ARE YOU PREPARED?
Living with earthquakes: a fact of life on the North Coast
What to do right now to prepare
How to reduce earthquake damage
Why we have earthquakes on the North Coast
Potential earthquake hazard along the North Coast
By taking action now, we can drastically reduce future earthquake losses.
For additional copies, write to:
Humboldt Earthquake Education Center HUMBOLDT STATE UNIVERSITY Arcata, California 95521-8299
The California Office of Emergency Services recommends
Injuries and deaths during earthquakes are caused by falling objects and collapsing structures. Show children safe areas to duck and cover. Practice counting how many seconds your test earthquake lasts. This will help you keep calm when a real earthquake strikes.
If an earthquake hits during the day, family members may be separated for hours or even days. Your local chapter of the American Red Cross can help you develop a plan at home, at work, and in your neighborhood. A family plan should include:
Falling objects and toppling furniture can be dangerous and expensive to replace or repair.
Free-standing wood burning stoves pose an additional risk in an earthquake to residents on the North Coast. A Humboldt State University survey found that over half the wood stoves in the area near the epicenter moved during the April, 1992, earthquakes and several fell over. Heavy objects such as stoves are actually more likely to move during strong ground shaking than lighter objects. Fire codes leave the stoves unsupported on all four sides and vulnerable to sliding or overturning in an earthquake. If the stove were to tip and/or separate from the stove pipe, cinders or sparks might easily cause a fire in the home.
In order to reduce the potential fire hazard following an earthquake, the stove should be anchored to the floor and stove pipe sections secured. It is important that the seismic anchors or braces do not conduct heat from the stove. Although there are many types of stoves in use, the following recommendations can be used for common installations:
Many residents of the North Coast have above-ground propane tanks. These tanks may move, slide or topple during heavy ground shaking and are potentially damaging unless both the tank and the pipe are properly secured. Gas leaks are frequently the cause of earthquake related fires. The following recommendations can be used to reduce the post-earthquake fire hazard associated with propane tanks.
Earthquakes may be traumatic events for all of us, but they are especially frightening for children, who may have to leave their homes and all that is familiar to them. A child does not usually understand such events and feels anxious and confused. After an earthquake, a childs fears are those of reoccurrence, injury, death, or of being alone, separated from the rest of the family. Aftershocks can increase these fears.
Parents sometimes ignore the emotional needs of a child once assured of their physical safety. A childs persistent fears may generate disruptive behavior, surprising and frustrating a parent who is trying to continue with the daily family routine. How a parent can help:
To find out more about how children react to emergencies, and what you can do to help, contact your local Mental Health Department.
Most people on the North Coast are safe at home if they live in a well braced wood-frame building of one or two stories. These buildings are unlikely to collapse completely during earthquakes. Common damage in these structures is light cracking of interior walls, cracking of brick chimneys in older homes may be required to prevent toppling during earthquakes. Have your chimney inspected by a qualified professional before using the fireplace after the earthquake.
Some wood-frame buildings can be hazardous, especially those built before 1940. Older wooden structures can fail at or near ground level if not adequately bolted to the foundation, or if the pier-and-post foundation or short cripple walls (often found between the foundation and the first floor) are not adequately braced. Your local community planning or building inspection office has information on adding foundation bolts and bracing cripple walls. Correcting these problems will drastically reduce earthquake risk in older homes.
Special considerations are needed for mobile homes and modular buildings not attached to permanent foundations. These structures can slide off their foundations is not properly secured to resist horizontal motion. The California Department of Housing and Community Development recommends the tall manufactured homes be equipped with certified Earthquake Resistant Bracing Systems (ERBS). For more information regarding mobile home support systems contact the
Department of Housing and Community Development (916) 255 - 2501
Modern public elementary and high schools and community college buildings must meet Field Act standards for seismic safety and have generally performed well during earthquakes. Following severe damage to many schools during the Long Beach earthquake of 1933, the Field Act was passed, requiring special seismic design standards for public school buildings. The provisions of the Field Act do not apply to colleges and private schools. Ask school officials whether your school buildings have been evaluated for earthquake safety recently.
Earthquakes will happen again here on the North Coast. The effects need not be disastrous. Most of the damage to wood-frame houses in the April, 1992, earthquakes could have been prevented.
