A study has highlighted the potential threat currently faced by California as it has experienced a record number of earthquakes recently. The study noted that in Long Beach and Seal Beach areas, a thousand tiny tremors have been recorded over eight months.
They noted that the majority of the quakes were shallow, and reached depths less than two kilometres below the surface, but the region is at risk of bigger tremors.
The study – published in Seismological Research Letters – stated that the researchers used three dense nodal arrays consisting of thousands of sensors to detect and locate seismic events in the Long Beach–Seal Beach area of California. They said that small events can be detected at “sufficient signal‐to‐noise levels during the night when urban noise is relatively low”.
The study suggested that constant shallow earthquakes, on the other hand, could build up and generate the possibility of a surface earthquake. The Alquist-Priolo zone, which surrounds the surface of active faults, must be at least 50 feet wide.
In the media release, the study authors wrote: “Our results suggest the zone of high hazard at the surface may therefore be much wider than the Alquist-Priolo zone indicates.”
The study further mentioned that most of the located events are clustered at very shallow depths and the results have also supported earlier suggestions that the shallow Newport‐Inglewood fault is wide-splayed in this area.
According to the story, the “seismicity pattern also compares well with some newly identified faults from reflection seismic surveys” and the shallow events underscore the complex nature of the faults and their seismic hazard. The study mentioned that shallow events elude detection by the regional seismic network.
Some of the faults were discovered recently by oil company exploration studies, but Yang pointed out that there was no proof that the minor shallow earthquakes recorded in their investigation were triggered by oil and gas operations.
Caltech researchers Yan Yang and Robert Clayton wrote in the SRL study that the catastrophic Long Beach earthquake of 1933, magnitude 6.4, may have ruptured in part on the Newport-Inglewood fault. While big earthquakes, such as the one in 1933, would most likely begin six to ten kilometres below the surface, “shallow seismicity suggests that there are many possible paths for a rupture to propagate to the surface”.
As quoted, Yang said: “The Newport-Inglewood fault along its entire length, as well as the whole Los Angeles Basin, could benefit from such studies. This would help to see if there are faults that have not been detected with the permanent seismic network or through geologic mapping.”
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