Earthquake Risk
in New Jersey
by Daniel R. Dombroski, Jr.
 in Acrobat™ PDF
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What is an earthquake?
In New Jersey earthquakes usually occur when slowly accumulated
strain within the Earth's crust is suddenly released along
a fault. The
energy from this movement travels as seismic waves along
the ground
surface and within the crust. The arrival of this released
energy is felt
as an earthquake.
How are earthquakes measured?
The measure of an earthquake's strength is expressed as
magnitude,
and is determined by using an instrument called a seismograph.
Magnitude values are expressed according to a scale in which
an
increase of 1 represents a 10--fold increase in amplitude
of the seismic
wave.
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A 10--fold increase in amplitude represents about a 32--fold increase
in energy released for the
same duration of shaking. The best known magnitude scale is one
designed by C.F. Richter in 1935 for
west coast earthquakes.
In New Jersey, earthquakes are measured with seismographs operated
by the Lamont--Doherty Earth
Observatory of Columbia University and the Delaware Geological
Survey.
An earthquake's intensity is determined by observing its effects
at a particular place on the Earth's
surface. Intensity depends on the earthquake's magnitude, the
distance from the epicenter, and local
geology. These scales are based on reports of people awakening,
felt movements, sounds, and visible
effects on structures and landscapes. The most commonly used scale
in the United States is the
Modified Mercalli Intensity Scale,
and its values are usually reported in Roman numerals
to distinguish
them from magnitudes.
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Past damage in New Jersey
New Jersey doesn't get many earthquakes, but it does get some.
Fortunately most are small. A few
New Jersey earthquakes, as well as a few originating outside the
state, have produced enough damage
to warrant the concern of planners and emergency managers.
Damage in New Jersey from earthquakes has been minor: items
knocked off shelves, cracked plaster
and masonry, and fallen chimneys. Perhaps because no one was standing
under a chimney when it fell,
there are no recorded earthquake--related deaths in New Jersey.
We will probably not be so fortunate
in the future.
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Area Affected by Eastern Earthquakes
Although the United States east of the Rocky Mountains
has fewer
and generally smaller earthquakes than the West, at least
two factors
increase the earthquake risk in New Jersey and the East.
Due to
geologic differences, eastern earthquakes effect areas ten
times larger
than western ones of the same magnitude. Also, the eastern
United
States is more densely populated, and New Jersey is the
most densely
populated state in the nation.
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 Geologic
Faults and Earthquakes in New Jersey
Although there are many faults in New Jersey, the Ramapo Fault,
which
separates the Piedmont and Highlands Physiographic Provinces,
is the best
known. In 1884 it was blamed for a damaging New York City earthquake
simply because it was the only large fault mapped at the time.
Subsequent
investigations have shown the 1884 earthquake epicenter was actually
located in Brooklyn, New York, at least 25 miles from the Ramapo
Fault.
However, numerous minor earthquakes have been recorded in the
Ramapo
Fault Zone, a 10 to 20 mile wide area lying adjacent to, and west
of, the
actual fault.
More recently, in the 1970's and early 1980's, earthquake risk
along the Ramapo Fault received
attention because of its proximity to the Indian Point, New York,
Nuclear Power Generating Station.
East of the Rocky Mountains (including New Jersey), earthquakes
do not break the ground surface.
Their focuses lie at least a few miles below the Earth's surface,
and their locations are determined by
interpreting seismographic records. Geologic fault lines seen
on the surface today are evidence of
ancient events. The presence or absence of mapped faults (fault
lines) does not denote either a seismic
hazard or the lack of one, and earthquakes can occur anywhere
in New Jersey.
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Frequency of Damaging Earthquakes in
New Jersey
Records for the New York City area, which have been kept for
300 years, provide good information
for estimating the frequency of earthquakes in New Jersey.
Earthquakes with a maximum intensity of VII (see table Damaging
Earthquakes Felt in New Jersey )
have occurred in the New York City area in 1737, 1783, and 1884.
One intensity VI, four intensity V's,
and at least three intensity III shocks have also occurred in
the New York area over the last 300 years.
The time--spans between the intensity VII earthquakes were 46
and 101 years. This, and data for the
smaller--intensity quakes, implies a return period of 100 years
or less, and suggests New Jersey is
overdue for a moderate earthquake like the one of 1884.
