Dr. Kazu Z. Nanjo, Leader of Division for Earthquake Prediction Research, Global Center for Asia and Regional Research, and Associate Professor of University of Shizuoka, Japan, announced that a new paper on the July 2019 earthquake sequence in Ridgecrest (California) was published in “Nature Communications” on June 17, 5:00 a.m. U.S. Eastern Time. This result implies importance to reconfirm regular-basis preparedness for an earthquake.

The Ridgecrest earthquake sequence, which occurred near the town of Ridgecrest, California, included a magnitude (“M”) 7.1 quake on July 4, 2019, as well as active foreshocks and aftershocks. A M6.4 event preceded the M7.1 quake 34 hours later. Crustal deformation due to this sequence caused stress perturbation in a nearby region, but implications of future seismicity are still uncertain.

Nanjo mainly focused on using a seismological parameter “b-value” describing the relative occurrence of large and small earthquakes. Generally, it is known that when a high stress is applied to the Earth’s crust, the number of large earthquakes relatively increases and the b-value tends to decrease. b-value helps image highly stressed patches in faults where future ruptures are likely. The Ridgecrest case allowed to test this idea.

The study supports the idea, showing that the rupture initiation from a low-b-value area, indicative of high stress, was common to both M6.4 and M7.1 earthquakes. The post-M7.1-earthquake sequence reveals that another low-b-value zone fell into an area near the Garlock fault that stretches more than 300 kilometers across Southern California and hosted past large earthquakes. The values are not as low as those immediately before the M6.4 and M7.1 quakes, but contribute the most recent values in a decreasing trend of the b-value. If this area were more stressed, there would be a high likelihood of further activation of seismicity that might influence the Garlock fault.

This study is not earthquake prediction, but rather demonstrates a likelihood of future rupture. Local residents can freely read his open-access paper to better understand the nature of their living area revealed by Nanjo’s work.

Important implication is that considering that earthquakes occur suddenly is a major premise to implement disaster prevention measures. The result of this new work encourages local residents to reconfirm preparedness for an earthquake on a regular basis.

Journal Reference: “Were changes in stress state responsible for the 2019 Ridgecrest, California, earthquakes?”, K. Z. Nanjo (2020), Nature Communications, DOI: 10.1038/s41467-020-16867-5

About University of Shizuoka

University of Shizuoka is a public university in Japan. It was created through the amalgamation of three former public colleges in 1987 and was expanded to comprise five colleges. The university maintains both undergraduate and graduate schools as well as a variety of research institutes.

Source: University of Shizuoka