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On September 17th and 18th, 2022, shallow earthquakes with magnitudes of 6.6 (ML) and 6.8 (ML) occurred in the eastern Taiwan Longitudinal Valley, which marks the collision zone between the Philippine Sea plate and the Eurasian plate, and led to noticeable surface deformation and ruptures within 70 km. This study primarily focuses on the southernmost section of the rupture zone—the Pingting Terrace. Surface rupture locations and behaviors correspond to changes in topography, providing mutual confirmation that the deformation behavior of Pingting Terrace is complex. Based on the distribution of surface ruptures and topography changes, this study roughly divides the Pingting Terrace into northern and southern segments, using the central concave feature as a boundary. The Riedel shear model analysis results show that the principal shear directions in the northern and southern segments are N–S trending and azimuth 20°, respectively. The maximum principal stress orientations are around 135° for the northern and 155° for the southern segments. These findings align with the fault mechanical investigation of the Lichi Mélange in the northern Muken River area of the Pingting Terrace. This suggests spatial changes in shear zone orientations within the Lichi Mélange, which contribute to developing pressure ridges due to transpressional forces. As a result, the Pingting Terrace experiences rapid uplift, causing the Luliao River to migrate southward into the Beinan River, while the eastern Beinan River turns to the eastward edge of the Pingting Terrace. Key points: Co-seismic surface rupture analysis using Riedel shear model. Flower structure of the Pingting Terrace. Relationship between the River channel change and the tectonic activity. [ABSTRACT FROM AUTHOR]

Earthquakes often occur in regions with complex material structure, such as sedimentary basins or mantle wedges. However, the majority of co-seismic modelling studies assume a simplified, often homogeneous elastic structure in order to expedite the process of model construction and speed up calculations. These co-seismic forward models are used to produce Green's functions for finite-fault inversions, so any assumptions made in the forward model may introduce bias into estimated slip models. In this study, we use a synthetic model of a sedimentary basin to investigate the impact of 3-D elastic structure on forward models of co-seismic surface deformation. We find that 3-D elastic structure can cause changes in the shape of surface deformation patterns. The magnitude of this effect appears to be primarily controlled by the magnitude of contrast in material properties, rather than the sharpness of contrast, the fault orientation, the location of the fault, or the slip orientation. As examples of real-world cases, we explore the impact of 3-D elastic structure with a model of the Taipei basin in Taiwan and a simulated earthquake on the Sanchaio fault, and with a 3-D geologic model of the San Francisco Bay Area and a slip model of the 1984 Morgan Hill earthquake on the Calaveras fault. Once again, we find that the presence of the basin leads to differences in the shape and amplitude of the surface deformation pattern, but we observe that the primary differences are in the magnitude of surface deformation and can be accounted for with a layered elastic structure. Our results imply that the use of homogeneous Green's functions may lead to bias in inferred slip models in regions with sedimentary basins, so, at a minimum, a layered velocity structure should be used. [ABSTRACT FROM AUTHOR]

Interseismic deformation describes the gradual accumulation of crustal strain within the tectonic plate and along the plate boundaries before the sudden release as earthquakes. In this study, we use 5 years of high spatial and temporal geodetic measurements, including Global Navigation Satellite System and Interferometric Synthetic Aperture Radar to monitor 3‐dimension interseismic crustal deformation and horizontal strain rate in Taiwan. We find significant deformation (strain rate >8 × $\times $ 10−6 yr−1) along the plate boundary between the Philippine Sea and the Eurasian Plates in east Taiwan. The high strain rate in the southern part of the Western Foothills is distributed along a few major fault systems, which reveals the geometry of the deformation front in west Taiwan. Our results help identify active faults in southwest and north Taiwan that were not identified before. These findings can be insightful in informing future seismic hazard models. Plain Language Summary: An earthquake cycle includes three phases: interseismic, coseismic, and postseismic. Interseismic deformation refers to the continuous crustal deformation that is built up by active tectonics. Depending on the relative motion between tectonic plates, the earthquake recurrence interval could vary by a few orders between different locations. As a result, knowing the crustal deformation rate and deformation accumulated in different fault zones can be useful for investigating future earthquake hazards. Using space geodesy tools like Global Navigation Satellite System and Interferometric Synthetic Aperture Radar, we can monitor surface deformation during the interseismic period. In this study, we monitor interseismic deformation in Taiwan using geodesy. We find that east Taiwan where two tectonic plates collide has the highest rate of deformation. In southwest and north Taiwan where most of the population resides, there is also high‐level of deformation distributed across a few different faults, indicating that some of the faults have a higher risk generating future earthquakes. As a result, knowing the amount of faults slip and amount of deformation built up during this interseismic period may inform us of potential future earthquake hazards. Key Points: This study combines Interferometric Synthetic Aperture Radar and Global Navigation Satellite System and produces high‐resolution 3‐D interseismic crustal velocities and strain rate estimates in TaiwanStrain rate measurements show high surface strain cumulation along east and southwest TaiwanThe surface strain rates and the earthquake hazard models based on seismology and field study are in good agreement [ABSTRACT FROM AUTHOR]

Uncertainties of the ground motions and structural parameters are the main factors that limit the accuracy of embankment seismic fragility assessment. In response to the uncertainties of the ground motions, artificial synthesizing method of the near-fault pulse-like ground motions was proposed, and 15 ground motions with the rupture fault distances ranging from 1 to 15 km were synthesized by taking the Chi-Chi earthquake in Taiwan, China, as an example. The Xi'an-Baoji expressway K1125 + 470 embankment was taken as the research object, and a total of 12 structural parameters were selected as the design variables, namely, the elastic modulus, bulk modulus, shear modulus, density, cohesion force, and internal friction angle of the embankment fill and soil foundation, respectively. In response to the uncertainties of these parameters, 3 principal components with large impacts on the embankment seismic fragility were extracted based on the principal component analysis. Mapping relationships among the principal components and embankment seismic damages were analyzed using the uniform design response surface method, and the seismic fragility assessment was carried out and the fragility curves were plotted. The research results are consistent with the actual embankment seismic damage conditions of the Chi-Chi earthquake, indicating that the proposed method is scientific and reasonable. It also shows that it would obviously overestimate the seismic performance in the embankment seismic fragility assessment without considering the uncertainties of the ground motions and structural parameters. [ABSTRACT FROM AUTHOR]

We investigate the coseismic and post-seismic deformation due to the 6 February 2018 M w 6.4 Hualien earthquake to gain improved insights into the fault geometries and complex regional tectonics in this structural transition zone. We generate coseismic deformation fields using ascending and descending Sentinel-1A/B InSAR data and GPS data. Analysis of the aftershocks and InSAR measurements reveal complex multifault rupture during this event. We compare two fault model joint inversions of SAR, GPS and teleseismic body waves data to illuminate the involved seismogenic faults, coseismic slip distributions and rupture processes. Our preferred fault model suggests that both well-known active faults, the dominantly left-lateral Milun and Lingding faults, and previously unrecognized oblique-reverse west-dipping and north-dipping detachment faults, ruptured during this event. The maximum slip of ∼1.6 m occurred on the Milun fault at a depth of ∼2–5 km. We compute post-seismic displacement time series using the persistent scatterer method. The post-seismic range-change fields reveal large surface displacements mainly in the near-field of the Milun fault. Kinematic inversions constrained by cumulative InSAR displacements along two tracks indicate that the afterslip occurred on the Milun and Lingding faults and the west-dipping fault just to the east. The maximum cumulative afterslip of 0.4–0.6 m occurred along the Milun fault within ∼7 months of the main shock. The main shock-induced static Coulomb stress changes may have played an important role in driving the afterslip adjacent to coseismic high-slip zones on the Milun, Lingding and west-dipping faults. [ABSTRACT FROM AUTHOR]

