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We proposed earthquake forecasting models for Albania, one of the most seismogenic regions in Europe, to give an overview of seismic activity by implementing area source and smoothing approaches. The earthquake catalogue was first declustered to remove foreshocks and aftershocks when they are within the derived distance windows and time windows of mainshocks. Considering catalogue completeness, the events with M≥4.1 during the period of 1960–2006 were implemented for the learning forecast model. The forecasting is implemented into an area source model that includes 20 sub-regions and a smoothing model with a cell size of 0.2 ∘ × 0.2 ∘ to forecast the seismicity in Albania. Both models show high seismic rates along the western coastline and in the southern part of the study area, consistent with previous studies that discussed seismicity in the area and currently active regions. To further validate the forecast performance of the two models, we introduced the Molchan diagram to quantify the correlation between models and observations. The Molchan diagram suggests that the models are significantly better than a random distribution, confirming their forecasting abilities. Our results provide crucial information for subsequent research on seismic activity, such as probabilistic seismic hazard assessment. [ABSTRACT FROM AUTHOR]

We proposed earthquake forecasting models for Albania, one of the most seismogenic regions in Europe, to give an overview of seismic activity by implementing area source and smoothing approaches. The earthquake catalog was firstly declustered to remove foreshocks and aftershocks when they are within the derived distance- and time-windows of mainshocks. Considering catalog completeness, the events with M≥4.0 during the period of 1960–2006 were implemented for the forecast model learning. The forecasting is implemented into an area source model that includes 20 sub-regions and a smoothing model with a cell size of 0.2° x 0.2° to forecast the seismicity in Albania. Both models show high seismic rate along the western coastline and at the southern part of the study area, consistent with previous studies which discussed seismicity in the area and currently active regions. To further validate the forecast performance from the two models, we introduced the Molchan diagram to quantify the correlation between models and observations. The Molchan diagram suggests that both models are significantly better than a random distribution, confirming their forecasting abilities. Our results provide crucial information for subsequent research on the seismic activity, such as probabilistic seismic hazard assessment. [ABSTRACT FROM AUTHOR]

Temporal fault‐zone observations are important to better understand the evolution of fault structure and stress configuration. However, long‐term monitoring in the fault zone is rare after a large earthquake. Here, we use seismic data in the fault zone at 1‐km depth from the Taiwan Chelungpu‐fault Drilling Project to study long‐term anisotropy after the 1999 Mw7.6 Chi‐Chi earthquake. The direct S‐wave splitting measurements resolve the overall weak anisotropy in the shallow crust. In order to resolve fault damage zone anisotropy, we perform coda cross‐correlation technique for 794 microearthquakes between 2007 and 2013. We estimate the temporal change in background shear‐wave velocity, fast shear‐wave polarization direction (FSP), and strength of anisotropy (Aani) in the fault damage zone. We show the average FSP direction is N93°E with a significant Aani of about 12%, likely due to the pervasive vertical microcracks created after the earthquake. Temporal variations of anisotropy exhibit seasonal variation with periodicity every 9–12 months that correlates with rainfall events. Furthermore, long‐term anisotropy shows a gradual rotation of FSP direction of about 15° during the first 4 years of observation. At the same time, the strength of anisotropy reduced from 17% to 10% and shear‐wave velocity increased, suggesting the fault healed after the earthquake. This study reports in‐situ evidence for two key observations: (a) long‐term, fault‐zone healing after a major earthquake and (b) modulation of 1‐km deep fault‐zone properties by surficial hydrologic processes. These observations may provide constraints on the response of the fault damage zone in the interseismic period. Plain Language Summary: Earthquakes are generated when rocks slip quickly along a fault. Damage produced by fault rupture is often observed in an area with enhanced fracture networks called the fault damage zone. Fractures inside the damage zone can open or close in response to the background stress. Therefore, observing fault fractures may provide useful information on how the background stresses evolve in the fault. Here, we observe the time‐dependent change of fractures in the fault damage zone through seismic anisotropy analysis. We measure the velocity difference of a seismic shear‐wave in different azimuths to determine the anisotropy. The azimuth of the fastest shear‐wave indicates the dominant fracture orientation inside the fault damage zone. We use borehole seismometers installed within the Chelungpu fault in Taiwan to study the long‐term anisotropy after a major earthquake, and found the fault‐zone is highly damaged with fractures aligning with the tectonic stress direction. Anisotropy varies seasonally due to fluid pressure changes in fault zone fractures during and after rainfall events. In addition, we observe a long‐term anisotropy change that might indicate fracture closure. This study presents a unique observational dataset which allows us to better understand fault zone evolution after a large earthquake. Key Points: A 7‐year (2007–2013) fault‐zone seismic anisotropy is presented from the Taiwan Chelungpu‐fault Drilling Project borehole seismic arrayAverage fast shear‐wave direction of N93°E and anisotropy strength of 12% are observed after the Chi‐Chi earthquakeAnisotropy varies seasonally with rainfalls. Long‐term anisotropy from 17% to 10% with increasing velocity suggests fault healing [ABSTRACT FROM AUTHOR]