Table of Earthquake Frequency
Worldwide
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Buildings and Earthquakes
The 1995 earthquake in Kobe, Japan, is an example of what might
happen in New Jersey in a similar
quake. It registered a magnitude 7.2 on the Richter scale and
produced widespread destruction. But it
was the age of construction, soil and foundation condition, proximity
to the fault, and type of structure
that were the major determining factors in the performance of
each building. Newer structures, built to
the latest construction standards, appeared to perform relatively
well, generally ensuring the life safety
of occupants.
New Jersey's building code has some provisions for earthquake--resistant
design. But there are no
requirements for retrofitting existing buildings -- not even for
unreinforced masonry structures that are
most vulnerable to earthquake damage. Housing of this type is
common in New Jersey's crowded
urban areas. If an earthquake the size of New York City's 1884
quake (magnitude 5.5) were to
occur today, severe damage would result. Fatalities would be likely.
Structures have collapsed in New Jersey without earthquakes;
an earthquake would trigger many
more. Building and housing codes need to be updated and strictly
enforced to properly prepare for
inevitable future earthquakes.
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How to Prepare for an Earthquake
Supplies
- Maintain emergency supplies of food, water (at least one gallon
per day per person), prescribed
medicines, etc., sufficient for three days.
- Rotate these with normal stocks to keep supplies fresh.
- Replace water every 6 months.
Emergency items
- Emergency items to have on hand should include:
- Fire extinguishers
- First--aid supplies and manual
- Battery--powered radio
- Flashlights with extra batteries.
(Replace or rotate batteries periodically)
- Nonelectric can opener
Secure unstable furniture and other items
- Secure unstable furniture and heavy items that might fall
in a quake (book cases, files, shelving,
TV's, computers, vases, etc.).
- Put heavy and large objects on lower shelves.
- Store glass, china and bottled foods in low, closed cabinets
with latches.
- Heavy pictures and mirrors should be hung away from beds,
chairs and couches.
- Tightly fasten overhead light fixtures.
- Strap water heaters and furnaces to walls, because the plumbing
is not strong enough to support
them in an earthquake. A broken gas pipe could lead to a fire.
- Keep pesticides, herbicides and flammable items in closed,
latched cabinets on bottom shelves.
Identify safe places inside and outside
- Under sturdy furniture like a heavy table or desk.
- Against an inside wall.
- Away from glass that might shatter (windows, mirror, pictures)
or where heavy furniture like
bookcases could fall.
- Locate places outdoors away from buildings, trees, telephone
and electrical lines, overpasses, or
elevated roadways.
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What to do During a Strong Earthquake
Indoors
- Move away from windows, glass doors, furniture or other objects
that might fall.
- Get under a sturdy table, or under a doorway of an interior
wall.
- Stay in the building unless it is clearly unsafe, or you are
told to leave. Leaving a building during
the shaking of an earthquake is very dangerous because objects
can fall on you.
- Use stairs. Elevators may lose power or fall.
Outdoors
- Move away from buildings (glass and debris may fall).
- Move away from trees, utility poles, overhead electric wires.
- Avoid highway underpasses, and other structures that might
collapse.
In a car
- Drive to (or stay in) open areas away from danger of falling
objects, power lines, etc.
- Vacate bridges, tunnels and underpasses.
- Watch traffic, other drivers may be disoriented, or out of
control.
- Once in an open area, stay in your car.
What to do If You Think You Feel an Earthquake:
- Ask your neighbors if they felt it too.
- Check with your local police.
- Call your local newspapers and radio stations
or the
New Jersey Geological and Water Survey:
(609) 292-2576
or (609) 292-1185
or
USGS "Did You Feel It?" site.
What to do After an Earthquake
Expect and Check
- Expect aftershocks. They cause additional damage and can collapse
weakened structures.
- Expect to be on your own for some time, perhaps two days or
more.
- Check for injured or trapped. Render assistance and first
aid as needed.
- Check for gas or smoke odors.
- Check for building structural damage and general safety.
Remember
- Flipping an electric switch causes sparks that will trigger
explosions around gas leaks, as will
cigarettes, candles and matches.
- Use flashlights, they are safe.
- Remain calm, and beware of panic in crowded places.
- Open closet and cupboard doors cautiously.
- Clean up spills of hazardous liquids, such as medicines and
flammables.
- Clean up broken glass and other debris.
- Inspect chimneys carefully. Damage could lead to fire.
- Leave gas-main on unless you smell gas.
- Leave electric-main on unless you know the lines are damaged
and unsafe.
