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Abstract: The 900-km-long Red River shear zone (RRSZ) lends a compelling support to the continental extrusion model for the tectonic evolution of southeastern Asia, but has been challenged by many of views, as some new records mainly from northern Vietnam, suspecting the dimensions of RRSZ neither in depth nor in displacement are as large as we expected before. However, compared to the northwestern half of the RRSZ in Yunnan province better studied by many fields, the southeastern half in northern Vietnam is relatively poorly constrained by seismic study, due to insufficient stations and data in the past. This study, using a newly deployed portable broadband seismic network, obtained the first local seismic tomography with a stepwise inversion using P and Pn phases. Surface geology, major structures, and rock properties are well correlated and identified in our model, suggesting the RRSZ is a lithospheric structure at least penetrating to the uppermost mantle with mantle thermal anomalies. In general, the crust of northern Vietnam appears to be weak and sits on a relatively hot uppermost mantle, showing a long and complex thermo tectonic history. A mid-lower crustal segmentation of RRSZ is also proposed to compromise the discrepancies recently observed between Yunnan province and northern Vietnam. [Copyright &y& Elsevier]

Abstract: This study develops a probabilistic approach for examining site-specific seismic hazards using data from 438 M w ≥5.5 earthquakes and attenuation relationships local to the region around Taiwan. These are combined to generate semi-observed peak ground accelerations, in a form which can be modeled by a double-lognormal distribution. This model, which satisfies statistical goodness-of-fit, provides the relationship between the exceedance probability and a given ground motion level. The study includes site-specific seismic hazard analysis for four nuclear power plant sites in Taiwan. The results show that the seismic hazards at the four sites are not the same. While no seismic hazard analysis is without challenge, a troublesome trend appears that many applications of decision making are being influenced by the complexity of the calculation, instead of how well the fundamentals of the analysis are understood and can be verified. The study provides an analysis which is not overly complicated, is in good agreement with an empirical control, and offers transparency, traceability, and verifiability. [Copyright &y& Elsevier]

Abstract: The first attenuation relationships of peak ground acceleration (PGA) and peak ground velocity (PGV) for northern Vietnam are obtained in this study. Ground motion data are collected by a portable broadband seismic network in northern Vietnam as a part of cooperation between the Institute of Geophysics, Vietnamese Academy of Science and Technology, Vietnam and Institute of Earth Sciences, Academia Sinica, Taiwan. The database comprises a total of 330 amplitude records by 14 broadband stations from 53 shallow earthquakes, which were occurred in and around northern Vietnam in the period between 01/2006 and 12/2009. These earthquakes are of local magnitudes between 1.6 and 4.6, focal depths less than 30km, and epicentral distances less than 500km. The new attenuation relationships for PGA and PGV are: where PGA is in cm/s2, PGV is in cm/s, and R is the epicentral distance in kilometers. The site corrections are also derived in this study. These site corrections are very suitable with the station corrections for ML and imply the qualification of the resulting attenuation relationships. [Copyright &y& Elsevier]

Abstract: Two recent catastrophic earthquakes that struck the Marmara Region on 17 August 1999 (Mw =7.4) and 12 November 1999 (Mw =7.2) caused major concern about future earthquake occurrences in Istanbul and the Marmara Region. As a result of the preparations for an expected earthquake may occur around Istanbul region, an earthquake early warning system has been established in 2002 with a simple and robust algorithm, based on the exceedance of specified thresholds of time domain amplitudes and the cumulative absolute velocity (CAV) levels (Erdik et al., 2003 ). In order to improve the capability of Istanbul earthquake early warning system (IEEWS) for giving early warning of a damaging earthquake in the Marmara Region, we explored an alternative approach with the use of a period parameter (τc ) and a high-pass filtered vertical displacement amplitude parameter (Pd) from the initial 3s of the P waveforms as proposed by Kanamori (2005) and Wu and Kanamori (2005) . The empirical relationships both between τc and moment magnitude (Mw ), and between Pd and peak ground velocity (PGV) for the Marmara Region are presented. These relationships can be used to detect a damaging earthquake within seconds after the arrival of P waves, and can provide on-site warning in the Marmara Region. [ABSTRACT FROM AUTHOR]