High-resolution bathymetry collected with an autonomous underwater vehicle (AUV) along the flanks of three ridges of the accretionary prism offshore southwestern (SW) Taiwan revealed more than 650 elongated depressions in water depths ranging from 1155 to 1420 m. The depressions are between 12 and 129 m long, 5 to 70 m wide, and up 9 m deep at their center and shallowing downslope to about 1-m depth. Due to their shape in downslope cross section, they are termed comet-shaped depressions (CSD). The CSD occur in patches of more than 100 with densities of 53 to 98 CSD/km2. In addition, seven topographic mounds were mapped and interpreted as pingos, which remotely operate vehicle (ROV) observations and sampling show to be covered with authigenic carbonate. These features overlie areas where multichannel seismic reflection (MCS) profiles show bottom simulating reflectors (BSR) and dipping strata extending from below the BSR to near the seafloor. We consider comet-shaped depression, a new type of pockmark, forms on a sloping seafloor where fluids expulsion occurred. We also suggest that the two types of distinctive geomorphic features are attributed to fluid venting which occurs at different rates, with the mounds developing slowly over time, but the CSD forming in discrete events perhaps associated with large earthquakes. [ABSTRACT FROM AUTHOR]

Coseismic surface ruptures during desctructive earthquakes (1999 Kocaeli–Düzce, Turkey and 1999 Chi-Chi, Taiwan) have caused devastating effects on buildings and infrastructures. Surface rupture remains a complicated phenomenon involving variable movements along near surface traces of both primary and secondary faults. The surface rupture patterns observed in nature, the rupture zone width and the magnitude of the surface rupture displacements, depend on the type of faulting, the earthquake magnitude, the complexities of fault geometry, as well as on the thickness and nature of the materials above bedrock. Surface rupture hazard assessment for determining the width of the surface rupture and rupture displacements magnitudes for civil engineering design needs to be site specific and incorporate various geological and geotechnical investigations. The current research on laboratory and numerical simulations to evaluate the impact of surface rupture on structure foundations is promising. However, it may be misleading to conclude that such models are sufficient to simulate the surface rupture complexities as observed in nature. [ABSTRACT FROM AUTHOR]

On 6 February 2018, a ML 6.2 earthquake struck the east coast of Taiwan and caused more than 200 casualties in the Hualien area. The mainshock initiated at a shallow depth of 6.3 km and was accompanied by numbers of foreshocks and aftershocks. Coseismic ruptures widely occurred along the Milun Fault with various deformational structures and caused a variety of damage. Extensive field survey and drone-based images reveal the distribution of offsets and the characteristics of the surface ruptures. The major surface ruptures were mostly synthetic and antithetic Riedel shears interlinked by push-up moletracks or tensional fissures. The strike of the principal displacement zone evolved from northeast to northwest and defined a 7.5-km-long NS fault trace concave to the east. The vertical offset reached its maximum of 50 cm on the north segment and became insignificant toward south. Both on-fault single measurements and cumulative sinistral offsets estimated across the rupture zones showed the pattern of southward declining with the maximum offset of 77 cm in the north. The width of the damage zone increased from 1 - 20 m to maximum 270 m in the south where the overstepping segments occurred. The development of step-overs is significant in the south, especially in the downtown area where slip is distributed across a transpressional area. Overall, the distinct features of the coseismic ruptures reflect the complex near-surface geology along the Milun Fault and offer new insights for future hazard assessments of the Hualien area. [ABSTRACT FROM AUTHOR]

A deadly Mw 6.4 earthquake occurred in the Hualien area of eastern Taiwan on 6 February 2018. It caused severe damage to infrastructure and creating surface ruptures in several areas mostly near the Milun Fault in Hualien City. In this study, we investigated the distribution of co-seismic surface ruptures by measuring the orientations of the ruptures, classifying the fracture patterns, and measuring the fracture geometries to calculate the principal displacement zone (PDZ) and the regional stress directions. As a result, local PDZ is observed to rotate anti-clockwise along the Milun Fault from north to south. Considering the deformation behaviors of the fractures and their relative positions along the Milun Fault, the shear zone in Qixingtan area is a horsetail structure derived from the right side of the end of the left-lateral strike-slip fault. In addition, the 170° trending fault splay in the middle segment and the fault branch corresponding to the Beipu Fault are elements of the Riedel shear structure associated with the left-lateral moving Milun Fault. Our results show that Riedel shear structures are common within co-seismic surface rupture zones in this area, and the variations in the orientations of Riedel structures reflect the influence of the pre-existing Milun Fault. We can also determine the most recent geometry, kinematics, and displacement characteristics of the Milun Fault through co-seismic surface ruptures and macro-scale Riedel shear structure analysis. This study provides a good example of understanding the relationship between the outcrop scale and the macroscale of the Riedel shear model. [ABSTRACT FROM AUTHOR]

A new interpretation is proposed for the 2010 Jia-Shian earthquake (Mw 6.2) which occurred on March 4, 2010 in southwestern Taiwan. Among the few tens of CGPS stations which recorded the mainshock at a distance less than 50 km we identified a few stations displaying horizontal displacements in disagreement with the dominant pattern of horizontal coseismic motion. Based on this observation, we find that a secondary rupture plane explains these peculiar motions and helps improving the modeling of the overall set of CGPS stations. We also model the teleseismic and strong-motion records to refine the rupture model. Finally, we obtain the slip distribution on a two-fault model by the joint inversion of all three datasets (CGPS, teleseismic, and strong-motion). The main fault plane is consistent with previous studies. It corresponds to a N311°E fault dipping 33° to the NE, with left-lateral–reverse oblique slip, likely related to the Chishan transfer fault zone (CTFZ). We find that the earthquake started with a sharp slip patch localized at the hypocenter. The secondary rupture plane is a N20°E thrust fault related to the Lungchuan anticline. Faulting on the main NW–SE plane during the Jia-Shian earthquake and stress inversions from previous studies both suggest a direction of maximum horizontal compression oriented NE–SW in the area of the earthquake, very different from the dominant NW–SE compression direction in Taiwan. We propose that such a variation can be related to a stress deviation in the immediate vicinity of the CTFZ, in a similar way as observed along the San Andreas Fault in California. On the other hand, rupture on the secondary fault plane associated with the Lungchuan anticline appears to be compatible with the dominant compression in Taiwan. [ABSTRACT FROM AUTHOR]

An unexpectedly large uplift of 91 mm was detected by geodetic observations during the rupture of 2016 M w 6.4 Meinong earthquake, Taiwan, which has been attributed as either a triggered anelastic and hydrologic related deformation from a proposed duplex/mud diapir or a triggered aseismic slip on a proposed backthrust. Here, using both high-rate GNSS and free-field strong motion data, we first estimated the coseismic source model. The locations of high PGV were inferred to explain the unexpected distribution of damaged buildings approximately 25–30 km west of the epicenter. Then, the aseismic surface displacements during the earthquake was differentiated using the coseismic source model. The aseismic extension of 3.8 μstrain is inferred at the unexpectedly large uplift region. Combining with local geology, residual gravity anomaly, seismic tomography, interseismic leveling vertical velocities, coseismic leveling uplifts and proposed Coulomb stress changes, the accelerated mud diapirism during the earthquake triggered by slip on the deep seismogenic fault was identified as the cause of unexpectedly large coseismic uplift. • High-rate GNSS and strong motion data were inverted to estimate coseismic slips. • Inferred PGV locations explained the unexpected distribution of damaged buildings. • Unexpectedly anelastic large uplift was caused by the accelerated mud diapirism. [ABSTRACT FROM AUTHOR]