If you have earthquake damage, NOW is the time to strengthen your home correctly. If your home escaped damage, NOW is the time to evaluate you home and install strengthening measures -- before you forget, and before the next earthquake. Contact a licensed professional about making the necessary changes to you home or office. Contact your local building department to modify details to fit local building codes.
How do you locate professionals to advise you on the resistance of your building to earthquake shaking? Who should perform the needed construction?
Civil and structural engineers and architects are trained to provide such information about structures. Geologists, foundation engineers, and geotechnical engineers are trained and licensed to evaluate soil conditions and recommend appropriate action. A contractor has to implement the detailed plans and specifications prepared by an architect or engineer.
A good place to start is to call a professional organization and ask about the types of work that might be required; how to select an engineer, geologist, or architect; and a list of members in your area.
Contact several firms or individuals to determine whether they do the types of work you need. Make sure the firm has the necessary licenses and has experience in strengthening structures to resist earthquake shaking. Check to see how satisfied other clients were.
Recognize that the quality of the advice given and the work performed, as well as the price you pay, may depend on the care you take in making your selection.
Become informed. Even if you do not understand the technical details, ask enough questions to understand the concepts and relative importance of the issues involved. You have a right to understand what needs to be done and why.
State and federal agencies do not inspect individual buildings. Your local building department may be willing to inspect your building, but they are not authorized to recommend actions to be taken.
Earthquake damage is typically concentrated in locations that can be identified in advance:
Failure of the ground during an earthquake can happen in many ways. Faults may break the ground as they slip during an earthquake. Steep or unstable hill slopes may slide, especially is an earthquake hits during the rainy season. Cracks may open along ridge crests close to the earthquake epicenter. Soft ground - such as the margins of Lake Earl and Humboldt Bay, or land near the lagoons - may amplify the ground acceleration and settle during shaking. The ground may fail if a sandy layer at shallow depth is saturated with water and flows like a liquid during the earthquake shaking.
Communities most vulnerable to earthquake damage can be identified by studying the locations of active faults and the damage patterns of past earthquakes. The map on pages 12 and 13 provides a regional overview of areas where the greatest hazards exist. Soft materials such as mud and beach sands (shown in magenta) are likely to experience stronger shaking than harder rock (shown in pink and blue) nearby. Steep areas on unstable ground (shown in yellow and green) are more likely to slide during a strong earthquake than more stable rock types.
A house built over a fault can be torn apart is the ground ruptures during an earthquake. The map on pages 12 and 14 as active by the California Division of Mines and Geology under the Alquist-Priolo Special Studies Zones Act of 1972. Local city and county planning departments have Special Studies Zone maps that will tell you if your home is on or near one of these faults. You can learn more about these zones and how to obtain maps by ordering Special Publication 42 from the California Division of Mines and Geology (listed in the reference section).
The insurance company that insures your home for fire is required to offer you earthquake insurance. The most common type of earthquake insurance is normally added as an endorsement on a standard homeowners insurance policy. Typically, there is a deductible of 5 to 10 percent or more, of the value of the home. This means that for a home currently insured at $200,000, you would have to pay $10,000 to $30,000 on damages before the insurance company would pay anything. Separate deductibles may apply to contents and structure. An important coverage is temporary living expenses. This pays for motel and meals if you have to move out of your home. There is usually no deductible on this coverage. The yearly cost of residential earthquake insurance is normally about $1.50 to $3.00 per $1,000 of coverage on the structure.
To find out more about earthquake insurance, ask your insurance agent or call the
California State Department of Insurance at (800) 927-4357.
Tsunamis are large ocean waves generated by major earthquakes beneath the ocean floor. Tsunamis caused by nearby earthquakes may reach the coast within minutes. When the waves enter shallow water, they may rise to devastating force. People on the beach or in low coastal areas need to be aware that a tsunami could arrive within minutes after a severe earthquake.
Tsunamis also may be generated by very large earthquakes far away in other areas of the Pacific Ocean. Waves caused by these earthquakes travel at hundreds of miles per hour, reaching California several hours after the earthquake . The International Tsunami Warning System monitors ocean waves after any Pacific earthquake with a magnitude larger than 6.5. If waves are detected, warnings are issued to local authorities, who can order evacuation of low lying areas if necessary.
