Special Earthquake Report No: 99-014

Regional Location:

Preliminary Magnitude:

Moment Magnitude:

Greenwich Mean Date:
Greenwich Mean Time:
Focal Depth (km):
Analysis Quality (A-D):


4.9 ML (NCSN)

5.6 Mw (UCBSL, NEIC)

5.7 km

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An earthquake registering a preliminary magnitude of ML 4.9 (NCSN) Mw 5.6 (NCSN, NEIC) occurred at 13:22:10 UTC (6:22 a.m. PDT), May 15, along the eastern side of the Sierra Nevada at Mammoth Lakes, California. The focal point (area of first motion) was centered a depth of 5.7 km and the fault plane solution suggested strike-slip motion along a NW or NE (preferred) trending fault. (See UCBSL fault plane solution,NEIC moment tensor solution)
The quake was centered about 12 miles southeast of Mammoth Lakes and about 8 miles southwest of Toms Place in the High Sierra south of the Long Valley caldera rim, in the upper McGee Creek drainage near Steelhead Lake (See Map). It was followed by a robust aftershock sequence, which during the initial hours, reached as high as 57 eq/hr. During the first five days, 30 aftershocks registered in the M3+ range, including four in the M4 range. The Largest aftershock came at 10:54 a.m later that morning and measured M4.8.
The United States Geological Survey in Menlo Park, California, issued an aftershock warning for the region. In general, their statement said that based upon the statistics of past aftershock sequences for this region of California, that there is a 30 percent chance of a M5.0 or stronger quake to strike within the next 7 days. Furthermore, the statistics suggest there is about a 5-10 percent chance of a quake registering equal to or large than the main shock to strike with the next 7 days. They also expect between 10-30 quakes to register M3 - M5 within the same 7 day period. (see the Aftershock Forecast)
The main jolt was sharply felt in the region and as far away as the San Joaquin and Sacramento Valley on the western side of the Sierra. Seismo-Watch received reports from Bakersfield, Coalinga, Fresno, Davis, Sacramento, Stockton, as well as several from the epicentral area.
Most in Mammoth Lakes felt a long drawn out rolling motion with some sharp jitters which intensified and toppled a few light items like cereal boxes and knick-knacks. Vons grocery store in downtown Mammoth Lakes said only a few cereal and other box items fell from shelves.
However in Lake Crowley, just 7 miles from the epicenter, residents said the earthquake shook quite hard and caused some minor damage. The Lake Crowley General Store reported several items were knocked from tables, including jars of pickles and mayonnaise and other items, totaling about $100 in damage.
The Lake Crowley Condominiums reported items tossed from shelves, including TV's stereos, books and pictures. These are two and three story buildings with most of the damage reported from the upper floors. There has been no dollar figure given as of yet.
Residents in the Lake Crowley area reported landslides and rock falls in the backcountry of the Sierra and clouds of dust billowed the air. Some said massive boulders tumbled down slopes, kicking up other rocks and creating large rock falls and talus slides. Road crews were dispatched and are cleaning debris from Convict Lake and McGee Creek Roads.
The Mono County Sheriff Watch Commander inspected the McGee Creek Campground and although he found the campground host had their nerves thoroughly shaken, they were in good health. The campground has been closed for the winter and scheduled to open in a week or two.
This was the first M5 earthquake in the Mammoth Lakes area since a pair of M5.1's struck in June and July of last year and the largest since a M6.5 struck on July 21, 1986.

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Subtract 8 hours from Greenwich Mean Time to obtain PST or 5 hours for EDT
Location Quality: A (good), B (fair), C (poor), D (bad)
Magnitude: Ml (local or Richter magnitude), Lg (mblg), Md (duration), Mb (body wave), Ms (surface wave), Mw (moment)

Standard Sources Include:
U.S. Geological Survey, National Earthquake Information Center, Golden, CO (NEIC)
Harvard Geophysical Observatory, Harvard University, Cambridge, MA (HRV)
U.S. Geological Survey, Northern California Seismic Network (NCSN)
University of California, Berkeley, Seismological Laboratory (UCBSL)
Southern California Seismic Network (USGS & Caltech), Pasadena, California (SCSN)
California Institute of Technology, Pasadena, California (Caltech)
various wire reports and/or personal communications

All data are preliminary and subject to change.
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