The earthquake slip distribution self-similarity is investigated in this study. We complied finite fault slip models for earthquakes in the Taiwan orogenic belt and global earthquakes to determine the slip distribution self-similarity. Forty-one earthquakes (19 Taiwan earthquakes and 22 global earthquakes) in the Mw = 4.6 - 8.9 magnitude range were examined. The fault slip exhibited self-similar scaling between the rupture slip and area. The average area ratio (Rs) and slip ratio (Rd) follows a scaling of Rs = 10a-n(Rd) . Slip self-similarity implies that a fault rupture exhibits fractal behavior. The scaling exponent can be considered as a measure for the roughness degree of the slip distribution on the fault surface. This study suggests that the slip distribution for large earthquakes (Mw > 7.0) tends to have a more homogeneous slip. Scaling exponents can provide insight into earthquake rupture mechanics and the scaling of heterogeneous slips on the fault surface provides a basis for ground motion simulation for a finite fault for an earthquake scenario, particularly for near-fault motion. [ABSTRACT FROM AUTHOR]

Earthquakes can generate seismic disturbances that propagate vast distances and trigger landslides that can achieve high-speeds. It remains difficult to identify the co-seismic ground motion of these landslides and their triggering earthquakes. In this paper, we report on the analysis of co-seismic ground motions generated by the initiating fracture and deposition impact of the Tsaoling landslide. The landslide, with a source volume of 125 × 10 6 m 3 , was triggered by the 1999 Chi-Chi earthquake in Taiwan. The ground motion was recorded by a strong ground motion station, CHY080, near the scar area. The polarization of the seismic waves indicates that the peak acceleration was parallel to the dip direction. Modified ensemble empirical mode decomposition (EEMD), with additional clustering analysis, was applied to decompose the seismic signals. Two instances were found in the seismic records of a series of peculiar wave packets, with the first being associated with the landslide initiation and the second the landslide impact on the deposit valley. To confirm the first landslide breakage, the decomposed signals were compared with the predictions of the analytic elastic wave model and Newmark analysis. The landslide impact was verified with a computational fluid dynamic simulation. Comparison between the EEMD decomposed signals, elastic wave theory, Newmark rigid body analysis, and numerical simulation demonstrates the claimed landslide motion. [ABSTRACT FROM AUTHOR]

The 2010 March 4, Jia-Shian (Mw 6.3) earthquake in SW Taiwan caused moderate damage and no surface rupture was observed, reflecting a deep source that is relatively rare in western Taiwan. We develop finite-source models using a combination of seismic waveform (strong motion and broadband), Global Positioning System (GPS) and synthetic aperture radar interferometry (InSAR) data to understand the rupture process and slip distribution of this event. The rupture centroid source depth is 19 km based on a series of moment tensor solution tests with improved 1-D Green's functions. The preferred fault model strikes 322° and dips 27° to the NE and the mainshock is a thrust event with a small left-lateral component. The finite-source model shows a primary slip asperity that is about 20 km in diameter at a depth range from 22 to 13 km, with peak slip of 42.5 cm, a total scalar seismic moment of 3.25 × 1018 N m (Mw 6.34) and with an average static stress drop of 0.24 MPa. The rupture velocity of this event is faster than the mid-crustal shear wave velocity in Taiwan, which suggests the possibility of a supershear event which has not been previously observed in Taiwan. Systematic resolution and sensitivity tests are performed to confirm the slip distribution, rupture velocity, the choice of weighting and smoothing for the joint inversions, and the consistency of the slip distribution. The first 24 hours of aftershocks appeared along the upper periphery of the main coseismic slip asperity. Both the mainshock and aftershocks are located in a transition zone where the depth of seismicity and an inferred regional basal décollement increases from central to southern Taiwan. The difference between the current orientation of plate convergence in Taiwan (120º) and the P axis of this event (052º) and nearby measurements of recent crustal strain directions (050° to 080°), as well as the relatively low static stress drop, suggest that the Jia-Shian event involves the reactivation of a deep and weak pre-existing NW–SE geological structure. [ABSTRACT FROM AUTHOR]

We propose an approach to imaging earthquake source rupture process by direct back-projection of local high-frequency (0.1–2.5 Hz) P-wave displacements from strong-motion records. A series of synthetic experiments are performed which demonstrate that our approach is capable of recovering the spatial-temporal distribution of the source slip with a good station coverage and a high average coherence value between the target and template waveforms. We demonstrate the effectiveness of our approach by applying it to image the slip distribution of an earthquake occurred on 2010 March 4, in Jiasian (Mw = 6.0 and ML = 6.4) in southern Taiwan. The resulting moment-rate amplitude images show that the source rupture initiated at the vicinity of the hypocentre, followed by a moderate moment-rate release to the southeast of the hypocentre and a subsequent upward propagation, and finally propagated in the northwest direction, in agreement with the distribution of aftershocks. The majority of the slip at 17–20 km depth occurred to the west of the hypocentre, in a general agreement with the slip distributions obtained from dislocation model and finite-fault inversions. Our modified back-projection approach relies on seismic waveforms with the considerations of a recent 3-D structure model, high average coherence value, station correction factor and simplified amplitude correction. It is computationally efficient and allows for near real-time determinations of source slip distributions after earthquakes using strong-motion records. A quick result for the rupture model can be used in the calculation of strong ground-motion, providing important, useful and timely information for seismic hazard mitigation. [ABSTRACT FROM AUTHOR]

Abstract: 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 12.9cm for a maximum slip of 27.3cm leading to a M w 6.47 for this event. The rupture initiates in the deepest portion of the fault at 20km depth, and propagated upward up to 2km 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 16s. 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. [Copyright &y& Elsevier]

Through a proposed multiple-event analysis of teleseismic P-waves, this study investigated the radiated seismic energy and rupture process of the 2010 JiaSian earthquake. Results showed that the earthquake comprised at least two sub-events. The first sub-event was followed by the second sub-event, ˜1.7 s later. The entire source duration was 5.4 s. Let the two sub-events occur on the same fault plane with a strike of 304° and a dip of 28°; the first sub-event had a relatively smaller seismic moment (M0) and larger radiated seismic energy (Es) than the second sub-event, and this leads to the Es/M0 of the first sub-event larger than that of the second sub-event thus. This feature implies that the first sub-event probably had a higher static stress drop during faulting. The total M0 was estimated to be 2.17 x 1018 Nm, corresponding to Mw = 6.15, and the total Es was ˜2.91 x 1013 Nm, larger than that estimated only from a single source. Subsequently, the Es/M0 was approximately 1.3 x 10-5, lower than ordinary earthquakes. The low static stress drop was probably responsible for the low Es/M0. Overall, the 2010 JiaSian earthquake was characterized by a relatively low ES/M0 and low static stress drop, and then the partial stress drop model would be relatively appropriate to interpret its rupture process. [ABSTRACT FROM AUTHOR]

Abstract: The southern Ryukyus represents an area where different tectonic stress regimes result in high seismicity and increased seismic hazard for nearby areas such as Taiwan. On 18 December 2001 at 04:03 (GMT) a strong earthquake (Mw 6.8) occurred in the forearc area of the southern Ryukyu subduction zone. Revised moment tensor solutions published by GCMT and BATS groups show a normal faulting mechanism with some strike-slip component and also point to a shallow focal depth (∼12km). We use arrival times picked at both Taiwanese and Japanese stations along with a 3D geo-realistic a priori velocity model in order to obtain accurate absolute locations for the mainshock and 153 of its aftershocks. Locations are derived by using the Maximum intersection (MAXI) algorithm which has been used in many previous seismicity studies in the southern Ryukyus. These improved locations indicate that the mainshock was caused by the failure of a NE–SW oriented fault that extends from the edge of the Nanao forearc sedimentary basin to the Ryukyu arc basement. Far-field P and SH waveforms of the mainshock recorded at stations surrounding the source and at distances 30–100°, were inverted for the purpose of investigating its rupture process. A non-negative least-squares inversion technique utilizing multiple time windows was used to derive the spatio-temporal slip distribution. The preferred slip distribution model shows that there is one large area of high slip (∼0.9m) at 5–15km depth that essentially represents the crystalline rocks of the Ryukyu arc basement. Another smaller area with lower slip (∼0.4m) extends at 10–15km depth beneath the Nanao basin. Most aftershocks are located in areas of low slip (<0.4m) filling the regions of slip deficit. It is likely that the 18 December 2001 earthquake was caused by a stress field interaction generated by the oblique subduction of the Gagua ridge and the gravitational forces acting at its landward flank. [Copyright &y& Elsevier]

