Travel Reference
In-Depth Information
San Francisco's Earthquakes
San Francisco lies on the San Andreas Fault and is
under constant threat of earthquakes. The Loma
Prieta earthquake of October 17, 1989, named
after the hill close to its epicenter in the Santa
Cruz Mountains, was the worst to hit the area
since 1906
(see pp30-31)
. Many buildings are now
being strengthened to withstand tremors, and
shelters like the one at the Moscone Center
(see pp116-17)
are stocked as emergency relief
sites. In addition, most hotels have their own
evacuation procedures, and the local telephone
directory has four pages of advice.
The 1989 earthquake
measured 7.1 on the
Richter scale. It caused some of the houses
that were built on landfill in the Marina
District to shift off their foundations.
Berkeley
The San Andreas Fault
is a
major fracture in the Earth's
crust. It extends almost the full
length of California, some
600 miles (965 km).
San Francisco
lies near the
northern end of
the fault.
Pacific Plate Meets North
American Plate
The San Andreas Fault is the
result of friction where two
major plates of the Earth's
crust meet - the eastern
Pacific and the North
American plates
San
Andreas
Fault
North American
plate
Epicenter (point on the surface above the focus
of an earthquake)
L (long) waves
travel across the
surface.
Hypocenter (the focus of an earthquake)
S (secondary)
waves
travel
through solid
parts of the crust.
P waves
S waves
L waves
P (primary) waves
travel through the
Earth's core.
Pacific plate
Hypocenter
A seismograph printout
shows the intensity
of earthquake vibrations graphically. Inside
the seismograph a pen traces P (primary),
S (secondary) and L (long) waves on a
rotating drum.
Earthquake energy vibrations
travel like waves
through the Earth's crust. The interval between the
arrival of the P and S waves tells scientists how far
away the epicenter of the earthquake is.
