 |
Part B: Background Information
EARTHQUAKES |
To gain a better understanding of earthquakes, read through the following list
of Frequently Asked Questions and responses.
Page Index:
Beliefs:
- Q: Is there a time pattern to when an earthquake occurs?
A: No. earthquakes occur at all times of day. There have been damaging
earthquakes both in the a.m.
and the p.m.
- Q: Is there such a thing as "earthquake weather"?
A: No. Many people believe earthquakes are more common in certain kinds of weather. In fact,
no correlation with weather has been found. Earthquakes begin many kilometers below
the region affected by surface weather. [People tend to notice earthquakes that fit the pattern
and forget the ones that don't.
- Q: Will California eventually fall into the ocean?
A: No. The motion of plates will not make California sink--California is moving horizontally
along the San Andreas fault and up around the Transverse Ranges.
- Q: Are earthquakes really on the increase?
A: We continue to hear from people throughout the world that
earthquakes are on the increase,
and it may seem that we are having more earthquakes. In fact, earthquakes of magnitude 7.0 or greater
have remained fairly constant throughout this century and according to records have
actually seemed to decrease in recent years. EXPLANATION: In the last 20 years we
have been able to locate more earthquakes yearly because of the increase in the number of
seismograph stations in the world and improved global communications. [e.g., 1931
there were 350 stations operating in the world, today more than 4,000 stations and
the data comes in rapidly from these stations by telex, computer and satellite. This
increase has help us and other seismological centers to locate many small
earthquakes which
were undetected in earlier years.
Faults:
- Q: What is a fault?
A: A fault is a thin zone of crushed rock between two blocks of rock, and can be any
length, from centimeters to thousands of kilometers. It 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 displacement over a long period of time.
- Q: Name the 3 different kinds of faults?
A: (1) Normal, dip-slip fault. The fault plane of a normal fault dips away from the
uplifted crustal block. Faulting occurs in response to extension. (2) Reverse, dip-slip
fault. The fault plane of a reverse fault dips beneath the uplifted crustal block.
Faulting occurs in response to compression. (3) Strike-slip fault. Crustal blocks
slide past each other. The slip may be left lateral or right lateral.
- Q: What is a thrust fault?
A: A thrust fault is a reverse fault with a gently incline, or low angle of dip. If
the slip on a fault is partly strike-slip and partly dip-slip, the fault is called
an oblique-slip fault.
- Q: What happens to a fault when an earthquakes occurs?
A: Earthquakes occur on faults. When an EQ occurs on one of these faults, the rock on one side
of the fault slips with respect to the other. The fault surface can be vertical,
horizontal, or at some angle to the surface of the earth. The slip direction can
also be at any angle.
- Q: What are the names of the 2 basic earthquakes?
A: Strike-slip earthquakes--occurs on an approximately vertical fault plane as the rock on one
side of the fault slide horizontally past the other. Dip-slip earthquake --
the fault is at
an angle to the surface of the earth and the movement of the rock is up or down.
- Q: How do we know a fault exists?
A: (1) if the earthquake left surface evidence, such as surface ruptures or fault scarps (cliffs
made by earthquakes); (2) if a large earthquake has broken the fault since we began instrumental
recordings in 1932; and (3) if the faults produces small earthquakes that we can record with
the denser seimographic network established in the 1970s.
- Q: What is "surface rupture" in an earthquake?
A: Surface rupture occurs when movement on a fault deep within the earth breaks through
to the surface. NOT ALL earthquakes result in surface rupture. 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.
- Q: What is meant by "fault creep?"
A: Fault creep is the slow movement of faults in 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.]
General Info:
- Q: What is an earthquake?
A: An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer
push the sides of the fault together. Stress builds up and the rocks slip suddenly,
releasing energy in waves that travel through the rock to cause the shaking that
we feel during an earthquake.
- Q: What is the cause of an earthquake?
