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[SSS06-P03] A new method to predict big earthquakes based on Shindo-data, Meteorological Agency: Correction of aftershocks and swarms, contour of equal rate of occurrence on map of Japan
Keywords:Japan Meteorological Agency Shindo-data, Statistical analysis, Aftershocks
The headquarters for earthquake research promotion of Japan (HERP) publishes maps predicting the probability of occurrence of big earthquakes. As a broad tendency, the predicted probabilities are high in the Pacific coast side and low in the Japan and East China sea coast sides of our lands. A governmental survey shows that the ratio of people who consider that they will experience severe natural hazards in coming 30 years are high in Pacific coast side and low in Japan and East China sea coast sides. TVs and newspapers repeatedly feature Nankai Trough and Tokyo inland earthquakes. However, the intensity 7 earthquakes have occurred in Hokkaido, Miyagi, Niigata, Ishikawa, Hyogo, and Kumamoto (twice) prefectures, including areas where the risk of big earthquakes had been considered not high. Here, we see a discrepancy between the risk awareness and the real risk of earthquakes. We propose a method for narrowing this discrepancy, predicting the rete of occurrence of big earthquakes based on the recorded earthquake intensities.
We use the shindo-data (https://www.data.jma.go.jp/eqev/data/bulletin/shindo.html) of Japan Meteorological Agency. We extracted the intensity records station by station, and then counted the number of a certain intensity or higher during the available record periods. The intensity and the logarithm of rate of occurrence exhibited good linear relationship as suggested by Usami and Katsumata (1973); however, we found that the intensity-rate of occurrence relationship changes with time, for instance before and after the big quakes: aftershocks inflate the rate of occurrence, especially of low intensity tremors. From the viewpoint of risk assessment, this change due to aftershocks is a serious problem.
To address the issue, we developed a method for correcting the effect of aftershocks and swarms based on the distribution of inter-earthquake intervals. We plotted the logarithm of empirical survival function of inter-quake intervals, then the survival function shows sharp drop near interval zero, and then it decreases gradually and linearly according to interval length. We drew a regression line on the latter linear part, extrapolate it to interval zero, and got the factor for correcting the effect of aftershocks and swarms. We applied this method to several monitoring stations including the Nishiharamura-Komura, one of the intensity 7 recorded points and found that, after the correction, all the intensity-rate of occurrence plots are practically on the same line. This result shows that the correction successfully removed the effect of aftershocks and swarms. With this method, we analyzed the shindo-data, and created a map of contour of equal expected occurrence rate of intensity 6 plus earthquakes. All but Shikamachi-Kanou of stations recorded the intensity 7 earthquakes are estimated to have high rate (1/100 years) of occurrence, showing that the map captures some important features of risk of big earthquakes.
Reference
Usami T and Katumata M (1973) Quarterly Journal of Seismology, 38, 15—21.
Headquarters for earthquake research promotion of Japan, https://www.jishin.go.jp/evaluation/seismic_hazard_map/shm_report/shm_report_2020/
Cabinet Office (2016) https://www.bousai.go.jp/kaigirep/hakusho/h28/zuhyo/zuhyo00_23_00.html
We use the shindo-data (https://www.data.jma.go.jp/eqev/data/bulletin/shindo.html) of Japan Meteorological Agency. We extracted the intensity records station by station, and then counted the number of a certain intensity or higher during the available record periods. The intensity and the logarithm of rate of occurrence exhibited good linear relationship as suggested by Usami and Katsumata (1973); however, we found that the intensity-rate of occurrence relationship changes with time, for instance before and after the big quakes: aftershocks inflate the rate of occurrence, especially of low intensity tremors. From the viewpoint of risk assessment, this change due to aftershocks is a serious problem.
To address the issue, we developed a method for correcting the effect of aftershocks and swarms based on the distribution of inter-earthquake intervals. We plotted the logarithm of empirical survival function of inter-quake intervals, then the survival function shows sharp drop near interval zero, and then it decreases gradually and linearly according to interval length. We drew a regression line on the latter linear part, extrapolate it to interval zero, and got the factor for correcting the effect of aftershocks and swarms. We applied this method to several monitoring stations including the Nishiharamura-Komura, one of the intensity 7 recorded points and found that, after the correction, all the intensity-rate of occurrence plots are practically on the same line. This result shows that the correction successfully removed the effect of aftershocks and swarms. With this method, we analyzed the shindo-data, and created a map of contour of equal expected occurrence rate of intensity 6 plus earthquakes. All but Shikamachi-Kanou of stations recorded the intensity 7 earthquakes are estimated to have high rate (1/100 years) of occurrence, showing that the map captures some important features of risk of big earthquakes.
Reference
Usami T and Katumata M (1973) Quarterly Journal of Seismology, 38, 15—21.
Headquarters for earthquake research promotion of Japan, https://www.jishin.go.jp/evaluation/seismic_hazard_map/shm_report/shm_report_2020/
Cabinet Office (2016) https://www.bousai.go.jp/kaigirep/hakusho/h28/zuhyo/zuhyo00_23_00.html