Author(s): Kuo-Fong Ma (corresponding author) [1]; Hidemi Tanaka [2]; Sheng-Rong Song [3]; Chien-Ying Wang [1]; Jih-Hao Hung [1]; Yi-Ben Tsai [1]; Jim Mori [4]; Yen-Fang Song [5]; Eh-Chao Yeh [6]; [...]

The 2018 February 6 M w 6.3 Hualien earthquake caused severe localized damage in Hualien City, located 20 km away from the epicentre. The damage was due to strong (>70 cm s−1) and sharp (duration ∼2.5 s) velocity pulses. The observed peak ground-motion velocity in Hualien City symmetrically decays with distance from the nearby Milun fault. Waveforms observed on the opposite sides of the fault show reversed polarity on the vertical and N–S components while the E–W component is almost identical. None of the published finite-fault slip models can explain the spatially highly localized large velocity pulses. In this study, we show that an M w 5.9 strike-slip subevent on the Milun fault at 2.5 km depth, rupturing from north to south at ∼0.9 V s speed, combined with site effects caused by surficial layers with low S-wave speed, can explain the velocity pulses observed at the dense strong-motion network stations. This subevent contributes only 25 per cent of the total moment of the 2018 Hualien earthquake, suggesting that a small local slip patch near a metropolis can dominate the local hazard. Our result strongly suggests that seismic hazard assessments should consider large ground-motion variabilities caused by directivity and site effects, as observed in the 2018 Hualien earthquake. [ABSTRACT FROM AUTHOR]

The southernmost portion of the Ryukyu Trench near the island of Taiwan potentially generates tsunamigenic earthquakes with magnitudes from 7.5 to 8.7 through shallow rupture. The fault model for this potential region dips 10° northward with a rupture length of 120 km and a width of 70 km. An earthquake magnitude of Mw 8.15 is estimated by the fault geometry with an average slip of 8.25m as a constraint on the earthquake scenario. Heterogeneous slip distributions over the rupture surface are generated by a stochastic slip model, which represents the decaying slip spectrum according to k-2 in the wave number domain. These synthetic slip distributions are consistent with the abovementioned identical seismic conditions. The results from tsunami simulations illustrate that the propagation of tsunami waves and the peak wave heights largely vary in response to the slip distribution. Changes in the wave phase are possible as the waves propagate, even under the same seismic conditions. The tsunami energy path not only follows the bathymetry but also depends on the slip distribution. The probabilistic distributions of the peak tsunami amplitude calculated by 100 different slip patterns from 30 recording stations reveal that the uncertainty decreases with increasing distance from the tsunami source. The highest wave amplitude for 30 recording points is 7.32m at Hualien for 100 different slips. Compared with the stochastic-slip distributions, the uniform slip distribution will be highly underestimated, especially in the near field. In general, the uniform slip assumption only represents the average phenomenon and will consequently ignore the possibility of tsunami waves. These results indicate that considering the effects of heterogeneous slip distributions is necessary for assessing tsunami hazards to provide additional information about tsunami uncertainties and facilitate a more comprehensive estimation. [ABSTRACT FROM AUTHOR]

