1:45 PM - 2:00 PM
[SSS08-10] Characteristics of foreshock activity on inland large earthquakes in Japan
Keywords:Foreshock, Inland earthquake, Regional characteristics
1. Introduction
Foreshocks can sometimes be observed, which are defined as smaller earthquakes taking place around the source area of the following main shock. Some previous studies pointed out that foreshock occurrence would have regional characteristics (e.g., von Seggern et al., 1981), but little has been revealed on foreshock activity. We investigated seismicity prior to 13 large inland earthquakes with Mjma >= 6.4 in Japan and discussed its regional characteristics.
2. Methods
It is quite important that we observe all the earthquakes of interest in studying seismicity, or number of earthquakes. We first confirmed earthquakes equal to and larger than Mjma = 2.0 that occurred in 2004 and later follow the Gutenberg-Richter relationship, suggesting that all the earthquakes with Mjma >= 2.0 are recorded. We decided to investigate the number of earthquakes with Mjma >= 2.0.
We studied cumulative number of earthquakes shallower than 50 km as a function of time from Jan. 1, 2004 whose epicenters were within +-0.5 degree both in latitude and longitude directions from those of individual 13 inland earthquakes. We then estimated the best-fit linear relationship for two time periods; (1) 395 to 30 days and (2) the last 30 days prior to each main shock. We finally discussed the change on seismicity rate by comparing the slope of the two periods.
3. Results and Discussion
Fig. 1 shows two examples of our results for (a) Niigata Chuetsu-oki earthquake in 2007 and (b) Iwate-Miyagi Nairiku earthquake in 2008. We clearly observed (a) decrease and (b) increase of seismicity prior to each main shock. All the results for 13 inland main shocks are described in Fig. 2. This figure indicates that earthquakes around Niigata prefecture had decrease of seismicity before individual main shocks. These earthquakes are located in the Niigata-Kobe Tectonic Zone (Sagiya et al., 2000), which has a fast strain accumulation rate. We would like to study the background physical phenomena causing the change of seismicity prior to a large earthquake by carrying out further investigation, including numerical simulations.
Foreshocks can sometimes be observed, which are defined as smaller earthquakes taking place around the source area of the following main shock. Some previous studies pointed out that foreshock occurrence would have regional characteristics (e.g., von Seggern et al., 1981), but little has been revealed on foreshock activity. We investigated seismicity prior to 13 large inland earthquakes with Mjma >= 6.4 in Japan and discussed its regional characteristics.
2. Methods
It is quite important that we observe all the earthquakes of interest in studying seismicity, or number of earthquakes. We first confirmed earthquakes equal to and larger than Mjma = 2.0 that occurred in 2004 and later follow the Gutenberg-Richter relationship, suggesting that all the earthquakes with Mjma >= 2.0 are recorded. We decided to investigate the number of earthquakes with Mjma >= 2.0.
We studied cumulative number of earthquakes shallower than 50 km as a function of time from Jan. 1, 2004 whose epicenters were within +-0.5 degree both in latitude and longitude directions from those of individual 13 inland earthquakes. We then estimated the best-fit linear relationship for two time periods; (1) 395 to 30 days and (2) the last 30 days prior to each main shock. We finally discussed the change on seismicity rate by comparing the slope of the two periods.
3. Results and Discussion
Fig. 1 shows two examples of our results for (a) Niigata Chuetsu-oki earthquake in 2007 and (b) Iwate-Miyagi Nairiku earthquake in 2008. We clearly observed (a) decrease and (b) increase of seismicity prior to each main shock. All the results for 13 inland main shocks are described in Fig. 2. This figure indicates that earthquakes around Niigata prefecture had decrease of seismicity before individual main shocks. These earthquakes are located in the Niigata-Kobe Tectonic Zone (Sagiya et al., 2000), which has a fast strain accumulation rate. We would like to study the background physical phenomena causing the change of seismicity prior to a large earthquake by carrying out further investigation, including numerical simulations.