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[SSS07-P16] Seismicity change in association with the 2011 off the Pacific Coast of Tohoku Earthquake inferred from Coulomb stress changes with focal mechanism of small earthquakes
Keywords:2011 off the Pacific Coast of Tohoku Earthquake, Coulomb failure function
The 2011 off the Pacific Coast of Tohoku Earthquake (hereinafter referred to as "Tohoku-Oki Earthquake") caused widespread crustal deformation. After the Tohoku-Oki earthquake, the number of earthquakes increased throughout Japan, including large earthquakes. Many of these earthquakes are thought to have been induced by stress changes caused by the Tohoku-Oki earthquake. Coulomb failure function (ΔCFF) has been widely utilized to study such earthquake triggering phenomena. The ΔCFF computes the stress change on the fault plane of interest (hereinafter a receiver fault) due to the deformation caused by the mainshock. In this study, we statistically investigated the ΔCFF caused by the Tohoku-Oki earthquake in the eastern Honshu region.
Determination of the fault mechanism of the receiver fault is essential to calculate ΔCFF; however, only moderate-to-large earthquakes that the focal mechanism solutions have been well obtained. It was thus difficult to study earthquake triggering based on ΔCFF in areas only with microearthquakes. Recently, Uchide (2020) estimated the focal mechanisms for microearthquakes in island areas of Japan by picking the first-motion polarity of P-waves using a neural network model.
In this study, we first investigate the ΔCFF on the receiver fault of earthquakes that occurred in the period after the Tohoku-Oki earthquake by using this catalog. In the following, pre- and post-Tohoku-Oki earthquake periods are defined as the periods from 1997 to March 11, 2011, and from March 11, 2011 to March 11, 2012, respectively. We evaluated the overall characteristics of the ΔCFF of the Tohoku earthquake for the microearthquakes that occurred during post-Tohoku-Oki period, and its significance in contrast to that for the pre-Tohoku-Oki period for the comparison. As a result, about 72 % of receiver faults had positive ΔCFFs in the period after the Tohoku-Oki earthquake, while it was about 48% before the Tohoku-Oki earthquake, suggesting that the number of earthquakes with positive ΔCFF increased after the Tohoku-Oki earthquake.
Next, we partitioned the eastern Honshu region into grids of 0.4° latitude and longitude, and averaged the ΔCFF of receiver faults within each grid to investigate the regional characteristics of ΔCFF. In both pre- and post-Tohoku-Oki earthquake periods, ΔCFF was negative in the central Tohoku region. In the period after the Tohoku-Oki earthquake, ΔCFF was positive in the areas where seismic activity increased except for the central Tohoku.
The difference of ΔCFF between the grids at the same position after and before the Tohoku-Oki earthquake was investigated, to examine the change of ΔCFF value before and after the Tohoku-Oki earthquake. The results show that the ΔCFF values increased after the Tohoku-Oki earthquake in a wide area, including all of Akita prefecture, southern Fukushima prefecture, and northern Kanto area. Even in the central Tohoku region, which showed negative values of ΔCFF, there was a change in the value of ΔCFF to be closer to positive. We investigated the ΔCFF over a wide area, but the characteristics of seismicity considered to be different in each narrow region. Therefore, it is necessary to study the regional ΔCFF. In the future, we expect to be able to use the focal mechanisms for microearthquakes to study in more detail in regions where it has been difficult to use the ΔCFF.
Determination of the fault mechanism of the receiver fault is essential to calculate ΔCFF; however, only moderate-to-large earthquakes that the focal mechanism solutions have been well obtained. It was thus difficult to study earthquake triggering based on ΔCFF in areas only with microearthquakes. Recently, Uchide (2020) estimated the focal mechanisms for microearthquakes in island areas of Japan by picking the first-motion polarity of P-waves using a neural network model.
In this study, we first investigate the ΔCFF on the receiver fault of earthquakes that occurred in the period after the Tohoku-Oki earthquake by using this catalog. In the following, pre- and post-Tohoku-Oki earthquake periods are defined as the periods from 1997 to March 11, 2011, and from March 11, 2011 to March 11, 2012, respectively. We evaluated the overall characteristics of the ΔCFF of the Tohoku earthquake for the microearthquakes that occurred during post-Tohoku-Oki period, and its significance in contrast to that for the pre-Tohoku-Oki period for the comparison. As a result, about 72 % of receiver faults had positive ΔCFFs in the period after the Tohoku-Oki earthquake, while it was about 48% before the Tohoku-Oki earthquake, suggesting that the number of earthquakes with positive ΔCFF increased after the Tohoku-Oki earthquake.
Next, we partitioned the eastern Honshu region into grids of 0.4° latitude and longitude, and averaged the ΔCFF of receiver faults within each grid to investigate the regional characteristics of ΔCFF. In both pre- and post-Tohoku-Oki earthquake periods, ΔCFF was negative in the central Tohoku region. In the period after the Tohoku-Oki earthquake, ΔCFF was positive in the areas where seismic activity increased except for the central Tohoku.
The difference of ΔCFF between the grids at the same position after and before the Tohoku-Oki earthquake was investigated, to examine the change of ΔCFF value before and after the Tohoku-Oki earthquake. The results show that the ΔCFF values increased after the Tohoku-Oki earthquake in a wide area, including all of Akita prefecture, southern Fukushima prefecture, and northern Kanto area. Even in the central Tohoku region, which showed negative values of ΔCFF, there was a change in the value of ΔCFF to be closer to positive. We investigated the ΔCFF over a wide area, but the characteristics of seismicity considered to be different in each narrow region. Therefore, it is necessary to study the regional ΔCFF. In the future, we expect to be able to use the focal mechanisms for microearthquakes to study in more detail in regions where it has been difficult to use the ΔCFF.