Abstract: The first local magnitude scale (ML ) for Northern Vietnam has been derived using a portable broadband seismic network in Northern Vietnam as part of the cooperation between the Vietnam Institute of Geophysics and the Institute of Earth Sciences at Academia Sinica, Taiwan. The composite horizontal peak amplitude data used in this study is comprised a total of 202 amplitude records from 14 broadband stations, which measure 36 shallow earthquakes with focal depths less than 36km, occurred in and around north-western Vietnam during 01/2006–10/2007. The new distance-correction function obtained in this study is where A 0 and r are the empirically determined distance correction and hypocentral distance, respectively. This distance-correction relation for Northern Vietnam is quite similar to one for southern California, implying relative efficient attenuation with distances. The correlation between the new ML and duration magnitudes (Md ), which is still being used for making the official earthquake bulletin in Vietnam, is expressed as [ABSTRACT FROM AUTHOR]

Earthquake early warning system uses high-speed computer network to transmit earthquake information to population center ahead of the arrival of destructive earthquake waves. This short (10 s of seconds) lead time will allow emergency responses such as turning off gas pipeline valves to be activated to mitigate potential disaster and casualties. However, the excessive false alarm rate of such a system imposes heavy cost in terms of loss of services, undue panics, and diminishing credibility of such a warning system. At the current, the decision algorithm to issue an early warning of the onset of an earthquake is often based on empirically chosen features and heuristically set thresholds and suffers from excessive false alarm rate. In this paper, we experimented with three advanced machine learning algorithms, namely, $K$ -nearest neighbor (KNN), classification tree, and support vector machine (SVM) and compared their performance against a traditional criterion-based method. Using the seismic data collected by an experimental strong motion detection network in Taiwan for these experiments, we observed that the machine learning algorithms exhibit higher detection accuracy with much reduced false alarm rate. [ABSTRACT FROM AUTHOR]

In this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ1, σ2 and σ3) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (S H) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of S H from north to south across the study area, with the direction of S H remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counter-clockwise. The direction of the horizontal maximum compression strain rate (εH) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the S H, εH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east–northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. S H in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role. [ABSTRACT FROM AUTHOR]

Recent studies have shown that structural damage exhibits stronger correlations with cumulative absolute velocity (CAV) than other ground motion intensity measures (e.g., PGA). This paper presents a CAV site-effect assessment for the Taipei areas for the first time. The study was based on more than 1200 strong-motion data from 47 major earthquakes that had occurred around Taiwan. The results show that the site effects are more conspicuous in the western Taipei than the eastern areas, and it is also obvious that the site effect is strong in locations close to the rims of the basin, where seismic waves could be easily reflected, refracted and superimposed. Subsequently, a map showing the areas in Taipei subject to severe CAV amplification and/or high variability was developed for site-effect microzonation for the study area, on the basis of CAV that was considered better correlated with structural damage under earthquake condition. [ABSTRACT FROM AUTHOR]

Abstract: Variations in the fractal dimension of earthquakes have been suggested to be a precursor of a large earthquake. However, the physical characteristics and seismicity are always different along a large fault system, and it is difficult to segment a large fault for further investigating. Therefore, the fractal dimension of earthquakes on a given fault only reflects the average seismic characteristics of the area and may be unrelated to precursory activity. In this paper, the evolution of the temporal generalized fractal dimension (Dq) of a seismic cluster within a small fault system associated with the Chiayi earthquake (ML =6.4), Taiwan, is investigated. The earthquakes of this sequence are confined in small source volume and reflect the behavior of the local fault system. Our results show that the Dq curve of the background stage is smooth with a low multifractal degree (0.36) and the seismicity is nearly monofractal in the Chiayi region. During the foreshock stage, the seismicity becomes active such that not only the Dq curve exhibits greater variability especially for the steep slope of Dq curve at q=0 but also the temporal fractal dimension changes from nearly monofractal to multifractal. In the aftershock stage, the fluctuation of Dq is large and the multifractal degree is up to 1.0. The temporal multifractal property becomes more pronounced as well. Finally, when the seismicity returns to the background stage, the Dq curve becomes smooth and nearly monofractal again. Therefore, the fluctuations in the Dq spectrum of an earthquake cluster associated with a large earthquake not only give a clear picture of the temporal alterations in the seismogenic regimes but also illustrate their variability through time for a large earthquake. In addition, the variation of the slope of Dq curve at q=0 in the foreshocks may be used as a precursor of a large earthquake. [Copyright &y& Elsevier]

