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The empirical attenuation functions for Local Magnitude (ML) currently used in Taiwan have been known for overestimated magnitudes around 0.2 compared with moment magnitude (MW) for shallow earthquakes (depths ≤ 35 km). Moreover, for deep earthquakes (depths > 35 km), ML (> 6) can be larger than MW around 0.5. Based on global observations and seismological theory, ML is equal to MW for magnitudes 4 - 6, whereas ML is smaller than MW for magnitudes > 6. This indicates that the attenuation functions for the Taiwan region need to be re-calculated. In this study, we used the new data set collected at the upgraded Central Weather Bureau Seismic Network (CWBSN24) from January 2012 to April 2019 and totally there are 692 events with ML ≥ 3 and 35228 amplitude data used for analysis. To accommodate the complicated tectonic environment in Taiwan, there are four attenuation functions, logA0(Δ), based on the focal depths and hypocentral distances, which are: http://tao.cgu.org.tw/images/docimage/2020061601T_Eq1.png for shallow earthquakes (focal depth, h ≤ 35 km), and http://tao.cgu.org.tw/images/docimage/2020061601T_Eq2.png for deep earthquakes (h > 35 km) where Δ is epicentral distances, R (= http://tao.cgu.org.tw/images/docimage/2020061601T_Eq3.png) is hypocentral distance. ML calculated by using the old attenuation functions for deep and large (ML > 5.5) earthquakes is larger than that by using the new ones about 0.4. With the new empirical attenuation functions, the relationship between ML and MW follows the global observations and also the new ML avoids the confusion for the public when releasing the official earthquake reports.

The February 2018 M(w )6.4 earthquake in eastern Taiwan caused extensive damage in Hualien City. Although damaging earthquakes are common in this region, there are relatively few permanent seismic stations deployed. Two days after the main- shock, we deployed 70 temporary seismic stations around Hualien City for 12 days (8-19 February), and station spacing was 1-5 km. During this time, 2192 aftershocks were located from which 580 focal mechanisms were determined. The aftershock sequence extended about 25 km southwestward from the epicenter of the mainshock into the Longitudinal Valley and to depths between 5 and 15 km. Earthquake hypocenters indicate that the aftershocks took place along a near-vertical to steeply west-dipping plane in the north that becomes more diffuse in the south. Focal mechanisms are predominantly extensional, different than the left-lateral strike slip with thrust-component faulting of the mainshock. Very few events occurred in the uppermost crust at depths of less than 5 km, and their focal mechanisms are left-lateral strike-slip faulting. From the mainshock to the aftershocks, the stress changed rapidly from a north-northwest-oriented compressional axis (P axis) to the same direction for the extensional axis (T axis)., The authors thank Wen‐Yen Chang of National Dong Hua University, Yu‐Ting Kuo and Mr. Chin‐Shang Ku of the Insitute of Earth Sciences (IES), Academia Sinica, and Hsuan‐Yu Kuo of the National Central University for the assistance of the field deployment. H. K.‐C. is supported by MOST (MOST 104‐2628‐M‐008‐005‐MY3 and MOST 107‐2628‐M‐008 ‐004 ‐MY3).

The southwest Taiwan fold-and-thrust belt is forming on the outer shelf and slope of the Eurasian continental margin. It comprises a roughly N-S striking, west verging imbricate thrust system that has been developing since the Late Miocene. Here we present the results of new surface geological mapping from which we construct balanced and restored cross sections and along-strike sections. From these we compile maps of the basal thrust, thrust branch lines and, where possible, stratigraphic cutoffs. To interpret the structure in the subsurface and beneath the basal thrust, we use a P wave velocity of 5.2km/s as a proxy for the top of the Mesozoic basement. We divide the southwest Taiwan fold-and-thrust belt into a number of thrust sheets that form the basis of our description and interpretations. From these data we interpret the 3-D structure of the fold-and-thrust belt and the influence that the structure and morphology of the continental margin is having on its development. We show that there is a significant along-strike change in the structure. This change takes place across a transverse zone that is composed of a suite of structures at the surface. We suggest that this transverse zone has a causal relationship with variations in the geometry of the basal thrust which in turn is related to (possibly fault bounded) basement highs and lows that are inherited from the continental margin. Plain Language Summary The southwest Taiwan fold-and-thrust belt comprises a roughly N-S striking, west verging imbricate thrust system that has been developing since the Late Miocene. We present the results of new surface geological mapping from which we construct balanced and restored cross sections and along-strike sections. We compile maps of the basal thrust, thrust branch lines, and stratigraphic cutoffs. We divide the southwest Taiwan fold-and-thrust belt into a number of thrust sheets that form the basis of our description and interpretations. From these data we interpret the 3-D structure of the fold-and-thrust belt and the influence that the structure and morphology of the continental margin is having on its development. We show that there is a significant along-strike change in the structure. This change takes place across a transverse zone that is composed of a suite of structures at the surface. We suggest that this transverse zone has a causal relationship with variations in the geometry of the basal thrust which in turn is related to (possibly fault bounded) basement highs and lows inherited from the continental margin., D. B., J. A-M., and C. B. acknowledge funding provided by the Spanish Ministerio de Economia y Competividad grant CGL2013-43877-P. H. K-C. acknowledges funding by MOST 104-2628-M-008-005-MY3.

