| 1 |
|
Loma Prieta Earthquake, October 18, 1989, Part 1 |
On October 17, 1989, a 7.1 magnitude earthquake occurred near Loma Prieta
in the Santa Cruz Mountains. Movement occurred along a 40-km segment of the San Andreas
fault from southwestof Los Gatos to north of San Juan Bautista. This slide set includes
damage in the more rural areas affected by the quake including: Boulder Creek, Aptos,
Los Gatos, San Jose, Scott's Valley, and Watsonville. The slides also depict earth
cracks and structural damage to homes in the Santa Cruz Mountains.Effects in Loma
Prieta Vicinity - On October 17, 1989, at 5:04 P.M. (PDT), a 7.1 magnitude earthquake
occurred near Loma Prieta in the Santa Cruz mountains, California. Movement occurred
along a 40-km segment of the San Andreas fault from southwest of Los Gatos to north
of San Juan Bautista. Measurements along Earth's surface after the earthquake show
that the Pacific plate moved 1.9 m to the northwest and 1.3 m upward over the North
American plate. The upward motion resulted from deformation of the plate boundary
at the bend in the San Andreas fault. At the surface, the fault motion was evident
as a complexseries of cracks and fractures. This earthquake was not unexpected. During
the 1906 San Francisco earthquake, there was only about one meter of movement on the
Santa Cruz segment of the San Andreas fault. Farther north in the San Francisco area,
there was more than 2.5 m of movement. This indicated that all of thestrain had not
been released in the Santa Cruz segment in the 1906 earthquake so this segment was
likely to break before the northern segment. Thousands of landslides occurred throughout
the area blocking roads and highways, hampering rescue efforts and causing damage
to structures. Landslides were particularly prevalent in the Santa Cruz mountains,
where they occur regularly even without earthquakes. These slides resulted in at least
two deaths. One slump slide near Laurel took with it several dozen houses damaging
them severely. Thirty percent of the buildings in the Pacific Garden Mall in downtown
Santa Cruz were damaged severely by amplified ground shaking and ground deformation.
The mall lies on unconsolidated deposits. One hundred thirty buildings-many dating
from the 19th Century-were damaged in this historic section. Several hundred houses
were either severely damaged or destroyed. The worst ground shaking appeared to occur
in the Santa Cruz Mountains close to the epicenter. Many buildings were damaged or
destroyed by ground cracking and shaking and by landsliding. Scores of mountain homes
were also destroyed. Initial damages were estimated at $350 million in Santa Cruz.
In Watsonville, two adjacent buildings of a department store sustained extensive structural
damage due to a weak first story, insufficient shear reinforcement of the columns,
and possible pounding of the two structures. Recently constructed buildings with tilt-up
walls performed well. At the Stanford University campus, 30 miles northwest of the
epicenter, 60 buildings sustained varying degrees of damage, with an estimated repair
cost of $160 million. Concrete sidewalks and curbs were systematically fractured and
buckled on northeast trending streets throughout downtown Los Gatos. Hollister also
experienced severe damage. Sand boils appear in irrigated fields near Hollister. San
Jose and Gilroy also reported collapsed and damaged buildings. Boulder Creek, Redwood
Estates, Los Gatos, Scott's Valley, Santa Cruz, and Watsonville all experienced strong
ground shaking and had a high percentage of damaged structures. These towns were only
16 to 32 km from the epicenter. The older structures in these towns were vulnerable
for one or more of the following reasons: deterioration of the structure, lack of
ties to the foundation, unreinforced masonry (brick or stone), lack of shear resistance
in the ground floor, pounding of adjacent structures, and timber diaphragms not tied
to unreinforced masonry walls, which allowed separation or pushing out of the walls.
In the epicentral area most of the damage resulted from the strong ground shaking
and landsliding. Ground shaking primarily affected unreinforced masonry structures,
and was enhanced in areas of fine-grained sand. Landslides occurred on steep slopes
where ground shaking was most severe.
|
