Takehide Nakano

GPS observations of postseismic deformation for the 1995 Hyogo-Ken Nanbu Earthquake, Japan

11-months of GPS observations, which started within 2 days of the 1995 Hyogo-ken Nanbu earthquake, detected significant postseismic crustal deformation that reached 20 mm at some stations. A comparison of postseismic and coseismic deformation shows a similar pattern in the direction of displacements. We tried to determine the predominant relaxation mechanism by examining the time dependence of observed postseismic deformation. We considered three models: an exponential, a power-law, and a logarithmicaly decaying model. However, each of these three time-dependent models can explain the observed data, equally well. In case of the exponential relaxation model, the relaxation time constant was estimated to be about 50 days. In the power-law model, the index p was nearly 1.0; this is almost the same as the p value of Omoris decay law of aftershock sequences for this event. This suggests that the mechanism causing postseismic deformation may be related to that producing aftershocks.

Crustal deformation at the Nankai Subduction Zone, southwest Japan, derived from GPS measurements

The interseismic strain accumulation process at the Nankai subduction zone, the plate-boundary region between the Philippine Sea and the Eurasian plates off Southwest Japan, has been revealed by GPS measurements spanning five years. Large strains characterized by a contraction of 2.2 ∼ 3.4 × 10−7/yr have accumulated in a narrow region extending parallel to the strike of the plate boundary. The contraction axes lie in a northwest-southeast direction nearly parallel to the direction of relative motion of the Philippine Sea plate with respect to the Eurasian plate. GPS-derived deformation data, including both rate and direction, are consistent with the expected earthquake deformation cycle at the Nankai subduction zone. Current interplate coupling in this zone is strong; much of the relative plate motion has been taken up by deformation of the marginal parts of the Eurasian plate.

Comparison of Crustal Deformations Observed with GPS and Strainmeters/Tiltmeters

Daily solutions from GPS observations can give uniform tectonic deformation and relative motions of stations more than 100km apart to each other. More precise wet delay correction is necessary in order to monitor local or the order of 10km deformations. On the other hand observation with strainmeters and tiltmeters show irregular changes which are probably associated with local stress accumulation or irregular concentration, but it is not easy to distinguish the irregular change caused by tectonic stress from that caused by meteorological or ground water disturbances. Combined observations with GPS and high sensitive strainmeters/tiltmeters are important to make clear the nature of observed irregular changes, and further to get information relating to the occurrence of earthquakes of M7 class and accordingly to approach the long-term earthquake prediction.