Abstract: The Hualien mainshock–aftershock sequence from December 1990 to January 1991 was used to investigate seismicity, active faults, stress patterns, and complex rupture processes in the Hualien region of eastern Taiwan. A combined data set, compiled from observations by the regional seismic network (Taiwan Telemetered Seismograph Network, TTSN) and a dense aftershock monitoring array (Hualien Temporary Seismic Network, HTSN), provided high-quality seismic data. Using these data, earthquake source parameters could be well constrained by the joint hypocenter determination (JHD) and double-difference relocation methods. These two relocation methods presented similar results and produced improved estimations of earthquake location. Station corrections determined by JHD showed significant lateral velocity variations across the Yuehmei fault. The relocated seismicity indicated that a previously known active fault, the Chimei fault, extends to offshore. Earthquake relocations also revealed a northwest dipping seismicity probably related to the Chimei fault. In addition, three sets of focal mechanism solutions are presented and their discrepancies are attributed to the complex rupture process. At first, earthquakes were initiated by thrust motions. Later, the rupture pattern changed to strike–slip motions. Variations in slip may reflect the fault geometry or rupture boundary conditions in the region. The observed stress patterns are considered to relate not only to the regional collision and mountain-building processes, but also to active faults in local structures. [Copyright &y& Elsevier]

Abstract: The March 04, 2010, Jiashian, Taiwan earthquake (Mw 6.4) ruptured an unknown fault at depth in southwestern Taiwan. The main shock initiated near the town of Liuquei at 23km depth and the rupture propagated westward. Measurements of coseismic displacements from Taiwan Continuous GPS Array indicate horizontal displacements of 5–27mm in the NW–SW directions to the west of the epicenter; while horizontal movements to the east of the epicenter are absent. The GPS vertical displacements show an uplift motion of about 5–25mm near the epicenter, in contrast to a small movement of about 5–10mm observed in the far-field GPS sites. We use coseismic GPS displacements and an elastic half-space dislocation model to invert for fault geometries and coseismic slip distribution associated with the Jiashian earthquake. Our preferred model exhibits 0.05–0.1m of reverse slip and ~0.04m of left-lateral slip on a N324°-trending fault with dip of 40° to NE, consistent with the earthquake focal mechanisms from BATS, USGS/NEIC, and Global CMT. The highest slip of 0.12m mainly occurs to the west of the epicenter at a depth range of 15–20km. Given the rigidity modulus of 60GPa, the geodetic moment is 4.95×1018 N-m, equivalent to a Mw 6.4 earthquake and consistent with the seismic moment estimated from seismic waveform inversion. Additionally, we notice that the mainshock rupture area is surrounded by high seismicity between 1991 and 2007, suggesting that the Jiashian earthquake may be triggered by the high stress concentration in the vicinity. The calculated Coulomb stress changes on nearby fault systems imparted by the coseismic slip suggest that the Jiashian earthquake may encourage failures on the Chukou fault and inhibit ruptures on the Hsinhua fault. However, the Coulomb stress changes are more complicated on the Chaochou fault and Chishan fault with both positive and negative stress changes. [Copyright &y& Elsevier]

Abstract: We conducted a variety of measurements and analyses at Chihshang Active Fault Observatory in eastern Taiwan, including surface-rupture mapping, three shallow borehole core analysis and kinematic analysis of geodetic measurements. We found that the Chihshang fault exhibits a three-branch fault system in the Chinyuan alluvial fan, which is composed of at least 100 m thick gravel deposits. Outside of the Chinyuan River, the Chihshang fault shows a single fault system with a sharp lithological contact. Combining the leveling results and trench excavation, we interpret that the three fault branches are coupled with a 50−60-m-wide pop-up structure in the hangingwall. Based on the ratio between vertical and horizontal displacements, we obtained dip angles of 38°, 62° and 16° for two west-vergent thrusts and an east-vergent backthrust, respectively. This pop-up was estimated to develop at the uppermost 30–40 m unconsolidated gravels during the last few thousand years above the main fault with a dip angle of 42°. By compiling the available ages data, we obtained an uplift rate of 2.3 cm/yr of the Chihshang fault and an alluvial sedimentation rate of 1.1 cm/yr during the past thousands years. Consequently, the individual uplift rate for each fault branch at the Chihshang Observatory was slightly less than the deposition rate of the Chinyuan River. No geomorphic fault scarp can thus be observed in the three-branch fault system area. [Copyright &y& Elsevier]

Abstract: The rupture process of the disastrous Sumatra–Andaman earthquake of 26 December 2004 was analyzed by array processes for teleseismic P-waves recorded by a dense broadband seismic array in Taiwan with epicentral distances of close to 31°. The azimuthal variation from the BATS array center to both ends of the rupture fault is approximately 21°, which is larger than that reported previously for seismic arrays used to image the rupture process of this earthquake, thereby providing a high spatial resolution in studying the source rupture behavior. Two array-processing methods were used to analyze teleseismic P-wave trains. Both analyses were based on data recorded by a broadband network, covering a region of 200×400km, with the aim of evaluating the rupture behavior of the earthquake. Consistent results from both analyses indicate that the earthquake had a rupture duration exceeding 500s, with major asperities encountered at 80, 260, and 330s after the initiation of rupturing. We traced the ruptured fault for more than 1200km from the point of initial rupture. The average rupture velocity was approximately 3.0km/s and the major northward rupture propagation began at 80s after the initiation of rupturing. [ABSTRACT FROM AUTHOR]

The Chi-Chi earthquake ( M = 7.6) took place in central western Taiwan in 1999. The earthquake caused reactivation of the Chelungpu Fault and resulted in 100-km-long surface ruptures. The fault strikes mostly north-south to NNE-SSW; however, the northern tip of the southern segment of the surface ruptures rotates clockwise to define an east-west trend, then jumps to a shorter NNW-trending rupture. The largest vertical displacement is recorded in the Shihkang area of the Shihkang-Shangchi Fault Zone, where vertical slips are up to 8-10 m. The Shihkang-Shangchi Fault Zone displays a complex fault pattern as a linkage damage zone between two fault segments with the greatest concentration of faults and fractures. Our new interpretation, based on recent published geometric, kinematic, and geophysical studies on the Chi-Chi earthquake fault, suggests that the Shihkang-Shangchi Fault Zone is not a simple termination zone, but may be an 'overstep zone' or a 'transfer zone'. Slip analysis along the surface ruptures indicates that they are composed of three fault segments and the amount of slip partly depends on the intersection angle between slip direction and fault strike. Our numerical modeling for the area indicates that Coulomb stress changes are mainly concentrated on tips and bends of the surface ruptures. Slip patterns indicate that the fault propagates toward the northeast. Therefore, this study suggests high potential for future earthquake activity along the unruptured Shangchi segment. Hence, future geohazard studies should focus on the Shangchi segment to evaluate potential earthquakes, determine recurrence intervals, and reduce future earthquake hazards. [ABSTRACT FROM AUTHOR]

Abstract: The in-situ stress computations presented by Haimson et al. (2010) overestimated the maximum horizontal stress in the vicinity of the Chelungpu Fault, Taiwan. Our analyses from extended leak-off tests, drilling-induced tensile fractures, and Anderson''s faulting theory show considerable lower values for the maximum horizontal stress. Our results suggest that closer to the ruptured fault, more S H is released, implying a strong earth crest in the shallower section, which is far away from the ruptured fault. However, it would require more detailed and intensive work in order to achieve an accurate interpretation of the in-situ stress in this region. [Copyright &y& Elsevier]

