Small earthquakes: key to understand the mechanism of inland large earthquakes and their hazards

Small earthquakes: key to understand the mechanism of inland large earthquakes and their hazards —Paper published in Nature Communications—

Conducting research on inland large earthquakes is important, because these earthquakes in earthquake-prone regions such as Japan inflict catastrophic damage on societies in the affected regions. Dr. Kazu Z. Nanjo, Leader of Division for Earthquake Prediction, Global Center for Asian and Regional Research, and Project Associate Professor of University of Shizuoka, has studied small earthquakes to show how the ruptures of inland large earthquakes are nucleated, using an example from the July 2019 Ridgecrest earthquakes, which occurred near the town of Ridgecrest in California. This is a pronounced finding in terms of disaster mitigation, since it may lead to the development of a framework for evaluating whether a next inland earthquake is approaching.

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.

Author’s remarks

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

Summary

There are several active faults in Shizuoka Prefecture. The aforementioned Fujikawa-kako fault zone is one such example. This fault zone is categorized as the highest rank based on the probability of the earthquake occurrence within the next 30 years. We hope that the reader will be encouraged to reconfirm earthquake preparedness on a regular basis.

Paper

Nanjo, K. Z. (2020) Were changes in stress state responsible for the 2019 Ridgecrest, California earthquakes?, Nature Communications, Volume: 11, Article number: 3082, DOI: 10.1038/s41467-020-16867-5.
Link: https://doi.org/10.1038/s41467-020-16867-5