Studies of mountain belts worldwide show that along-strike changes are common in their foreland fold-and-thrust belts. These are typically caused by processes related to fault reactivation and/or fault focusing along changes in sedimentary sequences. The study of active orogens, like Taiwan, can also provide insights into how these processes influence transient features such as seismicity and topography. In this paper, we trace regional-scale features from the Eurasian continental margin in the Taiwan Strait into the south-central Taiwan fold-and-thrust belt. We then present newly mapped surface geology, P-wave velocity maps and sections, seismicity, and topography data to test the hypothesis of whether or not these regional-scale features of the margin are contributing to along-strike changes in structural style, and the distribution of seismicity and topography in this part of the Taiwan fold-and-thrust belt. These data show that the most important along-strike change takes place at the eastward prolongation of the upper part of the margins necking zone, where there is a causal link between fault reactivation, involvement of basement in the thrusting, concentration of seismicity, and the formation of high topography. On the area correlated with the necking zone, the strike-slip reactivation of east-northeast striking extensional faults is causing sigmoidal offset of structures and topography along two main zones. Here, basement is not involved in the thrusting, there is weak focusing of seismicity, and localized development of topography. We also show that there are important differences in structure, seismicity, and topography between the margins shelf and its necking zone.

EGU2020: Sharing Geoscience Online, 4-8 may 2020, The region of Taiwan is undergoing active, oblique arc-continent colision between the Luzon Arc on the Philippine Sea Plate and the continental margin of Eurasia. The Fold-and-Thrust Belt (FTB) in Taiwan passes southwards into a submarine accretionary wedge at the Manila subduction zone. The aim of this contribution is to examine how an on land FTB changes into a marine accretionary prism in the context of an oblique arc-continent collision. The Miocene pre-orogenic sediments of the continental margin are widespread in the FTB ca. 23° latitude while the offshore wedge is built up dominantly by Pliocene to recent syn-orogenic sediments. In the transition area from the marine accretionary wedge ca. 21° latitude to the on land FTB, the thrust wedge is climbing up the slope of the Eurasian continental margin. The deformation front is at sea floor depth of ca. 4 km in the south to less than 1 km as it reaches the coast line. Here we use the island surface geology, marine reflection seismic profiles, and seismic tomography models to construct contour maps of the basal thrust and the depth to the Moho across a transition area from near 23° to near 21° latitude. In this zone, the deformation front draws a convex curvature as the wedge widens from ca. 50 in the north and south, to more than 130 km near 22° latitude. The basal thrust surface shows a scoop shape as its dip changes from southeast near the coast line to east southward. The basal thrust reaches over 7 km deep beneath the rear of the FTB before ramping into de basement and merging into the Chaochou fault at 10 km depth. Offshore, it shows a gentler dip from 7 km to c. 10 km depth before getting steeper towards the east below the Hengchung Ridge. The basal cuts laterally along-strike through the margin¿s sedimentary cover to incorporate thicker Miocene pre-orogenic sediments onto its hanging wall as it passes from the offshore wedge to the on land FTB. In the offshore area, the Moho (we use a Vp proxy of 7.5 km/s extracted from the seismic tomography) shallows southeastward, from near 25 km depth below the shelf slope break to less than 17 km depth below the offshore wedge near 21.5° latitude before it starts to deep east towards beneath the Taiwan coast. The Moho dips northeast from near 25 km depth below the coast near Kaohsiung, to near 40 depth below the rear of the FTB at 23.5°, latitude. This complex morphology of the Moho may be related to the changes in crustal thickness and the obliquity of the collision. Because of this, crustal thickening is less pronounced beneath southern Taiwan where the thinner part of the margin is colliding with the arc., This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency from the Ministry of Science Innovation and Universities of Spain.

