A second earthquake swarm hits west of the Hilton Creek Fault, in Long Valley Caldera near Mammoth Lakes, CA

By Ross Stein, Temblor

Temblor map of the two swarms, the caldera, and the principal fault systems.
Temblor map of the two swarms, the caldera, and the principal fault systems.

The two recent swarms are 20 km (12 mi) apart. The 13 August swarm lies on the west side of the Hilton Creek Fault, about 5 km off the southern end of the Fault in what is known as the “Sierran Block.” “Block” seems to me a misnomer: Far from being intact, the Block is riddled with small quakes which align along northwest-trending, east-dipping planes, and so must be traversed by active faults that either do not reach the Earth’s surface, or have not been discovered. In fact, the Sierran Block experienced four M≥6 shocks in three days in 1980.

Map of the 1980 M≥6 swarm from Prejean et al (2002), together with the two recent swarms.
Map of the 1980 M≥6 swarm from Prejean et al (2002), together with the two recent swarms.

The 21 August 2016 swarm is in the southern Long Valley caldera, within a few kilometers of the Medial Graben Fault, with a focal mechanism consistent with tension perpendicular to the Fault. The 21 August 2016 quake was felt in the skiing, fishing, and cycling resort community of Mammoth Lakes, California.

While the USGS fault database labels this strand as part of the Hilton Creek Fault Zone, and includes end-to-end rupture of this system in its earthquake forecast model for California (“UCERF3”), that view is disputed by USGS seismologists David Hill and Emily Montgomery-Brown. They argue that the Medial Graben Fault is the product of local stretching as the “resurgent dome” of the in Long Valley caldera expands due to underlying magma pressure. While this may be true, faults with different tectonic roles and origins sometimes rupture together, and so it is hard to exclude the possibility of a rupture linking the Hilton Creek and Medial Graben Faults.

On the basis of its length, the Hilton Creek Fault is capable of a M~6.5 earthquake. Based on the 1980 events, earthquakes of almost this magnitude might also be possible in the Sierran Block and in Long Valley Caldera.

Data from Northern California Seismic System: U.C. Berkeley, USGS Menlo Park, the Nevada Seismological Laboratory.

We are grateful for discussion with David Shelly, David Hill, and William Ellsworth (USGS)

Hill, David P., and Emily Montgomery-Brown (2015), Long Valley Caldera and the UCERF Depiction of Sierra Nevada Range-Front Faults, Bull. Seismol. Soc. Amer., doi:10.1785/0120150149,

Stephanie Prejean, William Ellsworth, Mark Zoback, and Felix Waldhauser (2002), Fault structure and kinematics of the Long Valley Caldera region, California, revealed by high-accuracy earthquake hypocenters and focal mechanism stress inversions, J. Geophys. Res, 107, 2355, doi:10.1029/2001JB001168, https://pangea.stanford.edu/departments/geophysics/dropbox/STRESS/publications/MDZ%20PDF’s/2003/Prejean%20et%20al.%202002%20JGR.pdf

Double-Difference Realtime Viewer of Felix Waldhauser, Lamonth Doherty Earth Observatory of Columbia University, http://ddrt.ldeo.columbia.edu/

  • Robert H. Sydnor

    Many of us California geologists who have performed extensive field work in this particular region of the Sierra Nevada range (=Long Valley Caldera and Mammoth) feel that the 21August2016 earthquake swarm should be properly classified as *within* the Long Valley Caldera, and not necessarily on the Hilton Creek Fault Zone which is about 5 kilometers east. The focal mechanism is different than expected on the Hilton Creek Fault.

    The USGS seismologist who specializes in the Long Valley Caldera is Dr. David Hill, who has been a colleague of Dr. Ross Stein for over 30 years. So we earnestly need the sagacious opinion of Dave Hill before proceeding.

    In 1970 (=46 years ago) I first met Dr. Roy Bailey, USGS geologist who was performing the seminal geologic field work at Long Valley Caldera that was later published in a USGS Professional Paper. It was a blessing to be mentored in the field at Long Valley by Roy Bailey, who had just received his Ph.D. degree in volcanology from Johns Hopkins University. The occasion was a joint geology field-trip for graduate students in volcanology from both the University of California at Riverside (Dr. Paul Robinson) and the University of California at Santa Barbara (Dr. Cliff Hobson). In sum total, there were 45 graduate geology students present, each specializing in volcanology. Note that there was not one seismologist present in 1970 for this geologic field work.

    It is not simple, nor expedient, to geologically map additional fault strands that are (likely) parallel to the Hilton Creek Fault Zone. Here is an example of the glacial terrain with steep tectonic mountain fronts (from one of my 7,000 geology photos posted onto Google Earth) of the McGee Creek area that is a few miles southeast of Hilton Creek. http://www.panoramio.com/photo/112827808

    So: All of us will have to watch the Long Valley seismicity very carefully because of these two small epicenter swarms. Abiding thanks to the great seismologist Ross Stein for alerting us to these two events. Mammoth lakes is heavily populated (in both winter for skiiers, and in summer for mountain bikers and hikers) so we are grateful for these timely seismology alerts from Dr. Ross Stein, USGS emeritus.