A: An earthquake occurs when plates grind and scrape against each other. In California there
are 2 plates the Pacific Plate and the North American Plate. The Pacific Plate consists
of most of the Pacific Ocean floor and the California Coast line. The North American
Plate comprises most the North American Continent and parts of the Atlantic Ocean
floor. These primary boundary between these 2 plates is the San Andreas fault. The
San Andreas fault is more than 650 miles long and extends to depths of at least 10
miles. Many other smaller faults like the Hayward (Northern California) and the San
Jacinto (Southern California) branch from and join the San Andreas fault zone. The
Pacific plate grinds northwestward past the North American Plate at a rate of about
2 inches per year. Parts of the San Andreas fault system adapt to this movement by
constant "creep" resulting in many tiny shocks and a few moderate earth
tremors. In other areas where creep is NOT constant, strain can build up for hundreds
of years, producing great earthquakes when it finally releases.
- Q: Can we prevent earthquakes?
A: No. An earthquake can occur virtually anywhere at any time. They cannot be prevented, however,
damage, destruction and loss of life can be significantly reduced but because Mother
Nature provides us with regular reminders of the dangers, we can prepare and reduce
loss of life, injuries and property damage if everyone sufficiently prepared themselves,
their homes, work places and communities for a major earthquake.
- Q: What is the hypocenter?
A: The point where the earthquake rupture begins, usually deep down on the fault.
- Q: What is the epicenter?
A: The point on the surface directly above the hypocenter.
- Q: What is the name of the zone of earthquakes surrounding the Pacific Ocean?
A: The zone is called the Circum-Pacific belt--about 90% of the world's
earthquakes occur there.
The next most seismic region (5-6% of Earthquakes) is the Alpide belt (extends from
Mediterranean region, eastward through Turkey, Iran, and northern India.
- Q: How is the media currently dealing with Earthquakes ?
A: The press, radio, and television are doing an excellent job of alerting the public
to the hazard through news, reports, feature articles and television specials--this
has caused an increased awareness of the threat that has prompted many people to
seek additional info about earthquakes and to ask questions about what they should do. Overall,
increased awareness will help to reduce death, injuries and property damage in the
next big earthquake.
- Q: Why are we having so many earthquakes?
A: There is nothing unusual about what's happening, it's perfectly natural. Many parts
of the U.S. are known to be highly seismically active. Sometimes there is a short-term
increase in seismicity in that region, in an area where we commonly see quakes, just
usually not so many at once. These types of increases are not usually alarming or
cause for concern. It is a high level of activity that has since slowed and the norm
is just an occasional earthquake.
- Q: How many earthquakes does the National Earthquake Information Center report yearly?
A: The NEIC now locates about 12,000 to 14,000 earthquakes yearly or approximately ~35/day.
According to long-term records how many earthquakes are expected yearly? According to long-term
records (since about 1900) we expect about 18 major earthquakes (7.0-7.9) and one great
earthquake (8.0 or above) in any given year. [see National Earthquake Information Center
information.] Records show that 1992 (23 earthquakes) was the first time that we have reached or exceeded
the long-term average number of major earthquakes since 1971 (19 earthquakes
/ 1 major earthquake).
Geologic Hazards:
- Q: Who do I ask about earthquake insurance, ground shaking maps, specific soil sites, property
on fault zones, etc.?
A: Because soil varies from site-to-site it's recommended that the property be evaluated
by a professional geologist or geotechnical engineer [Some counties have a geologist
on staff]. Local government should have copies of updated hazard maps. (1) CALL the
USGS Earth Sci Information Center (415) 329-4390 for maps, books, reports, etc. (2)
CALL your Local Government Planning Department [San Mateo County Planning and Bldg.
Division--County Ofc. Bldg, 590 Hamilton, RWC, 363-4161] (3) CALL California Division
of Mines and Geology, Berry Street, Suite #210, San Francisco, CA 94107, 415-904-7707
or CDMG Library and Information Desk, Sacramento 916-327-1850 http://www.consrv.ca.gov/dmg/shezp
To purchase earthquake fault Hazard Zone maps [Special Publication #42] BPS Reprographic Services
149 Second Street SF 94105 415-512-6550 (4) CALL Association of Bay Area Governments
[PO Box 2050, Oakland, 94604-2050; 510-464-7900 (email: shaky@abag.ca.gov) http://www.abag.ca.gov
(5) Ask the property owner or real estate agent to see any geologic report prepared
for the site.
