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The Hualien city is located near the pole of rotation induced by the along-strike transition from Ryukyu subduction to the collision between the Luzon arc and the Chinese continental margin in northeastern Taiwan (Rau et al., J Geophy Res 113: B09404, 2008). Such a tectonic regime creates varied seismogenic structures that are responsible for the ruptures of the 1951 M L 7.3 Hualien-Taitung earthquake sequence (Chen et al., J Geophy Res 113: B02304, 2008), the 2018 M w 6.4 Hualien earthquake sequence (e.g., Rau and Tseng, Terr Atmos Ocean Sci 30: 281–283, 2019), the 2021 M L 6.2 Shoufeng earthquake sequence, and numerous and persistent moderate-to-small size earthquake swarms in the Hualien area. This special issue was introduced by the several month-long 2021 episode of moderate-to-small size earthquake swarms occurred in the Hualien area (Figs. 1 and 2, Central Weather Bureau, 2022). The 2021 Hualien earthquake episode includes small-to-moderate-size earthquakes that release minute strain energy on the small asperities and larger events (M > 5.5) that ruptured nearby asperities. Figure 3 shows the spatiotemporal distribution of the Hualien earthquake sequence between April 1 and December 31 of 2021. The 2021 Hualien earthquake sequence provides us a great opportunity to examine the earthquake physics, characteristics, seismogenic processes, and the early warning algorithm of an earthquake swarm, which are the key elements for the understanding of generation of earthquakes. In this special issue, we have collected six papers that detailed various aspects of the 2021 Hualien earthquake sequence, which provide us a better understanding of the nature of the Hualien earthquake swarms.

Citizen seismology encourages the public involvement in data collection, analysis, and reporting, and has the potential to greatly improve the emergency response to seismic hazard. We believe in the value of citizen seismology and started with distributing Quake-Catcher Network (QCN) sensors at schools in Taiwan. Unlike most of western countries, Earth Sciences is a required course in junior and senior high schools of Taiwan (Ministry of Education, 2014). Low-cost seismometer is potentially a powerful tool in classrooms to teach earthquake science, which makes school teachers our targeted users. We work closely with school teachers and students to understand how can a citizen seismology project can help them to encourage successful engagement. In this study, we establish the achievements gained and problems encountered in different phases of the project since 2013. The main tasks carried out here include (1) refinement of Citizen Seismology Literacy (CSL) into three dimensions: Awareness, and Willingness, and Technology fluency (2) development of an interactive competition platform called the Near-Real Time Earthquake Game Competition (EGCnrt) that allows citizen seismologists to report earthquake information by processing P- and S-wave arrivals, peak ground motion, and first motion of P-waves for every inland M≥4 earthquakes in Taiwan; (3) development of the formative assessment for the 1.5-hour-long, non-lecture-based game activities. Based on 565 student surveys completed after our 2016 summer outreach activity, we found that all three CSL dimensions have an influence on students’ score in the training activity. The final score in EGCnrt is also found to highly correlate with the performance in the three training sections. We therefore, propose that the CSL can act as a powerful indicator for the performance and engagement in earthquake learning activities. The game-based, non-lecture-based learning approach can be effective in promoting citizen seismology in the future.

Using intraslab earthquakes shallower than 150 km in the southernmost Ryukyu subduction zone, previous studies in Taiwan found the wave guide effect that typically shows a low-frequency (5 between 1997 and 2016. Event are classified based on the waveform characteristics and their frequency contents.To detect events with similar properties, we applied sliding-window cross-correlation (SCC)​ using three components of P waveform data simultaneously for a set of stations​. The time window used here was 10 s and traces were bandpass filtered in the frequency range 0.5–10 Hz. After the degree of similarity are calculated, e​vents containing comparable waveforms can be sorted into families. The events within a family would have been triggered because they came from the same source region and their paths to a particular receiver should produce similar waveforms. Our results show that most earthquakes are low in waveform similarity, implying no “repeating” behavior for those intermediate intraslab events. However, some events (cc>0.6 threshold) present enough charterers that can be grouped as a family.One important property is the frequency content of the arrivals that may be related to the speed of structure traveled. We have developed a work scheme to determine the delayed time of higher-frequency energy. On family of events show beautiful dispersion with arrival time smoothly increasing with frequency between 0.5 and 6 Hz.​ Another type of dispersive waveforms appear as two distinct arrivals: low frequency and then high-frequency energy, separated by around 1 s. The time delay seems to be independent of focal depth. The latter case has been reported in the previous study for shallower event and it was interpreted as effect from low-velocity layer or heterogeneity of the subducted slab. On the other hand, the continuous dispersion is a new feature observed by our study, which may infer a thinner layer and/or longer propagation for some kind of reflecting waves to develop such interference.In addition, we will classify the waveforms according to the frequency content and decay of coda. The variations in P coda properties can be associated with the way in which the seismic energy gets ducted into the stochastic waveguide associated with the lithosphere. With sufficient amount of data, it is possible to further identify the earthquakes with unusual source properties or structure anomaly along specific propagation paths. We expect the classification results can provide a reference for future numerical simulation analysis. Keywords: Ryukyu subduction zone, SCC, guide wave, waveform classification, intermediate-depth earthquakes

A new algorithm is developed in this study to automatically determine regional moment tensor (MT) and its centroid depth with real-time waveforms in 2–4 min after the Central Weather Bureau (CWB) issues an earthquake notice. The program selects 3–7 BATS (Broadband Array for Taiwan Seismology) stations under three criteria: best azimuthal distribution, highest signal-to-noise ratio, and shortest distances. It then inverts MT solutions in parallel with various settings including Moho depth of velocity models, frequency bands, and isotropic constraints. The optimal solution is determined by searching for the best waveform-fit with an acceptable non-double-couple component from the results of combinations of these inversion settings. Our new rapid MT report system greatly reduces computational resources and avoids human judgments. By applying this full-scanning approach on BATS (named AutoBATS), we re-determine the MTs for over 3000 regional earthquakes from 1996 to 2016 to provide the most up-to-date MT catalog for the Taiwan area. The AutoBATS MTs are overall consistent with the Global Centroid Moment Tensors with a mean difference in Kagan angle of 22.0 ± 16.6° and MW of − 0.08 ± 0.10. Those focal mechanisms better illuminate the tectonic structures owing to the much improved resolving ability for shallow ( 140 km) earthquakes. With the new regional MT catalog, we refine the relationship between moment and local magnitudes: MW = 0.87 ML + 0.23 for the Taiwan region.