Large, destructive historical earthquakes off the coast of China’s Fujian Province point to important tectonic activity in the western Taiwan Strait that, until recently, has received little attention. We present newly acquired reflection seismic data that is used to study the shallow crustal structure of the western Taiwan Strait. With these data we map the location of the Benhai fault for the first time, describe its upper crustal geometry and, in combination with seismicity and earthquake focal mechanisms, interpret its kinematics. These new data demonstrate that there is wide spread evidence of faulting that reaches the sea floor in the western Taiwan Strait, clearly indicating that the Benhai fault is active. Faults that cut up section from steep basin sidewalls to form flower structures or terraced sidewall fault zones, together with the fault and basin map pattern, are consistent with this fault zone being in the early developmental stages of a dextral strike-slip system. Earthquake focal mechanisms, although not definitive, support the model of an active dextral strike-slip fault system in the western Taiwan Strait. © 2020, The Author(s)., ZYF acknowledge funding supported by the National Natural Science Foundation of China (Grant No. 41774104). KC acknowledge funding supported by the Ministry of Science and Technology (MOST 108-2116-M-008-022-MY3). JAM and DB acknowledge funding provided by the Spanish Ministerio de Economia y Competividad grant PGC2018-094227-B-I00. DB also acknowledges financing through the Salvador de Madariaga program (project PRX17/00036) provided by the Spanish Ministerio de Educación, Cultura, y Deporte for a 6 month stay at NCU. We also thank C.-W. Ho of Central Weather Bureau for providing the earthquake catalogue.

EGU General Assembly 2020, Online, 4–8 May 2020, Studies of mountain belts worldwide have shown that the structural, mechanical, and kinematic evolution of their foreland fold-and-thrust belts are strongly influenced by the structure of the continental margins that are involved in the deformation. The area on and around the island of Taiwan provides an unparalleled opportunity to investigate this because the entire profile of the SE margin of the Eurasian plate, from the shelf in the north to the slope and continent-ocean transition in the south and the offshore, is currently involved in a collision with the Luzon arc on the Philippine Sea plate. Taiwan can, then, provide key insights into how such features as rift basins on the shelf, the extensional faults that form the shelf-slope break in the basement, or the structure of the extended crust and morphology of the sedimentary carapace of the slope can be directly reflected in the structural architecture, the location and pattern of seismicity, topography, and the contemporaneous stress and strain fields of a fold-and-thrust belt. For example, east-northeast striking faults that have been mapped on the necking zone of the Eurasian margin can be traced into the island of Taiwan where they are causing important along-strike changes in various aspects of the structural, mechanical, kinematic, and morphological behavior of the fold-and-thrust belt. In particular, across the upper part of the necking zone there is an abrupt north-south change in structure, an increase in the amount of seismicity, an increase in topography, a rotation of the direction of maximum compressive horizontal stress, of the GPS displacement vectors, the compressional strain rate, and the maximum shear strain rate. These changes are interpreted to be caused by east-northeast striking, dextral strike-slip faulting in the basement that is taking place as a result of the reactivation of pre-existing faults along the upper part of the necking zone. The abrupt southeastward increase in topography across the upper part of the necking zone is the surface expression of the basal thrust of the fold-and-thrust belt ramping down into the basement, with maximum elevations reached in the basement-involved thrust sheets, suggesting a causal link between basement involvement in the thrusting and high topography. On the shelf, the roughly northeast-oriented Hsuehshan Trough is inverting along almost north-south striking basin bounding faults that penetrate into the middle crust and have well-clustered, deep seismicity. There are no substantial differences in the contemporaneous stress and strain field. There is, however, a clear relationship between basement involvement in the thrusting and the development of high topography in the Hsuehshan Range. Only the upper part of the slope is involved in the fold-and-thrust belt in southernmost Taiwan. In this area, there is a reduction of the amount of seismicity and lower topography. The largest part of the corresponding thrust wedge developed in the lower slope is offshore., This work is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades grant PGC2018-094227-B-I00.

