate the performance of three methods, including the commonly used maximum-likelihood estimation method and two relatively new b -value estimation methods, namely the b -positive and K–M slope methods, on calculating b -values for real-time seismic catalogues. We also apply these three methods to both observed seismic catalogues (the seismic sequence in the Costa Marchigiana, Italy, and a high-resolution early aftershock sequence of the 2023 two M w ∼7.8 earthquakes in Türkiye) and synthetic real-time seismic catalogues. The results in this study show that it seems difficult to obtain accurate b -values for real-time earthquake catalogues by each of these three methods, but the combination of these methods may give a better judgment—if all three methods suggest that the change in b -value is significant, the probability of making a correct decision is very high. Facing the uncertainty of b -value estimation that always exists, we advocate exploring the effectiveness of standard b -value estimation strategies in practical applications. [ABSTRACT FROM AUTHOR]

Full-waveform inversion (FWI) has emerged as the state-of-the art high resolution seismic imaging technique, both in seismology for global and regional scale imaging and in the industry for exploration purposes. While gaining in popularity, FWI, at an operational level, remains a heavy computational process involving the repeated solution of large-scale 3-D wave propagation problems. For this reason it is a common practice to focus the interpretation of the results on the final estimated model. This is forgetting FWI is an ill-posed inverse problem in a high dimensional space for which the solution is intrinsically non-unique. This is the reason why being able to qualify and quantify the uncertainty attached to a model estimated by FWI is key. To this end, we propose to extend at an operational level the concepts introduced in a previous study related to the coupling between ensemble Kalman filters (EnKFs) and FWI. These concepts had been developed for 2-D frequency-domain FWI. We extend it here to the case of 3-D time-domain FWI, relying on a source subsampling strategy to assimilate progressively the data within the Kalman filter. We apply our strategy to an ocean bottom cable field data set from the North Sea to illustrate its feasibility. We explore the convergence of the filter in terms of number of elements, and extract variance and covariance information showing which part of the model are well constrained and which are not. Analysing the variance helps to gain insight on how well the final estimated model is constrained by the whole FWI workflow. The variance maps appears as the superposition of a smooth trend related to the geometrical spreading and a high resolution trend related to reflectors. Mapping lines of the covariance (or correlation matrix) to the model space helps to gain insight on the local resolution. Through a wave propagation analysis, we are also able to relate variance peaks in the model space to variance peaks in the data space. Compared to other posterior-covariance approximation scheme, our combination between EnKF and FWI is intrinsically scalable, making it a good candidate for exploiting the recent exascale high performance computing machines. [ABSTRACT FROM AUTHOR]

A Bayesian linear regression to determine the bias in the Nafe–Drake relationship between compressional velocity and density provides an improved model for the density structure of Kīlauea volcano, Hawaiʻi. In previous work, we combined the results of seismic tomography with the Nafe–Drake relationship between compressional velocity and density to explain the large values of gravity disturbances overlying the summits and rift zones of the island's volcanoes. These results were used to determine mechanisms for gravitational instability of the island flanks. Here, we use laboratory measurements of the relationship of velocity and density for a wide range of Hawaiʻi island rocks as a prior in a Bayesian regression, with seismic tomography, to refine the 3-D density structure for Kīlauea volcano. This refined structure shows dense bodies (3220 kg m−3) between 5 and 8 km below sea level that underly regions of magma storage, found from geodetic and geophysical studies, beneath the summit and East Rift Zone of Kīlauea volcano. Above these bodies, density isosurfaces surround and cradle sources of pressure change determined from geodetic models, both at the summit and along the East Rift Zone. Continued subsidence of the summit following the 2018 eruption is aligned with a bowl-shaped density structure, formed primarily by density isosurfaces between 2800 and 2900 kg m−3 at 4–6 km depth. These surfaces underly the ∼3 km depth at which dyke injection initiates, are largely aseismic, and from their density values are inferred to contain high concentrations of olivine. Taken together, these density structures are consistent with an olivine-rich mush with variable porosity that increases in density with depth and provides a mechanism to form olivine cumulates both at the summit and along the rift zones. This structural framework for Kīlauea volcano is consistent with melt and mush transport occurring over a large range of depths to accommodate the growth and spreading of the volcano. [ABSTRACT FROM AUTHOR]