Taiwan is located at the convergent plate boundary between the Eurasian and the Philippine Sea plates. As a result, intense earthquake activity and associated surface deformation are manifesting in this region. To implement and promote citizen earthquake science in Taiwan, we have developed several web-based platforms with multi-purpose themes, such as earthquake science information, popular science education, and crowdsourcing systems. First, with the rapid earthquake report issued from the Taiwan Central Weather Bureau (CWB), the available near real-time scientific results obtained from the Taiwanese seismology community are collected and published to a platform, the Taiwan Earthquake Science Information System (TESIS). The scientific information archived at TESIS includes CWB earthquake reports, focal mechanisms, shake maps, and finite source models (for strong earthquakes). All real-time results are integrated into a GIS system with background geospatial information, such as geological maps, traces of active faults, background seismicity, and inter-seismic GPS velocity fields. Second, by collaborating with Stanford University to maintain a regional Quake-Catcher Network (QCN) server in Taiwan, we have promoted citizen seismology in Asia by bringing earthquake information and scientific knowledge to the public. More than 200 school teachers have already installed the QCN sensors in Internet-enabled computers. Through two web-based educational platforms, users are able to access the guidelines and further interact with the recorded waveforms. Third, we also developed an earthquake damage reporting system – the Taiwan Scientific Earthquake Reporting (TSER) system – to encourage the citizen to collect field observation for significant earthquake-induced ground damages such as surface fault rupture, landslide, rock fall, liquefaction, and landslide-triggered dam or lake. The TSER system is constructed under the Ushahidi mapping platform, which has been widely used in crowdsourcing for the geospatial archiving of events. Trained high school teachers and public volunteers can send their ground damage observations, including photographs, through the TSER system. Most of these products and online systems are now being operated by the Taiwan Earthquake Research Center (TEC). With these newly developed platforms and materials, we aim to not only raise earthquake awareness and preparedness, but also encourage public participation in earthquake science in Taiwan.

We report the near-surface ( V p ), shear wave velocity ( V s ), V p / V s ratio, Poisson's ratio and V s anisotropy of Taiwan by applying seismic coda interferometry to 34 borehole-surface station pairs. We find clear characteristic cos ⁡ 2 θ dependence of V s in all the determinations, and about half of the amplitudes of anisotropy are larger than 10%, with the largest amplitudes up to 34%. The patterns of anisotropy fall into two categories, OPA (Orogeny-Parallel Anisotropy) and SAA (Stress-Aligned Anisotropy). Both types of anisotropy fit well the local geological fabrics and/or the ambient stress, and show strong correlation with the Poisson's ratios at the borehole sites. With these new findings and reported tomographic results, we infer that the SAA are likely confined to the uppermost portion of the crust, in particular to the fluid-saturated late-Quaternary deposits. The strong near-surface anisotropy also implies that delay times contributed by the shallow crust might have been underestimated in studies of shear-wave splitting measurements using the direct arrivals of earthquake waves.

Despite a moderate magnitude, M_w = 6.4, the 5 February 2016 Meinong, Taiwan, earthquake caused significant damage in Tainan City and the surrounding areas. Several seismograms display an impulsive S-wave velocity pulse with an amplitude of about 1 m s-1, which is similar to large S-wave pulses recorded for the past several larger damaging earthquakes, such as the 1995 Kobe, Japan, earthquake (M_w = 6.9) and the 1994 Northridge, California, earthquake (M_w = 6.7). The observed PGV in the Tainan area is about 10 times larger than the median PGV of M_w = 6.4 crustal earthquakes in Taiwan. We investigate the cause of the localized strong ground motions. The peak-to-peak ground-motion displacement at the basin sites near Tainan is about 35 times larger than that at a mountain site with a similar epicentral distance. At some frequency bands (0.9 - 1.1 Hz), the amplitude ratio is as large as 200. Using the focal mechanism of this earthquake, typical “soft” and “hard” crustal structures, and directivity inferred from the observed waveforms and the slip distribution, we show that the combined effect yields an amplitude ratio of 17 to 34. The larger amplitude ratios at higher frequency bands can be probably due to the effects of complex 3-D basin structures. The result indicates that even from a moderate event, if these effects simultaneously work together toward amplifying ground motions, the extremely large ground motions as observed in Tainan can occur. Such occurrences should be taken into consideration in hazard mitigation measures in the place with frequent moderate earthquakes.

In this study we utilize regional and teleseismic earthquake moment tensor solutions in order to infer the contemporary crustal stress in the Greek region. We focus on crustal earthquakes and select only solutions with good waveform fits and well-resolved nodal planes. A data set of 1614 focal mechanisms is used as input to a regional-scale damped stress inversion algorithm over a grid whose node spacing is 0.35°. Several resolution and sensitivity tests are performed in order to ascertain the robustness of our results. Our findings show that for most of the Greek region the largest principal stress σ1 is vertically oriented and that the minimum principal stress axis σ3 are subhorizontal with a predominant N-S orientation. In the SW Peloponnese the orientation of σ3 axes rotates clockwise and in SE Aegean counterclockwise. These results are in agreement with the generally accepted model that slab rollback combined with gravitational spreading of the Aegean lithosphere are the main causes of the extension. Transitions between different faulting types in NW Greece or in the Aegean occur within narrow zones in the order of tens of kilometers. A visual comparison of the principal horizontal stress axes and the principal strain axes derived from GPS observations shows good agreement, suggesting that the crust in the Greek region behaves largely in an elastic manner.

