New insight into faults in the Long Valley Caldera reduces the predicted earthquake shaking intensity

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The shaking hazard posed by earthquakes on range-front faults near the town of Mammoth Lakes, CA is lower than previously estimated.

Long Valley Caldera Range Front Fault Diagram...

This is a simplified diagram of regional range front faults as they intersect the Long Valley Caldera. The Hartley Springs Fault and Hilton Creek Fault are range-front faults caused by regional Basin and Range extension (Earth's crust is pulling apart, and the Sierra Nevada mountains are lifting up and moving to the west as the Basin and Range drops down to the east). Fault movement is normal along the main segments of the Hartley Springs fault and Hilton Creek fault, but those faults end at the rim of the Long Valley Caldera; extension continues into the caldera with dike emplacement related to the Inyo Domes ("Inyo Dike"), and is accommodated by a series of "leaky" right-lateral oblique transform faults south of the resurgent dome in the caldera, causing earthquakes to occur in a west-northwest to east-southeast orientation (called the South Moat Seismic Zone, SMSZ), which is what Dave Hill and Emily Montgomery-Brown, USGS Research Geophysicist at CalVO, observed in seismic data from the Long Valley area in a 2015 publication. Their discovery suggests a lower predicted intensity of potential earthquakes in Mammoth Lakes.

(Credit: Hill, David. Public domain.)

A re-evaluation of geologic and geophysical data in Long Valley Caldera shows that two large Basin and Range faults (Hartley Springs and Hilton Creek) do not extend into the caldera, as previously thought. According to a Uniform California Earthquake Rupture Forecast based upon 2008 data, these two major Basin and Range faults are capable of producing magnitude 6.5 or greater earthquakes in the Long Valley volcanic region. However, recent CalVO research published in the Bulletin of the Seismological Society of America provides evidence that neither fault has ruptured within the caldera since its formation some 760,000 years ago. Earthquakes that do occur in the caldera are much smaller in magnitude, most likely caused by minor faults as well as ongoing volcanic resurgence.

The research by Dave Hill, Scientist Emeritus and former Scientist-in-Charge of the Long Valley Observatory (now CalVO), and Emily Montgomery-Brown, Research Geophysicist at CalVO, is the scientific basis for a new appendix to the scenario earthquake hazard report, which reduces the potential shaking hazard in the vicinity of Mammoth Lakes and Long Valley caldera. Evidence indicates that rupture along the Hartley Springs and Hilton Creek faults both end at the rim of the Long Valley Caldera. In the case of the Harley Springs fault, however, extension continues into the caldera, but it is driven by dike emplacement related to the Inyo Domes volcanic chain ("the Inyo Dike").

Those of us who love the Mammoth Lakes area are somewhat relieved to know that although it is still a geologically exciting and active place, Mammoth is slightly safer from earthquakes than previously predicted!