5:15 PM - 7:15 PM
[SSS12-P05] Elucidation of the normal faulting stress field in the coastal region of Tohoku based on focal mechanism estimations of microearthquakes using S-net data
Keywords:Normal faulting stress field , Tohoku coastal region, focal mechanism, S-net
Since the occurrence of the 2011 Mw9.0 Tohoku-Oki earthquake, seismic activity has increased in the coastal areas of the Tohoku region and within the overriding plate. Normal-faulting earthquakes, which are uncommon in the region, have also occurred more frequently. In particular, major normal-faulting earthquakes exceeding M7, such as the Mj7.0 earthquake in Iwaki City, Fukushima Prefecture, on April 11, 2011, and the Mj7.4 earthquake off the coast of Fukushima on November 22, 2016, highlight the necessity of assessing the potential for normal-faulting earthquakes in the region.
In this study, we utilized data from the high-sensitivity velocity-type seismometers installed in the Seafloor Seismic and Tsunami Observation Network (S-net) (Aoi et al., 2020; Kanazawa et al., 2016; Mochizuki et al., 2017; Uehira et al., 2016) to attempt a high-precision estimation of focal mechanisms for microearthquakes in the coastal areas of the Tohoku region. The focal mechanisms of microearthquakes are generally determined using the P-wave first-motion polarities. However, as the earthquake magnitude decreases, the number of observations capable of distinguishing the polarity of P-wave first motions becomes limited, making it difficult to determine the mechanisms uniquely. To overcome this problem, this study applied a method that also incorporates P- and S-wave amplitude data (Imanishi et al., 2011). The natural frequency of the S-net seismometer is high at 15 Hz, which results in attenuation of the low-frequency components that are important for estimating focal mechanisms. For instance, in the case of earthquakes with a magnitude of around 2, the amplitude near 2 Hz becomes nearly 100 times smaller. After reviewing a large number of earthquake datasets, it was found that, when the signal-to-noise ratio (S/N) is high, the true amplitude values can be restored by deconvolving the frequency response of the S-net seismometer, making them usable for estimating the focal mechanisms of microearthquakes.
As a result of determining the hypocenters and estimating the focal mechanisms of earthquakes that occurred off the coast of Fukushima and Ibaraki prefectures, it was confirmed that incorporating S-net data improved the accuracy of the solutions, allowing for the estimation of focal mechanisms for microearthquakes down to at least Mj2.0. Furthermore, the seismic activity associated with the branching fault extending from the plate interface toward the landward side, as suggested by Imanishi et al. (2012) off the coast of Ibaraki, was also confirmed through the analysis using S-net data. The stress field around the branching fault was found to promote normal-fault slip. In the future, comprehensive determination of focal mechanisms across the entire Japan Trench region should be carried out, along with the integration of various data sources, including seismic velocity structure, seismic activity, and acoustic survey results, to conduct a thorough examination of the mechanisms behind normal-faulting earthquake generation.
Acknowledgements. Seismograph stations used in this study include permanent stations operated by the National Research Institute for Earth Science and Disaster Prevention (Hi-net), the Japan Meteorological Agency, and Tohoku University.
In this study, we utilized data from the high-sensitivity velocity-type seismometers installed in the Seafloor Seismic and Tsunami Observation Network (S-net) (Aoi et al., 2020; Kanazawa et al., 2016; Mochizuki et al., 2017; Uehira et al., 2016) to attempt a high-precision estimation of focal mechanisms for microearthquakes in the coastal areas of the Tohoku region. The focal mechanisms of microearthquakes are generally determined using the P-wave first-motion polarities. However, as the earthquake magnitude decreases, the number of observations capable of distinguishing the polarity of P-wave first motions becomes limited, making it difficult to determine the mechanisms uniquely. To overcome this problem, this study applied a method that also incorporates P- and S-wave amplitude data (Imanishi et al., 2011). The natural frequency of the S-net seismometer is high at 15 Hz, which results in attenuation of the low-frequency components that are important for estimating focal mechanisms. For instance, in the case of earthquakes with a magnitude of around 2, the amplitude near 2 Hz becomes nearly 100 times smaller. After reviewing a large number of earthquake datasets, it was found that, when the signal-to-noise ratio (S/N) is high, the true amplitude values can be restored by deconvolving the frequency response of the S-net seismometer, making them usable for estimating the focal mechanisms of microearthquakes.
As a result of determining the hypocenters and estimating the focal mechanisms of earthquakes that occurred off the coast of Fukushima and Ibaraki prefectures, it was confirmed that incorporating S-net data improved the accuracy of the solutions, allowing for the estimation of focal mechanisms for microearthquakes down to at least Mj2.0. Furthermore, the seismic activity associated with the branching fault extending from the plate interface toward the landward side, as suggested by Imanishi et al. (2012) off the coast of Ibaraki, was also confirmed through the analysis using S-net data. The stress field around the branching fault was found to promote normal-fault slip. In the future, comprehensive determination of focal mechanisms across the entire Japan Trench region should be carried out, along with the integration of various data sources, including seismic velocity structure, seismic activity, and acoustic survey results, to conduct a thorough examination of the mechanisms behind normal-faulting earthquake generation.
Acknowledgements. Seismograph stations used in this study include permanent stations operated by the National Research Institute for Earth Science and Disaster Prevention (Hi-net), the Japan Meteorological Agency, and Tohoku University.