- Q: Who do I talk to about volcanic eruptions over the last year?
A: Volcano Hotlines [recordings]: HVO 808-967-7977 Mt. St. Helens 303-273-8600 (Steve
Brantley) Cascades Akutan Volcano Alaska 907-786-7478 http://vulcan.wr.usgs.gov
Ground Shaking:
- Q: What kind of damage does ground shaking cause?
A: Ground shaking causes 99% of the earthequake damage to California homes.
- Q: What system does the USGS use to characterize the severity of ground shaking
at different location?
A: The USGS uses a scale based on ground acceleration (as a percentage of the acceleration
due to the force of gravity) and the Modified Mercalli Intensity Scale (Mercalli
Scale). The 12-level intensity scale represents the effects of the shaking on people,
manmade structures, and on the landscape by a number. Seismologists use the list
to represent the severity of ground shaking in a community or part of a community
by a number. List of 1-12 levels of the Modified Mercalli Intensity Scale of 1931
(Abridged; Wood and Neumann, 1931)? See Earthquakes & Volcanoes, v. 25, no. 2,
1994, p. 87 for further details.
- Q: What is an isoseismal map?
A: A map showing intensities at individual locations may be contoured with isoseismal
lines, to provide a representation of the broad variations of the shaking over the
region surrounding the earthequake source. An isoseismal map shows the different levels of
intensity experienced in the affected area.
- Q: How do you measure the shaking that you feel during an earthquake?
A: What you feel is very complex--hard, gentle, long, short, jerky or rolling--and not
describable with one number. Motions are described by the PEAK VELOCITY (how fast
the ground is moving); PEAK ACCELERATION (how quickly the speed of the ground is
changing); FREQUENCY (energy is released in waves and these waves vibrate at different
frequencies just like sound waves); and DURATION (how long the strong shaking lasts).
- Q: What 3 factors primarily determine what you feel in an earthquake?
A: (1) Magnitude (you feel more intense shaking from a big earthquake than from a small one;
big earthquake also release their energy over a larger area and for a longer period of time.
In most cases, only 10-15 seconds of shaking that originate from the part of the
fault nearest you will be very strong; (2) distance from the fault (earthquake waves die
off as they travel through the earth so earthquake shaking becomes less intense farther from
the fault), and (3) local soil conditions (certain soils greatly amplify the shaking
in an earthquake. Seismic waves travel at different speeds in different types of rocks. Passing
from rock to soil, the waves slow down but get bigger. A soft, loose soil will shake
more intensely than hard rock at the same distance from the same earthquake. The looser and
thicker the soil is, the greater the amplification will be, (eg, Loma Prieta
earthquake damage area of Oakland and Marina (SF) were 100 km (60 mi) and most of the Bay Area
escaped serious damage.
Intensity:
- Q: What is earthquake intensity?
A: Earthquake intensity is a measure of the effects of ground shaking at a specific locality.
(See the discussion of intensity and magnitude under Measuring Earthquakes above).
Earthquake Hazards:
- Q: What are some of the factors you should consider when assessing potential hazards
for a particular area?
A: Proximity to active earthquake faults; Seismic history of the region (frequency of
earthquake; time
since last earthquake; Bldg. construction (type of bldg. and foundation; architectural layout;
materials used; quality of workmanship; extent to which earthquake resistance was considered
by the designer; local site conditions (type and condition of soil; slope of the
land; fill material; geologic structure of the earth beneath; annual rainfall.
Magnitude:
- What are the Earthquakes Magnitude classes?
