Abstract: Unprecedented large-scale silent slip was inferred from the movement of the earth's surface by GPS measurements in 2001 in the Tokai region, southwest Japan. Evaluating whether such movement is uniquely connected to the expected Tokai earthquake, or whether it repeatedly occurs in this area and is thus not clearly precursory becomes vitally important. Because of the short history of GPS observations and the limited land coverage surrounding the tectonic plate boundary beneath Suruga bay, we take advantage of continuously recorded seismicity that is presumed to be sensitive to the deformation at the depth at which large and great earthquakes nucleate.

Together with the well-maintained NIED earthquake data, we employ the seismicity-to-stress inversion approach of rate/state friction to infer the Changes in tress with time and location in and around the presumed nucleation zone of the future Tokai earthquake.

But how can this—or any—such forecast be validated? The long historical record in Kanto affords a rare opportunity to calculate the probability of shaking in an alternative manner, based almost exclusively on intensity observations. This approach permits robust estimates for the spatial distribution of expected shaking, even for sites with few observations. The resulting probability of severe shaking for an average 30-yr period is ~35% in Tokyo, Kawasaki, and Yokohama, and ~10% in Chiba, in good agreement with our independent estimate, and thus bolstering our view that Tokyo’s hazard looms large.

Mapping stress changes inverted from microseismicity year by year, we find that the stress under Lake Hamana, the western expected future Tokai source, has been decreasing since 1999, during which the GPS data showed a normal trend characteristic of a locked fault accumulating strain toward a future large earthquake. In contrast, stresses in the surrounding regions are calculated to have increased by transfer from Lake Hamana region. We interpret that this continuous process is associated with the 2000-2004 Tokai slow slip event.

The characteristic patterns related to aseismic stress release are also identified in the early 1980s and during 1987-1989, when slow events are inferred to have occurred on the basis of conventional geodetic measurements.

Revisiting the seismotectonics and taking into account the mechanical implications of the inversion results, we argue that the fault transition zone between a deep stable creeping zone and a locked zone undergoes episodic creep and plays an important role in the transfer of stress to the locked zone. Consequently, even though we speculate that the current (2000-to-present-day) silent slip event might be one of the repeating events, the inferred enlargement of the stress releasing area is significant and possibly raises the likelihood of the next Tokai earthquake.