Liquefaction-Induced Lateral Spreading in Oceano, California, During the 2003 San Simeon Earthquake

The December 22, 2003, San Simeon, California, (M6.5) earthquake caused damage
to houses, road surfaces, and underground utilities in Oceano, California. The
community of Oceano is approximately 50 miles (80 km) from the earthquake
epicenter. Damage at this distance from a M6.5 earthquake is unusual. To
understand the causes of this damage, the U.S. Geological Survey conducted
extensive subsurface exploration and monitoring of aftershocks in the months
after the earthquake. The investigation included 37 seismic cone penetration
tests, 5 soil borings, and aftershock monitoring from January 28 to March 7,
2004.

The USGS investigation identified two earthquake hazards in Oceano that explain
the San Simeon earthquake damage?site amplification and liquefaction. Site
amplification is a phenomenon observed in many earthquakes where the strength
of the shaking increases abnormally in areas where the seismic-wave velocity of
shallow geologic layers is low. As a result, earthquake shaking is felt more
strongly than in surrounding areas without similar geologic conditions. Site
amplification in Oceano is indicated by the physical properties of the geologic
layers beneath Oceano and was confirmed by monitoring aftershocks.

Liquefaction, which is also commonly observed during earthquakes, is a
phenomenon where saturated sands lose their strength during an earthquake and
become fluid-like and mobile. As a result, the ground may undergo large
permanent displacements that can damage underground utilities and well-built
surface structures. The type of displacement of major concern associated with
liquefaction is lateral spreading because it involves displacement of large
blocks of ground down gentle slopes or towards stream channels. The USGS
investigation indicates that the shallow geologic units beneath Oceano are very
susceptible to liquefaction. They include young sand dunes and clean sandy
artificial fill that was used to bury and convert marshes into developable
lots. Most of the 2003 damage was caused by lateral spreading in two separate
areas, one near Norswing Drive and the other near Juanita Avenue. The areas
coincided with areas with the highest liquefaction potential found in Oceano.

Areas with site amplification conditions similar to those in Oceano are
particularly vulnerable to earthquakes. Site amplification may cause shaking
from distant earthquakes, which normally would not cause damage, to increase
locally to damaging levels. The vulnerability in Oceano is compounded by the
widespread distribution of highly liquefiable soils that will reliquefy when
ground shaking is amplified as it was during the San Simeon earthquake. The
experience in Oceano can be expected to repeat because the region has many
active faults capable of generating large earthquakes. In addition,
liquefaction and lateral spreading will be more extensive for moderate-size
earthquakes that are closer to Oceano than was the 2003 San Simeon earthquake.

Site amplification and liquefaction can be mitigated. Shaking is typically
mitigated in California by adopting and enforcing up-to-date building codes.
Although not a guarantee of safety, application of these codes ensures that the
best practice is used in construction. Building codes, however, do not always
require the upgrading of older structures to new code requirements.
Consequently, many older structures may not be as resistant to earthquake
shaking as new ones. For older structures, retrofitting is required to bring
them up to code. Seismic provisions in codes also generally do not apply to
nonstructural elements such as drywall, heating systems, and shelving.
Frequently, nonstructural damage dominates the earthquake loss.

Mitigation of potential liquefaction in Oceano presently is voluntary for
existing buildings, but required by San Luis Obispo County for new
construction. Multiple mitigation procedures are available to individual
property owners. These procedures typically involve either changing the
physical state of the underlying sands so they cannot liquefy or building a
foundation that can resist the permanent displacement of the ground. Lateral
spreading, which is the major threat to underground utilities, is particularly
challenging to mitigate because typically large areas are involved and sizeable
volumes of soil must be prevented from moving. Procedures to prevent spreading
commonly require subsurface barrier walls. Prevention of lateral spreading may
also require community rather than individual efforts because of the scale and
cost of these mitigation measures.

[Summary provided by the USGS.]