The duration of strong shaking is particularly important for assessing building performance, potential landslides and liquefaction hazards. The results of this investigation can potentially help reduce related fatalities and economic losses. In this study, we analyzed the acceleration seismograms of the Taiwan strong motion network to characterize the strong shaking duration associated with earthquake sources, propagation paths and site effects. This study proposes a new definition for the strong shaking duration called 'effective shaking duration' (ESD), which considers the amplitude and radiation energy decays. We first consider the window of a time series during which the amplitude is ≥0.01 g, and we then defined the ESD as the length of the interval of the dissipated energy within 5-95 % of the total energy during this time frame. We calculated the strong shaking duration for 495 inter-plate events with magnitudes of M > 5.0 and focal depths <50 km in the Taiwan region from 1994 to 2012. Using a nonlinear regression procedure, we thus obtained an empirical equation for strong shaking durations. The equation is a function of earthquake magnitude, distance and site conditions, which are defined by the V30 value (the S-wave velocity structure of the top 30 m of the site). The results indicate that the shaking durations significantly increase with magnitude and also decrease with distance and V30. Compared with empirical equations from global datasets, our empirical equation is applicable to earthquakes in other regions and will produce smaller but more applicable duration values for smaller earthquakes. However, for larger events, our ESD values are comparable with those derived from other definitions (e.g., significant duration). Although the empirical relationship is mainly based on Taiwanese events, in view of the massive dataset, this empirical equation could provide important information to the global community regarding the ground shaking duration estimation in the ground motion prediction of future earthquakes. [ABSTRACT FROM AUTHOR]

Advances in observational, theoretical and computational technologies have made it possible for automatic, real-time solutions of the focal mechanisms of earthquake point sources. However, for earthquakes of moderate and greater magnitudes, the complexity of the source kinematic processes often requires additional characteristics on the source rupture in order to make seismotectonic inferences and to explain the observed directivity effects of the radiation of seismic energy. We develop an efficient and effective approach to determining the average finite-rupture models of moderate earthquakes by fitting synthetic and recorded broadband waveforms. A Green's tensor database is established using 3-D structural model with surface topography to enable rapid evaluations of accurate synthetic seismograms needed for source parameter inversions without the need for high-performance computing. We take a two-step strategy: In the first step, a point-source model is determined by a grid search for the best fault-plane solution. Then, taking the two nodal planes in the point-source model as candidates of the actual fault plane, a second grid search is carried out over a suite of simplified finite-rupture models to determine the optimal direction and speed of the integrated rupture of the finite source. We applied our method to four moderate events (MW ≈ 6) in southeastern Taiwan. Results show that our technique provides an effective choice in semi-automatic, near real-time determinations of finite-source parameters for earthquake hazard assessment and mitigation purposes. [ABSTRACT FROM AUTHOR]

