EARTHQUAKES
Photo: San Francisco in Flames, 5 hours after the 1906 earthquake.
INTRODUCTION
In most years more earthquakes are felt in California than any other state in the United States. To some extent this is due to the large population in California. Alaska is also earthquake prone, but there are fewer people there to report earthquakes.
California's fame as "earthquake country" results from the fact that the state straddles the juncture of two great crustal plates. The well known San Andreas fault is the common boundary between these two plates. The Pacific plate, on which sits Monterey, Santa Barbara, Los Angeles, and San Diego is moving northwestward past the North American plate, which includes all of California east of the San Andreas fault. The relative rate of movement is 5 to 6 cm (2 to 2½ inches) per year. To accommodate this steady movement, the earth's crust ruptures along zones of weakness, such as the San Andreas fault. Earthquakes are the result.
FAULTS
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 thousands of years. When displacement occurs suddenly, the movement generates waves that ripple outward through the earth producing the ground-shaking known as an earthquake.
Faults and earthquakes are the result of deep-seated forces, called tectonic stress, which gradually deform the rocks of the earth's crust until rupture occurs. The rock deformation, called strain, is largely elastic and is stored in the rocks as elastic strain energy. When the strength of the rock is exceeded, rupture occurs, usually along faults which are zones of weakness. 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.
Only a small percentage of the recognized faults in California are known to have a potential for causing earthquakes. In general, those faults which historically have been the source of earthquakes will, most likely, be the source of future earthquakes and are, therefore, called active faults. Faults that show evidence of rupture during Holocene time (the last 11,000 years) are also considered to be active. If movement during historic or Holocene time cannot be demonstrated, but movement may have occurred during Quaternary time (the last 1,800,000 years), the fault is classified as potentially active. Faults are not necessarily inactive if movement during Quaternary time cannot be demonstrated. The geologic evidence necessary to prove inactivity is difficult to obtain and in a given locality may not exist. The relative activity of most faults in California is not known.
PHOTO
Trace of the San Andreas fault through Skinner Ranch, Marin County, after the 1906 earthquake. At Skinner Ranch the average horizontal displacement was 5 m (16 feet). Photo by G. K. Gilbert.
PHOTO
Aerial view of the Owens Valley fault, running left to right across the picture. The total displacement is 9 to 15 m (30 to 50 feet) where the road crosses the scarp. The 1872 Owens Valley earthquake produced 4 m (13 feet) of net vertical offset on this fault near Lone Pine, Inyo County. Photo by David B. Slemmons.
PHOTO
Right-lateral slip (about 25 cm) on the Imperial fault following the October 1979 earthquake (M 6.5) is depicted by offset of white center stripe in road. Cumulative offset resulting from the 1979 event, May 1940 earthquake (M 6.7), and creep is documented by offset (about 1 meter) of the crack (near the center stripe) which represents the former center line of the underlying concrete road. This demonstrates the important concept that faulting generally occurs along the same trace location as in previous, recent fault events. Photo by Earl W. Hart.
Research of historical earthquakes has shown that ground shaking which accompanies large earthquakes is responsible for more damage than is ground rupture along the faults themselves. Damage to structures during earthquakes is more commonly related to the type and quality of construction and to the foundation materials on which they are built than solely to the proximity to the fault producing the earthquake. Although only a few structures have been ripped apart by fault rupture, this hazard is nevertheless very real for structures built across an active or potentially active fault.
HOW EARTHQUAKES ARE
MEASURED
Vibrations produced by earthquakes are detected, recorded, and measured by instruments called seismographs. These devices may amplify ground motions beneath the instruments to over one million times, transcribing the motion into a zig-zag trace called a seismogram. From the data expressed in seismograms, the time, epicenter, and focal depth of an earthquake can be determined, and estimates can be made of its relative size and the amount of energy that was released.
The point on the fault where rupture initiates is referred to as the focus or hypocenter of an earthquake. The hypocenter of an earthquake is described by its depth in kilometers, its map location in latitude and longitude, its time and date of occurrence, and its magnitude. The term epicenter, which is more commonly used to refer to an earthquake location, is the point on the earth's surface directly above the hypocenter. The description of an epicenter is the same as for a hypocenter except that the depth is omitted.
The severity of an earthquake is generally expressed in two ways, magnitude and intensity. The magnitude of an earthquake, as expressed by the Richter magnitude scale, is a relative measure that depends on the maximum trace amplitude registered on a standard instrument called a Wood-Anderson torsion seismograph. On this scale, the earthquake's magnitude is expressed in whole numbers and decimals. The intensity as expressed by the Modified Mercalli intensity scale, is a partly subjective measure which depends on the effects of a quake such as damage at a particular location. Although there is only one magnitude number for a selected earthquake, there may be many values of intensity.
