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Geophysical Journal International, vol. 177, n°2, pp. 609 - 623, 2009International audienceThe 2003 Chengkung earthquake (Mw 6.8) occurred on the east-dipping Chihshang fault of eastern Taiwan, a listric fault with dominant reverse motion. A joint inversion taking into account geodetic (GPS) and seismological (accelerometric and teleseismic) data is performed. This modelling highlights the rupture process behaviour on the fault plane at depth. The coseismic rupture developed essentially between 12 and 26 km depth, expanding laterally by about 30 km towards the SSW. Two slip patches dominated, with slip values exceeding 1.5 m. During the 30 s of the whole process, the rupture propagation decelerated with time, starting at 3.4 ± 0.3 km s–1 in the two main slip patches and ending with velocities below 1.6 ± 0.1 km s–1. The 48 hr of aftershocks that followed the main shock are distributed essentially inside the coseismic slip patches on the fault surface, except in the deepest, southern part of the fault where slip is less resolved. The results of our modelling illustrate the particular behaviour of the 2003 Chengkung earthquake where the coseismic slip rapidly diminished towards the surface, in good agreement with the observation of fast post-seismic slip followed by interseismic creep along the Chihshang fault

The inner core (IC) east-west hemispheric dichotomy is widely recognized, but the reported position of the hemispheric boundary varies among studies due to uneven sampling coverage and the data analyzed. This study investigates the sharpness of the western hemispheric boundary (WHB) near 180°W by analyzing differential time residuals of PKiKP-PKPdf and PKP(bc-df) for PKPdf phases that sample 155°E-130°W in various azimuthal directions. Using PKiKP-PKPdf observations, the WHB is located at 175°E-180°W in the southern hemisphere, based mainly on the lateral isotropy-anisotropy transition. However, based on the lateral isotropic velocity contrast and this isotropy-anisotropy transition between the two hemispheres, its location is 170-160°W in the northern hemisphere. These findings indicate that the WHB is sharp and exhibits a latitudinal dependence with a 10°-20° kink, as well as 1.75% anisotropy in the uppermost IC across the 180-155°W range of the western hemisphere. As suggested by PKP(bc-df), the WHB could remain at 160°W at depth. The isotropic velocity contrast near the WHB (160°W) between the eastern and western hemispheres is lower than previous estimates using PKPdf phases sampling the bulk part of each hemisphere.

International audienceOn 1 April 2006, the Taitung earthquake (Mw 6.1) occurred in Taiwan at the boundary between the Philippine Sea and Eurasian plates, where high convergence rates contributed to the development of Plio‐Pleistocene orogeny in the region. From the joint inversion of seismic and geodetic data, we identified the event's fault geometry and reconstructed the distribution of coseismic fault slip. We modeled fault geometries with increasing complexity and selected the model that best reproduced all datasets, simultaneously. Even though the earthquake magnitude was moderate, rupturing occurred in two steps. The initial rupture was generated on a listric, north-south‐trending fault (for which dip decreases with increasing depth), and was immediately followed by movement along a perpendicular structure that cross‐cuts the main fault at 5 km south of the earthquake hypocenter. The average slip along the rupture was 30 cm, with a maximum of 87 cm. Oblique‐reverse fault movement was characterized by a predominant left‐lateral component. The amount of slip is well constrained for offsets of more than 5 cm, with an associated uncertainty of 32%. For slip amounts greater than 5 cm, uncertainties on rake and rupture time are 11° and 0.54 s, respectively. The rupture propagated from the hypocenter bilaterally, moving slightly faster toward the south (2.5±0.4 km/s) than to the north (1.7±0.1 km/s). To the south, the rupture was rapidly transmitted upward at the junction with the cross‐cutting east-west segment, whereas in the north, the rupture remained confined to the lower segment of the main fault. From Global Positioning Systems (GPS) and seismic data (time window <1 min), we infer that the cross‐cutting segment was activated following coseismic rupture on the main north-south fault, yet close enough in time to be associated with coseismic movement acquired by GPS (daily solutions)

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