- If you turn off a utility-main after a disaster, only the
utility company is allowed to turn it on
again -- after they have determined it is safe. This may leave
you without heat or light for several
days.
Water
- Water can be obtained from canned vegetables, melted frozen
foods or ice, hot-water-heater
drain valves (even if water main is off), and from toilet tanks
(not bowls).
Pets
- There may be significant behavioral changes in pets after
an earthquake. Cats and dogs that are
normally friendly and quiet may become aggressive or defensive.
Watch them closely. Leash dogs
or place them in a fenced yard.
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Sources of Earthquake Information:
New Jersey Geological and Water Survey
State Geologist
P.O. Box 420
Mail Code:29-01
Trenton, NJ 08625-0420
Phone: (609) 292-2576 or 292-1185
FAX: (609) 633-1004
Internet: www.state.nj.us/dep/njgs/
Survey earthquake
publications:
- Digital Geodata Series DGS04-1 Earthquakes Epicentered in New Jersey
- Earthquakes in New Jersey, Dombroski, Daniel R., Jr., 1973,
revised 1977, 30 p., 1 illus.
- Catalog of New Jersey Earthquakes through 1990, Dombroski,
Daniel R., Jr., 1992, 30 p.,
2 illus. (GSR 31).
New Jersey State Police
Tom Giordano
Office of Emergency Management
Preparedness Unit
Box 7068 River Road
West Trenton, NJ 08628--0068
Phone: (609)538-6008
U.S. Geological Survey
Phone: 1-888-ASK-USGS
Internet: www.usgs.gov/ or earthquake.usgs.gov
Lamont-Doherty Cooperative Seismographic Network
Phone: 845-365-8554
Internet:
http://www.ldeo.columbia.edu/LCSN
County Offices of Emergency Management
see your local phone book's blue-pages for number.
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Glossary
Amplitude -- A seismic wave amplitude is how far the
ground shakes up and down or side to side.
To determine magnitudes, the amplitudes measured by seismographs
are adjusted for distance of the
seismograph to the epicenter.
Earthquake-- A sudden motion or trembling in the Earth
caused by the abrupt release of strain,
slowly accumulated by faulting, or volcanic activity.
Epicenter -- The point on the surface directly above
the focus.
Fault -- A fracture--surface in the Earth's crust along
which sliding motion has taken place in the past.
See also, fault line. Types of faults and motion.
Fault Line -- The trace of a fault on the surface, frequently
exposed by erosion. Often simply called a
fault.
Focus or Hypocenter -- The location of an earthquake
within the crust.
Highlands -- a relatively large area of elevated or mountainous
land standing above adjacent low
areas.
Intensity -- The severity of the effects of an earthquake.
The intensity of an earthquake is different
at different locations. Intensities are given in Roman numerals
to distinguish them from magnitudes.
Magnitude -- A measure of an earthquake's size. Magnitude
is determined by various methods.
Magnitude is an index of the amplitude of the seismic waves created
by an earthquake. An increase
of one magnitude unit represents a ten--fold amplitude increase.
This ten--fold amplitude increase
represents about a 32--fold energy increase, for the same duration
of shaking (larger quakes
generally last longer, releasing even more energy).
Modified Mercalli Intensity -- The intensity scale most
commonly used in the United States.
(see table above)
Physiographic province -- A region with similar geologic
structure and history, and whose features
and landforms differ significantly from that of adjacent regions.
Piedmont -- an area lying or formed at the base of a
mountain or mountain range.
Richter Magnitude -- The first widely used magnitude
scale. It was developed by Charles F.
Richter for west--coast earthquakes.
Return Period or Recurrence Rate -- The average amount
of time between earthquakes of similar
size and location. They are determined from our short 300--year
historical record. Return periods
of the larger, infrequent, earthquakes can be estimated from that
of the smaller ones, and sometimes
from geologic clues in sediments that were disturbed by ancient
shocks. Return periods are shorter
in the West than the East, and also shorter for smaller earthquakes.
A return period is an average,
and does not imply that earthquakes happen on any schedule.
Seismic -- caused by an earthquake
Seismograph -- An instrument that detects, magnifies,
and records vibrations of the Earth, especially
earthquakes
Tsunami -- (Japanese for harbor wave) A sea--wave caused
by an earthquake. It has a very low
profile in deep water, but grows vertically (up to 50 feet or
more) in the shallow water near land.
It often breaks, like a huge surf-wave. Popularly called a tidal
wave.
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