Keisuke Yoshida

Stress fields in inland areas of eastern Japan before and after the Tohoku‐oki earthquake were estimated by inverting focal mechanism data. Before the earthquake, σ 1 axis was oriented EW in Tohoku but NW‐SE in Kanto‐Chubu. The stress fields changed after the earthquake in northern Tohoku and in southeastern Tohoku near Iwaki city, where the orientations of the principal stresses became approximately the same as the orientations of the static stress change associated with the earthquake. This in

Abstract In this study, we investigated temporal variations in stress drop and b‐value in the earthquake swarm that occurred at the Yamagata‐Fukushima border, NE Japan, after the 2011 Tohoku‐Oki earthquake. In this swarm, frictional strengths were estimated to have changed with time due to fluid diffusion. We first estimated the source spectra for 1,800 earthquakes with 2.0 ≤ M JMA < 3.0 , by correcting the site‐amplification and attenuation effects determined using both S waves and coda wave

Abstract The spatiotemporal distributions of hypocenters and temporal change in seismicity patterns were investigated in details for events in the Yamagata‐Fukushima border earthquake swarm, which was a remotely triggered earthquake sequence by the 2011 Tohoku‐Oki earthquake. We relocated the hypocenters by applying the double‐difference location method to differential arrival time data obtained by waveform cross correlations together with the Japan Meteorological Agency catalogue data. The hypo

Abstract An M w 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the M w 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainsho

Abstract This study describes an ongoing intense earthquake swarm in the crust of the northeastern Noto Peninsula, Japan, that began around the end of 2019. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust. However, no volcanic activity has occurred in this region since the Middle Miocene (15.6 Ma). This study investigates the cause of this earthquake swarm based on the spatiotemporal evolution of earthquake hypocenters and seismic reflectors. The h

Abstract In order to investigate changes in the stress field following the 2008 M 7.2 Iwate‐Miyagi Nairiku earthquake in NE Japan, we determined many focal mechanisms within the focal area, using data from both the dense aftershock observation network deployed just after the main shock and other temporary and routinely operated stations in the surrounding area. Applying stress tensor inversions to these focal mechanism data, we estimated the detailed spatial distribution of the principal stress

Abstract A shallow M 7.3 event with a M 6.5 foreshock occurred along the Futagawa‐Hinagu fault zone in Kyushu, SW Japan. We investigated the spatiotemporal variation of the stress orientations in and around the source area of this 2016 Kumamoto earthquake sequence by inverting 1218 focal mechanisms. The results show that the σ 3 axis in the vicinity of the fault plane significantly rotated counterclockwise after the M 6.5 foreshock and rotated clockwise after the M 7.3 main shock in the Hinagu f

Abstract Temporal variations of the fault frictional strength was investigated based on the diversity of focal mechanisms in the source area of the Yamagata‐Fukushima border earthquake swarm, a significant earthquake swarm that occurred in central Tohoku, NE Japan, which started just after the 2011 M 9.0 Tohoku‐Oki earthquake. The focal mechanisms of events in this swarm activity were determined using P wave polarity data as well as short‐period (1.5–2.5 Hz) waveform data from the direct P wave.

In order to know whether principal stress orientations in the source area rotated after the 2011 April 11 Mw 6.6 Fukushima-Hamadori earthquake in NE Japan, we investigated detailed spatial distributions of stress orientations for both the pre- and post-main shock periods using a large amount of focal mechanism data. We applied stress tensor inversions to focal mechanism data from Japan's National Research Institute for Earth Science and Disaster Prevention's F-net broadband seismic network and t

Abstract The 2024 M w 7.5 Noto Peninsula, Japan, earthquake was initiated within the source region of intense swarm activity. To reveal the mainshock early process, we relocated the earthquake hypocenters and found that many key phenomena, including the mainshock initiation, foreshocks, swarm earthquakes, and deep aseismic slip, occurred at parts of a previously unrecognized fault in intricate fault network. This fault is subparallel (several kilometers deeper) to a known active fault, and the m

We investigated a detailed spatial distribution of principal stress axis orientations in the source area of the 2003 M6.4 Northern Miyagi Prefecture earthquake that occurred in the forearc of northeastern Japan. Aftershock hypocentres were precisely relocated by applying the double difference method to arrival time data obtained at temporary stations as well as at surrounding routine stations. We picked many P-wave polarity data from seismograms at these stations, which enabled us to obtain 312