Summary The distribution of historic earthquakes in the Sumatra subduction zone reveals, in the fore-arc region, the intense seismic activity and frequent occurrences of Mw > 8 earthquakes throughout the whole area. In contrast, the neighboring region has less dense seismicity, and no large earthquake greater than Mw8 has been observed in the Java subduction zone. Such different seismic behaviors may be due to distinct degrees of the stress accumulation and release. In this study, the strength of plate coupling inferred from mantle lithosphere buoyancy (Hm) estimation is used to explain the seismogenic behavior in the Sunda-Andaman subduction zone. Strong and weak plate coupling status are obtained in the Sumatra and Java subduction zones, respectively. These results can explain the significant differences in seismogenic behaviors in the Sunda-Andaman subduction zone. In assessing the global implications of this finding, we observe that uplifted serpentinized fore-arc mantle peridotite is the critical phenomenon in weak plate coupling cases and leads to a limit on the width of the coupling zone. Strong plate coupling can cause a relatively low gravity anomaly as well as a negative trench-parallel gravity anomaly (TPGA) in the fore-arc regions and correlates well with the occurrence of large earthquakes, whereas weak plate coupling can cause a positive TPGA and constrain the potential occurrence of large earthquakes.

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

In this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (sigma(1), sigma(2) and sigma(3)) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (S-H) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of SH from north to south across the study area, with the direction of SH remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45 degrees counter-clockwise. The direction of the horizontal maximum compression strain rate (epsilon(H)) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the SH, eH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east-northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. SH in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role., DB, JA-M and CB acknowledge funding provided by the Spanish Ministerio de Ciencia, Innovacion y Universidades (former Ministerio de Economia y Competitividad) grants CGL2013-43877-P. H. and PGC2018-094227-B-I00, with additional support from the Generalitat de Catalunya grant 2017SGR1022.

The interaction between two flipping subduction systems shapes the complicated lithospheric structures and dynamics around the Taiwan region. Whether and in what form the Eurasian Plate subducts/deforms under Taiwan Island is critical to the debate of tectonic models. Although an east dipping high-velocity anomaly down to a depth below 200 km has been reported previously, its detailed morphology remains uncertain and leads to different interpretations. With a two-step strategy of nonlinear joint inversion, the slab images of the Eurasian Plate were retrieved in a geometry that is hyperthin in the south, becoming massive and steeper in the central, and severely deformed in the north. The possible depth and dimension of a slab break were also investigated through synthetic tests of whether the slab had torn. Moreover, the slab deflection found at ~23.2°N latitude seems to correspond to where the nonvolcanic tremors and recent NW-SE striking structures have occurred in southern Taiwan.

The Alishan area of Taiwan spans the transition from the platform with full thickness of the Eurasian continental margin in the north to the thinning crust of its slope in the south. This part of the foreland thrust and fold belt includes important along-strike changes in structure, stratigraphy, and seismic velocities. In this paper we present the results of new geological mapping from which we build geological cross sections both across and along the regional structural trend. Fault contour, stratigraphic cutoff, and branch line maps provide 3-D consistency between the cross sections. Minimum shortening is estimated to be ~∼15km with displacement overall to the northwest. A P wave velocity model helps constrain the structure at depth by providing insight into the possible rock units that are present there. P wave velocities of ≥5.2km/s point toward the presence of basement rocks in the shallow subsurface throughout much of the southeastern part of the area, forming a basement culmination. The changes in strike of thrusts and fold axial traces, the changing elevation of thrusts and stratigraphic contacts, and the growing importance of Middle Miocene sediments that take place from north to south are interpreted to be associated with a roughly northeast striking lateral structure coincident with the northern flank of this basement culmination. These transverse structures appear to be associated with the inversion of Eocene- and Miocene-age extensional faults along what was the shelf-slope transition in the Early Oligocene, uplifting the margin sediments and their higher P wave velocity basement during Pliocene-Pleistocene thrusting. Key Points Mapping in Taiwan gives information on fault reactivation at shelf-slope break Identify an important, northeast striking lateral structure P wave model points to the presence of a basement culmination, This research was carried out with the aid of grants by CSIC—Proyectos Intramurales 2006 3 01 010 and MICINN: CGL2009-11843-BTE.

