Mammoth Lakes Region: Aftershocks continued to shake the region following the June 8 Hilton Creek M5.1 temblor. Ten of the 260 quakes registered M3.0+, including three in the M4 range.
The first M4 quake measured M4.0 and happened at 8:32 a.m. Sunday, June 21, about 12 mi. southeast of Mammoth Lakes and beneath the Lake Crowley Campground (depth: 6.9 km). The second M4 measured M4.1 and occurred the morning of June 24 north of the campground between HWY 395 and Lake Crowley. At 8.3 km deep, it was the deepest of the three M4's. Finally, a M4.3 was recorded early Friday afternoon, June 26, along the Sierra range front west of McGee Creek and at the northwest margin of the aftershock zone (depth: 6.2 km). All M4's were sharply felt but there were no reports of damage or injury.
Detailed analysis of the aftershock pattern shows a clear northwest trend along the range front to Mt. McGee. Although the epicenters plot close the surface trace of the Hilton Creek fault, this fault dips to the east, placing the earthquake focal points in the footwall or west side of the subsurface trace of the fault.
Elsewhere, a cluster of four shallow (0.2-3.8 km) M3's occurred about 11 mi. south of Mammoth Lakes in the Sierra near the Silver Divide and a M3.2 triggered near Bloody Mountain (depth: 18 km). Activity inside the caldera boundaries continued to be relaxed.

The USGS Volcanic Unrest Status is currently GREEN indicating weak to moderate unrest, posing no immediate risk. (more)

For more information on the Long Valley caldera and other volcanoes around the world, please see the following web sites:

Sources: USGS, University of California, Berkeley, Geophysics Laboratory; personal communications.


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