A temporary seismic network consisting of 295 land stations was deployed in 2009 in Taiwan to monitor airgun sources during an onshore-offshore experiment in Taiwan. We exploit the continuously recorded dataset to detect and map seismicity in the seismically very active Taiwan. By combining recent automatic detection and phase picking techniques, we successfully generate an initial earthquake catalog of over 8400 events. The hypoDD algorithm is used to relocate and filter these events. This network recorded smaller events than the permanent regional network because of the many stations around the high mountain ranges and the generally high station density along 6 lines. The results based on the 2009 data generally reproduce the dominant seismicity features from many years of earthquake monitoring in Taiwan. In addition, we map hitherto unknown dipping zones under the Foothills and the Central Range that may correspond to seismogenic structures.

Knowledge of the crustal structure is important for understanding the tectonic framework and geological evolution of southeastern China and adjacent areas. In this study, we integrated the datasets from the TAIGER (TAiwan Integrated GEodynamic Research) and ATSEE (Across Taiwan Strait Explosion Experiment) projects to resolve onshore-offshore deep crustal seismic profiles from the Wuyi-Yunkai orogen to the Taiwan orogen in southeastern China. Three seismic profiles were resolved, and the longest profile was 850 km. Unlike 2D and 3D first arrival travel-time tomography from previous studies, we used both refracted and reflected phases (Pg, Pn, PcP, and PmP) to model the crustal structures and the crustal reflectors. In total, data from 40 shots, 2 earthquakes, and approximately 1,950 stations were used; 15,612 arrivals were selected among three transects. Using these data, we determined the complex crustal evolution since the Paleozoic era , involving the closed Paleozoic rift basin in central Fujian, the Cenozoic extension due to the South China Sea opening beneath the coastline of southern Fujian, and the on-going collision of the Taiwan orogen. The shape of the Moho, which also reflects the crustal evolution, can be summarized as follows: ~ 30 km deep to the west of Fujian, deepening toward central Fujian (~ 35 km), becoming shallower toward the Taiwan Strait (~ 28 km), deepening again toward the mountain belt of Taiwan (~ 42 km), and becoming shallower toward the Pacific Ocean (~ 10 km).

On 5 February 2016, a moderate earthquake occurred in southwestern Taiwan with M L 6.4 and a focal depth of 16.7 km. This earthquake caused damage to a few buildings and 117 casualties. A low‐cost earthquake early warning (EEW) system ( P ‐alert) is in operation for the purpose of EEW and for providing near‐real‐time shake maps. During this event, a detailed shaking map was generated by the P ‐alert system within 2 min after the earthquake occurrence, and the high shaking regions strongly correlated with the locations in which the damage and casualties occurred. In the field, individual P ‐alert devices also serve as onsite EEW systems using P ‐wave information. The individual P ‐alert provided a 4–8 s lead time before the arrival of violent shaking in the damaged regions. For regional EEW, both the Central Weather Bureau (CWB, official agency) and the P ‐alert system responded very well. Currently, regional warnings in Taiwan are only provided to cities at epicentral distances of 50 km or more by the CWB. For cities within a 50‐km epicentral distance, the P ‐alert system could be useful for providing onsite EEW. The performance of the P ‐alert network during this earthquake proves the efficiency of this real‐time, low‐cost network in terms of early warning (regional and onsite), near‐real‐time shake maps, rapid reports, and strong‐motion data for research purposes.

H.K-C. is supported by Ministry of Science and Technology (grant MOST104-2628-M-008-005-MY3). D. Brown acknowledges funding by MINECO grant-GL2013-43877-P. Q. Li and Z. Ye acknowledge funding by Sinoprobe-02-03. S wave tomography model presented in all figures can be obtained from the lead author (Hao Kuo-Chen, kuochen@ncu.edu.tw).

It is hoped that through the cultivation of a crew of volunteer citizen seismologists, public involvement could be encouraged and the discovery and inquiry into earthquake knowledge could be promoted. These volunteers can contribute to data collection, analysis, and reporting, and have the potential to greatly improve the emergency response to earthquakes. The Citizen Seismologists in Taiwan Project (CSTaiwan) is designed to elevate the quality of earthquake science education by incorporating earthquake and tsunami stories and educational earthquake games into traditional school curricula. The project aims to build a cloud-based computing service incorporating an earthquake school (i.e., a website for online learning) where teachers can easily teach their students about earthquakes and children can learn about earthquakes in a fun environment. Here we demonstrate how students perform P -a ndS-wave picking and measure seismic intensity through an interactive learning platform, how scientists and school teachers work together, and how we create a near-realtime earthquake games competition to facilitate continuous learning while making earthquake science fun. We also develop 49 questions associated with participants’ preknowledge, attitude, and skills in earthquake sciences, called Citizen Seismological Literacy (CSL). The CSL model may serve as an example to quantify citizen’s background in earthquake sciences and could be applied as a framework for seismologists around the world who wish to approach the public for educational purposes, while considering promoting the public’s seismologic literacy.