On 20 September 1999, the Ms 7.6 Chi-Chi earthquake ruptured the Chelungpu fault in central Taiwan. After the earthquake, several boreholes of different depths were drilled. Those boreholes penetrated the fault plane. The physical (mechanical, thermal, hydraulic, electric, and magnetic) parameters were measured either on the core samples or through well-loggings. Results are significant for studies of the Chelungpu fault. However, the measured results are published in different articles and reports. It is not convenient for the earth scientists to take advantage of those results. Hence, those results are compiled and described in this paper. In addition, the correlations among a few parameters are also reported. [ABSTRACT FROM AUTHOR]

Earthquakes and bedrock river incision are fundamental processes in the evolution of tectonically active landscapes, yet little work has focused on understanding how a bedrock river responds to a single large earthquake. Data from the 1999 Mw = 7.6 Chi-Chi earthquake in central Taiwan show dramatic differences in river response that depend on the proximity to the rupture zone. Near the fault scarp, vertical ground deformation intensifies river incision on a time scale of years to decades, while distal to the fault, landslides induced by the earthquake mantle bedrock on the river bed with sediment, impeding incision for decades to centuries as the material is evacuated. This surprising spatial and temporal variability in bedrock incision caused by earthquakes has implications for the paleoseismic interpretation of bedrock terraces and for the evolution of tectonically active landscapes. [ABSTRACT FROM AUTHOR]

In the Taipei Metropolitan Area, there is a significant problem to be resolved: Can two major active faults, i.e., the Chinshan and Shanchiao faults fail simultaneously? The two faults are both normal faulting based upon recent observations. In order to explore this problem, the static stress transfer between the two faults has been analyzed. We compute the static Coulomb failure stress changes, ΔCFS, on a fault plane due to a failure of the neighboring fault. Numerical computations are conducted for the combinations of three different dip and rake angles of the two fault planes. Results show that the Chinshan fault can be more likely triggered by a failure of the Shanchiao fault. Rupture of a fault is more capable of triggering the neighboring fault, when the rake angle of the ruptured fault is toward than opposite to the striking direction of the to-be-triggered fault. [ABSTRACT FROM AUTHOR]

Abstract: The 2003 Chengkung earthquake has been debated because of inconsistent fault slips determined by seismic and geodetic observations. A pure thrust fault plane solution has been estimated by seismic waveform inversions form the mainshock and its major aftershocks; however, thrust slips with obvious northward horizontal motions are necessary to interpret GPS co-seismic deformation. Based on individual observation, its implication can be dramatically different. In this study, near source dense seismic array and continuous GPS observations were employed to examine the spatial-temporal variation of fault slip of this earthquake. And average rupture properties of major aftershocks were determined by stress inversion of earthquake mechanisms. Based upon strong motion observations, the near source two-dimensional ground motions were reconstructed from 54 free-field strong motion records with epicenter distances of less than 60 km. The reconstructed two-dimensional snapshots provide the visualization for seismic wave propagations near the source region. Based on the reconstructed seismic wave-fields, numerical modeling results confirmed that the dominant seismic energy of mainshock was released from a thrust fault slip and the same as its major aftershocks from stress inversions. No obvious seismic energy was released by some strike–slip type events. Observations from creepmeter and continuous GPS showed limited co-seismic deformations and obvious after-slips near the earthquake fault. The integrated analysis provided unique information to understand the quasi-static fault slips of an earthquake sequence. In this study, a model of sudden thrust slip with a horizontal creeping movement fault system has been proposed to interpret the rupture behavior of the Chengkung earthquake. During the process, the seismic energy was released from a thrust type fault rupture and following some strike–slip type slow fault movements. If the thrust slip is the nature of faulting in the Longitudinal Valley during major earthquakes, it should be significant to seismic hazards, source physics and its tectonic implications. However, the detailed horizontal creeping cannot be resolved by seismometers and low sampling rate GPS observations, presently. To completely understand the quasi-static faulting behavior of an earthquake, the installations of a high sampling rate strainmeter and continuous GPS monitoring near a fault are necessary. [Copyright &y& Elsevier]

We use GPS-derived coseismic and postseismic displacements of the 2003 Mw 6.8 Chengkung, Taiwan, earthquake to examine seismogenic behaviour of the southern Longitudinal Valley. We invert for fault slip on the Chihshang fault, a segment of the Longitudinal Valley fault, based on a simplified layered earth model and a listric-shape fault geometry. Our model infers that maximum coseismic slip of about 0.72 m occurred at a depth of 15 km. Conversely, postseismic slip of about 0.1 m over a 157 d period mainly occurred at depths less than 10 km. We find an anticorrelation between coseismic and postseismic slip distributions. However, the depth profiles of seismicity before and after the main shock are very similar and both show the peak seismicity at 15–25 km depth, consistent with the depth range of large coseismic slip. The potency distribution at depth reveals a coseismic slip deficit at shallow depths, which is compensated by postseismic and interseismic creep associated with the Lichi Mélange. Aseismic slip is an important component of the slip budget on the Chihshang fault. According to the historic earthquake records, interseismic slip rate and coseismic slip on the Chihshang fault, the recurrence interval of this type of event is between 12 and 36 yr. We use a rate-dependent friction model of afterslip to infer the frictional parameter, dτss/dln V= 0.03–0.5 MPa, at shallow depths on the Chihshang fault. Assuming effective normal stress of 100 MPa at 5 km depth, the rheological parameter, , is about 3 × 10−4–5 × 10−3, which is in good agreement with laboratory experiments. [ABSTRACT FROM AUTHOR]

On 26 December 2006, two ML = 7.0 events occurred offshore south of Pingtung; one is associated with a normal-faulting and the other with a strike-slip faulting. The area where these earthquakes were located is not usually expected to have large earthquakes. We deployed 11 short period OBSs over the source zone for one week and recorded a series of aftershocks which were also recorded on land at the CWB network stations. The joint dataset made it possible for us to perform a 3-D velocity tomography and earthquake relocation in this region, where the velocity structures were not well known and location of earthquakes with only land data was uncertain. The tomographic results show a prominent high Vp perturbation zone (HVPZ) that we consider as the uppermost mantle of the subducted plate dipping from SW to NE beneath southern Taiwan. Most of the relocated earthquakes are distributed just above the HVPZ or near and along the bottom of a relatively low velocity subducted crust. Our results show that the subducted and bent Eurasian plate off SW Taiwan could have been unbent and become an upwards concave geometry for the upper 30 km. The main shock is near the bottom of the inflected surface. The distribution of the earthquake sequence generally displays in a NW-SE direction, coinciding with the plate convergence orientation between the Philippine Sea Plate and Eurasian Plate. This orientation also follows a relatively low Bouguer gravity anomaly stripe that is due to a heavy loading of the Taiwan orogen on the east-dipping Eurasian Plate. Considering that the hypocenter of the first main-shock is near the bottom of the aftershocks, we suggest that the first normal faulting earthquake was caused by an unbending effect in the subducting crust and this event triggered the release of accumulated energy between the Philippine Sea Plate and Eurasian Plate. Thus, we suggest that the rupture surface of the Pingtung earthquake sequence had propagated upwards and northwestward in the direction of plate convergence. [ABSTRACT FROM AUTHOR]

