Foreshock migration preceding the 2016 M_w 7.0 Kumamoto earthquake, Japan

Abstract

We investigated the spatiotemporal evolution of an earthquake sequence following a series of large shallow intraplate earthquakes, including a Mw 6.2 foreshock and Mw 7.0 main shock, in the Kumamoto area of Kyushu, SW Japan. To more precisely characterize the evolution of the earthquake sequences, we applied a matched filter technique to continuous waveform data, using template events obtained via a double‐difference relocation algorithm. Migrations of seismicity fronts along the directions of fault strike and dip are clearly seen, starting immediately after the Mw 6.2 foreshock. These migrations are interpreted to result from aseismic slip triggered by the foreshock, propagating toward the nucleation point of the subsequent Mw 7.0 main shock rupture. When combined with static stress changes induced by the Mw 6.2 foreshock, it is likely that stress transfer from both aseismic and seismic slip during the foreshock sequence loaded stress onto the main shock rupture faults, bringing them closer to failure.

Characteristics of foreshock activity inferred from the JMA earthquake catalog

“…Two recent large earthquakes, the 2011 M9.0 earthquake off the Pacific coast of Tohoku (Tohoku-oki) and the 2016 M7.3 Kumamoto earthquakes, were accompanied by M6-7 foreshock activities (e.g., Kato et al 2012Kato et al , 2016Marsan and Enescu 2012). For the case of the 2016 Kumamoto sequence, the Japan Meteorological Agency (JMA) issued aftershock probabilities following the April 14 M6.5 earthquake based on the Earthquake Research Committee (ERC) protocol (ERC 1998(ERC , 2016.…”

Bayesian inference and interpretation of centroid moment tensors of the 2016 Kumamoto earthquake sequence, Kyushu, Japan

“…The strike-slip fault planes of Hinagu fault zone are noticeable also in the vertical cross sections in Fig. 9 showing hypocenters of the Kumamoto earthquakes as inferred by Kato et al (2016). The H-C method is useful, especially for the normal dip-slip aftershocks with unclear evidence of surface traces of faults; moreover, it supports also assumed orientation of strike-slip fault planes.…”

“…Additionally, we Fig. 10 Visualization of the assumed activated fault planes in top and south-side view; red squares = foreshocks' ruptures; blue squares = aftershocks' ruptures; yellow rectangles = mainshock's rupture adopted from Asano and Iwata (2016); green dots = relative locations of earthquakes inferred by Kato et al (2016) for period April 14-May 13, 2016. The scale in figure is only orientative, because of used perspective projection have implemented decomposition of MT with significant CLVD component into two shear MTs with preserved T-axis as a physical constraint.…”

“…Further surveys on imaging causative source faults beneath the surface are necessary to investigate the relationship between the surface ruptures and the geometry of earthquake source faults. The background seismicity is adopted from Kato et al (2016). The sizes of the circles are proportional to the JMA magnitude of the events.…”

“…It is characterized by many normal faults formed in the N-S extensional stress regime. According to Kamata and Kodama (1999), this extension can be related to the effect Kato et al (2016) for period April 14-May 13, 2016. The size of circles is proportional to the JMA magnitude of the events.…”