Right after the 2010 Chiashian earthquake, there have been five M ~6 mid- to lower crust events occurred inland Taiwan, in which the 2016 Meinong earthquake is the most devastated. The 6 February 2016 ML 6.4 Meinong earthquake (03:57:27 local time) occurred at about 35 km ESE of the Tainan city with a focal depth of 16.7 km. It is a moderate-sized event, however, produced widespread strong shaking in the 35-km-away Tainan city and caused about 10 buildings collapsed and 117 death. In addition, significant aftershocks occurred right beneath the Tainan city with focal depths reaching 30 km at the lower crust, which has never been observed in inland SW Taiwan. The Taiwan Earthquake Model (TEM) announced a seismic hazard map of Taiwan in the end of 2015 and indicated a relatively high seismic hazard in Tainan (Rau and Ma 2016; Wang et al. 2016). Although the TEM model does not account for the blind faults as shown by the 2016 Meinong event, such an event occurred at this location was considered as an area source in the TEM model and the extremely high strain rate, ~10-6 in SW Taiwan anticipates the reactivations of any pre-existing structures in this highly deformed crust. The scientific uniqueness and unexpectedly severe hazard in Tainan drive us to better understand the nature of the 2016 Meinong earthquake sequence in both scientific and engineering aspects.

article i nfo Mindoro Island is located where the Palawan Continental Block indented into the Philippine Mobile Belt during the Early Miocene and where the Manila Trench terminates, having ceased convergence due to collision. The seismotectonics of Mindoro are crucial to understanding the processes of arc-continent collision and slab dynamic responses of convergence cessation. This study was conducted using data from both the EHB and Global Centroid Moment Tensor catalogues. It is shown that following the southeasterly reduction of convergence rates from the Manila Trench offshore NW Mindoro to onshore SW Mindoro, the slab dipping angles steepen, were initiated at depth (~200 km) and propagate upwards. The horizontal distances of the trench and slab, as measured from the Wadati-Benioff zone at 200 km depth, also reduce in a southeasterly direction. Observations of intermediate-depth earthquakes that exhibit predominantly down-dip extensional stress patterns attest that the steepening of slab dipping angles is due to the negative buoyancy of the slab. In contrast, a broad region covered by central and south Mindoro, the Romblon group, and NW Panay, is characterized by a sporadic distribution of shallow earthquakes of mostly strike-slip type. Among these, the significantly sized events (Mw N 5.5) tend to dis- tribute around the borders suggesting that the region roughly acts as a unit block, moving or rotating coherently. Events in the vicinity of the Wawa-Mamburao valley exhibiting normal faulting of NW-SE horizontal extension suggest rifting of the Macolod Corridor extending southwestward to NW Mindoro.

Vertical ocean‐bottom seismograph (OBS) data at frequencies below 0.05 Hz are contaminated by noise induced by infragravity waves. We constructed the transfer function between pressure and velocity data from OBSs deployed in Taiwan waters to remove the wave pressure‐induced noise from seismic recordings. Data were analyzed from five portable broadband OBSs deployed each for 10 months at water depths from 1740 to 4600 m and from a cabled, shallow‐buried seismograph (EOS1) installed on the seafloor at 300 m depth. Removing long‐period noise from these OBS data improves the identification of teleseismic phases such as P , S , SS , P diff , and PKIKP that are otherwise ambiguous or unidentifiable. For EOS1, infragravity‐wave signals completely mask the P and S waveforms in the 10–50 s period band suitable for centroid moment tensor (CMT) solutions for most of the local events. Application of the transfer functions to predict and remove wave deformation yielded clean prominent P and S waveforms at these periods and aided in the CMT determination for small events jointly with land stations. The relative amplitudes of the wavenumber‐normalized transfer function for some of the OBSs are mostly determined by the thickness of the sediment at the OBS site.

Seismic data collected during explosion experiments performed as part of the TAiwan Integrated GEodynamics Research (TAIGER) project provide an excellent opportunity to obtain high‐resolution images of the structure of the crust and upper mantle beneath Taiwan. The most significant feature observed at near‐source stations located on the western coastal plain in Taiwan is high‐energy later arrivals. These high‐amplitude multiples almost completely mask the lower‐amplitude signals (seismic refraction and wide‐angle reflection) from the deep crust. The later arrivals are identified as free‐surface‐reflected multiples. The nature and generation of these high‐energy, multiple diving waves are demonstrated using synthetic examples. Their generation requires the presence of a steep velocity gradient in the shallow crust. A detailed analysis of the observation data provided information on the velocity gradients in this region. An accurate layer‐velocity model, including the boundary orientation and its depth, and velocity gradient, was constructed based on a 1D waveform simulation and 2D seismic raytracing modeling for travel times. The present results indicate that the thick sediment in the survey area dips shallowly to the east, has a surface P ‐wave velocity of ![Graphic][1] , and an average velocity gradient of about 0.72/s from the surface to 3.0‐km depth. The thick sediment of the 2D model shows lateral variations in velocity gradient, increasing from west to east. This velocity model may provide useful information for future data processing to reduce multiple diving waves with the aim of enhancing the deep‐surface refraction/reflection signal. The velocity gradient calculated for the thick sediment of the western coastal plain may require a revision of the regional seismic velocity model developed for southwestern Taiwan, to improve the accuracy of regional hypocenter determinations, and to predict the strong ground motions produced by large earthquakes beneath this region. [1]: /embed/inline-graphic-1.gif

The Jiashian earthquake ( M L 6.4) occurred on 4 March 2010. It was the largest inland event in southern Taiwan of 2010. The mainshock location was unexpected since it occurred in an area with relatively low background seismicity. In addition, reports of earthquake focal mechanisms do not fit with any known active fault geometry. In order to understand the origin of this earthquake, especially its rupture process, we perform a joint source inversion by using teleseismic body wave, GPS coseismic displacements and near field ground motion data. In this study, we considered a northwest–southeast trending fault with a northeast dip retrieved from GPS coseismic data and aftershocks distribution. To analyze the detailed slip distribution in space and time, we used near field 3D Green’s functions provided by spectral-element method and a full time–space inversion technique. We find a complex rupture process with several slip patches distributed inside two main asperities. The slip map reveals a mean slip of 12.9 cm for a maximum slip of 27.3 cm leading to a M w 6.47 for this event. The rupture initiates in the deepest portion of the fault at 20 km depth, and propagated upward up to 2 km depth to form the two asperities. The source time function of this event revealed two pulses corresponding to the two asperities, for a total duration time of about 16 s. Most aftershocks occurred near the upper boundary of the deepest asperity while no aftershocks are located close to the shallowest one. We infer that the locations of these slip patches are related to the surrounding fault systems that may have restricted the rupture propagation during the earthquake.

