Systematic Detections of Intermediate-Depth Earthquakes in the Subduction Zone of Central and Northeastern Japan

Abstract:

Intermediate-depth earthquakes (IDEQs), defined as between ~70 and 350 km depths, dominate in the number of all deep earthquakes below 70 km and pose seismic hazards in certain regions. However, mechanisms behind IDEQs are still in debate. With abundant seismicity and dense seismic instrumentation, Japan is an ideal place to study IDEQs. In this study, we focus on the Central and Northeastern Japan, where the double seismic layers of IDEQs and the slab geometry are well resolved. To obtain more details of the seismogenic fault structures that host IDEQs and better understand their statistical behaviors and physical mechanisms, we perform a systematic searching for possible missing IDEQs in our studied region with a Matched Filter Technique (MFT). Specifically, we use the 37523 IDEQs listed in the Japan Meteorological Agency (JMA) catalog between 2004 and 2018 as templates to scan through the continuous waveforms recorded by ~800 borehole stations of high-sensitivity seismograph network (Hi-net) operated by National Research Institute for Earth Science and Disaster Resilience (NIED). Given the scale of detecting computation, we will start with the time windows one month before and after the 2011 M9 Tohoku-Oki earthquake as well as one month before and after each one of sixteen M>=5 IDEQs listed in the JMA catalog. Our preliminary results on a small test dataset show that the new MFT catalog has at least 3 times more events than listed in the JMA catalog. Our ultimate goal is to perform detection for the entire time period between 2004 and 2018. We will use the double-difference based method to relocate these newly detected IDEQs and compare the results with other independent estimations of the subducting slab geometry. The event temporal-spatial distribution, frequency-magnitude statistics, aftershock productivities, source properties, fault orientation, and stress drops revealed by our complete IDEQ catalog will help us identify which mechanism(s) is most responsible for IDEQs in this classic subduction-zone setting.