Search

The Bayesian earthquake analysis tool (BEAT) is an open-source Python software to conduct source-parameter estimation studies for crustal deformation events, such as earthquakes and magma intrusions, by employing a Bayesian framework with a flexible problem definition. The software features functionality to calculate Green’s functions for a homogeneous or a layered elastic half-space. Furthermore, algorithm(s) that explore the solution space may be selected from a suite of implemented samplers. If desired, BEAT’s modular architecture allows for easy implementation of additional features, for example, alternative sampling algorithms. We demonstrate the functionality and performance of the package using five earthquake source estimation examples: a full moment-tensor estimation; a double-couple moment-tensor estimation; an estimation for a rectangular finite source; a static finite-fault estimation with variable slip; and a full kinematic finite-fault estimation with variable hypocenter location, rupture velocity, and rupture duration. This software integrates many aspects of source studies and provides an extensive framework for joint use of geodetic and seismic data for nonlinear source- and noise-covariance estimation within layered elastic half-spaces. Furthermore, the software also provides an open platform for further methodological development and for reproducible source studies in the geophysical community.

SUMMARYThe investigation of stresses, faults, structure and seismic hazards requires a good understanding and mapping of earthquake rupture and slip. Constraining the finite source of earthquakes from seismic and geodetic waveforms is challenging because the directional effects of the rupture itself are small and dynamic numerical solutions often include a large number of free parameters. The computational effort is large and therefore difficult to use in an exploratory forward modelling or inversion approach. Here, we use a simplified self-similar fracture model with only a few parameters, where the propagation of the fracture front is decoupled from the calculation of the slip. The approximative method is flexible and computationally efficient. We discuss the strengths and limitations of the model with real-case examples of well-studied earthquakes. These include the Mw 8.3 2015 Illapel, Chile, megathrust earthquake at the plate interface of a subduction zone and examples of continental intraplate strike-slip earthquakes like the Mw 7.1 2016 Kumamoto, Japan, multisegment variable slip event or the Mw 7.5 2018 Palu, Indonesia, supershear earthquake. Despite the simplicity of the model, a large number of observational features ranging from different rupture-front isochrones and slip distributions to directional waveform effects or high slip patches are easy to model. The temporal evolution of slip rate and rise time are derived from the incremental growth of the rupture and the stress drop without imposing other constraints. The new model is fast and implemented in the open-source Python seismology toolbox Pyrocko, ready to study the physics of rupture and to be used in finite source inversions.

Inferring the geometry and evolution of an earthquake sequence is crucial to understand how fault systems are segmented and interact. However, structural geological models are often poorly constrained in remote areas and fault inference is an ill-posed problem with a reliability that depends on many factors. Here, we investigate the geometry of the Mw 6.3 2008 and 2009 Qaidam earthquakes, in northeast Tibet, by combining InSAR time series and teleseismic data. We conduct a multi-array back-projection analysis from broadband teleseismic data and process three overlapping Envisat tracks covering the two earthquakes to extract the spatio-temporal evolution of seismic ruptures. We then integrate both geodetic and seismological data into a self-consistent kinematic model of the earthquake sequence. Our results constrain the depth and along-strike segmentation of the thrust-faulting sequence. The 2008 earthquake ruptured a ∼32∘ north-dipping fault that roots under the Olongbulak pop-up structure at ∼12 km depth and fault slip evolved post-seismically in a downdip direction. The 2009 earthquake ruptured three south-dipping high-angle thrusts and propagated from ∼9 km depth to the surface and bilaterally along the south-dipping segmented 55–75∘ high-angle faults of the Olonbulak pop-up structure that displace basin deformed sedimentary sequences above Paleozoic bedrock. Our analysis reveals that the inclusion of the post-seismic afterslip into modelling is beneficial in the determination of fault geometry, while teleseismic back-projection appears to be a robust tool for identifying rupture segmentation for moderate-sized earthquakes. These findings support the hypothesis that the Qilian Shan is expanding southward along a low-angle décollement that partitions the oblique convergence along multiple flower and pop-up structures.