Here we tackle a tectonically important question – the upper mantle velocity structure beneath central Taiwan – with seismically interesting observations – receiver-side slab waveform effects. We use teleseismic P waveforms of the NS broadband array deployed by the TAIGER project to examine patterns of variation in arrival time, pulse width, and amplitude – measuring the first two by Gaussian fitting – and contrast measurements of earthquakes to the southeast (SE earthquakes) with those of one Sumatra earthquake in order to focus on upper mantle heterogeneities. Overall variation patterns as a function of earthquake are compatible with ray-tracing predictions. Relative reduced arrival times and amplitudes at central Taiwan stations suggest the existence of a deep aseismic slab below. From simulations of 2-D wave propagation, we conclude that lateral heterogeneity of crust and uppermost mantle primarily contributes to variations in arrival time and only secondarily to variations in amplitude and pulse width. Furthermore, discrepancies between source-side and receiver-side waveform effects, where the latter are not always amplitude-reduced, are explained by constructive interference between the fast and slow phase. Thus, the use of full waveform information can provide independent constraints to complement results of previous studies. A future extension will be to incorporate S waves and apply waveform inversion to yield quantitative constraints.

Landslides induced by typhoon Morakot during its passage across Taiwan on 7–9 Aug 2009 claimed more than 700 lives and caused heavy economic loss. Unlike earthquake monitoring, precise locations of landslides could not be determined in near-real time because their seismic phases are difficult to identify. Here, we show that large, damaging landslide events are characterized seismically by a distinct waveform pattern of frequent intermixes of P and S waves over a time window of several tens of seconds. The predominant frequency band during these time windows ranges from 0.5 to 5 Hz. The high-frequency content is clearly deficient relative to that of local earthquakes by about one to two orders. We also demonstrate that large landslide events can be located and monitored with algorithms specifically designed for real-time seismic applications. This near-real-time monitoring capability would be particularly useful for emergency responders and government organizations to coordinate effective relief-and-rescue operations.

Using the ambient noise cross-correlation technique, we derived empirical Green’s functions (EGFs) of short-period Rayleigh waves from the continuous vertical component of data recorded by 34 seismic stations in northern Taiwan. We measured the group velocity of the EGFs in the frequency range of 0.2–0.5 Hz. The derived EGFs and group velocity measurements were carefully examined, and only quality results were used for the tomographic inversion. We implemented a wavelet-based multiscale inversion technique to construct the group velocity map for 0.35-Hz Rayleigh waves in northern Taiwan. The resulting model shows excellent correlation to the surface topography and geological units. To compare with models derived from traditional body-wave tomography, we calculated the group velocity maps and their fit to the noise-derived EGFs for two recent body-wave models. The comparison indicates that body-wave velocities, particularly V S , at shallow depth in the west coast plain are overestimated in two recent body-wave-derived models, and/or a much higher V P / V S ratio may exist for this area. Our results suggest that the noise-derived short-period EGFs may provide important constraints on shallow Earth structure, in complement to that provided by body waves.

SUMMARY The Ryukyu subduction is known to generate very few earthquakes in its central segment contrarily to its two extremities. We focus in this study on the southernmost part of the Ryukyu subduction zone offshore east Taiwan. Our first task was to build a homogeneous earthquake cataloguefortheperiod1900–2007.Thenewcatalogueprovideshomogenized M �

article i nfo Many researchers studied the relationships between appearances of geomagnetic anomalies and their distances to earthquake epicenters or faults. Yet, occasionally some magnetometer stations located nearby earthquake epicenters and/or faults do not observe geomagnetic anomalies. In this paper, a new hybrid system which simultaneously takes the hypocenter and fault plane solution into account is constructed to examine 38 earthquakes interpreted to be associated with geomagnetic anomalies during the period 1988- 2001 in Taiwan. The Surface Magnetic Anomaly Reference Tip (SMART) of the new system is used instead of the epicenter or the fault to investigate statistically the distance relationship between the anomalies and the earthquake parameters. Results show that the anomalies gather along the fault and in the belt zone to the SMART. Possible mechanisms causing the anomalies in the two zones are proposed and discussed. Characteristics of the anomaly might shed some light on locations of faults before earthquake occurrences.