Material density remains poorly constrained in seismic imaging problems, yet knowledge of density would provide important insight into physical material properties for the interpretation of subsurface structures. We test the sensitivity to subsurface density contrasts of spatial and temporal gradients of seismic ambient noise wavefields, using wave equation inversion (WEI), a form of seismic gradiometry. Synthetic results for 3-D acoustic media suggest that it is possible to estimate relative density structure with WEI by using a full acoustic formulation for wave propagation and gradiometry. We show that imposing a constant density assumption on the medium can be detrimental to subsurface seismic velocity images. By contrast, the full acoustic formulation allows us to estimate density as an additional material parameter, as well as to improve phase velocity estimates. In 3-D elastic media, severe approximations in the governing wave physics are necessary in order to invert for density using only an array of receivers on the Earth's free surface. It is then not straightforward to isolate the comparatively weak density signal from the influence of phase velocity using gradiometric WEI. However, by using receivers both at the surface and in the shallow subsurface we show that it is possible to estimate density using fully elastic volumetric WEI. [ABSTRACT FROM AUTHOR]

Equations of motion are derived for (visco)elastic, self-gravitating and variably rotating planets. The equations are written using a decomposition of the elastic motion that separates the body's elastic deformation from its net translational and rotational motion as far as possible. This separation is achieved by introducing degrees of freedom that represent the body's rigid motions; it is made precise by imposing constraints that are physically motivated and that should be practically useful. In essence, a Tisserand frame is introduced exactly into the equations of solid mechanics. The necessary concepts are first introduced in the context of a solid body, motivated by symmetries and conservation laws, and the corresponding equations of motion are derived. Next, it is shown how those ideas and equations of motion can readily be extended to describe a layered fluid–solid body. A possibly new conservation law concerning inviscid fluids is then stated. The equilibria and linearization of the fluid–solid equations of motion are discussed thereafter, along with new equations for use within normal-mode coupling calculations and other Galerkin methods. Finally, the extension of these ideas to the description of multiple, interacting fluid–solid planets is qualitatively discussed. [ABSTRACT FROM AUTHOR]

Multicomponent seismic exploration offers advantages in precisely describing complicated geological structures compared with conventional PP-wave exploration. For multicomponent ocean bottom node (OBN) seismic data, due to the complication of the seismic-wave propagation path and the diffuse scattering of wavefield energy in rugged seabeds, the reflections show distortion and crossover characteristics, resulting in the distortion of underlying strata in migration sections. Furthermore, the common midpoint stacking approach based on the uniform datum is no longer applicable because of the large elevation differences between sources and receivers. In this study, we propose a PP- and PS-waves prestack time migration (PSTM) method for OBN acquisition with rugged seabeds. The proposed method can significantly improve the imaging quality by correcting the downward P-wave traveltime in seawater and adopting an optimized migration aperture from the formation dip-angle model. Moreover, we develop feasible strategies to establish formation dip-angle and PP- and PS-migration velocity models. The typical feature of the improvements we make on PSTM is that the non-hyperbolic moveouts of both PP- and PS-waves on common image gathers induced by the non-uniform elevations of shot and receiver points and the asymmetry ray path of PS-wave are corrected to be quasi-hyperbolic. Numerical tests and field data applications demonstrate that our method can effectively image underlying structures with rugged seabeds and has potential in multicomponent OBN exploration. [ABSTRACT FROM AUTHOR]

