Spatial variation in the crustal anisotropy and its temporal variation associated with a moderate-sized earthquake in the Tokai region, central Japan

Abstract

SUMMARY Spatial and temporal variations in seismic anisotropy in the crust are investigated using earthquakes in the crust and the subducting Philippine Sea Plate beneath the Tokai region, central Japan. We use a total of 351 high‐quality waveform data recorded from 1986 December until 1999 August by the microearthquake observation network of the Research Center for Seismology and Volcanology, Nagoya University. The leading shear waves are polarized in an approximately E–W direction, independent of the focal depth, showing consistency with the direction of regional horizontal maximum compressive stress in the Tokai region. The time delay increases for greater focal depth down to 30 km. These results indicate that the regional compressive stress controls anisotropy not only in the upper crust but also in the lower crust. Assuming a uniform distribution of anisotropy, the degree of anisotropy is estimated to be 0.6 per cent in the lower crust. An increase in time delay is found after the Aichi‐ken Tobu earthquake (M= 5.7) in 1997 at station STN. This variation is statistically significant and is not an apparent change due to the variation in hypocentre distribution or any kind of artificial effect. On the other hand, no temporal variation in time delay is found at station INU. The spatial pattern of the temporal variation in time delay is consistent with the change in the Coulomb failure function (ΔCFF) caused by both coseismic and post‐seismic fault slips. We therefore conclude that the observed temporal change in time delays might be caused by increasing crack density as well as pore fluid pressure due to the static stress change accompanied with the Aichi‐ken Tobu earthquake.

Spatial variation in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan

“…The results of the temporal change in the normalized time delay due to the static stress change by the Aichi-ken Tobu earthquake (Saiga et al, 2003;Hiramatsu et al, 2005) provides δt n of 1.6 ms/km as the steady state value and δt n of 1.4 ms/km as the increase of the normalized time delay due to σ of 1.0 kPa as the increase of the effective static stress for opening or enlargement of micro-cracks. Based on these values, RSC SP is estimated to be 880 (MPa) −1 .…”

“…Liu et al, 1997;Hiramatsu et al, 2005) and a volcanic eruption (Miller and Savage, 2001). Recently, Savage et al (2010) reported that a clear correlation between δt and baseline length observed by GPS at Mt. Asama, Japan.…”

Continuous observation of seismic wave velocity and apparent velocity using a precise seismic array and ACROSS seismic source

“…This phenomenon has been explained by applying a theory of extensive dilatancy anisotropy (Crampin, 1987) in which the differential stress field dictates the preferential alignment of newly formed or growing cracks. Saiga et al (2003) reported temporal variation in the time delay of split shear waves resulting from static stress changes associated with a moderate magnitude earthquake. These observations demonstrate the sensitivity of the propagation properties of seismic waves to changes in applied stress; however, these researchers were not always able to analyze events of similar source parameters.…”

Stress state in the upper crust around the source region of the 1891 Nobi earthquake through shear wave polarization anisotropy

“…Following Hiramatsu et al (2010), we adopt here RSC SP = 880/(MPa) from the change in normalized time delays caused by the step-wise static stress change of a moderate size earthquake (Saiga et al 2003;Hiramatsu et al 2005) and T C = 2 years from the recovery of crack conditions to a step-wise static stress change (Hiramatsu et al 2005;Sugaya et al 2009). …”

“…Shear wave splitting is, therefore, expressed by two splitting parameters: the polarization orientation of the faster wavelet ϕ and the time delay between the two wavelets δt. In the case of the preferred orientation of microcracks, a temporal variation in shear wave splitting induced by a stress change in the crust is often accompanied by earthquakes (e.g., Liu et al 1997;Saiga et al 2003) or volcanic activities (e.g., Miller and Savage 2001;Savage et al 2010;Roman et al 2011;Honda et al 2014). Furthermore, detailed observations have revealed a healing process of microcracks in the crust, after an earthquake, through shear wave splitting (Tadokoro and Ando 2002;Hiramatsu et al 2005).…”