Transcript The Transverse Ranges Lecture Notes Page

The Transverse Ranges
LA in the Squeeze
• The coastline from
Ventura to Point
Conception beyond Santa
Barbara is unusual in that
it faces south, not west,
toward the mountainous
Northern Channel
Islands, a submerged
mountain range that
projects westward from
the Santa Monica
Mountains north of Los
Angeles.
• The Santa Ynez Range and
the Topa Topa Mountains
rise north of Santa Barbara
and Ventura, and farther
east are the San Gabriel and
San Bernardino Mountains,
raised up because the Big
Bend of the San Andreas
Fault causes continental
crust of the North American
Plate to collide with crust of
the Pacific Plate. These are
collectively known as the
Transverse Ranges, so called
because they are transverse
to the normal north-south
trend of ranges along the
Pacific margin of North
America.
• The Transverse Ranges
are one of the Earth’s
most rapidly uplifting
areas. The steep,
rugged mountains along
the Big Bend are some of
the evidence of rapid
uplift. The stepped
marine terraces near
Ventura and Palos
Verdes provide further
proof of rapid uplift: the
terraces there are rising
by as much as 5 to 10
mm per year.
Damage resulting from 1971 San Fernando Earthquake
Damage resulting from
1994 Northridge
Earthquake
• Sudden dramatic
uplift takes place
during some
earthquakes. During
the 1994 Northridge
earthquake, the Santa
Susana Mountains rose
70 cm, and during the
1971 San Fernando
Earthquake, the same
mountains rose about
2 meters.
• The mountain ranges are
raised to the highest
elevations in southern
California, and between
them the crust is dropped
down to depths of many
miles – although this fact
is hidden by great
thicknesses of sediment
beneath the valleys.
From west to east, these
thick basins of sediment
include the Santa Barbara
Channel, the Ventura
Basin, the San Fernando
Valley, the Los Angeles
Basin, and the San Gabriel
and San Bernardino
valleys.
• The great thicknesses
of sediment resulted
in vast accumulations
of oil and gas in the
Ventura and Los
Angeles basins and in
the Santa Barbara
Channel, adding to the
wealth of southern
California.
• The borders
between basins and
mountains are
typically active
faults – either
reverse faults or
strike-slip related
to the San Andreas
Fault System.
• The 1994 Northridge,
1987 Whittier Narrows
(ML 5.9) , and 1971 San
Fernando earthquakes
were all centered on
reverse faults buried
beneath the Los Angeles
Basin. The 1983
Coalinga Earthquake (ML
6.5) was also centered
on a blind fault, one of
several bordering the
western edge of the San
Joaquin Valley.
Wall Damage to Building in
Downtown Coalinga
Damage to State Theater Building, Coalinga
Second Floor Wall Failure on
Coalinga House
The Coalinga Earthquake of May 2, 1983
had a magnitude of 6.7, caused $31 million
in damage, but no deaths. The most
severe damage occurred to the old
(usually pre-1930) one- and two-story
buildings of unreinforced brick masonry
wall construction, with floors and roofs of
wood.
Total Collapse of Front of Residence,
Coalinga
Partial Collapse of Parking Garage, Whittier,
CA, 1987 Earthquake
The October 1, 1987 Whittier
Earthquake (ML 5.9) killed 3 people
and caused $350 million in damage
Collapse of Wall of Store in
1987 Whittier, CA Earthquake
The January 17, 1994
Northridge
Earthquake (ML 6.7)
was one of the worst
natural disasters in
the history of the
United States.
Partial collapse of Bullocks
Department Store
Collapsed roof near
Northridge Mall
Partially collapsed parking structure, Cal
State Univ. - Northridge
Northridge statistics include 56
dead, 25,000 dwellings uninhabitable,
and $10 billion in damage.
I-5 and SR-14 Freeway collapse
Collapse of apartment
building over garage in
Reseda
Damage to Kaiser Clinic in
Granada Hills
On February 9, 1971, at
6:01 A.M. local time, a
magnitude 6.5 earthquake
occurred in the San
Fernando Valley, 20 miles
north of Los Angeles. The
San Fernando earthquake
was felt over an area of
80,000 square miles. The
statistics include 58 dead
and $497.8 million in
damage.
Damage at I-5 and I-210
Freeway Interchange
Damaged home in
Crestview Park
Aerial view of damaged
Olive View Hospital
Aerial view of damage to
San Fernando VA Hospital
• In metropolitan Los
Angeles, numerous other
active faults lie beneath
the surface. These
include left-lateral
faults such as the Santa
Monica Fault, Hollywood
Fault, Raymond Fault,
and San Jose Fault.
Reverse and thrust
faults are likewise
numerous and include
the Sierra MadreCucamonga Fault, Elysian
Park Blind Thrust Fault,
Las Cienegas Blind
Thurst Fault, and
Compton-Los Alamitos
Thrust.
It’s clear that metropolitan Los Angeles, including the Los Angeles
Basin, San Gabriel Valley, and San Fernando Valley, is laced with active
faults. The majority of these faults appear to be slipping very slowly,
which suggests that the recurrence interval for earthquakes on these
faults is measured in thousands of years, in comparison with hundreds
of years for the San Andreas Fault System. The reason these faults
are so important is that earthquakes on them would affect millions of
people in Los Angeles. There is evidence from the Sierra Madre Fault
that larger earthquakes than those of the Coalinga, Whittier, San
Fernando and Northridge earthquakes – as large as M 7.6 – are
possible. Such an event would be catastrophic for southern California.
The situation in the Ventura Basin is a little different. Several
faults that include the Oak Ridge, San Cayetano and Santa
Susana faults – and quite possibly the Red Mountain Fault as
well – have slip rates that are quite a bit higher than those to
the south. This suggests a shorter recurrence interval for
earthquakes along these faults; a thousand years or less.