Institute of Earth Sciences

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.

Figure 1. The inner core east–west hemispheric boundary near 180W. The boundary is indicated by the yellow solid black dotted lines. Details are explained in Yu et al. (2017), The inner core hemispheric boundary near 180°W, Physics of the Earth and Planetary Interiors, 272, 1–16, doi:10.1016/j.pepi.2017.09.002.

Figure 2. Repeating earthquakes in the Tonga–Kermadec–Vanuatu subduction zones. Repeating earthquakes are indicated by black small stars. Details are explained in Yu (2013), Shallow-focus repeating earthquakes in the Tonga–Kermadec–Vanuatu subduction zones, Bulletin of the Seismological Society of America, 103 (1), 463–486, doi:10.1785/0120120123.

Figure 3. Repeating earthquakes (small stars) in the Sumatra subduction zones. Color area indicate co-seismic slip of the 2005 Mw 8.6 Nias and the 2007 Mw 8.4 and Mw 7.9 Bengkulu earthquakes. Seismic station PSI is indicated by the triangle. Black solid and black dotted lines indicate the trench and slab contour at 50 km depth, respectively.

Figure 4. Temporal velocity changes in (a) high frequency S coda waves (δVs) and (b) long period Rayleigh waves (δVLR) of the repeating earthquake sequence N1 recorded by station PSI over 2005−2015 since the 2005 Mw 8.6 Nias earthquake. The reference event is indicated by black open triangle and placed at the zero baseline. Vertical dotted lines indicate the occurrence of major earthquakes near the Sumatra subduction zone since 2004.

Figure 5. Inference and observation of temporal changes in δVs and δVLR induced by the 2004, 2005, 2007, 2008, 2010, and 2012 earthquakes for the Banda Aceh−Nias−Simeulue segment.