General Plan Update - City of Inglewood

Chapter 6 Community Safety and Hazards
On March 11, 1933, the Long Beach earthquake of M 6.4 occurred on the Newport-
Inglewood fault zone. The Long Beach earthquake produced extensive damage to
unreinforced masonry structures in the Long Beach area while less extensive damage was
reported in other parts of the Los Angeles basin. This event produced numerous ground
failures in the form of liquefaction, seismic settlement, and lateral spreading from
Newport Beach to Long Beach and as far inland as Garden Grove. Recent analysis of the
Long Beach earthquake shows that the earthquake had a subsurface rupture length of
approximately 13-16 kilometers (km). The earthquake was centered in Huntington Beach
at a 13 km depth and had a strike-slip focal mechanism consistent with the Newport-
Inglewood fault zone.
The M 6.7 San Fernando earthquake occurred on February 9, 1971. The epicenter was
approximately 30 miles north of the City and surface rupture was experienced on various
strands of the San Fernando fault zone, including the Tujunga and Sylmar faults, as a
result of this earthquake. Damage caused by the 1971 quake led to the modification of the
state building code and seismic safety standards to reduce structural damage and bodily
injury during maximum credible earthquakes.
The M 5.9 Whittier Narrows earthquake, related to the Elysian Park Fold and Thrust
Belt, occurred on October 1, 1987 on a previously unrecognized fault. The earthquake
epicenter was located approximately 20 miles north of the City. New studies have shown
that this event occurred on the Puente Hills blind trust fault.
The Sierra Madre earthquake occurred on June 28, 1991 along the Clamshell-Sawpit fault
of the Sierra Madre fault zone. This M 5.8 earthquake was located in the San Gabriel
Mountains, approximately 30 miles northeast of the City.
The most recent earthquake of significance in Southern California affecting the City was
the 1994 Northridge Earthquake, a magnitude 6.7 earthquake that occurred in the San
Fernando Valley. Approximately 15,000 structures experienced moderate to severe
damage and several bridges and overpasses collapsed. The epicenter for this earthquake
was the San Andreas Fault, which is located approximately 45 miles from the City.
Soils
The soil types within the City consist of alluvium. Generally, these soils offer poor
resistance to ground shaking and can amplify some surface motion. According to the
USGS, mean subsurface shear-wave velocities through Inglewood soils are calculated to
be 350 to 375 meters per second which is relatively low (loose wet sand is 230 m/second
while solid bedrock is over 1,600 m/sec). Lower subsurface shear-wave velocities signify
a propensity for greater amplification of ground motion.
Geologic Hazards
The amount of damage to a building does not depend solely on how hard it is shaken. In
general, smaller buildings such as houses are damaged more by higher frequencies, so
6.2-6
City of Inglewood General Plan Update Technical Background Report