We present various geological models to examine the geothermal energy potential at the Rahat volcanic field, Saudi Arabia. The method involves the application of geophysical techniques of gravity and seismic tomography results to derive a 3D subsurface geological structure representing possible subsurface structural information beneath the northern Rahat volcanic field. Results show a secluded subsurface structure within the basement crust with low densities (< 2500 kg / m 3 ) and a slightly high P-wave velocity, enclosed by a high density (> 2800 kg / m 3) and low P-wave velocity structure. Identified secluded subsurface structure could depict a magma intrusion within the basement and its depth proximity to the basalt deposition in the region signals the viability of this location for geothermal exploration. Deductions also show that the geothermal reservoir could be reached at starting depths of ~ 1.3 km, whereas its effect could be accessed at depths < 1 km. Within the geothermal active region of northern Harrat Rahat, this study has suggested possible locations with high potential for further geothermal exploration. [ABSTRACT FROM AUTHOR]

We establish Lipschitz stability properties for a class of inverse problems. In that class, the associated direct problem is formulated by an integral operator A m depending nonlinearly on a parameter 푚 and operating on a function 푢. In the inversion step, both 푢 and 푚 are unknown, but we are only interested in recovering 푚. We discuss examples of such inverse problems for the elasticity equation with applications to seismology and for the inverse scattering problem in electromagnetic theory. Assuming a few injectivity and regularity properties for A m , we prove that the inverse problem with a finite number of data points is solvable and that the solution is Lipschitz stable in the data. We show a reconstruction example illustrating the use of neural networks. [ABSTRACT FROM AUTHOR]

Ground-motion models (GMMs) are frequently used in engineering seismology to estimate ground motion intensities. The majority of GMMs predict the response spectral ordinates (such as spectral acceleration) of a single-degree-of-freedom oscillator because of their common application in engineering design practices. Response spectra show how an idealized structure reacts to applied ground motion; however, they do not necessarily represent the physics of ground motion. The functional forms of the response spectra GMMs are built around ideas taken from the Fourier spectral concept. Assuming the validity of Fourier spectral concepts in the response spectral domain could cause physically inexplainable effects. In this study, using a mixed-effects regression technique, we introduce four models capable of predicting the Fourier amplitude spectrum that investigates the impact of incorporating random-effect event and station terms and variations in using a mixed-effects regression technique in one or two steps using truncated dataset or all data (nontruncated dataset). All data consists of 2581 three-component strong ground motion data resulting from 424 events with magnitude ranging from 4.0 up to 7.4, from 1976 to 2020, and 706 stations. The truncated dataset's records, events, and stations are reduced to 2071, 408, and 636, respectively. As part of this study, we develop GMMs to predict the Fourier amplitude spectrum for the Iranian plateau within the frequency range of 0.3–30 Hz. We adopted simple, functional forms for four models, and we included a limited number of predictors, namely Mw (moment magnitude), Rjb (Joyner–Boore distance), and VS30 (time-averaged shear-wave velocity in the top 30 m). Due to statistical analyses, the style-of-faulting term was excluded from the final functional forms. The robustness of the derived models is indicated by unbiased residual variation with predictor variables. [ABSTRACT FROM AUTHOR]

This paper introduces SEISMONOISY, an application designed for monitoring the spatiotemporal characteristic and variability of the seismic noise of an entire seismic network with a quasi-real-time monitoring approach. Actually, we have applied the developed system to monitor 12 seismic networks distributed throughout the Italian territory. These networks include the Rete Sismica Nazionale (RSN) as well as other regional networks with smaller coverage areas. Our noise monitoring system uses the methods of Spectral Power Density (PSD) and Probability Density Function (PDF) applied to 12 h long seismic traces in a 24 h cycle for each station, enabling the extrapolation of noise characteristics at seismic stations after a Seismic Noise Level Index (SNLI), which takes into account the global seismic noise model, is derived. The SNLI value can be used for different applications, including network performance evaluation, the identification of operational problems, site selection for new installations, and for scientific research applications (e.g., volcano monitoring, identification of active seismic sequences, etc.). Additionally, it aids in studying the main noise sources across different frequency bands and changes in the characteristics of background seismic noise over time. [ABSTRACT FROM AUTHOR]