The 26 December 2006 Pingtung earthquake doublet (both about Mw 7.0) were the two largest events with magnitudes close to 7.0, following the 1999 Chi-Chi earthquake. The locations and focal mechanisms of the Pingtung earthquake sequence provide good opportunities for scientists to investigate plate movement and deep tectonic structure in southern offshore of Taiwan. However, precise hypocenters and source mechanisms of this sequence are not definite due to poor station azimuthal coverage and a lack of information from historical seismicity in this area. Here we use regional ground motion full waveform records to investigate source rupture processes and determine fault planes of the Pingtung earthquake doublet. The finite fault inversion result of the first event shows two large asperities occurred near the epicenter. The rupture mainly originated from the south then propagated northward on an eastern dipping fault plane. The second earthquake is a bilateral rupture along NW-SE direction having a large asperity close to the epicenter which released most of its seismic energy within the first 15 seconds. The slips of the two earthquakes show compensative patterns when mapped on the earth's surface. We show that the Pingtung earthquake doublet are large normal and strike-slip faulting which dips toward the east and west, respectively. The mechanisms determined from inversion results infer that the first event might relate to the bending of the South China Sea Plate during its subduction to the Philippine Sea Plate. The second event could happen at a pre-existing weak zone that was triggered by transferred stress after the first event. [ABSTRACT FROM AUTHOR]

Teleseicmic vertical P waves of an earthquake doublet occurring on 26 December 2006 offshore southwestern Taiwan are analyzed to recover spatial slip distribution. Ground-displacement records for period 0.01 - 0.5 Hz are inverted using linear inversion for finite fault analysis. Waveform inversion results show that a dual segment fault of different strike is necessary for the first event to explain teleseismic waveforms. The trend in fault geometry, thus, inferred was coincidentally consistent with the bathymetry in the fault rupture area, suggesting the possible tectonic structure controlling fault propagation. Both earthquakes show larger slip in the asperities near the hypocenter and there are two main asperities of similar moments. The first event ruptured along a high angle eastern dipping plane while the second event ruptured along a low-angle western dipping fault plane. The spatial distribution of the Pingtung earthquake doublet shows a compensative and conjugate relationship, and the seismic moments of the earthquake doublet were 2.7 x 1026 and 1.8 x 1026 dyne-cm, respectively. The stress drops corresponding to the asperities of the two earthquakes have values of 3 to 4 bars, individually, similar to the values observed in other subduction zones. [ABSTRACT FROM AUTHOR]

The magnitude 7.1 Pingtung offshore earthquake occurred on 26 December 2006 at a depth of 40 - 60 km within the subduction zone of the Eurasian plate. This event exhibits an emergent onset. The first large amplitude arrivals were delayed by several seconds with respect to the origin time and were preceded by small-scale slip. This event is one of a few large intermediate depth earthquakes well recorded in a near source region. Based on beamforming analysis, some near source recordings were used in this study to investigate the initial rupture process of this event. Analyzed results indicate that initial fault rupture of this event at the hypocenter was of short duration with initial resultant fault slip extending southward toward the shallower portion of the fault plane. Thereafter, a detected rupture front propagated both northward and downward. Following small-scale initial slip, a major rupture arose. A detailed rupture process has been reconstructed using backward projection of array seismograms from the fault plane to image fault slip. The spatial and temporal evolution of initial onset presents the same rupture behavior determined by beamforming analysis. Our analysis appears to indicate the distinct possibility of an initial seismic nucleation phase; however, uncertainties remain and a cascade process cannot be ruled out completely. [ABSTRACT FROM AUTHOR]

[Display omitted] • Coseismic surface pop-up of the 2016 Meinong earthquake is due to crust volumetric strain change. • The coseismic strain change is characterized by shallow dilatation and underlying contraction. • Mud diapiring under pop-up is likely triggered by 'squeezing" contract strain at base of reservoir. We investigated the 2016 Meinong earthquake (Mw 6.4) in southwestern Taiwan, which caused surface pop-up in an area of 10x15 km2 with maximum uplift of 12 cm, where lies an array of mud volcanoes and possible underlying mud diapir. We calculated 3D strain tensor in a 3D mesh with 5x5x2 km grids in the epicentral area induced by the Coulomb stress change due to coseismic fault slip. We obtained substantial contraction strain (10−5–10−6) that occurred in a lobe showing "squeezing" at the depth of 5–14 km below the surface pop-up area. Dilatation strain (10−5–10−6) occurred at shallow level (0–3 km) with a radial pattern around the surface pop-up area. Combining with local geology, which is composed of Mio-Pliocene ~5-km-thick mudstone in a fold-thrust belt, we interpret that the 2016 Meinong coseismic surface pop-up was closely related to mud diapirs/volcanoes, which were likely reactivated by sudden increase of fluid pore-pressure in the basal reservoir (at 5–6 km depth) and dilatation in the shallow level. We also explored the potential effects of the Coulomb stress transfer on nearby receiver faults – including three arrays of mud diapir, the regional decollement, a suspected backthrust and one thrust close to the pop-up area. Our results show that the Coulomb stress transfer a) favors NNE-trending mud diapirs in the coseismic pop-up area, with a combination of clamping stress changes at 5–6 km depth and unclamping stress changes at 0–4 km depth, and b) it does not favor triggered thrust slip on the regional thrusts. [ABSTRACT FROM AUTHOR]

Abstract: The northern segment of the Chelungpu Fault shows an unusually large co-seismic displacement from the event of the Mw 7.6 Chi-Chi earthquake in western Taiwan. Part of the northern segment near the Fengyuan City provides an excellent opportunity for characterizing active thrust-related structures due to a dense geodetic-benchmark network. We reproduced co-seismic deformation patterns of a small segment of this Chelungpu Fault using 924 geodetic benchmarks. According to the estimated displacement vectors, we identified secondary deformations, such as local rigid-block rotation and significant shortening within the hanging wall. The data set also allows us to determine accurately a 3D model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip, and the horizontal and vertical displacements at the surface. The predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area. The successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations. [Copyright &y& Elsevier]

There are two active faults, i.e., the Chinshan and Shangjiao faults in the Taipei Metropolitan Area. The moment magnitude (M), maximum displacement (Dmax), and average displacement (Dave) of potential earthquakes, which would rupture the two faults, are evaluated. The fault lengths are 25 and 20 km, respectively, for the Chinshan and Shangjiao faults. The optimum values of the three parameters are: (1) M = 6.6, Dmax = 0.97 m, and Dave = 0.42 m for the Shangjiao fault; and (2) M = 6.7, Dmax = 1.35 m, and Dave = 0.55 m for the Chinshan fault; and (3) M = 7.0, Dmax = 3.28 m, and Dave = 1.15 m for the Chinshan-Shangjiao fault system. [ABSTRACT FROM AUTHOR]

The 1999 Chiayi M 6.4 earthquake sequence that occurred about one month after the 1999 Chi-Chi M 7.6 earthquake is remarkably similar to the Ruey-Li M 6.2 earthquake sequence which occurred one year before the big Chi-Chi event. Both sequences were energetic and short in duration; with events confined in small source volumes, and the fault-plane orientation of individual events in these sequences varied abruptly during the development of the dynamic rupture. Both occurred on the southern extension of the large Chelungpu rupture causal to the Chi-Chi event, and in an area of highly fractured upper crust. We consider that this faulting behavior is due to the rupture-induced dynamic stress that interacts with the pre-existing highly fractured upper crust and continually perturbs the local stress field. This phenomenon appears to be typical of particularly active plate boundary regions with a highly fractured upper crust under intense tectonic loading. [ABSTRACT FROM AUTHOR]

Earthquake-related groundwater level changes have been recorded by a dense network of monitoring well stations in Taiwan. At most multiple-well stations, the direction and magnitude of coseismic water level changes vary in wells of different depths. Comprehensive water level data recorded in 209 wells in the vicinity of the seismogenic fault during the 1999 ML7.3 earthquake demonstrate a preliminary 3-D distribution of coseismic changes. The largest coseismic rise at a station observed typically in a confined gravel aquifer indicates the association of the magnitude of coseismic change with characteristics, rather than depth, of the aquifer. The variation of coseismic changes in the vertical direction implies possible inconsistency between the observed water level changes and the volumetric strains calculated from simple dislocation models. At a given hypocentral distance, the coseismic change in the footwall of the ruptured segment of the fault was much greater than that of the unruptured segment. This phenomenon suggests that fault displacement plays an important role in the generation of coseismic changes. While hypocentral distance correlates well with coseismic rise or fall in the vicinity of the ruptured seismogenic fault, poor correlation is found for coseismic change further from the earthquake epicentre. [ABSTRACT FROM AUTHOR]