SUMMARY Coseismic signals of groundwater levels are generally obtained by subtracting the responses of atmospheric pressure, Earth tides and precipitation from the observed data. However, if the observations are conducted without nearby barometers, pluviometers or Earth tide data for correction, coseismic signals are often difficult to extract. In this case, the Hilbert–Huang transform (HHT) is used to obtain the instantaneous frequencies and amplitudes for every point of the decomposed intrinsic mode functions (IMFs) from groundwater level data for differentiating the related frequency-dependent responses without a further auxiliary input. The extracted coseismic signals show intense amplitude pulses that are clearly seen in the third IMF. In addition, two types of coseismic signals can be readily distinguished in the results from the HHT transform. One is an instantaneous short-time signal induced by the passing of seismic waves. Another coseismic signal is a sustained signal induced by the nearfield earthquake occurring near the Hualien station, Taiwan and shows a positive correlation between the earthquake distance and magnitude. Our results generally show that using the HHT transform improves our understanding of automatic detection of the coseismic signals from the groundwater level.

article i nfo The Tatun Volcano Group (TVG) represents the main volcanic center in northern Taiwan and based on various observations it is considered a potentially active volcano. TVG has been monitored since 2003 by a seismic network that consists of eight stations equipped with three-component, short-period and broadband seismometers. In this study, we use waveform data of high frequency earthquakes in order to investigate the stress field orientation beneath the TVG area. The focal mechanisms of 35 selected events have been derived using P-wave polarities and amplitude ratios, assuming a double-couple source. These fault plane solutions and various subsamples derived from them, were subsequently inverted for the best fitting stress tensor using a linear inversion method. The results show stress homogeneity beneath TVG for depths larger than 3 km while the stress tensor is characterized by a subvertical σ1 and a subhorizontal NW-SE trending σ3 axis consistent with the regional stress field in northern Taiwan. On the other hand, Chihsinshan which is an area of vigorous hydrothermal activity, exhibits a localized stress field with horizontal NW-SE trending σ3 axis and NE-SW trending horizontal σ1. Such an axes orientation is likely to be causing opening of microcracks and thus favour the ascent and circulation of fluids in the upper crust. Shear wave splitting measurements seem to confirm these results, showing fast polarization directions along NNE-NE, subparallel to the main fracture system in TVG. Shear wave anisotropy averages about 2.9% and is probably caused by an anisotropic volume of fluid-saturated cracks within the upper 2.5 km of the crust.

SUMMARY This study conducts a comprehensive investigation of crustal seismic anisotropy over varied geological regimes of Taiwan. With a large amount of earthquake data, the lateral variation of seismic shear wave splitting (SWS) is fully examined in terms of crustal deformation process. As the well-known vigorous orogeny subjected to the Eurasian–Philippine plate collision, tectonic convergence of Taiwan is presumably propagating from east to west. The acquired SWS waveform data cover areas from the slightly deformed Western Plains to the intermediate-to-high metamorphic Western Foothills and central mountain ranges. By means of waveform cross-correlation, the SWS parameters—the fast-wave polarization orientation and delay time—infer that the mechanism of lithologic deformation of Taiwan convergence can be classified into two domains: the convergence-parallel laminating west of the Deformation Front and the convergence-perpendicular striking east of the Deformation Front. The convergence-parallel SWS measurement presents the internal fabrics consisting of microfractures subject to lateral compression before the yielding of the lithologic strength, whereas the convergence-perpendicular measurements reveal the lateral accommodation of deformation as the stress/strain surpass the yielding strength of rock, where the predominant SWS polarization is in the NE–SW direction similar to the general trend of Taiwan's mountain ranges. It is remarkable that there is no correlation between metamorphic degrees with SWS parameters. The geological province which corresponds to higher metamorphism is not consistent with large SWS parameters. This may be because of anisotropic weakness caused by multiple tectonic processes at considerable metamorphic zone. Furthermore, comparison of the SWS delay times with corresponding focal depths suggests that seismic anisotropy in the upper crust may come from multiple layers, and the fabric lamination causing the anisotropy may be confined only within the shallow crust.

We compile a comprehensive list of hypocentral locations and source parameters for earthquakes occurred in the Taiwan region. The collected database is used to construct 3D patterns of seismic deformation that are associated with various tectonic processes such as subduction and collision between the Philippine Sea plate and Eurasia. Specifically, we divide the entire Taiwan region into a 3D grid of 10km interval and the amount of seismic moment released at each point is the sum of all individual events in the vicinity. In case when the physical dimension of an earthquake is larger than 10km, the corresponding seismic moment is distributed across multiple grid points. This quantitative approach gives a better depiction on many first-order features in the region. The most intriguing one is that a significant deficit in the total amount of released seismic moment can be clearly identified around the Chi-Chi source region before the big earthquake occurred in 1999. The deficit trough is then filled by the seismic moment of the Chi-Chi earthquake sequence. This deficit-then-fill pattern in a region's seismic moment distribution can be used as an indicator to pinpoint the locations of large earthquakes in the foreseeable future. Following this argument, we suggest that disastrous earthquakes with magnitudes comparable to that of the Chi-Chi earthquake are due for the two regions adjacent to the Chi-Chi source area (i.e., the Miaoli-Hsinchu domain to the north and the Chiayi domain to the south).

We have taken the seismic moment (M o) values of 79 earthquakes occurring in the Taiwan region that have been published in the Global centroid-moment tensor (CMT) and regional Broadband Array in Taiwan for Seismology (BATS) catalogues for the period 1996–2005 and compared the values determined from the global and regional networks, respectively. M oG and M oB are used to denote the M o values published in the Global CMT and regional BATS catalogues, respectively. Our results show that M oB linearly correlates with M oG and that M oB is, on average, approximately equal to 0.37M oG. This difference may be caused by the use of shorter period seismic waves in BATS for estimating M oB. The moment magnitude evaluated from regional BATS seismograms is about 0.3 less than that estimated from global data.