We have developed a real-time moment tensor monitoring system (RMT) which takes advantage of a grid-based moment tensor inversion technique and real-time broad-band seismic recordings to automatically monitor earthquake activities in the vicinity of Taiwan. The centroid moment tensor (CMT) inversion technique and a grid search scheme are applied to obtain the information of earthquake source parameters, including the event origin time, hypocentral location, moment magnitude and focal mechanism. All of these source parameters can be determined simultaneously within 117 s after the occurrence of an earthquake. The monitoring area involves the entire Taiwan Island and the offshore region, which covers the area of 119.3°E to 123.0°E and 21.0°N to 26.0°N, with a depth from 6 to 136 km. A 3-D grid system is implemented in the monitoring area with a uniform horizontal interval of 0.1° and a vertical interval of 10 km. The inversion procedure is based on a 1-D Green's function database calculated by the frequency–wavenumber (fk) method. We compare our results with the Central Weather Bureau (CWB) catalogue data for earthquakes occurred between 2010 and 2012. The average differences between event origin time and hypocentral location are less than 2 s and 10 km, respectively. The focal mechanisms determined by RMT are also comparable with the Broadband Array in Taiwan for Seismology (BATS) CMT solutions. These results indicate that the RMT system is realizable and efficient to monitor local seismic activities. In addition, the time needed to obtain all the point source parameters is reduced substantially compared to routine earthquake reports. By connecting RMT with a real-time online earthquake simulation (ROS) system, all the source parameters will be forwarded to the ROS to make the real-time earthquake simulation feasible. The RMT has operated offline (2010–2011) and online (since January 2012 to present) at the Institute of Earth Sciences (IES), Academia Sinica (http://rmt.earth.sinica.edu.tw). The long-term goal of this system is to provide real-time source information for rapid seismic hazard assessment during large earthquakes. [ABSTRACT FROM AUTHOR]

SUMMARY The 1909 April 14 Taiwan earthquake caused significant damage in Taipei. Most of the information on this earthquake available until now is from the written reports on its macro-seismic effects and from seismic station bulletins. In view of the importance of this event for assessing the shaking hazard in the present-day Taipei, we collected historical seismograms and station bulletins of this event and investigated them in conjunction with other seismological data. We compared the observed seismograms with those from recent earthquakes in similar tectonic environments to characterize the 1909 earthquake. Despite the inevitably large uncertainties associated with old data, we conclude that the 1909 Taipei earthquake is a relatively deep (50-100 km) intraplate earthquake that occurred within the subducting Philippine Sea Plate beneath Taipei with an estimated MW of 7 ± 0.3. Some intraplate events elsewhere in the world are enriched in high-frequency energy and the resulting ground motions can be very strong. Thus, despite its relatively large depth and a moderately large magnitude, it would be prudent to review the safety of the existing structures in Taipei against large intraplate earthquakes like the 1909 Taipei earthquake. [ABSTRACT FROM AUTHOR]

The rupture process and tectonic surroundings of the 2008 Wenchuan, China, earthquake are both complex in a way that might be related to the heterogeneous stress field of the Longmen Shan region. In this study, we construct dynamic models with heterogeneous initial stress to reproduce a kinematic inversion result by Wen et al. (2012) and investigate the physical mechanisms of the variable slip pattern and rupture velocity in this event. The results show that, for the 2008 Wenchuan earthquake, (1) the rupture behavior and slip pattern can be explained by heterogeneities in initial stress and (2) the variations of rupture velocity and ground-motion pattern are strongly affected by both stress drop and fault strength. Our preferred dynamic model agrees with the kinematic model that the 2008 Wenchuan earthquake initiated with a slow velocity in the southwest, sped up in the segment with the largest slip, and then slowed down for the remaining propagation to the northeast. [ABSTRACT FROM AUTHOR]

SUMMARY Microearthquakes with magnitude down to 0.3 were detected by the Taiwan Chelungpu-fault Drilling Project Borehole Seismometers (TCDPBHS). Despite the large coseismic slip of 12 m at the drill site during the 1999 Chi-Chi earthquake, our studies show very little seismicity near the TCDPBHS drill site 6 yr after the Chi-Chi main shock. The microearthquakes clustered at a depth of 9-12 km, where the Chelungpu thrust fault turns from a 30° dipping into the horizontal decollement of the Taiwan fold-and-thrust tectonic structure. Continuous GPS surveys did not observe post-slip deformation at the larger slip region and no seismicity was observed near the drill site. Therefore we suggest that the thrust belt above the decollement is locked during this interseismic period. We further investigated source parameters of 242 microearthquakes by fitting ω−2-shaped Brune source spectra to our observation data using a frequency-independent Q model. We find that the static stress drop increases significantly with increasing seismic moment. However, due to the intense debate on this topic of scaling-relations and the related self-similarity of earthquakes, we further improve the data analysis and correct for path and site effects using the Projected Landweber Deconvolution (PLD) method for events within some clusters. The PLD method analyses the source time functions of the larger and the smaller event by an iterative technique. As a result we received source dimensions and stress drops of larger events including path and site effect corrections. The results from the PLD method are less scattered and also show a positive relation between static stress drop and seismic moment. We find a similar positive trend for the apparent stress scaling with seismic moment. [ABSTRACT FROM AUTHOR]