We present the first high resolution global MHD with coupled inner magnetosphere simulation results of an observed theta aurora event. We use the Space Weather Modeling Framework in the Geospace configuration, which produces accurate field aligned current closure in the ionosphere that is integral to theta aurora formation. At the location of the observed theta aurora, the simulation produces a narrow channel of Joule heating along both open and closed field lines, and between a pair of oppositely directed field‐aligned current sheets in the ionosphere. We demonstrate that this Joule heating pattern that we identify as theta aurora maps to a reconnection region at the magnetotail flanks as well as in the distant magnetotail. The theta aurora maps to a cross‐tail current disruption and field‐aligned current source region in a highly twisted magnetotail. Plain Language Summary: The light of the aurora observed in the Earth's atmosphere is a signature of magnetic processes in outer space. The Sun produces hot magnetized plasma that blows through the solar system like a wind. The Earth's magnetic field shields us from the harmful impacts of the Sun's magnetized plasma wind. The interaction between the Earth's magnetic field and the Sun's plasma wind causes the ring of auroral light we see in the polar regions. Sometimes the ring of auroral light grows a bar of auroral light across its center (over the Earth's magnetic poles), transforming the ring into a structure like the Greek letter theta. We call this the theta aurora. It is unknown what physical processes in the Earth's magnetic field create the theta aurora. In this study we show the first realistic simulation that uses solar wind driver conditions of an observed theta aurora event, and demonstrate that we can successfully produce the theta aurora structure in the simulation. We find that the theta aurora's center bar of auroral light comes from multiple regions in the vast Earth's magnetotail. This new result helps us better understand the dynamics of the Earth's space environment and better protect ourselves against the Sun's hazardous plasma wind effects. Key Points: We show first global MHD simulation event study that successfully produces a Joule heating signature that we identify as theta auroraSimulation associates transpolar arc signatures with reconnection at the magnetospheric flank and in the distant magnetotailThe transpolar arc maps to a cross‐tail current disruption and field‐aligned current source region in a highly twisted magnetotail [ABSTRACT FROM AUTHOR]

The epidemic type aftershock sequence (ETAS) model is used as a baseline model both for earthquake clustering and earthquake prediction. In most forecast experiments, the ETAS parameters are estimated based on a short and local catalog, therefore the model parameter optimization carried out by means of a maximum likelihood estimation may be not as robust as expected. We use Bayesian forecast techniques to solve this problem, where non-informative flat prior distributions of the parameters is adopted to perform forecast experiments on 3 mainshocks occurred in Southern California. A Metropolis–Hastings algorithm is employed to sample the model parameters and earthquake events. We also show, through forecast experiments, how the Bayesian inference allows to obtain a probabilistic forecast, differently from one obtained via MLE. [ABSTRACT FROM AUTHOR]

Young α-rich (YAR) stars within the old Galactic thick disc exhibit a dual characteristic of relative youth determined with asteroseismology and abundance enhancement in α elements measured from high-resolution spectroscopy. The youth origin of YAR stars has been proposed to be binary evolution via mass transfer or stellar mergers. If that is the case, YAR stars should spin rapidly and thus be magnetically active, because they are mass and angular momentum gainers. In this study, to seek this binary footprint, we select YAR stars on the main-sequence turn-off or the subgiant branch (MSTO-SGB) from APOGEE DR17, whose ages and projected rotation velocities (v sin  i) can be precisely measured. With APOGEE v sin  i and LAMOST spectra, we find that YAR stars are indeed fast rotators and magnetically active. In addition, we observe low [C/N] ratios and high Gaia RUWE in some YAR stars, suggesting that these MSTO-SGB stars probably have experienced mass transfer from red-giant companions. Our findings underscore that magnetic activity can serve as a valuable tool for probing the binary evolution for other chemically peculiar stars, such as red giants with lithium anomalies and carbon-enhanced metal-poor stars. [ABSTRACT FROM AUTHOR]

