14:30 〜 14:45
[SCG40-34] Simultaneous hypocenter determination of shallow tremors and regular earthquakes in the northern Japan Trench
キーワード：微動、Double Difference、震源決定、日本海溝
In the northern Japan Trench, regular earthquakes (Fast earthquakes) and shallow tectonic tremors (Slow earthquakes) are considered to be spatiotemporally coupled. To discuss the interactions between tremors and regular earthquakes more in detail, this study aims to improve the accuracy of hypocenter locations by using a novel approach that enables us to process arrival time data of tremors and regular earthquakes simultaneously.
To quantitatively evaluate the interaction between tremors and earthquakes, it is desirable to use the same method for the detection and location of these events, having different waveform characteristics. Since the onsets of the tremor signals are not clear, the location method with absolute arrival times cannot be used, whereas the method is standard for studies of regular earthquakes. The Envelope Cross-correlation Method (ECM; e.g., Obara, 2002), which measures travel time differences at multiple stations by assuming the similarity of envelope waveforms among the stations, is widely used. However, in particular, in the source zone of the shallower tremors, scattered waves caused by complex geological structures may degrade the envelope similarities to reduce the accuracy of arrival time measurements. To overcome this issue, Hendriyana & Tsuji (2021) developed the Characteristic Function (CF) that is sensitive to the arrival of the S-waves and improved the measurement accuracy of arrival time differences of tremors by cross-correlating the CFs at different stations.
Minamoto et al. (2023) applied their CF-based method to the short-period OBS records obtained in the northern Japan Trench and showed that it can detect and locate tremors as well as regular earthquakes. The location accuracy was the same level as that of ECM. On the other hand, the constraint on the focal depths was still low, the relationship between the hypocenters of slow and fast earthquakes was not clear, and the relation between the activity and the geological structure was difficult to discuss.
In this study, we aim to further improve hypocenter determination accuracy by introducing the "Double-pair Double Difference Method" (Guo & Zhang, 2017), which is based on the Double Difference algorithm (Waldhauser & Ellsworth, 2000) but for the travel time difference between stations. The double-pair double difference method is expected to improve the accuracy of the determination because it cancels travel time fluctuations due to structure heterogeneity along ray paths by further taking differences in the travel times between stations and seismic events. This method is suitable for analyzing arrival time differences obtained by correlating the CFs, obtained both for tremors and earthquakes. The introduction of the method is expected to improve hypocenter accuracies by reducing the effects of complex heterogeneity of the sub-seafloor structure and to enable quantitative discussion of the relationship between the locations of slow and fast earthquakes by comparing the hypocenters of them determined simultaneously.
To quantitatively evaluate the interaction between tremors and earthquakes, it is desirable to use the same method for the detection and location of these events, having different waveform characteristics. Since the onsets of the tremor signals are not clear, the location method with absolute arrival times cannot be used, whereas the method is standard for studies of regular earthquakes. The Envelope Cross-correlation Method (ECM; e.g., Obara, 2002), which measures travel time differences at multiple stations by assuming the similarity of envelope waveforms among the stations, is widely used. However, in particular, in the source zone of the shallower tremors, scattered waves caused by complex geological structures may degrade the envelope similarities to reduce the accuracy of arrival time measurements. To overcome this issue, Hendriyana & Tsuji (2021) developed the Characteristic Function (CF) that is sensitive to the arrival of the S-waves and improved the measurement accuracy of arrival time differences of tremors by cross-correlating the CFs at different stations.
Minamoto et al. (2023) applied their CF-based method to the short-period OBS records obtained in the northern Japan Trench and showed that it can detect and locate tremors as well as regular earthquakes. The location accuracy was the same level as that of ECM. On the other hand, the constraint on the focal depths was still low, the relationship between the hypocenters of slow and fast earthquakes was not clear, and the relation between the activity and the geological structure was difficult to discuss.
In this study, we aim to further improve hypocenter determination accuracy by introducing the "Double-pair Double Difference Method" (Guo & Zhang, 2017), which is based on the Double Difference algorithm (Waldhauser & Ellsworth, 2000) but for the travel time difference between stations. The double-pair double difference method is expected to improve the accuracy of the determination because it cancels travel time fluctuations due to structure heterogeneity along ray paths by further taking differences in the travel times between stations and seismic events. This method is suitable for analyzing arrival time differences obtained by correlating the CFs, obtained both for tremors and earthquakes. The introduction of the method is expected to improve hypocenter accuracies by reducing the effects of complex heterogeneity of the sub-seafloor structure and to enable quantitative discussion of the relationship between the locations of slow and fast earthquakes by comparing the hypocenters of them determined simultaneously.