Twenty-four tsunamis have caused damage in the United States and its territories during the last 204 years. Just since 1946, six tsunamis have killed more than 350 people and damaged a half billion dollars of property on Hawaii, Alaska and the West Coast.
Tsunamis have historically been rare in California. Since 1812, California has experienced fourteen tsunamis with wave heights greater than 3 feet; six of these were destructive. Ten of these were generated by distant earthquakes near Alaska, Chile or Japan. The worst damage resulted from the 1964 Alaskan earthquake. Four came from earthquakes nearby, one of which caused some damage.
The Cape Mendocino earthquake produced a tsunami that reached Eureka in about 20 minutes with wave heights of about one foot. The tsunami reached Crescent City in 50 minutes and was detected in Oregon, the San Francisco Bay Area, Santa Barbara, and Hawaii.
Although not destructive, the April tsunami is important because it illustrates both how quickly a wave can arrive at nearby coastal communities and how long the danger period can last. The first wave arrived at Crescent City in less than an hour, but the highest waves, about one-and-a-half-feet, arrived nearly four hours later. Abnormally large waves continued for more than eight hours. The April tsunami provided important for the National Oceanographic and Atmospheric Administration, which is currently studying how a larger tsunami may affect the North Coast.
The geology of the North Coast is distinct from the rest of California. Geologists and seismologists find this region of special interest because it coincides with a "triple junction," a place where three plates of the earth's outer shell are in contact. As a result of this unique geologic setting, the North Coast is vulnerable to several types of earthquakes from a variety of sources.
South of the triple junction, the Pacific plate is moving at a rate of about two inches per year to the northwest (relative to North America). Most of this motion takes place along the San Andreas fault system. The sliding motion is neither smooth nor constant. The motion of the plates deforms the rocks along the plate boundary until the rocks can no longer withstand the strain. Then a sudden slip along the fault releases energy, causing earthquake shaking. The relative motion of the two plates is almost entirely horizontal, as the Pacific plate moves north relative to the North American plate with each successive earthquake.
The forces are very different north of the triple junction. Here the Gorda plate and its northern extension, the Juan de Fuca plate. Where the plates collide, the Gorda plate slowly descends beneath the North American plate along the Cascadia subduction zone that extends eastward under the edge of North America.
The collision between the North American and Gorda plates has created a zone of folded and faulted crustal rocks. Along most of the Cascadia subduction zone, these deformed rocks are offshore. Near its southern end, however the zone curves onshore, exposing nine major thrust faults along the Humboldt County coastline. Thrust faults differ from the horizontally-moving San Andreas fault. The rocks on the other side. Geologists have shown that during the last million years the rocks on top of this group of North Coast thrust faults have been pushed a mile or more to the northeast relative to the rocks beneath.
Until recently, the Cascadia subduction zone was not regarded as a major earthquake threat. Indeed prior to the April 25, 1992 Cape Mendocino earthquake, the Cascadia plate boundary was not known to have produced a major earthquake during the past 150 years. New evidence, however, indicates that the subduction zone is active and capable of producing great earthquakes. Offshore studies show a zone of folds and faults that deform the youthful sea floor sediments of southern Canada, Washington, and Oregon and join up with similarly deformed rocks exposed on land in northern California. Recent geologic studies of faulting along the Humboldt County coast identified at least five large earthquakes during the past 1,700 years. Each of these earthquakes changed the elevation of large parts of the Humboldt County coast by several feet or more and may have been accompanied by movement of one or more local thrust faults.
The North Coast has accounted for about one-quarter of all the earthquake energy released in California during historic times. The size, location, and frequency of past earthquakes give an indication of what to expect in the future.
The instrumental record of earthquakes on the North Coast extends back only into the early Twentieth Century. However, it is possible to learn about older earthquakes from written accounts in newspapers, church logs, and diaries. From these accounts, seismologists can sometimes reconstruct the pattern of ground shaking and estimate the location and magnitude of early earthquakes. Although locations are imprecise, the record of damage to North Coast communities, fourteen with strength similar to the April, 1992, earthquakes.
