Small earthquakes: key to understand the mechanism of inland large earthquakes and their hazards —Paper published in Nature Communications—
On June 17, 2020 (JST), the paper has been published in "Nature Communications" (2-year Impact Factor: 11.878; 5-year Impact Factor: 13.811). This study was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research) and by JSPS KAKENHI Grant Number JP 20K05050.
The main points
- This research focused on the “b-value,” which represents the ratio of small earthquakes to large ones. It is well known that when a high stress is being applied on the Earth’s crust, the proportion of large earthquakes increases, causing the b-value to fall. Seismic monitoring networks in California have being improved for detection capability of small earthquakes with a high degree of accuracy. So, we came to a realization that we could carry out our detailed and comprehensive investigation into b-values.
- By analyzing the vast number of large and small events that had occurred since 1980’s (Fig.1), we observed the low b-values prior to the two large events of magnitudes M6.4 and M7.1 in the Ridgecrest earthquake sequence around their rupture initiation points. This is interpreted as an indication that stress that had been built up near both of the rupture initiation points caused these earthquakes to relax the stress.
- We still need to keep a close eye on future activation of seismicity near the southern end of the causative faults, at which the b-values are currently decreasing, indicative of stress increasing. The 300-km long Garlock fault that lies near this end, hosted large earthquakes in the past. If the southern end were more stressed, there would be a high-likelihood of further activation of seismicity that might influence the Garlock fault.
Fig. 1: The Ridgecrest earthquake sequence and b-values. SB: Santa Barbara; LA: Los Angeles; eq: earthquake. See Nanjo (2020) for details.
- There is a consensus among researchers that predicting earthquakes with a high degree of accuracy is difficult, at least for the present. In that sense, this study is not earthquake prediction, but rather demonstrates a likelihood of future rupture.
- With vast improvement of seismic monitoring networks in recent years, more and more microseismicity can now be detected, enabling us to conduct detailed and comprehensive analysis of seismicity. For example, our recent research developed new methods to monitor the plate boundary coupling along the Nankai Trough megathrust subduction zone,※1 and to estimate the stress state on and around the Kumamoto earthquake fault zone.※2
- Our findings could also be extended to be applicable to the Fujikawa-kako fault zone, the Izu-Tobu volcanoes, and Mt. Fuji’s underground. Projects are already underway to pursue these possibilities.
※1 A new method for monitoring the Nankai Trough megathrust subduction zone~Paper published in "Nature Communications"~
※2 A new method to monitor the state of stress on and around the Kumamoto earthquake fault zone: Hazard identification and assessment of the fault zone
Contact for this study and press release
Global Center for Asia and Regional Research, University of Shizuoka