5:15 PM - 6:30 PM
[SCG39-P26] Gravity data analysis to extract temporal gravity anomalies associated with slow slip events in the Ryukyu Trench
Keywords:slow earthquakes, slow slip events, gravity, groundwater, crustal deformation, geodesy
Slow slip events (SSEs) have been detected at depths of 30-40 km along the plate subduction boundaries around the world, where different evidences have suggested the presence of high-pressure fluids such as seismic velocity structures, laboratory experiments and numerical simulations. Theoretically, temporal variations in the fluid pressure can change recurrence intervals of SSEs. However, few observational facts have been reported to support transient fluid flow during an SSE. 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, gravity anomalies associated with the two long-term SSEs which occurred during 2001-2005 and 2013-2017 were detected by absolute gravimeters, suggesting that high-pressure fluids could migrate toward the shallow part of the plate interface during the SSEs. In the Ryukyu Trench, where a long-term SSE has occurred approximately every six months, relative gravimeters gPhoneX with a higher temporal resolution were installed last year. In this study, we analyzed one-year gravity data from January 2020 and examined various corrections to extract gravity changes during the long-term SSEs. We removed a long-term drift by a polynomial regression and tides and atmospheric pressure loading by BAYTAP-G. The residual short-term component with periods from a few to ten days was synchronized with precipitation and non-tidal sea level variations. The long-term component with periods longer than ten days included gravity changes with the order of magnitude of 1 microGal, which could not be explained by precipitation and crustal movements alone. To extract gravity anomalies during SSEs, we need to improve the method to remove the long-term drift, model the influence of groundwater which has not yet been considered, and correct for the effects of sea level changes including long- period tides.
In the Tokai area, gravity anomalies associated with the two long-term SSEs which occurred during 2001-2005 and 2013-2017 were detected by absolute gravimeters, suggesting that high-pressure fluids could migrate toward the shallow part of the plate interface during the SSEs. In the Ryukyu Trench, where a long-term SSE has occurred approximately every six months, relative gravimeters gPhoneX with a higher temporal resolution were installed last year. In this study, we analyzed one-year gravity data from January 2020 and examined various corrections to extract gravity changes during the long-term SSEs. We removed a long-term drift by a polynomial regression and tides and atmospheric pressure loading by BAYTAP-G. The residual short-term component with periods from a few to ten days was synchronized with precipitation and non-tidal sea level variations. The long-term component with periods longer than ten days included gravity changes with the order of magnitude of 1 microGal, which could not be explained by precipitation and crustal movements alone. To extract gravity anomalies during SSEs, we need to improve the method to remove the long-term drift, model the influence of groundwater which has not yet been considered, and correct for the effects of sea level changes including long- period tides.