Great = M > 8
Major = 7 M 7.9
Strong = 6 M 6.9
Moderate = 5 M 5.9
Light = 4 M 4.9
Minor = 3 M 3.9
Historic Earthquakes:
Northridge, CA
DATE: Jan. 17, 1994
TIME: 4:31 a.m.
LAT 37.036¡ N., Long 121.883¡ W.
[20 mi west-northwest of Los Angeles]
MAGNITUDE: 6.7 Mw
DURATION:
DEPTH: 11 mi
DEATHS: 57
INJURIES: 9,000
PROPERTY DAMAGE: $13-15 Bil
COMMENTS: Blind fault (occurred on an unknown "thrust fault");
7 freeway sites closed due to damage; 2 hospitals closed
150,000 w/o water; 40,000 w/o natural gas; 25,000 people left homeless;
18,480 injuries and 1,533 hospitalized
Loma Prieta, CA
DATE: Oct. 17, 1989 TIME: 5:04 pm PDT (15.24 seconds after)
MAGNITUDE: 7.1
DURATION: 15 seconds
DEATHS: 62
INJURIES: 3,757
PROPERTY DAMAGE: >$6 Bil
COMMENTS: rupture of 40 km fault stretch
(25 mi [40 km] segment of San Andreas fault SW of San Jose slipped ~ 7 ft];
SF Bay Bridge unusable for 1 month; 18,306 homes damaged;
2,575 business damaged; 12,053 people displaced
San Francisco, CA 1906Q
DATE: April 18, 1906
TIME: 5:12 am
MAGNITUDE: 8.25
DURATION: 40 seconds
DEATHS: 700-2,500 people (debated)
PROPERTY DAMAGE: $
COMMENTS: 400 km fault segment released;
San Andreas fault slipped as much as 15 ft along 270 mi (434 km) segment
from south of San Jose to northwestward to Cape Mendocino
Measuring Earthquakes:
- Q: How are earthquakes recorded?
A: By a seismographic network. Each seismic station in the network measures the movement
of the ground at the site. The slip of a block of rock over another in an
earthquake releases
energy that makes the ground vibrate. That vibration pushes the adjoining piece of
ground and causes it to vibrate, and thus the energy travels out from the
earthquake in a
wave.
- Q: What is the most common method used to measure an earthquake?
A: Magnitude is the most common measure of earthquake size (used to tell the public). Magnitude
is a measure of the energy produced by an earthquake and is not what you feel during the
event. In the 1930, Beno Gutenberg and Charles Richter borrowed the idea from astronomers.
- Q: What is moment magnitude?
A: Moment is a physical quantity proportional to the slip on the fault times the area
of the fault surface that slips; it is related to the total energy released in the
earthquake. The moment can be estimated from seismograms (and also from geodetic measurements).
The moment is then converted into a number similar to other earthquake magnitudes
by a standard formula. The result is called the moment magnitude. The moment magnitude
provides an estimate of earthquake size that is valid over the complete range of
magnitudes, a characteristic that was lacking in other magnitude scales.
- Q: What is the Mercalli Scale of earthquake intensity based on?
A: The Mercalli Scale is based on observable earthquake damage. From a scientifc standpoint,
the Richter scale is based on seismic records while the Mercalli is based on observable
data which can be subjective. Thus, the Richter scale is considered scientifically
more objective and therefore more accurate. For example a level I-V on the Mercalli
scale would represent a small amount of observable damage. At this level doors would
rattle, dishes break and weak or poor plaster would crack. As the level rises toward
the larger numbers, the amount of damage increases considerably. The top number,
12, represents total damage.
- Q: What is the difference between intensity scales and magnitude scales?
A: Intensity scales, like the Modified Mercalli Scale and the Rossi-Forel scale, measure
the amount of shaking at a particular location. So the intensity of an earthquake
will vary depending on where you are. Sometimes earthquakes are referred to by the
maximum intensity they produce. Magnitude scales, like the Richter magnitude and
moment magnitude, measure the size of the earthquake at its source. So they do not
depend on where the measurement is made. Often, several slightly different magnitudes
are reported for an earthquake. This happens because the relation between the seismic
measurements and the magnitude is complex and different procedures will often give
slightly different magnitudes for the same earthquake.
