Two current seismic quiescences within 40 km of Tokyo

Abstract

Two highly significant seismic quiescence anomalies that started in 1993.0 exist currently within about 40 km of Tokyo. They are located in central Saitama province at 35.99°N/139.56°E and in Tokyo Bay, south of Funabashi, at 35.59°N/l39.94°E. The radii of the anomalous volumes are approximately 15 km, centred at about 25 and 20 km depth, respectively. The seismicity rate during the quiescence was less than 10 per cent background rate in both cases. The significance of these quiescences, measured by the standard deviate Z-test, comparing the rate of the last three years to the backgound rate, is higher than for any other rate changes present in the catalogue (Zmax=7.5 and 7.2 respectively). However, episodes of highly significant quiescences, which could have been interpreted as precursors but were not followed by main shocks, also exist in the data set. A third seismic quiescence was found to be in progress since 1993.3 ± 0.5 in NW Saitama province at 36.18°N/139.22°E. It has a radius of 14 km and is less significant than the other two anomalies (Zmax=6.8). In a polygon that covers the areas of all three quiescences defined above, the seismicity rate since 1994 is only 30 per cent of the previous background rate (i.e. 100 earthquakes that would normally have occurred did not). This analysis is based on the earthquake catalogue of the National Institute for Earth Science and Disaster Prevention for the years 1980–1996.2 and Mmin=1.0, as well as for the years 1986.2–1996.2 with Mmin=0.1. The anomalies are equally clear in the clustered and in the full catalogues. To avoid contamination by explosions, only nighttime data (19:00 to 7:00 hours local time) were used. The frequency-magnitude distributions in the quiescent volumes at central Saitama. NW Saitama and Funabashi have b-values of 1.3, 1.5 and 1.1, respectively. Based on the hypothesis that crustal volumes with high b-values may not be capable of M6 + earthquakes, these three quiescent volumes may be ruled out as source volumes for main shocks of these magnitudes. Using lilt seismic quiescence hypothesis alone, it is a reasonable interpretation to suggest that M=6.5 ± 0.5 earthquakes are expected to terminate each of the three quiescences within 1 year ± 1 year from now, with a 50 per cent probability each. The interpretation that all three quiet volumes could rupture together in a large earthquake seems less likely. However, based on the information that the quiescences are located in volumes with b-values above the average b=0.8, it can be argued that the three current quiescences are false alarms. The fact that these two lines of evidence allow opposing interpretations in these cases shows that neither hypothesis is sufficiently tested. Thus, the quiescence hypothesis is not advanced enough for issuing earthquake predictions. Nevertheless, it is important that scientific tests of prediction scenarios are formulated in real time, so we can learn how to modify our hypotheses to make them useful for prediction in the future.