3:45 PM - 4:00 PM
[SSS11-25] Source characteristics of earthquakes occurring in the aseismic front of the Pacific Plate off shore of Miyagi Prefecture
Keywords:Off Miyagi Prefecture earthquakes, Aseismic front, Pacific Plate, Strong motion generation area, Source characteristics
Seismic activity with large inter-plate and intra-plate earthquakes is high in the area where the Pacific Plate subducts beneath the Tohoku region, especially off the coast of Miyagi Prefecture. In recent years, inter-plate earthquakes occurred off Miyagi Prefecture on March 20, 2021 (MJMA6.9), and May 1, 2021 (MJMA6.8), both caused the strong ground motions with a maximum seismic intensity of 5+ in Miyagi Prefecture. These earthquakes, known as aseismic front (AF) earthquakes, occurred in relatively deeper depths and closer to land compared to other inter-plate earthquakes. The variety of earthquake depths in this region, including both inter-plate and intra-plate earthquakes, provides us valuable data to study the diverse seismic events associated with the Pacific Plate. However, AF earthquakes are not yet fully understood, and their source characteristics require further investigation to predict the accurate ground motions from these earthquakes. Therefore, it is essential to quantitatively evaluate their source characteristics using observational records. This study aims to elucidate the source characteristics of AF earthquakes by (1) estimating the Strong Motion Generation Area (SMGA) model for the AF earthquake on March 20, 2021, and (2) estimating the stress drop of small-to-moderate earthquakes, including AF earthquakes, that occurred around the Pacific Plate off Miyagi Prefecture.
The SMGA model for the March 2021 earthquake was estimated using the empirical Green's function (EGF) method. We used the strong motion records from K-NET, KiK-net, and the Onagawa Nuclear Power Plant to simulate the ground motions in 0.1-10 Hz. Two major waveform packets originating from the source were identified in the observed records. Using their observed and theoretical travel times, the spatiotemporal locations of the two SMGA rupture initiation points on the fault plane were estimated. The EGF event was selected as the Mw5.1 earthquake on May 18, 2020, and the scaling parameters (N and C) were determined through spectral ratio analysis. The SMGA parameters, including size, rise time, rupture velocity, and relative rupture initiation points, were optimized using residual function minimization and a grid search approach based on records from four stations near the source. The synthetic waveforms generated from the estimated SMGAs reproduced the observed waveforms well. The stress drops for SMGA1 and SMGA2 was estimated to be 22 MPa. Comparisons of short-period levels of acceleration source spectra derived from the SMGA model with the existing scaling laws and past earthquake parameters indicated consistency with the inter-plate earthquake scaling relationships for eastern Japan.
Stress drops were estimated for 273 earthquakes (Mw: 3.6-7.0) occurring around the Pacific Plate off Miyagi Prefecture, including the AF earthquakes. The S-wave coda spectral ratio analysis (Somei et al., 2014) was applied to strong motion records to extract source spectral ratios, which were modeled using an ω-2 source spectrum to estimate corner frequencies and stress drops. The use of S-wave coda waves reduced station-dependent variations compared to direct S-wave observations. The estimated stress drops ranged from approximately 0.1 to 10 MPa. AF earthquakes exhibited slightly larger stress drops than shallower inter-plate earthquakes. There are no clear differences of stress drop between inter-plate and intraplate earthquakes, with both showing depth-dependent variations.
These results suggest that while the SMGA model for the March 20, 2021 AF earthquake aligns well with inter-plate earthquake scaling laws and past earthquake parameters in eastern Japan, AF earthquakes tend to have slightly larger stress drops and larger short-period levels compared to shallower inter-plate earthquakes when we consider the results of small-to-moderate AF earthquakes. This study provides quantitative insights into the source characteristics of AF earthquakes, contributing to improved seismic hazard assessment for the region.
The SMGA model for the March 2021 earthquake was estimated using the empirical Green's function (EGF) method. We used the strong motion records from K-NET, KiK-net, and the Onagawa Nuclear Power Plant to simulate the ground motions in 0.1-10 Hz. Two major waveform packets originating from the source were identified in the observed records. Using their observed and theoretical travel times, the spatiotemporal locations of the two SMGA rupture initiation points on the fault plane were estimated. The EGF event was selected as the Mw5.1 earthquake on May 18, 2020, and the scaling parameters (N and C) were determined through spectral ratio analysis. The SMGA parameters, including size, rise time, rupture velocity, and relative rupture initiation points, were optimized using residual function minimization and a grid search approach based on records from four stations near the source. The synthetic waveforms generated from the estimated SMGAs reproduced the observed waveforms well. The stress drops for SMGA1 and SMGA2 was estimated to be 22 MPa. Comparisons of short-period levels of acceleration source spectra derived from the SMGA model with the existing scaling laws and past earthquake parameters indicated consistency with the inter-plate earthquake scaling relationships for eastern Japan.
Stress drops were estimated for 273 earthquakes (Mw: 3.6-7.0) occurring around the Pacific Plate off Miyagi Prefecture, including the AF earthquakes. The S-wave coda spectral ratio analysis (Somei et al., 2014) was applied to strong motion records to extract source spectral ratios, which were modeled using an ω-2 source spectrum to estimate corner frequencies and stress drops. The use of S-wave coda waves reduced station-dependent variations compared to direct S-wave observations. The estimated stress drops ranged from approximately 0.1 to 10 MPa. AF earthquakes exhibited slightly larger stress drops than shallower inter-plate earthquakes. There are no clear differences of stress drop between inter-plate and intraplate earthquakes, with both showing depth-dependent variations.
These results suggest that while the SMGA model for the March 20, 2021 AF earthquake aligns well with inter-plate earthquake scaling laws and past earthquake parameters in eastern Japan, AF earthquakes tend to have slightly larger stress drops and larger short-period levels compared to shallower inter-plate earthquakes when we consider the results of small-to-moderate AF earthquakes. This study provides quantitative insights into the source characteristics of AF earthquakes, contributing to improved seismic hazard assessment for the region.