11:00 〜 13:00
[SCG44-P16] Preliminary analysis of continuous gravity measurement data obtained at a long-term slow slip area in the Ryukyu Trench
キーワード：スロー地震、スロースリップ、重力、地下水、地殻変動、間隙流体
Slow slip events (SSEs) have been detected along the circum-Pacific plate subduction zones, where the presence of high-pressure pore fluids has been suggested by seismic wave velocity structures, laboratory experiments and numerical simulations. The presence of high-pressure fluids reduces the effective normal stress and lowers frictional strength of the fault, which causes SSEs. Transient fluid pressure changes (or fluid migration) have been theoretically studied, but supporting observational facts are still limited. Fluid migrations cause a subsurface mass redistribution and change the surface gravity. Therefore, this study is aimed at detecting fluid migrations by observing gravity.
In the Tokai area along the Nankai Trough, gravity anomalies synchronized with two long-term SSEs which occurred during 2001-2005 and 2013-2017 were detected by absolute gravimeters, suggesting a possibility that fluids migrated up dip along the plate interface during the SSEs. These observations have been conducted approximately once a year, which is insufficient to confirm whether a gravity anomaly occurs during SSEs with shorter durations. To improve the temporal resolution of the gravity measurement, we used gPhoneX relative gravimeters which allows continuous measurement.
At Ishigaki Island in the Ryukyu Trench, long-term SSEs with durations of ~1 month occur approximately twice a year. We initiated gravity observation in January 2020 at the Ishigaki Meteorological Office of the Japan Meteorological Agency, using two gPhoneXs to confirm a reliability of the obtained gravity data. In this presentation, we report a preliminary analysis result of the first two-year gravity data. We removed the effect of tides using BAYTAP-G and GOTIC2 and correct for the effect of polar motion and vertical crustal movements. The entire observation period was divided into a few months and an instrumental drift in each period was removed by fitting a quadratic function. Consequently, the corrected gravity data from both gravimeters agreed within ~1 microGal. The observation spans of the gravity data included the four SSEs.
To geophysically interpret the observed gravity changes, we first investigated effects of precipitation. The gravity changes estimated with both physical and empirical models well explained the overall tendency of the observed gravity changes (RMS < 1 microGal). However, some of the gravity changes with amplitudes of a few microGal and periods of a few tens of days could not be explained by the effects of precipitation.
We will initiate groundwater level observation near the gravity measurement site this spring. We will model gravity changes due to non-tidal sea level and groundwater level changes as well as precipitation and quantify the model uncertainties. Based on the model uncertainties and results of stacking, we will evaluate gravity changes during SSEs.
In the Tokai area along the Nankai Trough, gravity anomalies synchronized with two long-term SSEs which occurred during 2001-2005 and 2013-2017 were detected by absolute gravimeters, suggesting a possibility that fluids migrated up dip along the plate interface during the SSEs. These observations have been conducted approximately once a year, which is insufficient to confirm whether a gravity anomaly occurs during SSEs with shorter durations. To improve the temporal resolution of the gravity measurement, we used gPhoneX relative gravimeters which allows continuous measurement.
At Ishigaki Island in the Ryukyu Trench, long-term SSEs with durations of ~1 month occur approximately twice a year. We initiated gravity observation in January 2020 at the Ishigaki Meteorological Office of the Japan Meteorological Agency, using two gPhoneXs to confirm a reliability of the obtained gravity data. In this presentation, we report a preliminary analysis result of the first two-year gravity data. We removed the effect of tides using BAYTAP-G and GOTIC2 and correct for the effect of polar motion and vertical crustal movements. The entire observation period was divided into a few months and an instrumental drift in each period was removed by fitting a quadratic function. Consequently, the corrected gravity data from both gravimeters agreed within ~1 microGal. The observation spans of the gravity data included the four SSEs.
To geophysically interpret the observed gravity changes, we first investigated effects of precipitation. The gravity changes estimated with both physical and empirical models well explained the overall tendency of the observed gravity changes (RMS < 1 microGal). However, some of the gravity changes with amplitudes of a few microGal and periods of a few tens of days could not be explained by the effects of precipitation.
We will initiate groundwater level observation near the gravity measurement site this spring. We will model gravity changes due to non-tidal sea level and groundwater level changes as well as precipitation and quantify the model uncertainties. Based on the model uncertainties and results of stacking, we will evaluate gravity changes during SSEs.