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Landers and Big Bear California Earthquakes, June 28, 1992 |
California residents were rudely awakened early Sunday morning June 28, 1992,
by an earthquake of magnitude 7.6 (Ms) followed by a smaller 6.7 (s) magnitude earthquake
about three hours later. The largest shock occurred approximately six miles southwest
of Landers, California and 110 miles east of Los Angeles. The second earthquake was
centered approximately eight miles southeast of Big Bear City in the San Bernardino
Mountains near Barton Flats. A distance of seventeen miles and 7,000 feet in elevation
separate the two earthquake locations. In addition to depicting the differences in
terrain of the two locations, examples of structural damage, liquefaction, surface
faulting, and earthquake generated landslides are illustrated. Statistics - The largest
shock occurred at 4:58 AM PST approximately six miles southwest of Landers, California,
and 110 miles east of Los Angeles. Seismicity and Geology - The first shock was the
most powerful earthquake in the contiguous 48 states in forty years. The initial earthquake
was reported felt throughout Southern California from the San Luis Obispo to the Mexico
Border. It was felt outside California in Idaho, Colorado, New Mexico, and Nevada.
The first shock occurred on the north-north-west striking Camp Rock, Emerson, and
Johnson Valley faults and produced a forty-three mile-long surface rupture zone. In
the Landers area there was about ten feet of right-lateral strike slip movement. The
second shock occurred on a separate, previously unknown northeast trending left-lateral
strike-slip fault that intersects the Camp Rock-Emerson fault. Scientists are investigating
the possibility that these earthquakes may be related to a future event on the San
Andreas fault, which together with the faults involved in this series of earthquakes
forms a triangle of faults.Damage - Both earthquakes caused buildings to collapse,
ignited fires, buckled roads, damaged water tanks and crippled water delivery systems,
produced numerous rock slides in the San Bernardino Mountains, and derailed standing
railroad boxcars near Barstow. The Landers earthquake was directly responsible for
the death of a small child who was killed by the collapse of a rock fireplace. Together,
the twin earthquakes caused two fatal heart attacks, more than 400 injuries, and left
750 people homeless. Preliminary estimates of property damage in San Bernardino County
by officials of the State Office of Emergency Services for the two earthquakes was
nearly $100 million, with more than half of the damage occurring in the Big Bear Lake
region. The earthquakes and aftershocks damaged 4,446 homes and 166 businesses; destroyed
77 homes and 10 businesses. Total damage to private property was estimated at $64.5
million; public facilities reported $25.5 million damage. Roads and bridges in San
Bernardino and Los Angeles counties sustained $500,000 in damage according to California
Department of Transportation. In Riverside County, the losses were estimated at $950,000;
most all of the damage was reported from the City of San Jacinto. Electricity was
knocked out for several hours in the Landers area; in other communities power and
communication links performed well with temporary outages. The damage to the water
storage tanks and the breaks in the distribution system caused water shortages in
many areas. A number of fires resulted from the earth shaking: Two residences burned
down completely in Landers. In Yucca Valley two mobile homes caught fire and one burned
down completely. Earth Effects - Priminary field measurements of movement along the
fault line of the 7.5 magnitude earthquake indicate that sections of the fault slipped
a maximum of eighteen feet horizontally and six feet vertically. The width of the
rupture zone was reported to range from eleven feet at the narrowest point to 140
feet across at the widest point. Cracks in a dirt road about a mile from the South
Fork campground entrance occurred as a result of the 6.7 magnitude earthquake. Small
water spouts were also observed. The low death toll and structural damage for this
series of earthquakes may be attributed to strengthened building code standards, firm
ground, and wide-open spaces. A single factor that contributed to the low property
damage and casualties was the fact that the force of the earthquake ruptured away
from the major population centers. The Landers-Big Bear earthquake sequence shows
that an earthquake on one fault system may trigger large tremors on other faults in
the same area.
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