Spatiotemporal variations of seismicity before major earthquakes in the Japanese area and their relation with the epicentral locations

Abstract

Using the Japan Meteorological Agency earthquake catalog, we investigate the seismicity variations before major earthquakes in the Japanese region. We apply natural time, the new time frame, for calculating the fluctuations, termed β, of a certain parameter of seismicity, termed κ1. In an earlier study, we found that β calculated for the entire Japanese region showed a minimum a few months before the shallow major earthquakes (magnitude larger than 7.6) that occurred in the region during the period from 1 January 1984 to 11 March 2011. In this study, by dividing the Japanese region into small areas, we carry out the β calculation on them. It was found that some small areas show β minimum almost simultaneously with the large area and such small areas clustered within a few hundred kilometers from the actual epicenter of the related main shocks. These results suggest that the present approach may help estimation of the epicentral location of forthcoming major earthquakes.

Identifying the occurrence time of an impending major earthquake: a review

“…This, which is remarkably the deepest minimum ever observed during the period 1984-2011 investigated, almost coincides with the initiation of an SES activity-being in accordance with the findings by Varotsos et al (2013)since anomalous magnetic field variations appeared during the period 4-14 January 2011 on the z-component at two measuring sites (Esashi and Mizusawa) lying at epicentral distances of around 130 km (Xu et al 2013;Han et al 2015Han et al , 2016. Concerning the estimation of the epicentral location of the impending mainshock without making use of SES data, we follow the procedure developed by Sarlis et al (2015): By dividing the entire Japanese region N 46 25 E 148 125 (large area) into small areas, a calculation of the fluctuations of j 1 of seismicity is carried out on them. Some small areas show a minimum of the fluctuations almost simultaneously with the minimum in the entire Japanese region (on 5 January 2011) and such small areas cluster within a few hundred kilometers from the actual epicenter.…”

“…In addition, beyond this simultaneous appearance of two different geophysical observables (SES activity and seismicity), Varotsos et al (2013) showed that these two phenomena are also linked closely in space, which opened the window for a reliable estimation of the epicentral area of an impending major earthquake. This has been subsequently confirmed by Sarlis et al (2015) for all major mainshocks of magnitude 7.6 or larger that occurred in Japan during 1984-2011 by applying a procedure that will be shortly described later in Sect. 4.…”

“…In case that geoelectrical data are not available, and in order to determine the initiation time of an SES activity we recall the finding of Varotsos et al (2013) mentioned in the Introduction that this initiation occurs almost at the time at which the fluctuations of the order parameter of seismicity in a large area exhibit a minimum. In addition, we recall that a spatiotemporal study of this minimum, as also mentioned in the Introduction and briefly described in the next subsection, enables an estimate of the epicentral area of the impending major earthquake (Sarlis et al 2015).…”

M W9 Tohoku earthquake in 2011 in Japan: precursors uncovered by natural time analysis

“…Second, generalizing a procedure employed by Varotsos et al (2013) to show that the two phenomena, SES activity and minimum of the fluctuations of the order parameter of seismicity, are also linked in space, we have recently ascertained (Sarlis et al 2015) that a spatiotemporal study of the minima b W,min observed before the six M JMA C 7.6 mainshocks during 1984-2011 in Japan leads to an estimation of their epicentral areas (see also Huang 2015, on this finding). The extent of the usefulness of this result for the case of the M W 9 Tohoku EQ in 2011 and in particular for the determination of its occurrence time has been recently investigated by Varotsos et al (2017).…”

“…that corresponds to this natural time window of length W. To compute the time evolution of b W , we apply the following procedure (Varotsos et al 2011a, c;Sarlis et al 2013Sarlis et al , 2015: First, take an excerpt comprising W(C100) successive earthquakes from the seismic catalog. We call it excerpt W. Second, since at least 6 earthquakes are needed for calculating reliable j 1 (Varotsos et al 2005), as mentioned above, we form a window of length 6 (consisting of the 1st to the 6th earthquake in the excerpt W) and compute j 1 for this window.…”