SUMMARY Theoretical and observational evidence implies that variable rupture velocity may be associated with high fault slip and high moment release rates during earthquakes. We investigate this relationship for the 2008 Mw 7.9 Wenchuan China, Earthquake, which appears to have experienced a highly variable moment release rate. We used an empirical Green's function (EGF) deconvolution analysis of teleseismic waveforms to retrieve the primary rupture characters for this event. Based on field observations and the deconvolved source-time function from thrust and strike-slip EGFs, we divided the ruptured fault into three segments, allowing us to determine the spatial slip distribution with different average rupture velocities on each segment. We deployed a grid search analysis, in which we integrated the teleseismic waveform inversion and forward modelling of the regional surface wave to determine the optimum rupture speed in each fault segment. Our result shows that the 2008 Wenchuan Earthquake had slip amplitude, direction and rupture velocity that were highly spatially variable. The earthquake initially ruptured with nearly pure thrust motion and a slow rupture velocity of 1.7 km s-1. Then, the rupture speed increased up to 3.1-3.3 km s-1 and produced the largest slip on the second segment, where two parallel faults ruptured simultaneously. Rupture velocity then slowed down to 2.5-2.9 km s-1 on the final segment, which underwent primarily strike-slip motion. In total, the fault extended 300 km with an average rupture speed in the range of 2.6-2.9 km s-1. The high rupture speed (close to the shear wave velocity) in the second segment may be related to the simultaneous rupture of the two parallel faults on well-established pre-existing structures. The spatial distribution of rupture velocity and slip are likely related to the strongly rotational and 3-D deformation in the eastern Tibetan Plateau margin, which leads to the heterogeneous stress field of the Longmen Shan region. [ABSTRACT FROM AUTHOR]

Abstract: Continuous cores and a suit of geophysical measurements were collected in two scientific drill holes to understand physical mechanisms involved in the large displacements during the 1999 Chi-Chi earthquake. Physical properties obtained from wire-line logs including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature, are primarily dependent on parameters such as lithology, depth and fault zones. The average dip of bedding, identified from both cores and FMI (or FMS) logs, is about 30° towards SE. Nevertheless, local azimuthal variations and increasing or decreasing bedding dips appear across fault zones. A prominent increase of structural dip to 60°–80° below 1856 m could be due to deformation associated with propagation of the Sanyi fault. A total of 12 fault zones identified in hole-A are located in the Plio-Pleistocene Cholan Formation, Pliocene Chinshui Shale and Miocene Kueichulin Formation. The shallowest fault zone occurs at 1111 m depth (FZ1111). It is a 1 m gouge zone including 12 cm of thick indurate black material. We interpreted this zone as the slip zone during Chi-Chi earthquake. FZ1111 is characterized by: 1) bedding-parallel thrust fault with 30-degree dip; 2) the lowest resistivity; 3) low density, V p and V s, 4) high V p/V s ratio and Poisson''s ratio; 5) low energy and velocity anisotropy, and low permeability within the homogeneous 1 m gouge zone; 6) increasing gas (CO2 and CH4) emissions, and 7) appearance of smectite within the primary slip zone. In situ stresses at the drill site were inferred from leak-off tests, borehole breakouts and drilling-induced tensile fractures from borehole FMS/FMI logs, and shear seismic wave anisotropy from DSI logs. The dominant fast shear-wave polarization direction is in good agreement with regional maximum horizontal stress axis, particularly within the strongly anisotropic Kueichulin Formation. A conjugate set of secondary directions are parallel to microcrack orientations. A drastic change of orientation of fast shear-wave polarization across the Sanyi thrust fault at the depth of 1712 m reflects the change of stratigraphy, physical properties and structural geometry. [Copyright &y& Elsevier]

