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郁文哲 Yu, Wen-che



Research interest
Seismology places the fundamental constraints on the interior of the Earth and earthquake sources. I improve the resolution of the seismic structures of the Earth’s interior and detect repeating earthquakes and use them to detect time-varying crustal and inner core structures (4D seismology) and estimate slip rate at the plate boundary.

Complex inner core structures of the Earth
The Earth’s inner core is composed of nearly pure iron and isothermal. The temperature and composition variations are small in the inner core, and it would be expected that the seismic structures are uniform. Seismology reveals that the inner core possesses rather complex structures: (1) east-west hemispheric scale in inner core anisotropy, velocity, attenuation; (2) small-scale heterogeneities comparable to the wavelength of P wave that generate scattered wavefield; (3) inner core anisotropy changes its form at the center; and (4) time-varying velocity structures (0.1 second time shift per decade) due to differential rotation of the inner core relative to the mantle. The challenges are: because the inner core lies under the complex mantle, differential times and amplitude ratios of inner core P wave (PKIKP, PKPdf) are adopted to lower the impact due to the complex mantle structures. Some of the related topics were investigated.

Repeating earthquakes
Repeating earthquakes are characterized by highly correlated seismic waveforms, and are generally interpreted as reflecting repeated slip at small asperities on the same fault, surrounded by stably sliding regions on the plate boundary. Repeating earthquakes can be used to estimate the slip rate at the plate boundary (based on the empirical scaling relationship between slip and seismic moment), and plate coupling. The recurrence interval of repeating earthquakes can also be used to assess the scaling relationship between recurrence interval and seismic moment. Repeating earthquakes search is carried out for the southwest Pacific (Tonga-Kermadec-Vanuatu) and Sumatra subduction zones. Search for repeating earthquakes occurred in other plate boundary will be carried out.

Earthquake induced time-varying crustal structures
Repeating earthquakes are applied to detect the 2004 Mw 9.2 Sumatra and 2005 Mw 8.6 Nias subduction zone earthquakes in the proximity of Sumatra. Near the rupture zones of the 2004 earthquake, temporal velocity changes of S coda waves recover steadily since the 2004 event throughout 2005-2015; whereas phase velocity changes of Rayleigh waves display transient recovery since the reduction induced by the 2004 until late 2007, followed by additional stronger reduction. Logarithmic recovery of velocity of S coda likely corresponds to deep processes of post-seismic afterslip or viscoelastic relaxation, whereas temporal breaks of phase velocity changes of Rayleigh waves near 2007 reflect surface damage and healing induced by strong ground motion of the 2004 and 2005 great earthquakes, and repeating damage by strong ground motion of the 2008 Mw 7.3 Simeulue and 2007 Mw 8.4 and Mw 7.9 Bengkulu earthquakes.
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