1986 Chalfant Valley earthquake
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The 1986 Chalfant Valley earthquake struck southern Mono County near Bishop and Chalfant, California at Pacific Daylight Time on July 21. With a moment magnitude of 6.2 and a maximum Mercalli intensity of VI (''Strong''), the shock injured two people and caused property damage estimated at $2.7 million in the affected areas. There was a significant foreshock and aftershock sequence that included a few moderate events, and was the last in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986.
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stations at the Long Valley Dam and an instrumented building in Bishop captured light to moderate readings from the event. The faulting reached the surface of the ground, but the maximum amount of measurable slip was limited to about of strike-slip motion along the White Mountain Fault Zone. Other destructive and surface-rupturing earthquakes, as well as large volume volcanic eruptions, have occurred in the area. Although the series of shocks occurred within the White Mountain seismic gap, the region continues to have potential for a very large earthquake.


Preface

July 1986 was an unusually active month for moderate to strong earthquakes in California, with three events occurring in less than two weeks, each with mild to moderate effects. The first of these shocks came on July 8 with a M6.0 event on the Banning fault near Palm Springs and the second event occurred off the coast of Oceanside as a M5.8 shock on July 13. While the earthquake off the southern California coast occurred in an area thought to be capable of generating a tsunami, the earthquake near Palm Springs occurred on a portion of the southern San Andreas Fault system that has been designated a seismic gap, and is a likely location for a very large earthquake. The same seismic gap theory (regarding the potential for a future large earthquake) had also been presented for the White Mountains area near Chalfant.


Tectonic setting

The Owens Valley, located at the western boundary of the Basin and Range Province, is confined by the
Sierra Nevada The Sierra Nevada () is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primarily ...
mountain range to the west and the White- Inyo Mountains to the east. The valley was brought about by active tectonics over the last 2–4 million years, and was the scene of a very large earthquake in 1872 that generated surface rupture from Lone Pine in the south to as far north as Big Pine, a distance of approximately . Other large, surface rupturing events in
1915 Events Below, the events of World War I have the "WWI" prefix. January * January – British physicist Sir Joseph Larmor publishes his observations on "The Influence of Local Atmospheric Cooling on Astronomical Refraction". *January 1 ...
,
1932 Events January * January 4 – The British authorities in India arrest and intern Mahatma Gandhi and Vallabhbhai Patel. * January 9 – Sakuradamon Incident (1932), Sakuradamon Incident: Korean nationalist Lee Bong-chang fails in his effort ...
, and
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delineate the Eastern California–Central Nevada seismic zone. As no large earthquake has occurred between the southern extent of the 1932 Cedar Mountains (Nevada) earthquake's rupture and the northern extent of the
1872 Lone Pine earthquake The 1872 Owens Valley earthquake – also known as the Lone Pine earthquake – struck on March 26 at in the Owens Valley (California, along the east side of the Sierra Nevada), with the epicenter near the town of Lone Pine. Its magnitude has ...
's rupture in the south (a distance of ), the zone has been labeled the White Mountain seismic gap. The 1934 M6.3 Excelsior Mountains earthquake and the 1986 Chalfant Valley event were several smaller earthquakes that have occurred within the gap, and both generated limited surface faulting and some surface cracking in the Volcanic Tableland, which was created .7 million years ago from a major volcanic eruption that also formed the Long Valley Caldera northwest of Bishop. An estimated 500 cubic kilometers of material ( tephra) produced in the event covers the northern Owens Valley as a rocky landform. The surface of the layered plateau is known as Bishop Tuff and features fumarole mounds and hundreds of north-south oriented fault scarps, many of which are visible on topographic maps, via aerial photography, and
satellite imagery Satellite images (also Earth observation imagery, spaceborne photography, or simply satellite photo) are images of Earth collected by imaging satellites operated by governments and businesses around the world. Satellite imaging companies sell ima ...
.


Foreshocks

The University of Nevada, Reno (UNR) and the United States Geological Survey (USGS) both operated seismograph networks in the region where the Chalfant Valley earthquake sequence occurred. The data that was captured was transmitted in real time to Reno, Nevada and Menlo Park, California and was digitized for later examination. The foreshock activity was found to have begun eighteen days prior to the main event on July 3, with the onset of a 3.5 event (
local magnitude The Richter scale —also called the Richter magnitude scale, Richter's magnitude scale, and the Gutenberg–Richter scale—is a measure of the strength of earthquakes, developed by Charles Francis Richter and presented in his landmark 1935 p ...
). Activity increased on July 18 with several doublets then reduced again in the hours prior to the primary M5.7 (intensity V (''Moderate'')) foreshock on July 20. All forty events that were recorded by the UNR and USGS stations before that event were within of its epicenter, and another 132 events that were considered aftershocks were documented in the remaining 24 hours leading up to the mainshock. One of these events, just over two hours after the primary foreshock, measured M4.7 with an intensity of IV (''Light'').


Earthquake

The mainshock occurred at on July 21 and caused the most damage in the sequence of events. The shock measured 6.2 as measured by the International Seismological Centre and originated at a depth of approximately . The
focal mechanism The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is ...
s of the three largest events (the July 20 foreshock, the mainshock, and the largest aftershock) were primarily
strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
with a minor amount of normal-slip and were especially well-constrained due to the dense array of seismometers in the area that were operated by the UNR and the USGS. The primary foreshock's preferred fault plane solution indicated left-lateral slip on a fault
striking Strike may refer to: People *Strike (surname) Physical confrontation or removal *Strike (attack), attack with an inanimate object or a part of the human body intended to cause harm *Airstrike, military strike by air forces on either a suspected ...
N25°E, but neither the mainshock nor the largest aftershock conformed with their right-lateral slip striking N25°W and N15°W respectively.