We investigate the thermo-mechanical properties beneath the young orogenic belt of Taiwan by constructing a shear strength profile from a vertical stratified rheological structure. The stratified rheological structure is estimated based on the recently developed thermal structure and its likely composition. Subduction–collision in the young orogenic belts and the thick accretionary wedge make a significant contribution to the growth of sialic crust in the hinterland. The sialic bulk crust not only results in a low seismic velocity but also produces weak crust in the hinterland. The earthquake depth–frequency distribution in the foreland and hinterland correlates very well with the regimes of the brittle/ductile transition revealed in the strength profile. Our results show that the observed two-layer seismicity in the foreland is due to a moderate geotherm and an intermediate mafic bulk composition; while single-layer seismicity in the hinterland is due to its felsic bulk composition. In the foreland, the mechanically strong crust (MSC) and the mechanically strong lithosphere (MSL) coincide with frequent seismicity. The shallow MSC in the hinterland is consistent with the 20- to 25-km seismicity occurring there. The total lithospheric integrated strength (LIS) in the hinterland is only about half of that in the foreland, suggesting a weak lower crust and lithosphere mantle in the hinterland. The results confirm that the earthquake cutoff depth is a proxy for temperature. The calculated decrease of effective elastic thickness (EET) from the orogenic margin (foreland) to the center (hinterland) is consistent with the results of flexure modeling in most orogenic belts. Due to the weak LIS in the hinterland, crustal thinning and rifting may occur in the future. Our results, thus, suggest that the mechanical structure is also closely related to the composition and is not directly reflected in the thermal structure. [Copyright &y& Elsevier]

Abstract: The attenuation factor, Q, at a fault zone is an important parameter for understanding the physical properties. In this study, we investigated the Q value of the Chelungpu Fault, the main rupture of the Mw 7.6 Chi-Chi earthquake, using the 7-level TCDP borehole seismometer array (TCDPBHS). The TCDPBHS was deployed at depths from 945 to 1270m throughout the 1999 ruptured slip zone at 1111m. Three borehole seismometers (BHS1–BHS3) were placed in the hanging wall, and the remaining three (BHS5–BHS7) were placed in the foot wall, with BHS4 near the slip zone. The configuration allowed us to estimate the Q-structure of the recent ruptured fault zone. In this study, we estimated Q values between BHS1 and BHS4, Qs 1 (Qp 1) at the fault zone and between BHS4 to 2km in depth, Qs 4 (Qp 4) beneath the fault zone. We utilized two independent methods, the spectral ratio and spectral fitting analyses, for calculating the Q value of Qs 1 (Qp 1) in order to provide a reliability check. After analyzing 26 micro-events for Qs and 17 micro-events for Qp, we obtained consistent Q values from the two independent methods. The values of Qs 1 and Qp 1 were 21–22 and 27–35, respectively. The investigation for the value of Qs 4 was close to 45, and Qp 4 was 85. These Qp and Qs values are quiet consistent with observations obtained for the San Andreas Fault at the corresponding depth. A low Qs 1 value for the recent Chelungpu Fault zone suggests that this fault zone has been highly fractured. Qs values within the Chelungpu Fault, similar to those within the San Andreas Fault, suggest that the Q structure within the fault zone is sedimentary rock independent. However, the possible existence of fluids, fractures, and cracks dominates the attenuation feature in the fault zone. [Copyright &y& Elsevier]