Earthquakes & Faults:
California straddles the juncture of two great crustal plates: the Pacific plate and the North
American plate. Monterey, Santa Barbara, Los Angeles and San Diego are on the Pacific plate,
which is constantly moving northwestward past the North American plate. The North American
plate includes the rest of California east of the San Andreas Fault. The relative rate of movement
is about 2 inches (50 mm) per year. Although the San Andreas Fault is considered the boundary of
the two great crustal plates, some of the motion is taken up on faults as far away as central Utah. In
California, about 40 mm/ year of the slip occurs on the faults of the San Andreas system, and about
10 mm/year of slip occurs on faults in the Mojave Desert and Basin and Range area, east of the
Sierra Nevada (a system known as the eastern California shear zone).
The constant motion of the crustal plates causes stress in the brittle upper crust of the earth.
These tectonic stresses build up as the rocks are gradually deformed. This rock deformation, or
strain, is stored in the rocks as elastic strain energy. When the strength of the rock is exceeded,
rupture occurs along a fault. The rocks on opposite sides of the fault slide past each other
as the rocks spring back to a relaxed position. The strain energy is released partly as heat and partly
as elastic waves called seismic waves. The passage of these seismic waves produces the
ground shaking of an earthquake.
Fractures in the earth’s crust along which the rocks on one side have shifted relative to those on
the other side are called faults. The total amount of displacement along a fault may be a few inches
or many miles if it has accumulated over millions of years. Faults are more likely to have future
earthquakes if they have had more recent earthquakes along them, have had greater total
displacement, and are aligned so that movement can relieve the accumulating tectonic stresses.
There are thousands of recognized faults in California, hundreds of which have been given
formal names, but only a very small number of these pose significant hazards. The motion
between the Pacific and North American plates occurs primarily on the faults of the San Andreas
Fault system and the eastern California shear zone. Other faults have much lower rates of
movement, and correspondingly longer times between significant earthquakes.
In order to classify faults by their relative hazards, geologists have found they can recognize
faults that have been active since the last ice age (during the “Holocene,” about the last
11,000 years). These include the most active faults, and represent most of the earthquake
hazard. Faults that have ruptured to the ground surface in Holocene time are “active” and are
included in Alquist-Priolo earthquake fault zones.
There are several other definitions of activity, used for other purposes, such as the
siting and design of large dams, power plants and other facilities. Another commonly used
designation is for faults that have offset geologic units from the Quaternary period (the last
1,800,000 years). Faults that offset Quaternary units are sometimes called “potentially active.”
Determining if a fault is “active” or “potentially active” depends on geologic evidence, which
may not be available for every fault. Although there are probably still some unrecognized
active faults, nearly all the movement between the two crustal plates, and therefore
the majority of the seismic hazards, are on the well-known active faults.
Ground shaking from large earthquakes is responsible for most of the damage caused by earthquakes. Damage to structures
is related to the type and quality of construction, and foundation materials. Building codes have been periodically
revised to account for our current understanding of how earthquake shaking can damage buildings. Other earthquake
hazards, including the surface rupture of a fault, and liquefaction and landslides that can be caused by the shaking, are
significant hazards. California state law gives the State Geologist the responsibility of establishing zones where
detailed geologic studies must be done before development projects can begin.
HOW OFTEN DO EARTHQUAKES OCCUR?
Every day earthquakes are detected in California by sensitive seismographs that record the minute vibrations of the earth.
Each year 100 to 150 earthquakes occur in the state that are big enough to be felt, but few of these cause any damage.
Earthquakes large enough to cause moderate damage to structures in the vicinity of the epicenter—those of M5 or
larger—occur 3 or 4 times a year. The M6 Whittier Narrows earthquakes, caused by a buried thrust fault, resulted in
hundreds of millions of dollars worth of property damage. On the average of once every 2 or 3 years, a moderate
earthquake (M6 to 6.9) strikes somewhere in the state. An earthquake of this size, such as the Northridge earthquake of
17 January 1994 (M6.7, buried thrust fault) or the Coalinga earthquake of 2 May 1983 (M6.5, buried thrust fault) is
capable of causing major damage if the epicenter is near a densely populated area.
Major earthquakes (M7 to 7.9) occur in California about every 10 years on average. Two recent major earthquakes,
the Landers earthquake of June 1992 (M7.3) and the Hector Mine earthquake of October 1999 (M7.1) caused extensive
surface fault rupture, but relatively little damage because they occurred in lightly populated areas of the Mojave
Desert. Earthquakes of similar size, such as the M6.9 Loma Prieta earthquake of October 17, 1989, cause extensive
damage over large areas when they occur in densely populated regions. The two largest earthquakes in California, the
Fort Tejon earthquake of 1857 and the famous San Francisco earthquake of 1906 were similar in magnitude (M7.9 and
M7.8) and resulted from movement along the San Andreas Fault. Earthquakes of this size (M7.7- 7.9) can cause more
extensive damage over a larger area than the M7.1 - 7.4 earthquakes that have stricken California in recent decades.
Great earthquakes (M greater than 8) have not occurred in California in historic time, but one earthquake in January
1700 may have been this large. Based on Native American oral histories, tree-ring studies, geological studies that show
the uplift or subsidence of large areas of coastal land, and records of a tsunami that struck Japan and cannot be correlated
with an earthquake anywhere else around the Pacific, a great (M9) earthquake occurred January 26, 1700 on the Cascadia
subduction zone extending north from Cape Mendocino to British Columbia. An earthquake of this size is similar to the
one that struck Alaska in 1964 and is capable of extensive damage over a very broad region.
Source - California Geological Survey
Basin and Range Region
The province is characterized by interior drainage with lakes and playas, and the typical ...
The Mojave is a broad interior region of isolated mountain ranges separated by expanses of ...
San Andreas Fault
The presence of the San Andreas fault was brought dramatically to world attention on April 18, 1906, when sudden ...
The Garlock Fault is the defining fault between the Mojave Desert and Sierra Nevada, Basin and Range geomorphic provinces.
What is a fault?
A fault is a fracture in the crust of the earth along which rocks on one side have moved relative to those
on the other side. Most faults are the result of repeated displacements over a long period of time. A fault
trace is the line on the earth's surface defining the fault. An active fault is one that has ruptured in
the last 11,000 years.
What is "surface rupture" in an earthquake?
Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. Surface
ruptures associated with the 1992 Landers Earthquake, in San Bernardino County, extended for 50 miles with
displacements of an inch to 20 feet. Not all earthquakes result in surface rupture. The Loma Prieta Earthquake
of 1989 caused major damage in the San Francisco Bay Area but the movement deep in the earth did not break
through to the surface.
Fault rupture almost always follows preexisting faults, which are zones of weakness. Rupture may occur
suddenly during an earthquake or slowly in the form of fault creep. Sudden displacements are more damaging
to structures because they are accompanied by shaking.
Fault creep is the slow rupture of the earth's crust. Examples of creep are well known along the Hayward Fault
where it crosses highly developed areas in Contra Costa and Alameda Counties. Although the Hayward Fault
ruptured suddenly in the 1868 earthquake, it also exhibits slow surface creep which offsets and deforms
curbs, streets, buildings, and other structures that lie on top of the fault.
Types of Faults
The three fundamental fault types are normal, reverse, and strike-slip. Normal faults involve a ...