Characteristics of the foreshock occurrence for Mj3.0 to 7.2 shallow onshore earthquakes in Japan

Abstract

We use the Japan Meteorological Agency (JMA) earthquake catalogue from January, 2001 to February, 2021 to investigate the spatiotemporal foreshock occurrence for shallow (within 30 km depth) onshore earthquakes (Mj3.0 to 7.2). We find clear peaks for the numbers of small earthquakes within 10 days and 3 km prior to the larger earthquakes, which are considered as our definition of foreshocks. After removing the aftershocks, earthquake swarms and possible triggered earthquakes by the 2011 Mw9.0 Tohoku-oki earthquake, we find that for the 2066 earthquakes (mainshocks), 783 (38%) have one or more foreshocks. There is a decreasing trend of foreshock occurrence rate with mainshock depth. Also, normal faulting earthquakes have higher foreshock occurrence rate than reverse faulting earthquakes. We calculate the earthquake occurrence rate as a function of the magnitudes of foreshocks and mainshocks, and we have found no clear trend between the magnitudes of foreshocks and mainshocks. Graphical Abstract: [Figure not available: see fulltext.]