Normal Pn waves are commonly observed in Taiwan from shallow regional earthquakes at epicentral distances larger than 120 km, similar to the observations in many other continental regions. However, the critical distances to observe Pn waves for shallow eastern Taiwan earthquakes vary with azimuth corresponding to a significant variation of crustal thickness. In particular, anomalous Pn waves are commonly observed for shallow eastern Taiwan earthquakes recorded on seismic stations at epicentral distances as small as 60 km along the collision zone suture, the Longitudinal Valley. For the same event, normal Pn waves are observed at other seismic stations elsewhere on the island. The apparent velocity of the anomalous and normal Pn waves from the same event is 7.8 ± 0.15 km/sec, which is consistent with the average Pn velocity in the Taiwan area. Thus, the unusually short critical distance for Pn waves in eastern Taiwan suggests that the crust beneath the collision zone suture must be very thin and the upper mantle beneath the Longitudinal Valley must be relatively elevated compared with that beneath the other parts of Taiwan. Assuming a simple 1D layered velocity model, the Moho depth beneath the suture zone can thus be estimated at ∼23 ± 2 km. This observation is consistent with the recent report from a high-resolution 3D tomographic inversion that a narrowly confined, anomalously elevated, and north-northeast–south-southwest elongated oceanic upper mantle was imaged beneath the Longitudinal Valley from Hualien in the north to Taitung in the south (Kim et al. , 2005, 2006). Furthermore, the preceding observations may also support the interpretation that the conduction of excess heat supply from the elevated hot oceanic upper mantle into the adjacent mid- to-lower continental crust over a long period of geological time may play an important role in the crustal deformation beneath the continent, including metamorphism, thickening, and uplifting.

The Tatun Volcano Group (TVG) is located at the northern tip of Taiwan, near the capital Taipei and close to two nuclear power plants. Because of lack of any activity in historical times it has been classified as an extinct volcano, even though more recent studies suggestthatTVGmighthavebeenactiveduringthelast20ka.InMay2003aseismicmonitoringprojectattheTVGareawasinitiated by deploying eight three-component seismic stations some of them equipped with both short-period and broadband sensors. During the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events, or as a continuous sequence in the form of spasmodic bursts. Mixed and low frequency events were also present during the same period,eventhoughtheyoccurredonlyrarely.ArrivaltimesfromeventswithclearP-/S-wavephaseswereinvertedinordertoobtaina minimum 1Dvelocity modelwithstation corrections.Probabilistic nonlinear earthquakelocationswere calculated for allthese events using the newly derived velocity model. Most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity, forming tight clusters at depths shallower than 4 km. Relative locations, calculated using the double-difference method and utilising catalogue and cross-correlation differential traveltimes, showed insignificant differences when compared to the nonlinear probabilistic locations. In general, seismicity in the TVG area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts, mixed/low frequency events and a b-value (1.17±0.1) higher than in any other part of Taiwan. These observations, that are similar to those reported in other dormant Quaternary volcanoes, indicate that a magma chamber may still exist beneath TVG and that a future eruption or period of unrest should not be considered unlikely.

Knowing whether there is a slab of subducted lithosphere beneath central Taiwan is important for our understanding of the regional tectonic evolution. In this study, we aim at resolving this issue by investigating the seismic effects of a potential near-receiver slab, using data from a local broadband array. Rays from deep and intermediate-depth earthquakes in the Tonga–Kermadec subduction zone to central Taiwan stations exhibit reduced amplitudes and travel times relative to KMNB in the 0.1–0.4 Hz band, with a correlation between the degree of reduction and earthquake latitudes. The overall amplitude reduction observed at central Taiwan stations is not observed for earthquakes from the Hindu Kush. Our analysis suggests that the observations are not caused by crustal structure but by receiver-side heterogeneous mantle or, more specifically, an eastern dipping aseismic slab beneath central Taiwan. The extent of the slab is determined by jointly modeling amplitude and travel time observations at SSLB and TPUB using a 2-D pseudospectral method.

We test a recent suggestion that orbital eccentricity has influenced the geomagnetic field by performing a range of spectral analyses, including estimation of statistical confidence limits, on a 2.2-million-year paleomagnetic inclination record from the western Caroline Basin (WCB). Our analyses indicate that the claimed ∼100-kyr signal, while present over discrete portions of the WCB paleomagnetic record, is not statistically significant for the entire record and is not modulated by the 404-kyr eccentricity component as would be expected if the signal was influenced by orbital eccentricity. This signal also has highly variable phase compared to orbital eccentricity variations and the paleomagnetic record shows no statistically significant coherency with the orbital signal. Our results from a nearby coeval western Philippine Sea record support the present analysis. Together, these data suggest that the hypothesis of orbital energization of the geomagnetic field has yet to be convincingly demonstrated.

SUMMARY Large earthquakes near active volcanoes, that exhibit non-double-couple source properties are usually interpreted as the result of either magma intrusion or geometrical complexity along the fault plane. Such an earthquake occurred in 1996 September 29 at Bardarbunga volcano in central Iceland, to be followed 2 days later by a major volcanic eruption at the area between Bardarbunga and the nearby Grimsvotn volcano. Both of these active volcanic centres lie underneath the Vatnajokull glacier, a permanent ice cap that covers a large area of central Iceland. This event was recorded by a temporary network (HOTSPOT) that consisted of 30 broad-band three-component seismometers covering most of Iceland. The waveforms of this event at all stations show an emergent, low-amplitude, high-frequency onset that is superposed on a longer-period signal. The corresponding amplitude spectra show a low-frequency content (

A model parameterization based on a multiresolution wavelet representation is proposed for geophysical inverse problems in this study. The matrix equation for the wavelet representation of the model is constructed by dual wavelet transforms of a readily built matrix based on the pixel parameterization. We demonstrate the merits of the proposed parameterization using the simple examples of geophysical data gridding and 3D seismic tomography, and comparing the results with those obtained by the conventional pixel parameterization. It is shown that for the conventional damped least‐squares solutions of pixel‐parameterized models, regularization by norm damping deteriorates the underlying model correlation due to sparse and heterogeneous sampling, whereas roughness damping schemes impose a priori arbitrary correlation length irrespective of the heterogeneous sampling. The scale hierarchy embedded in the proposed multiresolution parameterization facilitates the compromise between the dual spatial and scale resolution. Depending on the local richness of the data constraints across different scales at a site, model variation of various scale spectra can be robustly resolved through the scale hierarchy. There is thus no need to invoke additional smoothness regularization.

