Deep seismic reflection experiment using a dense receiver and sparse shot technique for imaging the deep structure of the Median Tectonic Line (MTL) in east Shikoku, Japan

Abstract

A seismic experiment was carried out in east Shikoku, Japan, to detect deep reflections across the Median Tectonic Line (MTL), which juxtaposes low-P/T metamorphic rocks with high-P/T metamorphic rocks. Our experiment employed an unconventional technique: sparse shot spacing, a strong energy source (dynamite) and a dense array of seismometers. The above specifications produce only single fold coverage without common midpoint (CMP) stacking. Nevertheless, the reflection profile provides essential information on the deep structure of the MTL, of other major faults, and of the Moho in east Shikoku. On the MTL, this profile is the first to delineate the MTL from the surface to about 12 km depth. The following three factors were essential to the success of our experiment. First, the receiver interval was sufficiently small to provide horizontal resolution that was able to detect deep reflectors. Second, the simple crustal structure does not require CMP stacking to enhance data quality. Third, a thin weathering layer at the surface reduced the attenuation of seismic waves and minimized the generation of the surface waves that often obscure deep reflectors. In these conditions, the technique can be an effective means of probing the deep crust while substantially reducing survey costs.

Subsurface structures derived from receiver function analysis and relation to hypocenter distributions in the region from the eastern Shikoku to the northern Chugoku districts, Southwest Japan

“…Ito et al (1996) found a sharp reflector dipping [30][31][32][33][34][35][36][37][38][39][40] • north in the upper crust by a seismic reflection survey. Kawamura et al (2003) and Sato et al (2005) also found that the sharp reflector extended to the lower crust from a recent seismic explosion survey. Although the subsurface structures in these regions have become clearer by analyzing geophysical data, the relations among the structures or between the structures and hypocenter distributions are not still clear.…”

“…Although the clear R1 cells in this study locate at similar depths to the oceanic Moho discontinuity, it seems improbable that they correspond to the obscure reflectors of Kurashimo et al (2002). Kodaira et al (2002) pointed out the existence of water at the PHS plate boundary by analyzing reflected phases with large amplitudes recorded in an onshore-offshore deep seismic survey conducted in the western Nankai Trough seismogenic zone, and estimated the very thin layer (about 300 m) with very low velocity at the plate boundary. Kurashimo et al (2002) also suggested a very low velocity layer with 1 km thickness at the upper most part of the PHS slab obtained from the same waveform calculation for very clear reflected phases as Kodaira et al (2002).…”

“…Kodaira et al (2002) pointed out the existence of water at the PHS plate boundary by analyzing reflected phases with large amplitudes recorded in an onshore-offshore deep seismic survey conducted in the western Nankai Trough seismogenic zone, and estimated the very thin layer (about 300 m) with very low velocity at the plate boundary. Kurashimo et al (2002) also suggested a very low velocity layer with 1 km thickness at the upper most part of the PHS slab obtained from the same waveform calculation for very clear reflected phases as Kodaira et al (2002). Although there are slight differences between their low velocity models (i.e., the location and the thickness of the low velocity layer), they both suggested the existence of the low velocity layers around the plate boundary.…”

Using large dynamite shots to image the structure of the Moho from deep seismic reflection experiment between the Sichuan basin and Qinling orogen

“…Developing the correct processes and selecting the correct parameters based on the characteristics of the data from the large dynamite shots are the key to imaging the lower crust and Moho. Many geophysicists have achieved good results in processing data from dynamite shots (e.g., Levander et al 1994;Klemperer et al 1986;Kawamura et al 2003;Stern et al 2015).…”

“…For example, Klemperer et al (1986) determined the structure of the Moho in Nevada, USA, using large dynamite shots. Kawamura et al (2003) revealed the deep crustal structure of the eastern central tectonic line using two large dynamite shots. Stern et al (2015) determined the structure of the Moho and the lithosphere-asthenosphere boundary beneath North Island, New Zealand, using large dynamite shots.…”