The 24 January 2020 Mw 6.77 Elazığ-Sivrice earthquake (Turkey), responsible for 42 casualties and ~ 1600 injured people, is the largest earthquake affecting the East Anatolian Fault (EAF) since 1971. The earthquake partially ruptured a seismic gap. The mainshock was preceded by two foreshocks with Mw ≥ 4.9 and small seismicity clusters occurring in the previous months close to the nucleation point of the main rupture. The significant aftershock sequence comprises twelve earthquakes with Mw ≥ 4.5 within 60 days. We jointly model quasi co-seismic static surface displacements from Interferometric Synthetic Aperture Radar (InSAR) and high-frequency co-seismic data from seismological networks at local, regional and teleseismic distances to retrieve source parameters of the mainshock. We reconstruct the rupture process using a Bayesian bootstrap based probabilistic joint inversion scheme to obtain source parameters and their uncertainties. Full moment tensor for 18 fore-/after-shocks with Mw ≥ 4.3 are obtained based on the modeling of regional broadband data. The posterior mean model for the 2020 Elazığ-Sivrice mainshock shows that the earthquake, with a magnitude Mw 6.77, ruptured at shallow depth (5 ± 2 km) with a left-lateral strike-slip focal mechanism, with a dip angle of 74° ± 2° and a causative fault plane strike of 242° ± 1°, which is compatible with the orientation of the EAF at the centroid location. The rupture nucleated in the vicinity of small foreshock clusters and slowly propagated towards WSW, with a rupture velocity of ~ 2100 ± 130 m s−1 and ~ 27 s rupture duration. The main rupture area, with a length of ~ 26 ± 5 km, only covered 70 % of the former seismic gap, leaving a smaller, unbroken segment of ~ 30 km length to the SE with positive stress change. The subsequent aftershock sequence extended over a broader region of ~ 70 km in length, spreading to both sides of the mainshock rupture patch into the regions experiencing a stress increase according to our Coulomb stress modeling. Our results support the hypothesis of a shallow locking depth of the Anatolian micro-plate, which has a possible implication to the seismic bursts along the EAF and alternating seismic activity on the North Anatolian and the East Anatolian faults.

The dynamics of magma deep in the Earth’s crust are difficult to capture by geophysical monitoring. Since May 2018, a seismically quiet area offshore of Mayotte in the western Indian Ocean has been affected by complex seismic activity, including long-duration, very-long-period signals detected globally. Global Navigation Satellite System stations on Mayotte have also recorded a large surface deflation offshore. Here we analyse regional and global seismic and deformation data to provide a one-year-long detailed picture of a deep, rare magmatic process. We identify about 7,000 volcano-tectonic earthquakes and 407 very-long-period seismic signals. Early earthquakes migrated upward in response to a magmatic dyke propagating from Moho depth to the surface, whereas later events marked the progressive failure of the roof of a magma reservoir, triggering its resonance. An analysis of the very-long-period seismicity and deformation suggests that at least 1.3 km3 of magma drained from a reservoir of 10 to 15 km diameter at 25 to 35 km depth. We demonstrate that such deep offshore magmatic activity can be captured without any on-site monitoring. Recent seismicity near Mayotte in the Indian Ocean is due to dyke propagation from and drainage of a 25–35 km deep magma reservoir, according to an analysis of earthquake and deformation data.

We present an analysis of aftershocks with M L >3 from the 2011 Prague, Oklahoma, earthquake sequence, an intraplate sequence of moderate‐size earthquakes within the North American craton. We apply waveform analysis to seismograms from temporary local seismograph networks to relocate the aftershocks’ hypocenters. The relocations show that the hypocenters are confined to the Precambrian basement. The character of the recorded seismograms waveforms are reconstructed by finite‐difference forward modeling of seismic sources buried at different depths in 2D crustal models. The numerical modeling indicates that the observed complex waveforms, and particularly the coda, are created by waves scattering off numerous shallow crustal velocity anomalies. We infer that these anomalies correspond to paleoriver channels that are embedded in the Pennsylvanian limestone formation. The modeling indicates that the amount of energy within the coda depends on the depth of the source. Our waveform modeling of the observed seismograms is consistent with our relocations of the aftershocks to depths of 5–10 km, within the Precambrian basement. Electronic Supplement:Empirical and synthetic data, model parameters, and geological figures.

This study contributes to the development of a new ground‐motion model for Central and Eastern North America (CENA). We evaluate the impact of the seismic‐velocity structure for different crustal regions on ground‐motion measures. We present an overview of the regionalization of the crustal structure of CENA, the physical parameters used for the simulation codes, and the statistical method applied for the comparison of pseudoabsolute response spectral accelerations (PSAs) of four crustal regions defined within CENA. The four crustal regions are (1) the Atlantic Coastal Plain (ACP), (2) the Appalachians (APP), (3) Central North America (CNA), and (4) the Mississippi Embayment/Gulf Coast (MEM). For each region and its statistically representative velocity structure, PSA matrices were computed, covering response oscillator frequencies from 0.5 to 20 Hz and hypocentral distances up to 500 km. PSA matrices for earthquakes at four different focal depths (5, 10, 20, and 30 km) were calculated. The developed method of the normed mean differences gives a meaningful measure for inter‐regional differences of ground motions in comparison with intra‐regional variability. In this study, we compare the ln(PSA) of the APP, ACP, and MEM region with the reference region CNA, respectively. The percentage of ln(PSA) mean difference values that are smaller than the ln(PSA) standard deviation of the reference region CNA lies between 73% and 91% for the APP region, dependent on the focal depth. For the ACP region, it is about 70% for all focal depths, except for the 20 km source depth, which was excluded because a layer boundary at 20.5 km causes unrealistically strong reflections. The percentage for the MEM region is less than 50% for each of the focal depths. This analysis demonstrates that there are two distinct ground‐motion groups for CENA: Group 1: CNA, APP, and ACP Group 2: MEM. Electronic Supplement: Figures of crustal velocity structures, table of Q studies, and Q studies illustrated for one region.