Serpentinized peridotite in the fore-arc has been observed in a number of subduction zones, including the northern Manila subduction zone which terminates northward in the Taiwan arc-continent collision. How this zone of serpentinization changes northward from the subduction of thinned continental lithosphere to full arc-continent collision in the Taiwan orogeny is not well known. In this paper we present 2-D gravity modeling along three P-wave (Vp) transects across the Taiwan orogeny. Two of these transects were collected with ocean-bottom seismometers. These two transects provide good constraints on the velocity structure to the west of, and on land, southern Taiwan. Conversion of Vp to density in this area allows us to model the gravity anomaly with very little misfit. Along the subduction zone, however, the velocity models are poorly constrained in the upper mantle, where an anomalous density unit has to be used in order to model the short wavelength gravity anomaly in this area. A third transect across central Taiwan that is derived from the TAIGER local tomography data, provides good control on the crust and upper mantle Vp structure that we use to place provide density constraints for modeling the gravity anomaly in this part of the collision zone. In order to model the short wavelength gravity anomaly across the Longitudinal Valley and the southern Longitudinal trough, an anomalous density block is required beneath the fore-arc region. We interpret that the source of this anomalous density material could be serpentinized fore-arc mantle, similar to what is interpreted for the northern Manila subduction zone farther south. Water released from the subduction of the extended crust of the continental margin results in the serpentinization of the fore-arc area and may be driving the uplifting of the high-pressure rocks. © 2016 Elsevier B.V., Wen-Bin Doo is supported by Ministry of Science and Technology of Taiwan (Grant No.: MOST 105-2611-M-008-002). D. Brown is funded by MINECO grant CGL2013-43877-P.

The Mw = 6.8 Chengkung earthquake on December 10, 2003 in eastern Taiwan was associated with abundant aftershocks. To explore the seismogenic mechanism and its causative fault system, we relocated the aftershocks, determined the focal mechanisms from P-wave polarities and used the Finite Dimension Source Model method to identify the fault plane. Based on our results, the earthquake can be divided into three seismic clusters: one is located in the east of the southern Longitudinal Valley, one on the Chihshang fault, and the other on the western side of the Luyeh fault to the south. Both the seismic distribution and Finite Dimension Source Model method show that east-dipping fault planes with thrust mechanisms coincide with previous studies based on background seismicity. The third seismic cluster suggests thrust mechanism along west-dipping fault plane that was not recognized by background seismicity. This seismic cluster occurred along the Central Range Fault previously proposed as active by geomorphic and geologic evidence.

On December 10, 2003, an earthquake of magnitude (M) 6.8, the strongest since 1951, occurred near the Chengkung area in eastern Taiwan. Approximately 65 d prior to the 2003 Chengkung earthquake, precursory changes in the groundwater radon concentration were observed at the Antung radon-monitoring station located 20 km from the epicenter. The radon anomaly was a decrease from a background level of 28.9 Bq L −1 to a minimum of 12.2 Bq L −1 . Observations at the Antung hot spring suggest that the groundwater radon, when observed under suitable geological conditions, can be a sensitive tracer for strain changes in the crust preceding an earthquake.

The Taiwan Integrated Geodynamic Research program (TAIGER) collected two wide-angle and reflection seismic transects across the northern Manila subduction zone that provide constraints on the seismic velocity structure of the crust. Two-dimensional gravity modeling along these two transects shows a significant, relatively high density (3.12 and 3.02 g/cm3) in the fore-arc region, at the interface between the subducting Eurasian Plate and the accretionary prism in front of the Luzon arc on the overriding Philippine Sea Plate. The anomalous density in this zone is higher than that in the fore-arc crust and the accretionary prism but lower than that in mantle. Numerous geophysical and geological data, together with numerical models, have indicated that serpentinization of the fore-arc mantle is both expected and observed. Serpentinization of mantle rocks can dramatically reduce their seismic velocity and therefore their seismic velocity in a density to velocity conversion. Therefore, the source of the high-density material could be serpentinized fore-arc mantle, with serpentinization caused by the dehydration of the subducting Eurasian Plate. We interpret that positive buoyancy combined with weak plate coupling forces in the northern Manila subduction zone is resulting in this serpentinized fore-arc mantle peridotite being exhumed. Key Points Two-dimensional gravity modeling shows a significant high-density structure in the fore-arc region Positive buoyancy and weak plate coupling result in serpentinized peridotite exhumed Exhumed serpentinized materials in the northern Manila subduction zone are found, Wen-Bin Doo was supported by Ministry of Science and Technology of Taiwan (grant MOST 104-2111-M-008-006). H.K-C. was supported by National Science Council (grant NSC101-2116-M- 008-023-MY3). D. Brown acknowledges funding by MICINN grant CGL2013-43877-P.

