2c shows how the expected cumulative model numbers, in the entire Kanto region, were decomposed into those of the three components background rates, self-triggering rates (aftershock productivity), and the intensity factor induced by the M9 event. Also, we show the inverted 3D spatial images of such characteristic parameters illustrate the discriminative seismicity patterns in the crust and on the subducting plates boundaries.įigure 2b clearly shows that the model in Eq. ( 1) fits well throughout the entire period until 2015, and Fig. The model characteristic parameters are location dependent and we estimate them using an empirical Bayesian method (Akaike 1980) to forecast the probabilities of occurrence beneath the Kanto plain. Then, we analyze the seismic activity beneath the Kanto Plain using a 3D model taking the induced effects of the M9 Tohoku-Oki earthquake into consideration. In this manuscript, we first apply the temporal ETAS model to see the change of seismicity in the Kanto region, before and after the M9 Tohoku-Oki earthquake. They are also reasonably consistent with the recent boundaries based on repeating earthquakes. Our adopted plate models, covering the entire Kanto region, have been developed using cubic spline surfaces determined on the basis of tectonic loading mechanics and seismic, geodetic, geologic, and geomorphic data. Also, recently, the plate boundaries in some zones have been partly delineated using repeating earthquakes (e.g., Kimura et al. There have been many different depth models proposed for the PHS Plate configuration beneath the Kanto region, mainly based on the spatial distribution of the micro earthquakes since Kasahara ( 1985). ( 2004) and Hashimoto and Matsu’ura ( 2006). In this work, we adopted the Philippine Sea (PHS) Plate and the Pacific (PAC) Plate models due to Hashimoto et al.
1b, c), after the unification of catalogs post October 1997 (cf., Acknowledgements). The hypocenters well delineate the plate boundaries (Fig. This ensured the stationarity of the ETAS models in the target period. Our model-fitting target period was from 1926 to 2015, but we also considered the precursory period from 1923 to 1925, including the M7.9 great Kanto earthquake of September 1, 1923. We have used space–time locations and magnitudes of earthquakes of M ≥ 4 selected from the Hypocenter Catalog of the Japan Meteorological Agency (JMA 2018) from 1923 to 2015. Our focus was the Kanto cuboid bounded by 138.5°–141.5☎ and 34.5°–37.0°N, down to a depth of 100 km, as shown in Fig. Thus, new modeling should take this induced effect into consideration (e.g., Dietrich 1994 Parsons et al. ( 2011) reported increased seismicity rates in many subregions in the Kanto area. Furthermore, recent seismic activity beneath the Kanto region has been induced by the M9 Tohoku-Oki earthquake of March 11, 2011. Hence, we here define a flexible 3D hierarchical space–time ETAS model that allows for such effects. However, it is known that earthquake occurrence rates are significantly dependent on the configuration of the interacting and colliding plates beneath the Kanto Plain. They separated depth effects from the horizontal 2D space, and showed that such a separable 3D model fits observations significantly better than the 2D model. ( 2018) applied a 3D space–time ETAS model in the Kanto region. 1), their interactions, and hence forecasting of the occurrence of inter-plate and intraplate earthquakes, are too complex for approaches such as the 2D space–time epidemic-type aftershock sequence (ETAS) models, as proposed by Ogata ( 2011), which ignores the depths of earthquakes. Because three tectonic plates meet beneath Kanto Plain (see Fig. The Kanto region urgently requires a 3D short- and medium-term seismicity forecasting model.
However, there has been little prior study of the spatial distribution of event probabilities associated with such a forecast. Utsu 2002), assumption of a stationary temporal Poisson process, and application of the Gutenberg–Richter (GR) law in the focal area. This estimate is based on earthquake records since 1885 (Utsu 1982), a number of historical disastrous earthquakes (e.g. The estimated occurrence probability of such an earthquake, during the next 30 years, is 70–80%. The dense population of the Tokyo metropolis prompted the government’s Earthquake Research Committee ( 2004) to predict, and subsequently update, the long-term probability of an M7 class earthquake beneath the southern Kanto Plain.