Strong near-fault ground motion, usually caused by the fault-rupture and characterized by a pulse-like velocity-wave form, often causes dramatic instantaneous seismic energy (Jadhav and Jangid 2006). Some reinforced concrete (RC) bridge columns, even those built according to ductile design principles, were damaged in the 1999 Chi-Chi earthquake. Thus, it is very important to evaluate the seismic response of a RC bridge column to improve its seismic design and prevent future damage. Nonlinear time history analysis using step-by-step integration is capable of tracing the dynamic response of a structure during the entire vibration period and is able to accommodate the pulsing wave form. However the accuracy of the numerical results is very sensitive to the modeling of the nonlinear load-deformation relationship of the structural member. FEMA 273 and ATC-40 provide the modeling parameters for structural nonlinear analyses of RC beams and RC columns. They use three parameters to define the plastic rotation angles and a residual strength ratio to describe the nonlinear load- deformation relationship of an RC member. Structural nonlinear analyses are performed based on these parameters. This method provides a convenient way to obtain the nonlinear seismic responses of RC structures. However, the accuracy of the numerical solutions might be further improved. For this purpose, results from a previous study on modeling of the static pushover analyses for RC bridge columns (Sung et al. 2005) is adopted for the nonlinear time history analysis presented herein to evaluate the structural responses excited by a near-fault ground motion. To ensure the reliability of this approach, the numerical results were compared to experimental results. The results confirm that the proposed approach is valid. [ABSTRACT FROM AUTHOR]

The 1999 September 20 Chi-Chi earthquake is the largest seismic event which occurred in the island during the twentieth century. Available seismic data relative to this earthquake are of high quality, and surface ruptures identified as features associated to the Chelungpu fault can be clearly observed at the surface and precisely mapped. We calculated the fractal dimension ( D) and b value of Gutenberg–Richter law for 6-month aftershocks of the Chi-Chi earthquake for the fault area, and find that the surface ruptures exhibit self-similar geometry only within specific ruler intervals. The D values of the surface ruptures reflect the fault slip and geometry at depth. More importantly, the small-size aftershocks seem more likely to occur within high D value and high b value areas, whereas small D value and small b value areas have a high potential for medium- and large-size aftershocks. [ABSTRACT FROM AUTHOR]

Abstract: We found active faults in the fold and thrust belt between Tunglo town and the Tachia River in northwestern Taiwan. The surface rupture occurred in 1999 and 1935 nearby the study area, but no historical surface rupture is recorded in this area, suggesting that the seismic energy has been accumulated during the recent time. Deformed fluvial terraces aid in understanding late Quaternary tectonics in this tectonically active area. This area contains newly identified faults that we group as the Tunglo Fault System, which formed after the area''s oldest fluvial terrace and appears at least 16km long in roughly N–S orientation. Its progressive deformations are all recorded in associated terraces developed during the middle to late Quaternary. In the north, the system consists of two subparallel active faults, the Tunglo Fault and Tunglo East Fault, striking N–S and facing each other from opposite sides of the northward flowing Hsihu River, whose course may be controlled by interactions of above-mentioned two active faults. The northern part of the Tunglo Fault, to the west of the river, is a reverse fault with upthrown side on the west; conversely the Tunglo East Fault, to the east, is also a reverse fault, but with upthrown side on the east. Both faults are marked by a flexural scarp or eastward tilting of fluvial terraces. Considering a Quaternary syncline lies subparallel to the east of this fault system, the Tunglo Fault might be originated as a bending moment fault and the Tunglo East Fault as a flexural slip fault. However, they have developed as obvious reverse faults, which have progressive deformation under E–W compressive stress field of Taiwan. Farther south, a west-facing high scarp, the Tunglo South Fault, strikes NNE–SSW, oblique to the region''s E–W direction of compression. Probably due to the strain partitioning, the Tunglo South Fault generates en echelon, elongated ridges and swales to accommodate right-lateral strike–slip displacement. Other structures in the area include eastward-striking portion of the Sanyi Fault, which has no evidence for late Quaternary surface rupture on this fault; perhaps slip on this part of Sanyi Fault ceased when the Tunglo Fault System became active. [Copyright &y& Elsevier]

The reactivation of the Chelungpu Fault triggered the 1999 Chi-Chi M w 7.6 earthquake, resulting in substantial ground surface ruptures and severe damage. After the earthquake, the surveys organized by the Central Geology Survey (CGS) of Taiwan and the National Center for Research on Earthquake Engineering (NCREE) immediately investigated a variety of cases related to engineering structures damaged by substantial ground deformation. The investigations gathered valuable in-situ data before human activities disturbed the local setting and altered co-seismic features of interest, providing opportunities for studying the interaction between surface rupture and engineering structures. This study outlines the reconnaissance of damaged engineering structures in Shigang District that cover three typical types of foundations and analyzes the fault geometry and fault slip at the local scale to construct a geomechanical model for numerical simulation. In addition, 3-dimensional (3D) modeling was performed for each case study to investigate the damage mechanisms of the structures subjected to the substantial ground deformations by using PFC3D (Particle Flow Code 3D), which is based on the discrete element method (DEM). The results of the DEM analyses, which have been calibrated with the in-situ data, showed the ability to evaluate the overall ground deformation, the detection of structural movement, and the critical elements of progressive collapse during faulting. This study demonstrates that the 3D DEM simulation is a reliable analysis tool for studying earthquake fault rupture-overburden layer-engineering structure interactions and can produce useful information for hazard assessment with an acceptable computation time. The 3D DEM analysis is therefore suggested to be applied to assess the potential damage scenarios for the engineering structures located in active fault zones. • Analysis of earthquake fault rupture-overburden layer-engineering structure interaction in the Chelungpu fault zone • Integrated field investigations, geomechanical analysis, and 3D DEM modeling • Damaged cases of engineering structures with different foundation types in the Shigang District were simulated. • Current abilities and limitations of the full-scale 3D DEM model are discussed. [ABSTRACT FROM AUTHOR]

The Coastal Range in eastern Taiwan, which has resulted from arc-continent collisions, involves well-developed Holocene uplifted marine terraces along a 140-km long stretch of the coast. This is the first study that unraveled the aseismic generation of marine terraces associated with coseismic uplift along the creeping fault. We organized DEMs/LiDAR mapping and field surveys to obtain detailed and systematic measurements of the inner and outer edges and wave-cut platform elevations along thirty-three stream profile transects, allowing us to locate the bedrock profile. Spatial and temporal variations in the 14C age are characterized by an offlapping built feature of significant seaward younging trend of ages across the marine terrace, caused by aseismic and coseismic uplift. Thus, heights of the bedrock riser and platform are caused by coseismic (large earthquake) and aseismic plus moderate coseismic vertical deformations during a large earthquake cycle, respectively. This indicates that each terrace duration and paleoshoreline ages can be used to infer the recurrence interval and the timing of large earthquake events, respectively. Results of the age of the paleoshoreline indicate six paleoseismic events at ∼8500, ∼6300, ∼4800, ∼3400, ∼1700, and ∼700 yr BP. This implies that the Chihshang creeping Fault could be capable of producing large earthquakes (greater than M7) at millennial time scales associated with larger coseismic deformations. The creep faulting associated with seismic and aseismic uplift is a dominant process constraining the Holocene marine terrace evolution. • This is the first study that unraveled the aseismic generation of marine terraces associated with coseismic uplift along the creeping fault. • The creeping fault could generate large earthquakes (greater than M7) at millennial time scales with larger coseismic displacements. • Offlapping built feature of seaward younging trend of ages across terrace caused by aseismic and coseismic uplift during the interseismic period. [ABSTRACT FROM AUTHOR]

