Abstract
We investigated the foreshock activity characteristics using the Japan Meteorological Agency Unified Earthquake Catalog for the last 20 years. Using the nearest-neighbor distance approach, we systematically and objectively classified the earthquakes into clustered and background seismicity. We further categorized the clustered events into foreshocks, mainshocks, and aftershocks and analyzed their statistical features such as the b-value of the frequency–magnitude distribution. We found that the b-values of the foreshocks are lower than those of the aftershocks. This b-value difference suggested that not only the stochastic cascade effect but also the stress changes/aseismic processes may contribute to the mainshock-triggering process. However, forecasting the mainshock based on b-value analysis may be difficult. In addition, the rate of foreshock occurrence in all clusters (with two or more events) was nearly constant (30–40%) over a wide magnitude range. The difference in the magnitude, time, and epicentral distance between the mainshock and largest foreshock followed a power law. We inferred that the distinctive characteristics of foreshocks can be better revealed using the improved catalog, which includes the micro-earthquake information.
Highlights
Foreshocks are earthquakes that occur prior to the mainshock, which is defined as the largest magnitude event in an earthquake sequence
The Japan Meteorological Agency (JMA) published and routinely updates an earthquake catalog (JMA unified catalog) in cooperation with the Ministry of Education, Culture, Sports, Science and Technology (MEXT) using seismic waveforms of stations belonging to the JMA, National Research Institute for Earth Science and Disaster Resilience (NIED), universities, and other institutes
The publication of the JMA unified catalog started in October 1997, and the data have been archived for approximately 20 years
Summary
Foreshocks are earthquakes that occur prior to the mainshock, which is defined as the largest magnitude event in an earthquake sequence. The foreshock activity often shows significant differences compared with the ordinary seismic activity such as a decreased b-value (e.g., Suyehiro 1966; Enescu and Ito 2001; Nanjo et al 2012; Tormann et al 2015) and migration and acceleration prior to the mainshock (e.g., Kato et al 2012; Marsan et al 2014) While these foreshock characteristics may reflect the accumulation of stress and/or the occurrence of slow slip within the seismogenic zone, Helmstetter et al (2003) showed that such features may be explained by using the ETAS statistical model of seismicity (Ogata 1988) and Gutenberg–Richter law (G–R law).
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