Oscillating neutron stars are sources of continuous gravitational waves. We study analytically the excitation of stellar oscillations by the mechanical impact on the stellar surface of 'clumps' of stochastically accreted matter. We calculate the waveform and spectrum of the gravitational wave signal emitted by the accretion-driven pulsations. Results are generated for an idealized model of a non-rotating, unmagnetized, one-component star with uniform polytropic index n poly assuming Newtonian gravity and the Cowling approximation. We find that the excited mode amplitudes grow with increasing n poly and mode order n. The gravitational wave signal forms a sequence of amplitude-modulated packets for n poly = 1, lasting ∼10−3 s after each impact. The gravitational wave strain increases with increasing n poly, but decreases with increasing n and increasing multipole order l for n poly = 1. In the observing band of current long-baseline interferometers, g modes emit higher, narrower peaks in the amplitude spectral density than f and p modes, with the highest peaks reaching ∼10−26 Hz−1/2 for modes with damping time τ nl ∼ 108 yr. The root-mean-square strain h rms, calculated by summing over modes with 2 ≤ l ≤ 4 and τ nl ≤ 108 yr, spans the range 10−33 ≤ h rms ≤ 10−32 for 1 ≤ n poly ≤ 2. [ABSTRACT FROM AUTHOR]

Asteroseismology can precisely characterize stars, but the accuracy of seismic inference is still an open question. In this work, we use three evolved stars, including one late subgiant (EPIC 211411922) and two young red giants (EPIC 211409560 and EPIC 211416749) in the open cluster M67 to validate seismic inferences of stellar mass and age. We start by measuring oscillation mode frequencies and identifying acoustic and mixed modes in all three stars. We then fit the stars to determine their masses and ages with a detailed modelling approach. Our age estimates for all stars agree well with the literature consensus age range of M67 (3.0–5.0 Gyr). Mass estimates for EPIC 211411922 and EPIC 211409560 are sensible because they are slightly higher than the typical turn-off mass of the cluster. The inferred mass for EPIC 211416749 is underestimated by 5–10 per cent compared with the turn-off mass, but this could be caused by poor data quality. Our results indicate that the detailed modelling can determine sensible masses and ages for evolved stars near the base of the red giant branch. [ABSTRACT FROM AUTHOR]

Datable graphical records of total solar eclipses allow us to assess contemporaneous variability of Earth's rotation speed, solar coronal structure, and solar-wind conditions. Such graphical records were studied back to the early 18th century. Here, we examine Matthew Paris' medieval manuscript, an eyewitness account of a total solar eclipse just after local sunrise on 1230 May 14, to analyse his drawings for this eclipse. We philologically identify his observational site as St. Albans and compute local eclipse visibility. To locate St. Albans in the totality path, our analysis requires an accumulative error in time due to the variation in the Earth's rate of rotation, Δ T , in the range 394 s < Δ T  < 764 s (loose scenario), slightly revising the latest published Δ T spline curve. The eclipsed Sun sould have been visible after local sunrise if we set the Δ T margins to 394 s < Δ T  < 659 s (strict scenario), which requires a further slight modification of the latest Δ T spline curve. Matthew Paris' drawing of the total solar eclipse showed streamer-like structures similar to solar-minimum-type coronal streamers, consistent with the local tilt of the apparent solar equator. This is morphologically consistent with the minimum-type solar coronal streamers predicted from the open solar flux reconstructed from the 14C data set. This record potentially demonstrates a similarity in solar cycles between the 'Medieval Grand Maximum' and modern solar cycles. Collectively, historical eclipse records could be used as spot references for Earth's rotation speed, solar coronal dynamics, and background solar dynamo activity on a millennial time-scale. [ABSTRACT FROM AUTHOR]