Taiwan is located at the convergent zone between the Eurasian plate and the Philippine Sea plate. Such a condition produces a series of geological structures striking along the northeast-southwest direction in the northeastern Taiwan. The coastal terrain is dominated by wave-cut platforms with many complex fractures, which may be associated with a fault structure throughout different time stages. Some secondary fractures may be created or continuous along existing ones with different directions.The unmanned aerial vehicle is used to take pictures above the wave-cut platform and the fracture lines are drawn for analyzing the pattern. Then with the field work data, the fracture geometry are consolidated into certain patterns to confirm the location of the existing fault system. The study also uses the principle of cross-cutting relationships and fault evolution to establish the fracture-formed sequence in this area. Preliminary results find that the fractures in the investigation area can be separated into at least three different time stages.In the first stage, a series of northwest-southeast fractures were formed. The fractures in the second stage linked the adjacent fractures in the first stage together and formed block shapes. In the third stage, N12˚E Riedel shear developed with a N18˚W-S18˚E maximum compressive principal stress(σ1).With previous stages fractures produced the damage zone and formed the fault structure. [ABSTRACT FROM AUTHOR]

The Longitudinal Valley in eastern Taiwan is characterized by frequent earthquakes and complex geologic structures. It is generally considered as the suture of the collision between the Philippine Sea plate and the Eurasian plate. When the M7.3 earthquake in October 1951 and the M6.4 earthquake in February 2018 happened, the rupture of the Milun fault was observed in the northernmost Longitudinal Valley (NLV). To understand the fault strike and geological formation in the NLV, we use gravity anomalies to model the density structures and then explored the tectonic structure mechanism.In this study, we collected the gravity data including onshore and offshore in NLV. After the gravity corrections, we obtained the gravity anomaly map of the NLV and proceeded to the 2-D density profiles modeling. Then using the Talwani technique to calculate the effect of the 2D density model. To reduce the non-unique problem in gravity modeling, we use the tomography, the reflection seismic data, the surface geological mapping and the previous results in this area to constrain the density model. After using tried and error method to modify the model, we finally get a reliable density structure that conforms with observed gravity data. We present four 2D density profiles perpendicular to the strike of the NLV. After the gravity modeling, we get the locations of strata and faults in the NLV. Because of the collision between the Philippine Sea plate and the Eurasian plates, the sediment in the NLV becomes thick and dipping to east. The lingding fault might be a sediment boundary made by the Coastal Range uplifted. Structural observations also show that the Luzon arc start to subduct in the NLV, caused the Milun tableland uplifted. This density model provides more details to understand the tectonic structure beneath the NLV. [ABSTRACT FROM AUTHOR]

We have developed a new procedure to detect ambient tremors along a low-seismic-efficiency transfer fault in southwestern Taiwan. The procedure includes bandpass filtering between 2-5Hz, noise reduction, modified normalized cross-correlation of waveform envelopes, amplitude contrast threshold, and waveform spectrogram analyses. The tremors were located by applying the grid-search method with a non-linear cross-correlation weighting technique. Totally 34 ambient tremor events during the year of 2012 with the maximum duration less than 10 minutes were detected in the transfer fault zone based on our new approaches. Our results show that transfer fault zone can be separated into three areas based on the distributions of tremors an earthquakes: the area I with depth of 0-6 km has few tremors and earthquakes; the area II with depth of 6-19 km has both tremors and earthquakes, and the area III with depth below 19 km has only a few tremors but no earthquakes at all. The coexistence of shallow-depth tremor with micro-earthquakes in the seismogenic zone (area II) has never been reported before and they clearly surround fault asperities. We interpreted the area I as the locking zone that agrees well with GPS observations and the area II as a seismogenic zone with mixing mechanical behaviors; i.e., both velocity-weakening friction and conditionally stable friction. The area III may represent an aseismic zone with some velocity-neutral patches embedded in massive velocity-strengthening fault rocks. The presence of short-duration tremor events coexisting with micro-earthquake in the seismogenic zone may be a common feature than a special case, especially in those faults with less seismic efficiency as the one we studied. Hence, they can act as good indicators of fault locking depth and asperities. [ABSTRACT FROM AUTHOR]

The seismic source spatiotemporal rupture process of events in Japan, Taiwan and Greece are imaged by backprojection of strong motion waveforms. Normalized high frequency (>2Hz) S-waveforms from recordings on dense strong-motion networks are used to scan a predefined 3D source volume over time. Backprojection is an alternative novel approach to image the spatiotemporal earthquake rupture. The method was firstly applied for large earthquakes at teleseismic distances but is nowadays used at local distances and higher frequencies. The greatest advantage of the method is that processing is done without any a-priori constraints on the geometry, dimension and size of the source. Thus, the spatiotemporal imaging of the rupture is feasible in higher frequencies (> 1Hz) than conventional source inversion studies even when the examined fault geometry is complex. This high frequency energy emitted during an earthquake is of great importance in seismic hazard assessment for certain critical infrastructures. The actual challenge in using high frequency local records is to distinguish the local site effects from the true earthquake source content. Otherwise, mapping them incorrectly to the source area limits the resolvability of the method. The real challenge is to remove the site effect part from all available records, or at least, to distinguish quantitatively good reference stations from hard-soil and rock sites. In this study, the advantages and limitation of the method are explored using waveform data from well recorded events in Japan (Hokkaido Eastern Iburi Earthquake, 2018, Kumamoto 7.1 Mw, 2016), Taiwan (Hualien 6.4 Mw, 2018) and Greece (Zakinthos 6.8 Mw, 2018). [ABSTRACT FROM AUTHOR]

We use seismic and geodetic measurements to optimize the fault geometry as well as coseismic slip distribution from the 2018 Mw 6.4 Hualien earthquake. We find that at least three faults were involved in the earthquake. The earthquake initiated from a south‐dipping fault in offshore Hualien, and slip transferred into the main west‐dipping oblique fault. The slip finally triggered movement of the east‐dipping Milun fault at shallower depth and caused surface rupture. Although there is no offshore data constraints, our inverted slip distribution shows that the majority of slip occurred between 5‐ and 10‐km depth on the main west‐dipping fault, and 1‐ to 3‐m slip on the shallower part of the Milun fault. Additionally, we process 5 months of postseismic deformation time series and find more than 5‐cm postseismic displacement occurred along the Milun fault but is insignificant near the Coastal Range located south of the Hualien City. Plain Language Summary: Taiwan is one of the most tectonically active regions in the world. In eastern Taiwan, the Longitudinal Valley is located between the Eurasian Plate and the Philippine Sea Plate, and the rapid plate collision has cause high surface deformation as well as earthquake hazard in Hualien, the biggest city in northern Longitudinal Valley. In this study, we use measurements during the earthquake to study the fault geometry and fault slip due to the Mw 6.4 Hualien earthquake occurred in February 2018. Our inversion results show that at least three faults are involved in the event, including a south‐dipping fault, a main west‐dipping fault, and the east‐dipping Milun fault located in the Hualien City. Our inverted fault slip at shallower depth is in agreement with field investigation after the earthquake. We also process 5 months of data to observe postearthquake surface deformation and find that there is more than 5 cm of displacement along the Milun fault, but insignificant displacement south of the Hualien City. Key Points: At least three faults were involved in the 2018 Mw 6.4 Hualien earthquakeMost of the slip occurred on the main west‐dipping fault close to the north Coastal RangeRapid postseismic transient along the Milun fault in the Hualien City, whereas insignificant displacement close to the Coastal Range [ABSTRACT FROM AUTHOR]