Stationarity of aftershock activities of the 2016 Central Tottori Prefecture earthquake revealed by dense seismic observation

Abstract

To clarify the relationship between earthquake occurrence and fluid, we analyzed data from a dense aftershock-observation network with 69 high-gain short-period seismographs installed immediately after the mainshock occurrence (October 21) in the aftershock area of the 2016 Central Tottori Prefecture earthquake. We determined the hypocenters and focal mechanisms of the aftershocks very precisely in the period from October 22 to December 15. We then investigated the temporal changes in the spatial distributions of hypocenters and Taxis azimuths of focal mechanisms. The distributions of aftershock hypocenters and Taxis azimuths are basically temporally stable, except those in limited portions in the shallow layer near the western edge of the aftershock area, where rapid decrease of aftershocks with Taxis azimuths of WSW to west was observed. If fluid rises from the lower crust due to fault rupture, the locations of aftershocks and focal mechanisms may change over time, especially in the deepest part of the aftershock region. However, the temporal change in these parameters was not apparent at depth. These observations suggest that the aftershock activity of the Central Tottori Prefecture earthquake was controlled mainly by stress concentration rather than strength reduction due to high fluid pressure.

Coseismic and Postseismic Deformation of the 2016 Central Tottori Earthquake and its Slip Model

“…Therefore, if hypocenter data are not correct, subsequently, slip distributions can have bias. Comparison of our model with the aftershock distribution 1 month after the event, relocated by Iio et al (), shows little overlapping of the largest main shock slip and active seismicity surrounding the large slip area (Figure c). Iio et al () used data from dense temporal seismic stations and reported that relocated aftershocks become shallower by 2 km than those estimated only with permanent stations (e.g., Kubo et al, ).…”

“…Comparison of our model with the aftershock distribution 1 month after the event, relocated by Iio et al (), shows little overlapping of the largest main shock slip and active seismicity surrounding the large slip area (Figure c). Iio et al () used data from dense temporal seismic stations and reported that relocated aftershocks become shallower by 2 km than those estimated only with permanent stations (e.g., Kubo et al, ). We also notice that the distributions of coseismic slip and aftershocks on Ross et al () seem to be shifted deeper than those of Iio et al ().…”

“…Iio et al () used data from dense temporal seismic stations and reported that relocated aftershocks become shallower by 2 km than those estimated only with permanent stations (e.g., Kubo et al, ). We also notice that the distributions of coseismic slip and aftershocks on Ross et al () seem to be shifted deeper than those of Iio et al (). Absolute location of slip distribution using Empirical Green's function method depends on the location of initial rupture (e.g., mainshock hypocenter), which might be the cause of inconsistencies between our model and Ross et al's.…”

“…The observed displacement is concentrated in the epicentral region (Figures a and b). Afterslip distribution indicates distributed afterslip in the shallow portion of the source fault, above the area of large coseismic slip, which is even shallower than aftershocks (Iio et al, ; Figure c). Maximum afterslip of 14 cm was observed near to the surface, with virtually no slip at depths greater than 7 km.…”

“…Topography data are taken from the Shuttle Radar Topography Mission digital elevation model (Jarvis et al, ). Aftershock distribution 1 month after the earthquake is shown by pink dots (Iio et al, ). Time series of selected stations are shown on Figures and .…”