Using non-linear force free field (NLFFF) extrapolation, 3D magnetic fields were modeled from the 12-min cadence Solar Dynamics Observatory Helioseismic and Magnetic Imager (HMI) photospheric vector magnetograms, spanning a time period of 1 hour before through 1 hour after the start of 18 X-class and 12 M-class solar flares. Several magnetic field parameters were calculated from the modeled fields directly, as well as from the power spectrum of surface maps generated by summing the fields along the vertical axis, for two different regions: areas with photospheric |6z| > 300 G (active region -- AR) and areas above the photosphere with the magnitude of the non-potential field (BNP) greater than three standard deviations above |BNP| of the AR field and either the unsigned twist number |Tw| > 1 turn or the shear angle ¥ > 80° (non-potential region -- NPR). Superposed epoch (SPE) plots of the magnetic field parameters were analyzed to investigate the evolution of the 3D solar field during the solar flare events and discern consistent trends across all solar flare events in the dataset, as well as across subsets of flare events categorized by their magnetic and sunspot classifications. The relationship between different flare properties and the magnetic field parameters was quantitatively described by the Spearman ranking correlation coefficient, rs. The parameters that showed the most consistent and discernable trends among the flare events, particularly for the hour leading up to the eruption, were the total unsigned flux 0), free magnetic energy (EFree), total unsigned magnetic twist (rTot), and total unsigned free magnetic twist (pTot). Strong (|rs| e [0.6, 0.8)) to very strong (|rs| e [0.8, 1.0]) correlations were found between the magnetic field parameters and the following flare properties: peak X-ray flux, duration, rise time, decay time, impulsiveness, and integrated flux; the strongest correlation coefficient calculated for each flare property was 0.62, 0.85, 0.73, 0.82, -0.81, and 0.82, respectively. [ABSTRACT FROM AUTHOR]

Updated formation and structure models of Jupiter predict a metal-poor envelope. This is at odds with the two to three times solar metallicity measured by the Galileo probe. Additionally, Juno data imply that water and ammonia are enriched. Here, we explore whether Jupiter could have a deep radiative layer separating the atmosphere from the deeper interior. The radiative layer could be caused by a hydrogen-transparency window or depletion of alkali metals. We show that heavy-element accretion during Jupiter's evolution could lead to the desired atmospheric enrichment and that this configuration would be stable over billions of years. The origin of the heavy elements could be cumulative small impacts or one large impact. The preferred scenario requires a deep radiative zone, due to a local reduction of the opacity at ∼2000 K by ∼90%, which is supported by Juno data, and vertical mixing through the boundary with an efficiency similar to that of molecular diffusion (D ≲ 10−2 cm2 s−1). Therefore, most of Jupiter's molecular envelope could have solar composition while its uppermost atmosphere is enriched with heavier elements. The enrichment likely originates from the accretion of solid objects. This possibility resolves the long-standing mismatch between Jupiter's interior models and atmospheric composition measurements. Furthermore, our results imply that the measured atmospheric composition of exoplanets does not necessarily reflect their bulk compositions. We also investigate whether the enrichment could be due to the erosion of a dilute core and show that this is highly unlikely. The core-erosion scenario is inconsistent with evolution calculations, the deep radiative layer, and published interior models. [ABSTRACT FROM AUTHOR]

Nanoflares are thought to be one of the prime candidates that can heat the solar corona to its multimillion kelvin temperature. Individual nanoflares are difficult to detect with the present generation of instruments, but their presence can be inferred by comparing simulated nanoflare-heated plasma emissions with the observed emission. Using HYDRAD coronal loop simulations, we model the emission from an X-ray bright point (XBP) observed by the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), along with the nearest available observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) and the X-Ray Telescope (XRT) on board the Hinode observatory. The length and magnetic field strength of the coronal loops are derived from the linear force-free extrapolation of the observed photospheric magnetogram by the Helioseismic and Magnetic Imager on board SDO. Each loop is assumed to be heated by random nanoflares, whose magnitude and frequency are determined by the loop length and magnetic field strength. The simulation results are then compared and matched against the measured intensity from AIA, XRT, and MaGIXS. Our model results indicate the observed emission from the XBP under study could be well matched by a distribution of nanoflares with average delay times 1500–3000 s. Further, we demonstrate the high sensitivity of MaGIXS and XRT for diagnosing the heating frequency using this method, while AIA passbands are found to be the least sensitive. [ABSTRACT FROM AUTHOR]