Damage

In Bishop, windows were shattered and ceiling tiles fell in several buildings, and a portion of the brick façade at the First Sierra bank building fell onto the sidewalk. A Burger King restaurant had part of its ceiling come down. Northwest of Bishop, a portion of
U.S. Route 395 U.S. Route 395 (US 395) is a U.S. Route in the western United States. The southern terminus of the route is in the Mojave Desert at Interstate 15 near Hesperia. The northern terminus is at the Canada–US border near Laurier, where the road be ...
was blocked temporarily by a
landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...
. Campers were briefly trapped at the Pleasant Valley Reservoir when a section of an access road was destroyed as a result of land movement. Both injuries that were reported (minor cuts and injuries from falling objects) occurred to the north in Chalfant Valley where 53
mobile home A mobile home (also known as a house trailer, park home, trailer, or trailer home) is a prefabricated structure, built in a factory on a permanently attached chassis before being transported to site (either by being towed or on a trailer). Us ...
s were knocked off their foundations and two homes were destroyed. Nearly all the buildings in that small town were affected, with water and sewer lines broken there as well. Many of the mobile homes were able to be remounted on their foundations, but 18–20 of the homes were unable to be repaired. The overall damage from the event was compared with the other events in southern California. While the Oceanside shock caused $200,000 in damage, the Palm Springs event caused an estimated $8 million in damage. Damage in from the Chalfant Valley event was estimated at $2.7 million.


Surface faulting

Ground fractures that were deemed to be tectonic in nature were documented on about of the White Mountain Fault Zone from Silver Creek to Piute Creek. The cracks trended 350° and a maximum right-lateral slip of about was measured, though other vertical (downslope) or extensional movement was also seen, especially on the -thick surface of the Bishop Tuff in the Volcanic Tableland. There, the largest displacements were seen in the vicinity of Casa Diablo Mountain. The cracks were found primarily in loose sand, which made measurements difficult.


Intensity

In Bishop, intensity VI effects included broken windows and cracked walls in commercial and government buildings. Items fell from store shelves at grocery markets, streets were cracked, and
wells Wells most commonly refers to: * Wells, Somerset, a cathedral city in Somerset, England * Well, an excavation or structure created in the ground * Wells (name) Wells may also refer to: Places Canada *Wells, British Columbia England * Wells ...
had changes in the flow of water. The damage to the mobile homes in Chalfant was also consistent with intensity VI shaking, with some of the homes moving laterally as much as 18 inches. Also in this classification was damage to pipe supports along a portion of the
Los Angeles Aqueduct The Los Angeles Aqueduct system, comprising the Los Angeles Aqueduct (Owens Valley aqueduct) and the Second Los Angeles Aqueduct, is a water conveyance system, built and operated by the Los Angeles Department of Water and Power. The Owens Valley ...
between the Upper, Middle, and Control Gorge Power Plants southeast of
Crowley Lake Crowley Lake is a reservoir on the upper Owens River in southern Mono County, California, in the United States. Crowley Lake is south of Mammoth Lakes. The lake was created in 1941 by the building of the Long Valley Dam by the Los Angeles Depa ...
. Intensity V (''Moderate'') effects (including cracked windows and broken underground pipes) were present in the Central Valley, Death Valley, and
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.


Strong motion

The foreshock, mainshock, and the two largest aftershocks were recorded by strong motion stations that were operated by the California Strong Motion Instrumentation Program (CSMIP) and the USGS. A total of 36 seismograms were captured from 11 stations, including a two-story steel frame building, the Long Valley Dam, and several free field stations. The Chalfant strong motion station recorded the largest horizontal accelerations for the foreshock, mainshock, and the July 21 M5.6 aftershock of 0.28 ''g'', 0.46 ''g'', and 0.17 ''g''. The instruments at the building on North Main street in Bishop recorded all four shocks and indicated 0.25 ''g'' at ground level and 0.4 ''g'' on the roof, both during the mainshock. The station at the earthen Long Valley Dam had produced many quality recordings since being put into operation in 1979 and saw mainshock accelerations of 0.09 ''g'' on
bedrock In geology, bedrock is solid Rock (geology), rock that lies under loose material (regolith) within the crust (geology), crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface mater ...
, and 0.24 ''g'' on an upper abutment.


Aftershocks

The day of the main event two aftershocks occurred (5.6 at 14:51 and 5.4 at 22:07) but the initial shock was V (''Moderate'') on the Mercalli intensity scale and the second event was felt only. Ten days following the mainshock on July 31, a stronger aftershock occurred with a local magnitude of 5.8 and a maximum Mercalli intensity of VI (''Strong''). That aftershock was felt in California and western Nevada, broke windows and knocked items off shelves in Bishop, and light fixtures were damaged at the National Weather Service office.


See also

*
List of earthquakes in California The earliest known California earthquake was documented in 1769 by the Spanish explorers and Catholic missionaries of the Portolá expedition as they traveled northward from San Diego along the Santa Ana River near the present site of Los Angeles ...
*
List of earthquakes in the United States The following is a list of notable earthquakes and tsunamis which had their epicenter in areas that are now part of the United States with the latter affecting areas of the United States. Those in ''italics'' were not part of the United States whe ...


References


External links


M 6.4 - 17km N of Dixon Lane-Meadow Creek, CA
– United States Geological Survey
Processed strong-motion data from the Chalfant Valley earthquakes of July 1986
California Strong Motion Instrumentation Program
Eastern California Shear Zone Puzzles Seismologists
Science Daily * {{Authority control Earthquakes in California 1986 earthquakes 1986 natural disasters in the United States