Abstract
Slow-slip events occur beneath Ishigaki and Iriomote Islands repeatedly once a half year. To detect gravity changes due to the slow-slip events, a superconducting gravimeter has been installed at a VLBI station in Ishigaki Island. Because this station is placed on sedimentary deposits in lowland near a mountain, rainfall around the station may fluctuate nearby underground density distribution. A three-component short-period seismometer has also been installed at the station since March 5, 2012. To explore density variation beneath the station due to rainfall, we analyze differential arrival times of direct P-wave at the station relative to a nearby permanent station for three regional or teleseismic earthquakes. After traveltime correction for the difference of epicentral distances, the net differential travel times are obtained from -0.01 to 0.00 s. These traveltimes only depend on the differences of station height and of seismic velocity structure just below the stations. The mean P-wave velocities in the sediment beneath the temporal station we obtained are between 0.9 and 2.1 km/s. These values are consistent with or close to the water-saturated velocity within a margin of error. These results thus suggest water-saturated sediment beneath the station during the observation period. Temporal variation of the differential times, although it lies within the error bar, seems to correlate to rainfall and soil moisture at the station. Although this method analyzes the discrete data in the time domain, it gives an average sedimentary P-wave velocity. Thus the method gives complementary data to other continuous methods such as soil moisture observation near the surface.
