The October 17, 1989 Loma Prieta earthquake was responsible for 62 deaths and 3,757 injuries. In addition, over $6 billion in damage was reported including damage to 18,306 houses and 2,575 businesses. Approximately 12,053 persons were displaced. The most intense damage was confined to areas where buildings and other structures where situated on top of loosely consolidated, water saturated soils. Loosely consolidated soils tend to amplify shaking and increase structural damage. Water saturated soils compound the problem due to their susceptibility to liquefaction and corresponding loss of bearing strength.
Liquefaction is a physical process that takes place during some earthquakes that may lead to ground failure. As a consequence of liquefaction, soft, young, water-saturated, well sorted, fine grain sands and silts behave as viscous fluids rather than solids. Liquifaction takes place when seismic shear waves pass through a saturated granular soil layer, distort its granular structure, and cause some of its pore spaces to collapse. The collapse of the granular structure increases pore space water pressure, and decreases the soil's shear strength. If pore space water pressure increases to the point where the soil's shear strength can no longer support the weight of the overlying soil, buildings, roads, houses, etc., then the soil will flow like a liquid and cause extensive surface damage. Some examples of this phenomena are shown below.
Fortunately,
areas susceptible to liquefaction can be readily identified
and the hazard can often be mitigated. Because of
the relative ease of
identifying hazardous areas, numerous liquefaction maps have been made
by govenrnment agencies. Liquefiable sediments are young, loose,
water saturated, well sorted and are either fine sands or silts. The
sediments are seldom older than Holocene, and are usually only present
on the modern floodplains of creeks and rivers. The map to your right
identifies areas likely to liquefy during a big quake (noted by the
letter "L")
Sand volcanoes, Moss Landing State Beach
(photo by Dan Orange): In
many low-lying coastal areas sand volcanoes formed where underlying
saturated sands liquefied during the seismic shaking and ejected
onto the
surface. During the 1906 earthquake, sand volcanoes spouted to a heights
around 20 feet in the Salinas Valley and near Moss Landing. During the Loma
Prieta earthquake, extensive liquefaction
occurred along the entire shoreline of the Monterey Bay, as well as
in San
Francisco's Marina District and along the bayshore in Oakland.
Collapsed highway bridge, State Highway 1, Struve Slough,
Watsonville (photo by Jeff Marshall).
Violent ground shaking combined with
liquefaction of unconsolidated slough muds led to the
spectacular failure
of this bridge on the famous Pacific Coast Highway 1 near
Watsonville. The
portions of the highway that collapsed are directly over
saturated slough
sediments. Upward acceleration of the bridge during the shaking
caused the
structure to separate from its support columns. As the bridge
then fell
back downward, the columns punctured through the concrete road
surface.
Collapsed highway bridge (close-up of
support columns), State Highway
1, Struve Slough, Watsonville (photo by Jeff Marshall): Visible to the
left of the columns in this photo are skid and scrape marks from a
California Highway Patrol car that flew onto the bridge at high speed
minutes after the earthquake. The driver, who survived the incident, was
unaware that the bridge had collapsed as he raced down the highway in
response to an earthquake emergency.
Collapsed highway bridge (close-up of support columns from below), State Highway 1, Struve Slough, Watsonville (photo by Dan Orange): This view from below the collapsed bridge shows the area around the base of a support column where soil was pushed away by the back and forth motion of the column. The rebar at the top of the column is still attached to the original location of contact between the column and the bridge. As mentioned above, upward acceleration of the bridge during the shaking caused the structure to separate from the support columns. As the bridge then fell back downward, the columns punctured through the concrete road surface.
Liquefaction induced road failure, Moss
Landing State Beach (photos by Dan Orange): This road is built across an
estuary and suffered extensive
damage due to liquefaction. It subsided several meters during the
earthquake, separating from the adjoining sections of road. The exposed
cross section of underlying sediments clearly shows a lower light colored
beach sand that liquefied and injected into an overlying dark orange silty
unit.
Failed river dike, San Lorenzo River, Beach Flats, Santa Cruz (photo by Jeff Marshall): Much of the earthen San Lorenzo River flood control levee in the Beach Flats area of Santa Cruz slumped and fractured as a result of extreme ground shaking and liquefaction of underlying unconsolidated beach and river sediments.
Failure and cracks induced by
liquefaction have been observed in the past (photographers unknown). These
images, probably from
the 1906 event, shows cracks formed by liquefaction at the San Lorenzo
River bed.