The majority of North Coast earthquakes have been centered offshore in the southeastern portion of the Gorda plate. These earthquakes recur frequently, causing some damage to North Coast communities about every two years. Communities in the coastal region from Cape Mendocino to Eureka have been struck far more frequently than the rest of Humboldt or Del Norte Counties. Northern Humboldt County, Del Norte County and coastal communities of southern Oregon have only rarely been affected by earthquakes in historic times. However, the historic record only gives a short-term view of earthquakes. Areas not affected historically may still be at risk.
Three powerful earthquakes struck the Cape Mendocino area on April 25th and 26th, 1992 (magnitudes 7.1, 6.6, and 6.7). The earthquakes caused 356 reported injuries, destroyed 202 buildings, and damaged an additional 906 structures. Fires triggered by the second earthquake destroyed most of the Scotia shopping district. Property losses were estimated at over $60 million and President Bush declared the region a Major Disaster Area. The earthquakes triggered numerous landslides and rock falls, and caused widespread liquefaction on the Mattole and Eel River Valleys. Many roads and bridges in the Cape Mendocino region suffered damage. The earthquakes were felt from southern Oregon, to Reno, Nevada, and as far south as Salinas.
The first earthquake, on Saturday, April 25th, is particularly important because:
Historic offshore damaging earthquakes have had magnitudes between 5.0 and 7.5. They are the most likely type of earthquake to recur in the near future. A future earthquake of this type will likely affect coastal communities between Cape Mendocino and Humboldt Bay with some damage possible as far north as the southern Oregon coast. These earthquakes may damage roads, bridges, chimneys and foundations, move heavy furniture and knock many items off shelves.
Onshore earthquakes have recurred on average every 20 years and, when centered near populated areas, have caused major damage. These earthquakes historically have had magnitudes between 5 and 7.1. Geologic evidence, however, suggests this type of earthquake may have magnitudes of 7.5 or larger. Such an earthquake could cause surface rupture and would produce stronger ground shaking than any historic earthquake. The likelihood of such an earthquake in the near future is unknown but is clearly less than the likelihood of offshore earthquakes.
Studies of the amount of strain released in the 1906 San Francisco earthquake and geologic studies of even older earthquakes suggest an average interval between earthquakes on the northern segment of the San Andreas fault of several hundred years. Recurrence of a 1906 type earthquake, although not as likely as an offshore earthquake, must be considered a possibility on the North Coast.
The April, 1992, earthquake broke a small part of the Cascadia subduction zone. Rupture of a longer segment would produce a very large earthquake under the more populated areas of the coast. Geologic evidence suggests that these earthquakes could have magnitudes of 8 or greater, produce strong ground motion lasting up to a minute or more, and generate local tsunamis affecting coastal areas of northern California, Oregon and Washington. Distant tsunamis might also be generated, that could affect other coastal regions of California and large areas of the Pacific basin.
Geologic studies suggest the last large subduction zone earthquake occurred approximately 300 years ago and that such very large earthquakes recur with intervals on the order of hundreds of years. The probability of such an earthquake within the near future is not well known, but less than that of offshore earthquakes in the Gorda plate. However, the increasing weight of scientific evidence supporting the eventual recurrence of such earthquakes and the enormity of their possible effects make planning and preparedness efforts essential for individuals, communities, and all levels of government.
Governors Office of Emergency Services Bay Area Earthquake Project. MetroCenter, 101 8th Street Suite 152, Oakland, CA 94607, (510) 893-0818, TDD (510) 893-0702. Publications, videotapes, and scripted slide shows on earthquake preparedness. Free catalog.
Humboldt Earthquake Education Center. Humboldt State University, Arcata, CA 95521, (707) 826-6019. A recorded message about local, regional and distant earthquakes is updated regularly (707) 826 6020.
USGS, U.S. Geological Survey, Earth Science Information Centers Menlo Park, CA 94025, 345 Middlefield Road, (415) 329-4390. Publications and maps concerning earthquake hazards. Mail orders to USGS Books and Report Sales, P.O. Box 25425, Denver, CO 80225. For orders less than $10.00, includes $1.00 P&H.
FEMA, Federal Emergency Management Agency, Building 105, The Presidio, San Francisco, CA 94129, (415) 923-7100. Documents should be ordered from FEMA, P.O. Box 70274, Washington, D.C. 20024.