International audience; We developed a Real-time Online earthquake Simulation system (ROS) to simulate regional earthquakes in Taiwan. The ROS uses a centroid moment tensor solution of seismic events from a Real-time Moment Tensor monitoring system (RMT), which provides all the point source parameters including the event origin time, hypocentral location, moment magnitude and focal mechanism within 2 minutes after the occurrence of an earthquake. Then, all of the source parameters are automatically forwarded to the ROS to perform an earthquake simulation, which is based on a spectral-element method (SEM). A new island-wide, high resolution SEM mesh model is developed for the whole Taiwan in this study. We have improved SEM mesh quality by introducing a thin high-resolution mesh layer near the surface to accommodate steep and rapidly varying topography. The mesh for the shallow sedimentary basin is adjusted to reflect its complex geometry and sharp lateral velocity contrasts. The grid resolution at the surface is about 545 m, which is sufficient to resolve topography and tomography data for simulations accurate up to 1.0 Hz. The ROS is also an infrastructural service, making online earthquake simulation feasible. Users can conduct their own earthquake simulation by providing a set of source parameters through the ROS webpage. For visualization, a ShakeMovie and ShakeMap are produced during the simulation. The time needed for one event is roughly 3 minutes for a 70 sec ground motion simulation. The ROS is operated online at the Institute of Earth Sciences, Academia Sinica (http://ros.earth.sinica.edu.tw/). Our long-term goal for the ROS system is to contribute to public earth science outreach and to realize seismic ground motion prediction in real-time.

Shear wave splitting in regional and teleseismic shear waves (ScS and S) has been analyzed for fast polarization directions and delay times at 33 short-period and 17 broad-band permanent and temporary seismic stations in the Taiwan region. The orientations of anisotropy for the Taiwan stations are generally parallel to the strike of the mountain belt, with a clockwise rotation following the trend of regional geologic fabric in north Taiwan. The delay times, on the other hand, vary greatly from 0.5 to 2.1 s; the largest values are from the southwestern Taiwan foothills, and an abrupt change in delay times is found in south Taiwan. The main source of shear wave splitting in Taiwan is compatible with an anisotropic region between 25 and 230 km depths. The observed splitting parameters are consistent with upper mantle anisotropy resulting from the collisional tectonics that built the Taiwan orogen.

The Santorini caldera has been the focus of several large explosive eruptions in the past, the last of which occurred in the early 1950s. The volcano was dormant until early 2011 when increasing number of earthquakes accompanied significant intra-caldera uplift. This seismic activity was recorded by 8 temporary as well as 19 permanent seismic stations that were installed on Santorini and nearby islands after the onset of the unrest. Using data from January 2011 until June 2012 we calculated accurate relative locations for 490 events utilizing both catalog and waveform cross-correlation differential travel times of P- and S-phases. The distribution of relocated events exhibits a large cluster between Thera and Nea Kameni islands along the caldera rim, suggesting the activation of a preexisting ring fault. All hypocenters are located between 5 and 11 km resulting in a sharp cutoff of seismicity above and below these depths. We also used P and S travel times in order to calculate average Vp/Vs ratios and estimated shear wave splitting parameters (fast direction φ, delay time δt) for events within the shear wave window. The Vp/Vs ratios at several stations exhibit a majority of values consistently below the regional one (~ 1.77). Their temporal variations can be explained as periods of gas influx and depletion in the upper crust beneath the caldera. A comparison of δt for a number of earthquake doublets shows a progressive decrease of delay times towards the end of the unrest probably as a result of cracks closing owing to stress relaxation. The seismological observations presented here are compatible with petrological models that suggest the existence of a deep (11–14 km) dacitic magma reservoir and a shallower (< 5 km) rhyolitic magma chamber.

The southernmost part of the Ryukyu subduction, where the Philippine Sea Plate is subducting under the Eurasian Plate, is known to be a very seismically active region of transition from a north-dipping subduction along the Ryukyu subduction to an similar to SE-NW collision along the Taiwanese orogenic wedge. In this paper, we will focus on the Ryukyu forearc area close to Taiwan where the deformation is paroxysmal. In order to decipher the nature of the seismic deformation in this region, a three month passive experiment, combining 22 Ocean Bottom Seismometers and 51 onland stations, has been led. Starting from an a-priori heterogeneous model, we have obtained 801 well-located earthquake hypocenters, a precise P-wave tomography model and 14 focal mechanisms. The seismicity along the Ryukyu forearc is mainly located not only in the vicinity of the Interplate Seismogenic Zone (ISZ) but also within both the subducting PSP and the overriding plate. Seismicity within the upper-plate is essentially localized east of Nanao basin where NW-SE extension occurs, and northwest of the Hoping basin where strike-slip dominates. As revealed by both the P-wave velocity structure and the newly derived seismicity, we argue that a sub-vertical step offsetting the subducting PSP around 10 km may support the presence of a trench-parallel tear. The PSP also undergoes extension in its upper part that is probably caused by buckling and slab pull. The P-wave velocity structure reveals three other major features: (1) a continuity between the Central Range and the Ryukyu Arc with a shallower Moho (similar to 30 km depth) between similar to 122.3 degrees N and similar to 122.5 degrees N along the Ryukyu Arc, (2) high P-wave velocities along the eastern side of the Central Range and, (3) two bodies with similar high crustal velocities (6.5-7.0 km/s) at 12-18 km depths, embedded within the Ryukyu arc basement, just north of Hoping Basin and north of the Nanao Basin.