At 5:23 AM Pacific Daylight time the Seismological Laboratory at UNR recorded a strong earthquake. The earthquake was centered 12 miles SSE of Gardnerville near Double Spring Flat, and had a local Richter magnitude of 6.0. It was located at latitude 38.82N, longitude 119.64W at shallow depth. The Seismological Laboratory recorded approximately 10 aftershocks having magni- tudes above 3.0 in the first eight hours following the earth- quake. The largest aftershock had magnitude 3.5. An earthquake of this size may cause significant damage. The earthquake was strongly felt in west-central Nevada and eastern California, including Minden, Gardnerville, Markleeville, Carson City, South Lake Tahoe, Reno, and Sparks. Felt reports have been received from as far east as Elko, and from as far west as Fresno and Sacramento. At present the Seismological Laboratory has received many reports of people feeling and being awakened by the earthquake, and of items knocked off shelves and broken. No reports of injury have been received. There have been very few reports of any struc- tural damage to buildings. Persons who observed the earthquake or its effects are encouraged to call (702) 784-4975 to make a report. In the past 40 years, eight earthquakes this size or larger have affected west-central Nevada. Seven of those were part of the 1954 earthquake sequence near Dixie Valley and Fairview Peak. The largest event in that sequence occurred in December 1954 and had a magnitude of 7.2. The last occurred exactly 28 years ago today. It was a magnitude 6.0 earthquake near Truckee, Califor- nia. The Seismological Laboratory has not determined which fault was responsible for today's earthquake. Its epicenter is located east of the Genoa fault and northwest of the Antelope Valley fault. Once the locations of aftershocks have been precisely determined, we hope to be able to associate the earthquake with the causative fault. There is now a chance (one in fifty) of a larger earthquake, which will decrease to normal over the next five days. Also, significant aftershocks capable of causing minor local damage usually follow an earthquake of this size. However, the fact that this event is located in a sparsely-populated area decreases the likelihood of further damage. The data reported here are preliminary. For more information, contact the University of Nevada Seismological Laboratory at (702) 784-4975. The free pamphlet "Earthquakes in Nevada and How to Survive Them," is available on request. Dr. John G. Anderson, Acting Director
The strongest aftershock so far from this morning's Double Spring Flat earthquake occurred at 4:58 PM, PDT. The preliminary magnitude determined by the UNR Seismological Laboratory is 5.3. The location is 14 miles south-southeast of Gardnerville, which is 2 miles south of the UNR epicenter for the main shock. Althogether, the Seismological Laboratory has detected thousands of small aftershocks. Since the first large shock this morning, the nearest station to the epicenter of these earthquakes has detected small aftershocks at a rate of several per minute. A few additional events from this continuing sequence ofaftershocks are likely to be strong enough to be felt. The data reported here are preliminary. For more information, contact the University of Nevada Seismological Laboratory at (702) 784-4975. The free pamphlet "Earthquakes in Nevada and How to Survive Them," is available on request. Dr. John G. Anderson Acting Director
Aftershocks from the Double Spring Flat earthquake continue, as expected. The rate on the nearest station is dropping, also as is normal for earthquakes in this region. That station is now detecting about one small aftershock per minute - down substantially from the rate of several per minute that was recorded shortly after the earthquake. The largest aftershocks to date, with magnitudes over 4 are listed below: Sept 12, 1994 4:57 pm 5.0 Sept 12, 1994 11:15 pm 4.1 Sept 13, 1994 2:22 pm 4.0 These magnitudes may be revised slightly from earlier announcements as new data has been added. The largest aftershock in last 24 hours occurred at 5:45 pm yesterday, September 14. Our preliminary magnitude for this event is 3.8.
Geologists from University of Nevada Department of Geology, Nevada Bureau of Mines and Geology, and US Geological Survey have been searching for evidence of primary faulting. Some minor cracks have been found, but no major fault rupture. This earthquake is of a size where major rupture is found about 50% of the time in Nevada, so the current earthquake is not unusual. Several of the aftershocks that we have located to date occur below the cracks found by the geologists. There is also a trend in the aftershocks that is nearly perpendicular to the trend of the minor cracks found by the geologists. Similar trends have been observed after other earthquakes in this region, so this also is not surprising. Field studies are continuing, and we expect to develop a more clear understanding of what happened over the next few days. The time window of increased probability for a larger earthquake to follow this event is essentially over. Western Nevada has had a history of large earthquakes, and it is possible for another to occur at any time. However, the chances of this event being an immediate foreshock of a larger earthquake are now rapidly approaching zero. The data reported here are preliminary. For more information, contact the University of Nevada Seismological Laboratory at (702) 784-4975. The free pamphlet "Earthquakes in Nevada and How to Survive Them," is available on request. Dr. John G. Anderson Acting Director
During the past 24 hours, there has been an increase in aftershock
activity compared to the weekend. This included six aftershocks with
magnitudes over 4. Specific times of these events are:
Sept 19, 8:17 pm, M=4.1
Sept 19, 8:37 pm, M=4.1
Sept 19, 8:45 pm, M=4.0
Sept 19, 10:17 pm, M=4.5
Sept 20, 8:38 am, M=4.5
Sept 20, 8:40 am, M=4.7
These large aftershocks had locations that are a few kilometers
southeast of the main shock, or about 2-3 miles northwest of
HolbrookJunction. They are all near the southeast end of the
aftershock zonefrom the Sept 12 main shock. Although these were
larger than most, continued aftershocks of this type are quite normal.
Residents should expect a few more of this size over the next several
weeks. These aftershocks do not increase the chance of a large
earthquake.
These most recent aftershocks are also near the northwest end of the
mapped location of the Antelope Valley fault. That fault is mapped to
extend south from Holbrook Junction, along the west side of Topaz Lake
and Antelope Valley for a total distance of about 17 miles. We are
actively investigating the hypothesis that the Double Spring Flat
earthquake occurred on a northwesterly continuation of the Antelope
Valley fault that was previously unmapped.
The Antelope Valley fault is believed to be capable of producing a
magnitude 7.2 to 7.4 earthquake once every 10,000 to 20,000 years.
The time of the last large earthquake on this fault is unknown.
It is quite common for small earthquakes to occur at the ends of
faults without precipitating a large earthquake, so the aftershocks
in this location are NOT a valid predictor of activity in the immediate
future.
The data reported here are preliminary. For more information, contact
the University of Nevada Seismological Laboratory at (702) 784-4975.
The free pamphlet, Earthquakes in Nevada and How to Survive Them, is
available on request.
Dr. John G. Anderson
Acting Director