Abstract: As part of a collaborative project involving researchers from the Vietnamese Academy of Science and Technology (Hanoi, Vietnam), National Taiwan University (Taipei, Taiwan), and Academia Sinica (Taipei, Taiwan), a 25-station broadband seismic array was installed in northern Vietnam during December of 2005 with the aim of acquiring a wide dynamic range of seismic data at a high spatial density. The stations were designed to image and interpret crustal and mantle structures beneath northern Vietnam, including information that would help to reconstruct the geodynamic evolution of the Red River Fault Zone. Since its deployment, the array has recorded significant local, regional, and teleseismic events. The high-quality distance and azimuth coverage of available events will help us to answer questions regarding the nature of deep seismic structures beneath the array and the regional tectonic evolution. As an unexpected bonus, the array recorded near-antipodal seismic waves from significant seismic events in South America, thereby providing valuable seismic data for analysis of the Earth’s deep interior. A similar network is planned for South Vietnam in the near future with the aim of studying the deep structure of the South China Sea. The proposed stations are scheduled for upgrades to enable the real-time transmission of data for earthquake monitoring and tsunami warnings for the South China Sea. [Copyright &y& Elsevier]

[Display omitted] • Two earthquakes having almost the same magnitude and focal mechanism in Hualien County, Taiwan, provide different damage scenarios. • The peak ground velocity (PGV) is found a better indicator of damage scenario as compared to peak ground acceleration (PGA) using shakemaps. • Our system provides PGV shakemap that complements the PGA issued by the official agency in Taiwan. • The dense P-Alert network in Taiwan plots near real-time PGA and PGV shakemaps, and these maps are posted regularly on social media. Two earthquakes having almost the same magnitude occurred in the Hualien area of Taiwan in 2018 and 2019. The 2018 earthquake had a magnitude M L 6.2 produced severe destruction; however, the 2019 earthquake (M L = 6.3) did not cause any severe damage. The P-Alert Strong Motion Network provides real-time shakemaps, in addition, to earthquake early warning (EEW) in terms of lead-time. Each instrument provides a different lead-time using peak ground acceleration (PGA) and peak ground velocity (PGV). During both the events, the instruments reported a lead-time of 1.5 to 8.0 s in the epicentral region. This network system also generated high-quality shakemaps during both earthquakes. The shakemaps showed that the higher PGAs are concentrated in the epicentral region for the 2018 and 2019 earthquakes. The lower PGA contour (≥25 Gal) extended to a broader area, including Taipei, during the 2019 earthquake compared to the 2018 earthquake. However, PGV shakemaps display a different pattern. The higher PGV values (more than 17 cm/s) are observed in the epicentral region during the 2018 earthquake (locations suffering building collapse) compared to the 2019 earthquake, suggesting that PGV correlates better with damage distribution as compared to the PGA. The PGV shakemap, currently only available for the P-Alert network, provides crucial information that complements the PGA issued by the official agency in Taiwan. [ABSTRACT FROM AUTHOR]

