A seismic swarm is in progress between southern Cascadia and northern San Andreas Faults since yesterday afternoon, with four shallow quakes – M4.5, M3.4, M4.1, M3.7 – in two hours west of Petrolia, California. A fifth M2.8 very shallow earthquake struck the same area a few hours later.
According to Tremblor’s map, the 5 earthquakes occurred on a right-lateral reverse fault that connects the San Andreas-Mendocino Fracture Zone with the shallow portion of the Cascadia Subduction Zone.
It is in this region that the two fastest-slipping faults in the western U.S. join, or nearly join. It is also where both the San Andreas and Cascadia Faults bend, meaning that the crust is being crushed, distorted, and fractured here.
What this event means for the Cascadia and San Andreas Faults is unknown. We however know that both faults are overdue for a cataclysmic earthquake. Cascadia by more than 30 years and San Andreas by more than 40 years.
Managing earthquake risk requires a resilient system of social awareness, education and communications, coupled with effective short- and long-term responses and implemented within an optimally safe built environment.
As California prepares for large earthquakes after a hiatus of more than a century, the clock is ticking. Yes, get ready and be prepared the the next Big Ones!
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