Seismic reflection imaging of deep crustal structures using local earthquakes in the Kanto region, Japan

Abstract

Abstract We applied a novel method of passive seismic reflection imaging to actual local earthquake data collected by a dense seismic network in the Kanto region, Japan. This method, which implements reverse time migration (RTM), is based on the cross-correlation of wavefields that are extrapolated forward and backward in time from receiver locations using passively observed seismic records. Using multiple reflections between the Earth’s surface and subsurface boundaries, internal structures are imaged using many earthquakes without well-defined source information. The objective of this case study is to evaluate the possibility of acquiring seismic reflection images of the deep crustal structure by applying the RTM-based method using P-wave reflections in the earthquake data collected by a dense seismic network. The P-wave reflection profile along a 191-km-long pseudo-survey line down to a 100 km depth is obtained using the seismic records of hundreds of local earthquakes observed at 72 receiver stations. A P-wave velocity model for RTM imaging is extracted from an existing 3D model obtained by seismic tomography in a previous study. The resulting image shows several continuous reflectors at depths of 15–70 km. These reflectors correspond to the spatially variable velocity and suggest deep structures related to dual plate subduction in this region. Two eastward-dipping reflectors imaged at depths of 15–50 km are likely the top and bottom surfaces of the crust of the Philippine Sea slab, and the westward-dipping reflector at depths of 50–70 km implies the top surface of the Pacific slab. The en-échelon reflectors at depths of 15–20 km may be reflective boundaries between the upper and lower crust in the overlying Okhotsk plate. Our case study results confirm the possibility of obtaining profiles at higher resolutions than are typically obtained by earthquake-based seismic tomography and of imaging at depths beyond the limits of artificially controlled-source seismic surveys. Further implementation of the RTM-based imaging method will improve its potential use for subsurface imaging and monitoring from dense passive seismic data. Graphical Abstract

Keywords

• Seismic reflection imaging
• Crustal structure
• Local earthquake
• Reverse time migration