RICHTER MAGNITUDE SCALE
The Richter magnitude scale, named after Dr. Charles F. Richter, Professor Emeritus of the California Institute of Technology, is the scale most commonly used. However, Richter magnitudes can be confusing and misleading unless the mathematical basis for the scale is understood. When an earthquake is recorded, the greatest excursion of the zig-zag trace is measured and compared with that of a standard reference earthquake, and a correction is made for epicenter to station distance. The result is a number the size of which directly corresponds to the size of the earthquake relative to the standard earthquake. The standard reference earthquake is defined in a way such that a magnitude zero earthquake produces a maximum trace amplitude of .001 millimeter at a distance from the epicenter of 100 kilometers. With appropriate distance corrections, the magnitude value is an effective means of size classification.
It is important to recognize that magnitude varies logarithmically with the wave amplitude of the quake recorded by the seismograph. Each whole number step of magnitude on the scale represents an increase of ten times in the measured wave amplitude of an earthquake. Thus, the amplitude of an 8.3 magnitude earthquake is not twice as large as a shock of magnitude 4.3 but 10,000 times as large.
Richter magnitude can also provide an estimate of the amount of energy released during the quake. For every unit increase in magnitude, there is about a 31-fold increase in energy. For the previous example, a magnitude 8.3 earthquake releases almost one million times more energy than one of magnitude 4.3.
The Richter magnitude scale has no fixed maximum or minimum; observations have placed the largest recorded earthquakes in the world at about 8.9, and the smallest at -3. Earthquakes with magnitudes smaller than about 2 are called "micro-earthquakes."
Richter magnitudes are not used to estimate damage. An earthquake in a densely populated area, which results in many deaths and considerable damage, may have the same magnitude as an earthquake that occurs in a barren, remote area, that may do nothing more than frighten the wildlife.
Figure: Diagram Showing Richter Magnitude Determination
EARTHQUAKE INTENSITY
The first scale to reflect earthquake intensities was developed by de Rossi of Italy, and Forel of Switzerland, in the 1880s and is known as the Rossi-Forel Scale. This scale, with values from I to X, was used for about two decades. A need for a more refined scale increased with the advancement of the science of seismology, and in 1902 the Italian seismologist, Mercalli, devised a new scale on a I to XII range. The Mercalli Scale was modified in 1931 by American seismologists Harry O. Wood and Frank Neumann to take into account modern structural features.
The Modified Mercalli intensity scale measures the intensity of an earthquake's effects in a given locality, and is perhaps much more meaningful to the layman because it is based on actual observations of earthquake effects at specific places. It should be noted that because the data used for assigning intensities can be obtained only from direct firsthand reports, considerable time---weeks or months---is sometimes needed before an intensity map can be assembled for a particular earthquake.
On the Modified Mercalli intensity scale, values range from I to XII. The most commonly used adaptation covers the range of intensity from the conditions of "I---not felt except by very few, favorably situated," to "XII---damage total, lines of sight disturbed, objects thrown into the air." While an earthquake has only one magnitude, it can have many intensities, which decrease with distance from the epicenter.
It is difficult to compare magnitude and intensity because intensity is linked with the particular ground and structural conditions of a given area, as well as distance from the earthquake epicenter, while magnitude depends on the energy released at the focus of the earthquake.
ILLUSTRATION: RELATIONSHIP BETWEEN EARTHQUAKE MAGNITUDE AND ENERGY
The volumes of the spheres are roughly proportional to the amount of energy released by earthquakes of the magnitudes given, and illustrate the exponential relationship between magnitude and energy. At the same scale the energy released by the San Francisco earthquake of 1906 (Richter magnitude 8.3) would be represented by a sphere with a radius of 110 feet.
HOW OFTEN DO EARTHQUAKES OCCUR?
Earthquakes are detected every day in California by sensitive seismographs, which record the minute vibrations of the earth. Each year 100 to 150 earthquakes occur in the state that are big enough to be felt by people, but few of these cause any damage. Earthquakes large enough to cause moderate damage to structures in the vicinity of the epicenter---those of magnitude 5 to 5.9 on the Richter scale---occur three or four times a year. The Santa Barbara earthquake of August 13, 1978 (a magnitude 5.1 event caused by rupture along an undetermined offshore fault) resulted in an estimated $12 to $15 million damage. The Livermore earthquake of January 24, 1980 (M 5.5, Mt. Diablo-Greenville fault) caused $3.9 million in damage. Often the epicenter is located in a remote area and little if any damage is done.
PHOTO
Masonary structures with little or no reinforcement, such as those in the Coalinga business district, usually sustain the greatest damage during an earthquake. Photo by James Stratta.
PLATE: MODIFIED MERCALLI INTENSITY SCALE OF 1931
The first scale to reflect earthquake intensities was developed by de Rossi of Italy, and Forel of Switzerland, in the 1880s. This scale, with values from I to X, was used for about two decades. A need for a more refined scale increased with the advancement of the science of seismology, and in 1902 the Italian seismologist, Mercalli, devised a new scale on a I to XII range. The Mercalli Scale was modified in 1931 by American seismologists Harry O. Wood and Frank Neumann to take into account modern structural features:
I Not felt except by a very few under especially favorable circumstances.
II Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motorcars may rock slightly. Vibration like passing of truck. Duration estimated.
IV During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V Felt by nearly everyone, many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.
VI Felt by all, many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.
VII Everybody runs outdoors. Damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars.
VIII Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motor cars disturbed.
IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.
X Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.
XI Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.
XII Damage total. Practically all works of construction are damaged greatly or destroyed. Waves seen on ground surface. Lines of sight and level are distorted. Objects are thrown upward into the air.
PLATE
COMPARISON OF
RICHTER MAGNITUDE
AND
MODIFIED MERCALLI INTENSITY
Richter Expected Modified MercalliSEE PHOTOMegnitude Maximum Intensity (at epicenter)
2 1-11 Usually detected only by instruments
3 111 Felt indoors
4 IV-V Felt by most people slight demege
5 VI-VII Felt by all: many frightened and run outdoors; damage minor to moderate
6 VII-VIII Everybody runs outdoors; damage moderate to major
7 IX-X Major damage
8+ X-XII Total And major damages
After Cherles F Richter, 1958 Elementart Seismology
Single story wood frame houses usually stand up well to earthquake shaking. Even though fault rupture extends beneath this structure, it did not collapse during the 1971 San Fernando earthquake. The garage, however, with the large opening in front, did yield to the horizontal shaking. Photo by R. Castle, courtesy of USGS.
On the average of once every two or three years, a moderate earthquake (M 6 to 6.9) strikes somewhere in the state. An earthquake of this size, such as San Fernando earthquake of February 9, 1971 (M 6.4, San Fernando fault) or the Coalinga earthquake of May. 2, 1983 (M 6.5, previously unknown fault) is capable of causing major damage if the epicenter is near a heavily populated area. The San Fernando earthquake caused $511 million in damage and took 64 lives.
The last major earthquake (M 7 to 7.9) in California was the Kern County earthquake of July 21, 1952 (M 7.7, White Wolf fault). Twelve people were killed and damage was estimated at $50 million. Earthquakes of magnitude 7 or larger cause extensive damage over large areas.
Since 1850 there have been three great earthquakes (M 8 or greater) in California. Both the Fort Tejon earthquake of 1857 and the famous San Francisco earthquake of 1906 resulted from movement along the San Andreas fault. The Owens Valley earthquake of 1872 was produced by movement along the Owens Valley fault on the east side of the Sierra Nevada Mountains. This great earthquake killed twenty-seven people and damaged or destroyed nearly every building in the towns of Lone Pine and Independence. This earthquake occurred at 2:30 in the morning, and Californians from Red Bluff to San Diego were awakened by the shaking of their beds.
EARTHQUAKE SAFETY TIPS
Although there is no way to eliminate all earthquake dangers, injury and damage can be reduced substantially If the following steps are taken before, during and after the quake.
BEFORE
1. Store emergency supplies: food, water, first aid kit, flashlight and battery-powered radio.
2. Take a practical first aid course.
3. Locate main switches and valves that control the flow of water, gas, and electricity into your house. Know how to operate them.
4. Support community programs that inform the public and emergency personnel about earthquake preparedness.
5. Take action to help strengthen or eliminate structures that are not earthquake-resistant.
6. Support "parapet ordinances" that would remove dangerous, unreinforced overhangs and cornices from buildings.
7. Support building codes that require earthquake-resistant construction and careful foundation preparation and grading.
8. Support land-use policies that recognize and allow for the potential dangers of active fault zones.
9. Heavy furniture above the fifth floor in tall buildings should be bolted to the floor.
10. Require guard rails across the inside of plate glass windows that extend to the floor.
11. Support basic research into the cause and mechanism of earthquakes and fault movement.
DURING
1. Don't panic even if you are frightened.
2. If you are indoors, stay there. Get under a desk, table, or doorway.
3. Do not rush outside. Falling debris has caused many deaths.
4. Watch for falling plaster, bricks, and other objects.
5. If you are outside, move away from buildings and power lines; stay in the open.
6. If you are in a moving car, stop as soon as it is safe. Remain in the car.
AFTER
1. Check your family, or the people near you for injuries.
2. Inspect your utilities for damage to water, gas, or electrical conduits. If they are damaged, turn them off.
3. Extinguish open flames.
4. Do not use the telephone except to report an emergency.
5. Turn on your battery-powered radio for emergency information.
6. Don't go sightseeing.
7. Stay away from damaged structures; aftershocks can cause the collapse of weakened structures.
8. Stay away from beaches and waterfront areas subject to seismic sea waves (commonly called "tidal waves").
From DMG NOTE 3, Faults and earthquakes;
and DMG NOTE 23, How earthquakes are measured.