Probability:
- Q: Does the USGS or Caltech predict earthquakes?
A: No. Neither the USGS or CalTech or scientists have ever predicted a major
earthquake (Parkfield
was not a major earthquake). Nor do they know how or expect to know how any time
in the foreseeable future. However, based on scientific data scientists estimate
that over the next 30 years the probability of a major earthquake ocuring in the SF Bay
area is 67% and 60% in Southern California.
- Q: Are probabilities the same as prediction?
A: No. Probabilities are estimated from the rate of aftershocks and these are sometimes
confused with the prediction of a particular event.
- Q: What is the probability that an earthquake will occur in the Bay Area?
A: Within the next 30 years the probability is 67% that an earthquake measuring M>6.7 will
occur in the Bay Area. The probability in Southern California is 60%.
Safety:
- Q: Is it true that there is nothing I can do with regards to earthquakes?
A: NOT TRUE. Earthquakes can be terrifying, and while no one can control earthquakes,
each of us can take actions to reduce the damage they do. We hope to provide concerned
citizens, like yourself, with some basic understanding and preventive measures to
reduce future damage.
- Q: During an earthquake should you head for the doorway?
A: Only true if you live in an old, unreinforced adobe house. In modern homes doorways
are no stronger than any other parts of the house and usually have doors that will
swing and can injure you. YOU ARE SAFER PRACTICING THE DUCK, COVER, AND HOLD under
sturdy piece of furniture.
- Q: What emergency supplies do I need?
Answer:
- Fire extinguisher;
- Adequate supplies of medications that you or family members are taking;
- Crescent and pipe wrenches to turn off gas and water supplies;
- First-aid kit and handbook;
- Flashlights with extra bulbs and batteries;
- Portable radio with extra batteries;
- Water for each family member for at least 3 days (allow at least 1 gallon per
person per day) and purification tablets or chlorine bleach to purify drinking water
from other sources;
- Canned and package foods, enough for several days and a MECHANICAL can opener.
Extra food for pets if necessary;
- Camp stove or barbecue to cook on outdoors (store fuel out of the reach of children);
- Waterproof, heavy-duty plastic bags for waste disposal.
- Q: What should I do during an earthquake?
Answer:
- If you are INDOORS--STAY THERE! (Get under a desk or table and hang on to it,
or move into a hallway or get against an inside wall. STAY CLEAR of windows, fireplaces,
and heavy furniture or appliances. GET OUT of the kitchen, which is a dangerous place
(things can fall on you). DON'T run downstairs or rush outside while the bldg is
shaking or while there is danger of falling and hurting yourself or being hit by
falling glass or debris.
- If you are OUTSIDE-- get into the OPEN, away from bldgs, pwr lines, chimneys,
and anything else that might fall on you.
- If you are DRIVING--stop, but carefully. Move your car as far out of traffic
as possible. DO NOT stop on or under a bridge or overpass or under trees, light posts,
power lines, or signs. STAY INSIDE your car until the shaking stops. When you RESUME
driving watch for breaks in the pavement, fallen rocks, and bumps in the road at
bridge approaches.
- If you are in a MOUNTAINOUS AREA--watch out for falling rock, landslides, trees,
and other debris that could be loosened by quakes.
- Things NOT to do during earthquake?
- DO NOT turn on the gas again if you turned it off; let the gas company do it;
- DO NOT use matches, lighters, camp stoves or barbecues, electrical equipment,
appliances UNTIL you are sure there are no gas leaks. They may create a spark that
could ignite leaking gas and cause an explosion and fire;
- DO NOT use your telephone, EXCEPT for a medical or fire emergency. You could
tie up the lines needed for emergency response. If the phone doesn't work send someone
for help;
- DO NOT expect firefighters, police or paramedics to help you. They may not be
available.
- Q: What to expect in the home when earthquake occurs? How to identify it? What can be done?