This study explores the seismotectonics of Kachchh in western India, a region with a low-to-moderate strain rate and a history of significant earthquakes, notably the 1819, Mw 7.8 Allah Bund, and the 2001, Mw 7.6 Bhuj. Despite its substantial seismic risk, comprehensive studies on Kachchh's seismogenic sources are scarce. This is attributed to the concealed nature of active structures, hindering definitive age constraints in paleoseismological research. Our research comprises a detailed paleoseismic analysis of the north-verging, reverse Jhura Fault underlying the Jhura anticline, a segment of the Kachchh Mainland Fault. This fault segment shows evidence of surface-rupturing earthquakes in the area south of the Great Rann of Kachchh. The investigation reveals three paleoseismic events: Event I before 9.72 ka B.P., Event II between 8.63–8.20 ka B.P., and Event III between 6.20–6.09 ka B.P. The elapsed time since the last event on this fault is > 8000 years, suggesting that the area is exposed to a significant earthquake hazard. This highlights the need for more precise characterization of individual seismogenic sources for future earthquake preparedness. [ABSTRACT FROM AUTHOR]

The mechanisms governing a commonly observed seismic velocity drop in the cratonic lithosphere, referred to as the mid‐lithospheric discontinuity (MLD), have been widely debated. To identify the composition and seismic structure of MLDs, we have analyzed Sp receiver functions (SRF) and mantle xenocrysts for six regions across Australia. We utilize locations where seismic stations and kimberlite‐hosted mantle xenocrysts are both available, allowing for comparison between seismological and petrological constraints. Our results show negative SRF phases indicative of the MLD coincide with clinopyroxene‐depleted zones at 60–140 km depth. Clinopyroxenes with different chemical compositions across the MLD define a litho‐chemical discontinuity. Modeling and experimental data show that MLDs may be explained by modified lherzolite with 10%–20% modal pargasite. Pargasite MLDs may form when rising H2O‐bearing melts cross the amphibole dehydration curve and react with clinopyroxene in lherzolite. Because the amphibole dehydration curve is isobaric at 80–120 km, pargasite will be precipitated as horizontal channels. Plain Language Summary: Seismic imaging of Earth's lithosphere has revealed a seismic velocity drop at 60–120 km depth, termed the mid‐lithosphere discontinuity (MLD). The mechanisms governing the MLD have been extensively debated. To advance the understanding of the MLD we carried out an interdisciplinary study to investigate the seismic structure and composition of MLDs identified in the cratonic lithosphere of Australia. We integrated analysis of seismic converted signals from discontinuities recorded by permanent seismometers with geochemical data for kimberlite‐hosted mantle xenocrysts collected in the vicinity. The method allows direct comparison between seismological and petrological constraints. Our results suggest that the MLD comprises anomalously low abundances of clinopyroxene and separates geochemically distinct layers within the lithospheric mantle. Experimental results and modeling suggests that the observed decrease in seismic velocity and absence of clinopyroxene may relate to the formation of pargasite channels in modified lherzolite. Key Points: Sp receiver functions and mantle xenocrysts are used to study mid‐lithosphere discontinuities beneath Australian cratonsMid‐lithosphere discontinuity at 60–120 km depth corresponds with mantle xenocryst populations that are depleted in clinopyroxeneModeling and experimental data suggests the mid‐lithosphere discontinuity is caused by pargasite‐bearing lherzolite [ABSTRACT FROM AUTHOR]