5-1. Seismic observations at the plate boundaries around Japanese islands

 

 The chemical composition and physical properties of the subduction plate boundary is thought to be a major factor to control the interplate earthquake generation.

By the seismic experiment in 1996, we found the relation between aseismic zone and intense PP reflection from the plate boundary.  Using travel time inversion and analysis of high reflectivity, we suggested a high fluid content and/or hydrous mineral layer at the plate boundary. 

To confirm the above relation between seismicity and reflectivity, we carried out an extensive experiment in 2001(Fig. 1). We observed intense PP reflections in the aseismic zone (see Highlight Research Section). Using clear appearance of PS converted waves, we estimated the S wave structure. The sediment layers has VpVs >3, and the layers below the sediments has Vp/Vs ~1.7. Fig. 2 is an example of transverse component of records. We compared data to the theoretical travel times for SS reflections, but we could not identify SS phases. This may suggest unusual characteristics for the plate boundary materials.

By the seismic experiment in the Izu-Bonin subduction zone, we suggested the presence of a layer as sepentinite continuing from the mantle wedge to the Torishima Serpentine Seamount. Sepentinite is characterized by low Vp and low frictional strength. These properties may cause aseismic slip along the plate boundary in the Izu-Bonin seduction zone.

Fig. 1. Seven profiles. Records in Fig. 2 are on the N-S pick line. Results for the PP reflection is described in the Highlight Research Section.

 

Fig. 2. Transverse records along Line 3. Theoretical travel time for SS reflected waves are shown by green.