Answer:
Contents of home may be damaged and can be dangerous:
- Shaking can make light fixtures fall, refrigerators and other large items move
across the floor, and bookcases and television sets topple over. IDENTIFY: Look around
your house for things that could fall or move;
- Ask yourself if your cupboard doors fly open (allowing dishes to shatter on the
floor);
- Is TV and stereo fastened down and are shelves fastened to wall? Do you have
hanging plants or light fixtures that might fall? Is there a heavy picture or mirror
on the wall over your bed?
WHAT CAN BE DONE: You can install door latches, braces and fasteners to fix most
of these hazards yourself.
- Q: What do you do AFTER an earthquake?
Answer:
- WEAR STURDY SHOES to avoid injury from broken glass and debris. Expect aftershocks:
- CHECK FOR INJURIES (if a person is bleeding, put direct pressure on the wound,
use clean gauze or cloth if available; If a person is not breathing, administer CPR;
DO NOT attempt to move seriously injured persons unless they are in further danger
of injury; COVER injured persons with blankets to keep themwarm; SEEK medical help
for serious injuries;
- CHECK FOR HAZARDS (FIRE HAZARDS--put out fires in your home or neighborhood immediately,
and/or call for help; GAS LEAKS--shut off main gas valve ONLY if you suspect a leak
because of broken pipes or odor; DAMAGED ELECTRICAL WIRING--Shut off power at the
control box if there is any danger to house wiring; DOWNED OR DAMAGED UTILITY LINES--do
not touch downed power lines or any objects in contact with them; SPILLS--clean up
any spilled medicines, drugs, or other harmful materials such as bleach, lye, gas;
DOWNED OR DAMAGED CHIMNEYS--Approach with caution--don't use damaged chimney (it
could start fire or let poisonous gases into your house; FALLEN ITEMS--beware of
items tumbling off shelves when you open doors of closets and cupboards;
- CHECK FOOD AND WATER SUPPLIES--Do not eat or drink anything from open containers
near shattered glass; If power is off, plan meals to use up foods that will spoil
quickly or frozen foods (food in the freezer should be good for at least a couple
of days; Don't light your kitchen stove if you suspect a gas leak; USE BBQ or camp
stoves, outdoors only for emergency cooking; If your water is off you can drink supplies
from water heaters, melted ice cubes or canned vegetables (AVOID drinking water from
swimming pools or especially spas--it may have too many chemicals in it to be safe.)
- Q: What are the steps to earthquake safety/awareness?
Answer:
- Estimate what earthquakes of what size are likely to occur (geology);
- Given the earthquake size we then estimate what the shaking will be (seismology);
- Given the shaking we estimate the response of different types of buildings
(earthquake
engineering). Only with all these steps can we take steps as society to enact bldg.
codes and retrofitting programs to make our community safer.
Seismographs:
- Q: What are seismographs?
A: Seismographs are instruments used to record the motion of the ground during an
earthquake--installed
in the ground throughout the world and operating as a seismographic network; 1st
developed ~1890.
- Q: What are seismograms?
A: The records (paper copy) produced by seismographs used to calculate the location
and magnitude of an earthquake. Shows how the ground moves with the passage of time. SEISMO
How do you read seismograms? The HORIZONTAL axis = time (measured in seconds); VERTICAL
axis= ground displacement (usually measured in millimeters) When there is NO
earthquake reading
= straight line except for small wiggles caused by local disturbance or "noise."
- Q: What is a seismometer?
A: An internal part of the seismograph which may be a pendulum or a mass mounted on
a spring.
- Q: How is the movement of the seismometer converted into a seismogram?
A: There are several ways:
- a pen drawing an ink line on paper revolving on a drum;
- a light beam making a trace on a moving photographic film or
- electromagnetic system generating a current that is recorded electronically on
tape.
Source: United States Geological Survey (USGS): http://wr.usgs.gov/more/eqfaq.html
Copyright ©
1997 Regents of the University of California