CDMG, California Department of Conservation, Division of Mines and Geology, P.O. Box 2980, Sacramento, CA 95812-2980, (916) 445-5716. Publications and maps concerning faults. Scenarios describing the likely effects of future earthquakes.
California Seismic Safety Commission, 1900 K Street, Suite 100, Sacramento, CA 95814-4186. Primarily concerned with encouraging hazard reduction and emergency planning. Information about legislation, state agency programs and unreinforced masonry building programs.
Earthquakes and Volcanoes. A bimonthly publication of the U.S. Geological Survey available yearly for $6.50 from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402, or by credit card at (202) 783-3238.
California Geology. A bi-monthly publication of CDMG, California Geology, P.O. Box 2980, Sacramento, CA 95812-2980 ($8.00 per 1 year subscription).
Earthquakes: Newly Revised and Expanded. By Bruce Bolt, W.H. Freeman, 41 Madison Ave. New York, N.Y. 1993, 317 pages.
On Shaky Ground: Americas Earthquake Alert. By John J. Nance, 1989, Avon Books, New York, 440 pages ($4.95).
Terra Non Firma. By J. M. Gere and H.C. Shah. 1984, W.H. Freeman, New York, 203 pages ($12.95).
Available in public libraries in Humboldt County and in the Humboldt State University Library
Earthquakes and Volcanoes of April 25-26, 1992, volume 23, number 3 U.S. Government Printing Office, Washington, D.C. 20402, 1992.
California Geology, March - April 1992, California Department fo Conservation, Division of Mines and Geology
The Humboldt Historian, The Humboldt Historical Society
Surviving the Big One, How to prepare for a Major Earthquake. A video developed for public television. KCET Video, P.O. Box 310, San Fernando, Ca 91341 (revised), 1 hour, (800) 228-5238 ($19.95 + $3.50 P&H if in California include Calif. sales tax).
General Preparedness Information Kit. ABAG, Publications, P.O. Box 2050, Oakland, CA 94604-2050, 1988 (ABAG P87059BAR, $2.00 including P&H).
Safety and Survival in an Earthquake. American Red Cross, 1989, 52 pages ($3.00 + $2.00 P&H from ARC).
Reducing Losses From Earthquakes Through Personal Preparedness. By W.J. Kockelman, 1984, U.S. Geological Survey Open File Report 84-765, 21 pages (USGS, $2.75).
Earthquake Ready. By Virginia Kimball, Roundtable Publishing, Inc., Santa Monica, California, 225 pages, 1992 ($14.95).
The Humboldt Earthquake, Impact on Schools. A 12 minute long video of some of our local schools after the April 25-26 earthquakes. Available through the Humboldt County Office of Education.
Earthquake Preparedness Activities for Child-Care Providers. 1989 54 pages. This guide is for child-care providers. ABAG, Publications, P.O. Box 2050, Oakland, CA 94604-2050, ($8 + $2 P&H) (P89002BAR).
Earthquakes: A Teachers Package for K-6 Grades. By the National Science Teachers Association, 1988, 280 pages ($17.50 + $2.50 P&H) (800) 722-6782.
Earthquake Ready: Preparedness Planning for Schools. June, 1990, 76 pages. For administrators, faculty, and staff. ABAG, Publications, P.O. Box 2050, Oakland, CA 95604-2050 ($7 + $3 P&H) (P90002BAR).).
Earthquake Preparedness: A Key to Small Business Survival. 1985, 8 pages. ABAG, Publications, P.O. Box 2050, Oakland, CA 94604-2050 ($3 + $1 P&H) (P87055BAR).
Earthquake Preparedness Policy: Considerations for School Governing Boards. January, 1991, 12 pages. ABAG, Publications, P.O. Box 2050, Oakland, CA 94604-2050, Oakland, CA 94604-2050 ($3 + $1 P&H) (P91001BAR).
Corporate Comprehensive Earthquake Preparedness Planning Guidelines. 1985, 48 pages, ABAG, Publications, P.O. Box 2050, Oakland, CA 94604-2050 ($2.00 + $.90 P&H) (ABAG P85055BAR).
LORI DENGLER (DENGLERL@AXE.HUMBOLDT.EDU) KATHY MOLEY (MOLEYK@AXE.HUMBOLDT.EDU)