Temporal variation of frictional strength in an earthquake swarm in NE Japan caused by fluid migration

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 M9.0 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. The stress field in the source area of this swarm was estimated by applying stress tensor inversions to these focal mechanism data. Based on the estimated stress field, and under the assumption of uniform stress, we calculated relative frictional strengths for individual focal mechanisms. The calculated relative frictional strengths vary over a wide range, but their average value exhibits a characteristic temporal variation, which is at first small, but steadily increases with time for 100 to 150 days, and then becomes approximately constant. We confirmed this characteristic temporal variation of the average relative frictional strength by assuming the stress to be nonuniform. Similar temporal variations of the average relative frictional strength are obtained for even these cases, confirming the variation. The most likely cause for the observed temporal variation of the average relative frictional strength is the temporal variation of the pore fluid pressure in the source area of the swarm, facilitated by the Tohoku‐Oki earthquake and the subsequent fluid diffusion.

Temporal Changes in Stress Drop, Frictional Strength, and Earthquake Size Distribution in the 2011 Yamagata‐Fukushima, NE Japan, Earthquake Swarm, Caused by Fluid Migration

“…The remarkable earthquake swarm activity that occurred near the border between Yamagata and Fukushima Prefectures in NE Japan (Figure ) after the 2011 M 9.0 Tohoku‐Oki earthquake is one example of such migrating swarms. Previous studies (Okada et al, ; Terakawa, Hashimoto, & Matsu'ura, ; Yoshida, Hasegawa, & Yoshida, ) have suggested that this swarm occurred in response to an increase in pore pressure due to fluids rising from below, induced by the EW extension caused by the Tohoku‐Oki earthquake (Yoshida et al, ). The source area of this swarm is located just beneath the caldera structure, which is believed to include shallow igneous bodies with hydrothermal fluids immediately below (Yoshida et al, ).…”

“…The initiation of seismicity in this swarm was delayed until 7 days after the occurrence of the Tohoku‐Oki earthquake, possibly the time needed for the fluids to move upward and increase the pore pressure. Temporal variation in the relative magnitude of frictional strength was estimated, based on a diversity of focal mechanisms, by Yoshida, Hasegawa, and Yoshida (), as shown in Figure b. The frictional strength was at first small, but steadily increased with time, and then became approximately constant.…”

“…The frictional strength was at first small, but steadily increased with time, and then became approximately constant. Yoshida, Hasegawa, and Yoshida () explained this temporal variation in frictional strength as a consequence of the temporal change in pore pressure due to fluid diffusion.…”

“…The black curve indicates the cumulative number of earthquakes with magnitudes equal to or greater than 2.0. (b) Temporal change in frictional strength estimated by Yoshida, Hasegawa, and Yoshida (). The bars indicate the 95% confidence range.…”

Seismological evidence of fault weakening due to erosion by fluids from observations of intraplate earthquake swarms

“…Heinicke et al [] speculate that this interaction leads to an extensive hydrothermal alteration of crustal rocks at focal depth, erosion of faults, and gradual fault weakening, resulting finally in fault failure. A variation of frictional strength of faults caused by fluid migration is also reported by Yoshida et al [] for an earthquake swarm in NE Japan. The fault weakening processes are recognized to be important also for standard seismic sequences and can span various time scales.…”