IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS015 Oral Presentation 2332
Implications of transitional structure across Central Taiwan from studies
of wide-aperture seismic, receiver functions, stress field and earthquake
data
Prof. How-Wei Chen
Institute of Geophysics Professor
We present our integrated study on delineating the transitional structure related to the subductioncollision
tectonic features beneath Central Taiwan. Four types of studies were conducted including: a
wide-angle active source experiment; teleseismic receiver function analysis to identify regional crust
structure features; spatio-temporal stress field analysis from BATS and CWB data and co-seismic Chi-Chi
earthquake source rupture and strong motion analysis. Four different types of datasets all reflect strong
coupling between source rupture and wave generation processes due to lateral variations of crust-scale
structure. Wide-aperture seismic experiment from a 500kg explosive source and datasets collected from
a fairly dense seismic array consisting of 150 IRIS stations, 33 portable 3-component strong motion
stations and island-wide broadband and short-period seismic monitoring stations are used. The data
processing is implemented including noise reduction; depth filtering for direct arrivals and surface
waves; multiple attenuation and prestack reverse-time depth imaging. From receiver function analysis,
the best estimated Moho depth from 41 broadband stations on average is 30 km. The results reveal a
westward crustal thinning from 37.5 km in eastern Taiwan to 30 km in the east coast of mainland
China. In the north, the averaged Moho depth is 25 km and deepens to 34 km in southern Taiwan.
Notable crustal thinning under northern Taiwan may associate with back-arc opening and magmatic
activities. Variation of H and Vp/Vs ratio (κ) across SSLB (52 km, 1.77), YULB (39.5 km, 1.59) marks the
strong lateral change in the Moho topography. Relatively low κ at YULB and LYUB (29.49km, 1.56)
together with SSLB may indicate a possible trace of active collision and orogenic processes involved
beneath central Taiwan. The overall thicken crust in the south, thin crust in the north and possible
transition in the central Taiwan marks the different tectonic units between the subduction wedge and
the collision prism onshore. Spatiotemporal evolution of tectonic stress state within the seismogenic
zone associated with Chelungpu fault indicate that the maximum compressive stress (σ1) axis is highly
correlated with plate motion between Philippine Sean and Eurasian plates. The temporal variation of σ1
axis base on aftershock sequences has fairly unique short recurrence interval of 300 days or more.
Rather distinct temporal oscillatory pattern of σ1 axis directions indicate an under-damped forced
oscillation system associated with Chi-Chi earthquake rather than over-damped situation reported by
Zhao et al, (1997). Study of Chi-Chi earthquake including 3D fault plane geometry, co-seismic source
rupture processes follow by strong motion simulation and prediction. The largest Chi-Chi earthquake
over last century associated with others studies all indicate that the most damaged event is highly
influence by the crustal structure. All studies confirm that heterogeneous and possibly north extension
of subducted crust-scale structure have significant effects on waveform and travel-times.
Keywords: stress inversion, central taiwan, receiver function