2:45 PM - 3:00 PM
[SSS08-14] Determination of the epicenter of shallow very low-frequency earthquakes southeast off the Kii Peninsula using correlation methods
Keywords:shallow very low-frequency earthquake, slow earthquake, southeast off the Kii Peninsula, correlation method
Southeast off the Kii Peninsula is located in the northeastern part of the Nankai Trough, where the megathrust earthquake have occurred repeatedly at intervals of 100-150 years, and seismic phenomena called "slow earthquakes" with longer time constants than those of regular earthquakes also occur. Slow earthquakes occur in the transition zones adjacent to the seismogenic zone on the plate boundary, so that they may accumulate strain and trigger the megathrust earthquake (Obara & Kato, 2016). Shallow very low-frequency earthquakes (SVLFEs), a type of slow earthquake, radiate seismic waves with dominant frequencies between 0.01 and 0.1Hz, and are observed by broadband seismic networks on land and offshore. SVLFEs have been analyzed mainly using array analysis (e.g. Asano et al. 2008) and grid search (e.g. Asano et al. 2015). It has been revealed that SVLFEs southeast off the Kii Peninsula migrate along the strike direction of the plate boundary and tend to be concentrated around the stress accumulation peak. In this study, we attempted to determine the epicenter of SVLFE southeast off the Kii Peninsula using the correlation method, unlike previous studies, to obtain more detailed spatio-temporal characteristics.
Generally, the correlation method treats the time difference that gives the maximum of the cross-correlation coefficient (CC) calculated from waveforms of two stations as the observed travel-time difference. In this study, the objective function is the sum of the squared residuals of the observed and theoretical travel-time differences for a combination of stations, and the epicenter is determined by minimizing the objective function. However, a problem with the correlation methods is that the time-lag corresponding to the maximum CC does not always provide the true travel-time difference. Therefore, instead of simply using the time-lag with the maximum CC as the travel-time difference, the one with the minimum absolute residual of the travel-time difference among multiple time-lags corresponding to maxima of CC is used as a candidate for the observed travel-time difference. Furthermore, in order to minimize the effect of the three-dimensional seismic velocity structure, the observed travel-time difference data are selected conditional not only on CC but also on the difference in travel-time difference and that in epicentral distances.
In this study, we analyzed four periods of active SVLFEs: September 2004, March-April 2009, April 2016, and December 2020-January 2021. The data used were the vertical component of broadband seismometer records from 33 F-net stations. They were band-pass filtered at periods between 20 and 50 sec, and then resampled to 1 sec. The analysis was performed continuously on the waveforms reduced by the theoretical travel-time for a certain reference point southeast off the Kii Peninsula, shifting the analysis window of 300 seconds in length by 150 seconds. The apparent wave velocity was assumed to be 3.8 km/s. The obtained epicenters were adopted as the results if the bootstrap standard error was less than 10 km.
Figure shows the results of the analysis of activity during March and April 2009. The number of detected events is 258, and two clusters are seen in the epicenter map. The spatio-temporal plot shows that the active area migrated eastward along X-X' at a speed of about 9 km/day in the first half of the analysis period. This speed is similar to that observed in the Hyuga-nada region. In the latter half, the active area migrated from east to west and from downdip side to updip side. This study shows that the determination of epicenter using the correlation method is effective in detecting spatio-temporal characteristics of SVLFE activity southeast off the Kii Peninsula. Results using DONET data will also be shown in the presentation.
Acknowledgments: F-net and DONET records were used in this study. We thank members of the National Research Institute for Earth Science and Disaster Resilience and the Japan Agency for Marine-Earth Science and Technology for their assistance.
Generally, the correlation method treats the time difference that gives the maximum of the cross-correlation coefficient (CC) calculated from waveforms of two stations as the observed travel-time difference. In this study, the objective function is the sum of the squared residuals of the observed and theoretical travel-time differences for a combination of stations, and the epicenter is determined by minimizing the objective function. However, a problem with the correlation methods is that the time-lag corresponding to the maximum CC does not always provide the true travel-time difference. Therefore, instead of simply using the time-lag with the maximum CC as the travel-time difference, the one with the minimum absolute residual of the travel-time difference among multiple time-lags corresponding to maxima of CC is used as a candidate for the observed travel-time difference. Furthermore, in order to minimize the effect of the three-dimensional seismic velocity structure, the observed travel-time difference data are selected conditional not only on CC but also on the difference in travel-time difference and that in epicentral distances.
In this study, we analyzed four periods of active SVLFEs: September 2004, March-April 2009, April 2016, and December 2020-January 2021. The data used were the vertical component of broadband seismometer records from 33 F-net stations. They were band-pass filtered at periods between 20 and 50 sec, and then resampled to 1 sec. The analysis was performed continuously on the waveforms reduced by the theoretical travel-time for a certain reference point southeast off the Kii Peninsula, shifting the analysis window of 300 seconds in length by 150 seconds. The apparent wave velocity was assumed to be 3.8 km/s. The obtained epicenters were adopted as the results if the bootstrap standard error was less than 10 km.
Figure shows the results of the analysis of activity during March and April 2009. The number of detected events is 258, and two clusters are seen in the epicenter map. The spatio-temporal plot shows that the active area migrated eastward along X-X' at a speed of about 9 km/day in the first half of the analysis period. This speed is similar to that observed in the Hyuga-nada region. In the latter half, the active area migrated from east to west and from downdip side to updip side. This study shows that the determination of epicenter using the correlation method is effective in detecting spatio-temporal characteristics of SVLFE activity southeast off the Kii Peninsula. Results using DONET data will also be shown in the presentation.
Acknowledgments: F-net and DONET records were used in this study. We thank members of the National Research Institute for Earth Science and Disaster Resilience and the Japan Agency for Marine-Earth Science and Technology for their assistance.