• GNSS is used to infer the mean annual water-thickness change of 0.53 m in Taiwan. • Consistent spatio-temporal change of water cycle in geodetic and hydrological data. • Phase shifts among data sets reflect the complex nature of transient water storage. • Water storage variation is affected by infiltration rate, capacity, and landscape. We systematically investigate the spatiotemporal water storage changes in Taiwan using geodetic (GNSS and GRACE) and hydrological (precipitation, GLDAS and LSDM assimilation models, and in-situ groundwater level) datasets. We use GNSS-observed vertical deformation to estimate water storage changes based on elastic loading theory and weighted least-squares inversion, correcting for contributions from global loads using GRACE. The mean annual water-thickness change inferred from GNSS across Taiwan is 0.53 ± 0.17 m and the largest seasonal change of up to 0.91 m is estimated in southwest Taiwan. Comparison of the geodetic and hydrological data shows that the spatial pattern of annual water storage change estimated from GNSS, GLDAS, and precipitation data are generally consistent, indicating significant seasonal water-load fluctuations in Taiwan. However, the GRACE solution significantly underestimates the amplitude of water mass change in Taiwan due to leakage effect, but temporally correlates well with GNSS estimates. Hydrological assimilation model GLDAS, dominated by shallow soil moisture variations, predicts that the average seasonal variation of water thickness is only about 17% of GNSS estimates. This value is about half of the mean annual LSDM water storage change of 0.18 m including an estimate of both soil moisture and surface water. The discrepancy suggests that the contribution of groundwater is substantial and the total water storage change in the hydrological assimilation model is underestimated in Taiwan. The spatiotemporal distributions derived using independent component analysis (ICA) are generally consistent between the geodetic and hydrological data. However, comparisons of seasonal amplitudes and phases between all data pairs reveal different response times to precipitation, reflecting the complex nature of transient water storage due to variable rainfall patterns, infiltration rate, soil saturation, and runoff. The peak rainfall occurs in June-July, which is one-to-two months before the peak GNSS subsidence. Water storage of the GLDAS model also reaches its maximum in August, suggesting the water storage is controlled by the infiltration rate and capacity and the total water recharge from rainfall is generally larger than discharge in the summer. The highest groundwater levels lag one and two months behind the peak GNSS subsidence in western and eastern Taiwan, respectively, indicating a higher infiltration rate in western Taiwan. [ABSTRACT FROM AUTHOR]

The preliminary station-based seismic velocity models of the Western Solomon Islands are proposed for the first time in this study, and those are obtained from joint inversion of P-wave receiver functions and surface-wave group velocity dispersion curves. • The station-based seismic velocity models of the Western Solomon Islands are proposed in this study. • The low-velocity zones above the Moho depth are observed beneath several stations, and may be related to the lower crustal magma. • The Moho depths are approximately 25 km beneath four of seven seismic stations, and the remaining stations reveal a deeper Moho (35–40 km) might be due to subduction processes. An M W 8.1 earthquake occurred in 2007 and induced a tsunami that hit the Western Solomon Islands and caused casualties. This motivated us to deploy a seismic network around the rupture zone of the 2007 event. To investigate the seismic velocity structure of the Western Solomon Islands, we select seismograms of teleseismic events recorded by our seismic network. Joint inversion of P-wave receiver functions and surface-wave group velocity dispersion curves is used to estimate station-based 1-D velocity models. The resulting velocity models show a highly variable crustal structure across the region. The Moho depths beneath the stations range from 25 to 40 km. A low-velocity zone (LVZ) is observed at most seismic stations in this work. Our study provides preliminary station-based seismic velocity models for the study region, and more stations will be deployed in the continuing project. An integrated 3-D velocity model will be determined in the future. [ABSTRACT FROM AUTHOR]

Delineation of physical factors that contribute to earthquake triggering is a challenging issue in seismology. We analyze hydrological modulation of seismicity in Taiwan using groundwater level data and GNSS time series. In western Taiwan, the seismicity rate reaches peak levels in February to April and drops to its lowest values in July to September, exhibiting a direct correlation with annual water unloading. The elastic hydrological load cycle may be the primary driving mechanism for the observed synchronized modulation of earthquakes, as also evidenced by deep earthquakes in eastern Taiwan. However, shallow earthquakes in eastern Taiwan (<18 km) are anticorrelated with water unloading, which is not well explained by either hydrological loading, fluid transport, or pore pressure changes and suggests other time-dependent processes. The moderate correlation between stacked monthly trends of large historic earthquakes and present-day seismicity implies a modestly higher seismic hazard during the time of low annual hydrological loading., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)