Temporal change in the stress field after the 2011 Tohoku earthquake was observed by stress tensor inversions of focal mechanisms of earthquakes near the source region. The maximum compressive stress (σ 1) axis before the earthquake has a direction toward the plate convergence, dipping oceanward at an angle of 25-30 degrees. Its dip angle significantly increased by 30-35 degrees after the earthquake, and σ 1 axis came to intersect with the plate interface at a high angle of about 80 degrees. By using the observed rotation of σ 1 axis, we estimated the ratio of mainshock stress drop to the background deviatoric stress τ/τ to be 0.9-0.95. This shows that the deviatoric stress causing the M w 9.0 earthquake was mostly released by the earthquake, or the stress drop during the earthquake was nearly complete. Adopting the average stress drop obtained by GPS observation data, the deviatoric stress magnitude is estimated to be 21-22 MPa. This suggests the plate interface is weak. The nearly complete stress drop caused a high dip angle of σ 1 axis, which is the reason why not a small number of normal fault type aftershocks have occurred.
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,σ1axis 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 indicates that differential stress magnitudes in these areas before the earthquake were smaller than 1 MPa. The stress field did not change in central Tohoku, even though the stresses loaded after the earthquake had nearly reversed orientations, which indicates that the differential stress magnitudes there were significantly larger than 1 MPa. In Kanto‐Chubu, stresses having nearly the same orientations as the background stresses were loaded after the earthquake, and the stress fields did not change as expected. This may have caused very high induced seismicities in Kanto‐Chubu.
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 ≤ MJMA < 3.0, by correcting the site‐amplification and attenuation effects determined using both S waves and coda waves. We then determined corner frequency assuming the omega‐square model and estimated stress drop for 1,693 earthquakes. We found that the estimated stress drops tended to have values of 1–4 MPa and that stress drops significantly changed with time. In particular, the estimated stress drops were very small at the beginning, and increased with time for ~50 days. Similar temporal changes were obtained for b‐value; the b‐value was very high (b ~ 2) at the beginning, and decreased with time, becoming approximately constant (b ~ 1) after ~50 days. Patterns of temporal changes in stress drop and b‐value were similar to the patterns for frictional strength and earthquake occurrence rate, suggesting that the change in frictional strength due to migrating fluid not only triggered the swarm activity but also affected earthquake and seismicity characteristics. The estimated high Q−1 value, as well as the hypocenter migration, supports the presence of fluid, and its role in the generation and physical characteristics of the swarm.
After the occurrence of the 2011 magnitude 9 Tohoku earthquake, the seismicity in the overriding plate changed. The seismicity appears to form distinct belts. From the spatiotemporal distribution of hypocenters, we can quantify the evolution of seismicity after the 2011 Tohoku earthquake. In some earthquake swarms near Sendai (Nagamachi-Rifu fault), Moriyoshi-zan volcano, Senya fault, and the Yamagata-Fukushima border (Aizu-Kitakata area, west of Azuma volcano), we can observe temporal expansion of the focal area. This temporal expansion is attributed to fluid diffusion. Observed diffusivity would correspond to the permeability of about 10 À15 (m 2 ). We can detect the area from which fluid migrates as a seismic low-velocity area. In the lower crust, we found seismic low-velocity areas, which appear to be elongated along N-S or NE-SW, the strike of the island arc. These seismic low-velocity areas are located not only beneath the volcanic front but also beneath the fore-arc region. Seismic activity in the upper crust tends to be high above these low-velocity areas in the lower crust. Most of the shallow earthquakes after the 2011 Tohoku earthquake are located above the seismic low-velocity areas. We thus suggest fluid pressure changes are responsible for the belts of seismicity.
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