[1] Knowledge of the rock types and pressure-temperature conditions at crustal depths in an active orogeny is key to understanding the mechanism of mountain building and its associated modern deformation, erosion and earthquakes. Seismic-wave velocities by themselves generally do not have the sensitivity to discriminate one rock type from another or to decipher the P-T conditions at which they exist. But laboratory-measured ratios of velocities of P to S waves (Vp/Vs) have been shown to be effective. Results of 3-D Vp and Vp/Vs tomographic imaging based on dense seismic arrays in the highly seismic environment of Taiwan provides the first detailed Vp/Vs structures of the orogen. The sharp reduction in the observed Vp/Vs ratio in the felsic core of the mountain belts implies that theα-β quartz transition temperature is reached at a mean depth of 24 ± 3 km. The transition temperature is estimated to be 750 ± 25°C at this depth, yielding an average thermal gradient of 30 ± 3°C/km.

[1] The convergence of the Philippine Sea and the Eurasian plates in the Taiwan region led to the formation of a young collisional mountain between two subduction zones of nearly orthogonal polarities. The geological processes underlying the collision and its relation to subduction are the primary targets of TAIGER (Taiwan Integrated Geodynamic Research) project. Newly acquired passive and active sources data on land, supplemented by ocean bottom as well as permanent seismic network data, are used to derive a new 3-D tomographic velocity model. Using the 7.5 km/sec contour as a marker for crustal deformation, two trend-parallel “roots,” one under the Central Range with a maximum depth of 55 km and the other under the Coastal Range at 40 km, are found to extend from southern to central (∼24°N) Taiwan. Between the two roots and lying approximately beneath the Longitudinal Valley, the 7.5 km/sec contour rises to 25 km depth. In the upper mantle, a high velocity zone, east-dipping and its upper surface coinciding with a Wadati-Benioff zone in the south (∼22.8°N), becomes near vertical and ill-defined in the north (∼23.9°N). The crustal deformation as defined by the “roots” and the along-trend variations of the upper mantle structures under Taiwan provide key information for the orogeny. With the thickening of crust to 55 km or more processes such as eclogitization and delamination may come into play.

[1] We determine 58 sets of splitting parameters, fast direction (φ) and delay time (δt), from broadband SKS/SKKS waves recorded at temporary and permanent stations in Taiwan. These and earlier results, from S and ScS as well as SKS, show an island-wide pattern of φ alignment with N–NNE structural trends in southern and central Taiwan and then turn smoothly to EW following the structural bight in northern Taiwan. Remarkable differences in δt for eastern and southwestern events are found, resulting probably from a more anisotropic upper mantle in the east, under the Central Range. With δt an average of 1.3 seconds our observations are consistent with the hypothesis that the orogeny involves shear or flow in the upper mantle in central and southern Taiwan. In northern Taiwan, near the ends of the collision and subduction zones, the extrusion and the hydration of olivine may contribute to the orientation of φ.

SUMMARY A moderate earthquake of Mw= 6.8 occurred on 2003 December 10. It ruptured the Chihshang Fault in eastern Taiwan which is the most active segment of the Longitudinal fault as a plate suture fault between the Luzon arc of the Philippine Sea plate and the Eurasian plate. The largest coseismic displacements were 13 cm (horizontal) and 26 cm (vertical). We analyse 40 strong motion and 91 GPS data to model the fault geometry and coseismic dislocations. The most realistic shape of the Chihshang fault surface is listric in type. The dipping angle of the seismic zone is steep (about 60°–70°) at depths shallower than 10 km and then gradually decreases to 40°–50° at depths of 20–30 km. Thus the polygonal elements in Poly3D are well suited for modelling complex surfaces with curving boundaries. Using the strong motion data, the displacement reaches 1.2 m dip-slip on the Chihshang Fault and decreases to 0.1 m near surface. The slip averages 0.34 m, releasing a scalar moment of 1.6E26 dyne-cm. For GPS data, our model reveals that the maximal dislocation is 1.8 m dip-slip. The dislocations decrease to 0.1 m near the surface. The average slip is 0.48 m, giving a scalar moment of 2.2E26 dyne-cm. Regarding post-seismic deformation, a displacements of 0.5 m were observed near the Chihshang Fault, indicating the strain had not been totally released, as a probable result of near-surface locking of the fault zone.

