[SSS11-P05] Receiver function imaging of the Philippine Sea plate subducting beneath Shikoku (2)
Keywords:receiver function, Philippine Sea plate, Shikoku, Nankai Trough megaquake, seismological structure
We carried out receiver function analysis using the waveforms of teleseismic earthquakes observed at seismic stations on a linear array that crossed Shikoku from Tokushima Prefecture to Ehime Prefecture along the northeast east – west southwest direction. We successfully obtained clear images of the upper boundary and the oceanic Moho of the Philippine Sea plate subducting beneath Shikoku.
The purpose of this study is to estimate accurately the structure of the areas around the Philippine Sea slab, which are the source of the Nankai Trough magaquake, and the propagation path of strong seismic waves. Such information is necessary for accurately estimating the extent of the source area and the strong motion generation area, and improving the prediction of earthquake magnitude and strong motion.
In this presentation, we report the shape of the Philippine Sea slab which is being revealed by receiver function analysis in Shikoku.
2. Linear array observation
On the survey line from Tokushima City to Seiyo City, Ehime Prefecture, 14 temporary stations were deployed, including nearby permanent stations, so that the interval was about 5 km.
At each temporary station, the ground velocity output of a high-sensitivity seismometer (Sercel, L-4-3D) with a natural period of 1 s was continuously recorded to CF cards of a data logger. The data logger was powered by a car battery, which was charged by solar cells.
3. Receiver function analysis
The receiver function is a waveform obtained by extracting PS converted waves generated at S-wave velocity discontinuities below seismic stations.We converted the time axis of the receiver function to the depth axis by using the Japan Meteorological Agency's seismic velocity model JMA2001 (Ueno et al., 2002), and obtained an image of the S-wave velocity discontinuities by stacking the amplitude of the receiver functions obtained for many teleseismic events at the stations. The obtained receiver function image is shown in Fig. 1.
The series of red images behind the red line labeled OM is considered to be the top of a high-velocity layer and can be interpreted as the ocean Moho in the Philippine Sea slab. The blue line labeled ST above it is considered to be the top of the low-velocity oceanic crust, that is the top of the Philippine Sea slab. Here, we assume the thickness of the oceanic crust to be almost constant (about 7 to 8 km), and place the interpretation line (ST) in a series of blue images above the clear ocean Moho (OM). At -80 < x <50 km, dark blue images are seen above the slab upper surface (ST), which suggest the existence of a low velocity layer. It is thought to be due to slab-derived fluids. The Philippine Sea plate is subducting from the front to the back of the paper while tilting west-southwest.
Seismic waveform data from permanent stations at the National Research Institute for Earth Science and Disaster Prevention, the Japan Meteorological Agency, the National Institute of Advanced Industrial Science and Technology, Kochi University, and Kyoto University were used.