Earthquake Swarm off the Hilton Creek Fault, which last ruptured in 1980

Ross Stein and Volkan Sevilgen, Temblor (Click here to check your seismic risk for free)

An intense swarm, so far topped by a M=3.9 shock on 13 August 2016, is occurring just off the southern tip of the active Hilton Creek Fault (see the cluster of red shocks below), and has been felt at Mammoth Lakes and in Bishop, CA. The events are extensional, resembling the master fault, but lie several miles southwest of the mapped fault trace, amid 470 M≥0 shocks over the past month (green shocks below).

Temblor Map- Hilton-Creek-fault

In addition to active faults, Long Valley Caldera lies just to the north, which erupted 700,000 years ago, lofting volcanic ash into the stratosphere, and depositing it as far east as Kansas. The caldera remains volcanically activity, with magma (molten rocks) about 5-10 miles below the surface, creating hot springs and gas release. The Mammoth Lakes ski resort is perched on the southern edge of the caldera.

Oddly, the majority of the quakes lie south and west of the active faults, perhaps because faults are much more difficult to identify in the crystalline rocks of the High Sierra, or because the shocks are striking on a network of small faults that do not cut the earth’s surface. Our guess is that geologists have missed some major faults here. What a great thesis topic it would be to find them.

The Temblor Seismic Hazard Rank along the Hilton Creek Fault, 32, is significant. The Mammoth Lakes area experienced four M≥6 shocks in the 1980’s, which ruptured parts of the Hilton Creek Fault (Taylor and Bryant, 1980). Geologists assess the fault slip rate at 1-4 mm/yr, making it one of the most active faults of the Basin and Range, which stretches from the Eastern Sierra to the Wasatch Front of Utah.

Hilton Creek fault
Hilton Creek fault

The glacial moraines (the rocky debris swept out in front and along the sides of the glaciers) issuing from the Sierra cross the Hilton Creek Fault. The moraine crests are cut by the fault in a beautiful “Geology 1” illustration of how a ‘normal’ fault lifts up one side and down-drops the other (above, with North to the right). The ‘Tioga’ moraine crest is offset by 19 m (63 ft). The Tioga spans a cold spell lasting from 30,000-10,000 years ago, with the greatest ice advance 21,000 years ago. This gives a local slip rate of the Hilton Creek Fault 1-2 mm/yr. The location of the moraine can be seen in the Temblor map at the “re” in the word, “Creek.”

The Hilton Creek Fault is about 20 km long; if this were its true length at depth, then the largest earthquake one would expect on it is about M=6.5. But as illustrated here, what one sees on the surface can be deceiving.

Data from USGS, University of Reno Nevada Seismological Laboratory, and

Taylor, G.C., and Bryant, W.A., 1980, Surface rupture associated with Mammoth Lakes earthquakes of 25 and 27 May, 1980, in Sherburne, R.W., ed., Mammoth Lakes, California earthquakes of May, 1980: California Division of Mines and Geology Special Report 150, p. 49-67.

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