[1] The 2003 Chengkung earthquake (Mw 6.8) provided diagnostic evidence for a source model showing the deformation process of the seismogenic Chihshang fault in eastern Taiwan. The aftershocks show a fault-bend at a depth of 18 km. Coseismic ground displacements recorded by strong-motion records allow us to deduce instant rupturing of this event. Our resulting model shows a fault length of ∼33 km and dip-slip dominant rupture on fault-plane deeper than 18 km. Estimated coseismic displacements constrain two fault planes: one at 5–18 km depth dipping 60°E and 18–36 km depth dipping 45°E. The uppermost fault-plane of the Chihshang Fault (0–5 km) did not break immediately after the main shock; however, it may have a major role in after-slip and even interseismic ground deformation. The Taiyuan basin developed in the hanging wall is a geomorphic feature consistent with and adequately explained by coseismic ground displacements.

Mechanisms for interpreting anomalous decreases in radon in ground water prior to earthquakes are examined with the help of a case study to show that radon potentially is a sensitive tracer of strain changes in the crust preceding an earthquake. The 2003 Chengkung earthquake of magnitude (M) 6.8 on December 10, 2003, was the strongest earthquake near the Chengkung area in eastern Taiwan since 1951. The Antung radon-monitoring station was located 20 km from the epicenter. Approximately 65 d prior to the 2003 Chengkung earthquake, precursory changes in radon concentration in ground water were observed. Specifically, radon decreased from a background level of 780 pCi/L to a minimum of 330 pCi/L. The Antung hot spring is situated in a fractured block of tuffaceous sandstone surrounded by ductile mudstone. Given these geological conditions, we hypothesized that the dilation of brittle rock mass occurred at a rate faster than the recharge of pore water and gas saturation developed in newly created cracks preceding the earthquake. Radon partitioning into the gas phase may explain the anomalous decrease of radon precursory to the 2003 Chengkung earthquake. To support the hypothesis, vapor-liquid, two-phase radon-partitioning experiments were conducted at formation temperature (60 degrees C) using formation brine from the Antung hot spring. Experimental data indicated that the decrease in radon required a gas saturation of 10% developed in rock cracks. The observed decline in radon can be correlated with the increase in gas saturation and then with the volumetric strain change for a given fracture porosity.

We relocated 6,173 shallow earthquakes and determined focal mechanism solutions of thirty-three M(subscript L)≥5.0 events in the eastern Taiwan region (bounded by 22.7°N, 120.8°E; 24.5°N, 121.5°E; 24.3°N, 122.3°E and 22.5°N, 121.6°E) in order to investigate their tectonic implications. Our resuits suggest that the distribution of the relocated earthquakes generally agrees with the fault plane determined from the focal mechanism solutions. The Longitudinal Valley (LV) is an east-dipping seismic suture zone that separates two different tectonic regions. On the left-hand side of the LV, shallow earthquakes and normal fault-type focal mechanism were found in the Central Range region. But on the right-hand side of the LV, most events that occurred in the Philippine Sea Plate (PSP) were the thrust-type, especially for the southern portion and a few normal and strike-slip focal mechanism events were observed in the northern portion. The tectonics of eastern Taiwan can be classified from south and north into two regions, and there may exist a structure or boundary at about latitude 23.5°N with a strike of N110°E. In the northern portion of eastern Taiwan, the plate boundary appears to be an east-dipping plane defined by the earthquake distribution. The focal mechanism solutions determined in this zone show thrust type faulting and agreed with this boundary plane (strike N20°E and dip about 500). In this region, the Eurasia Plate (EP) dips to the east, and the PSP bends and dips to west. Thus, shallow earthquakes with normal and strike- slip focal mechanisms were observed in the offshore area of this region. On the other hand, the southern part of eastern Taiwan exhibits simple tectonics conditions. The EP subducts to the east under the PSP. The LV is a suture zone dipping to the east, and most events have occurred on the righthand side of the LV. Focal mechanism solutions of these events were thrust-type with strike about N25°E and dip 54° to the east.