Geologists use seismic waves to locate the center of an earthquake geologists use data from three or more data stations determined the location of the epicenter. A seismograph mmeasures the difference between the arrivals of P waves and S waves..
Two additional types of surveys, gravity and magnetic, often work hand-in-hand to characterize the subsurface. Gravity surveys identify density differences in rock formations. This is useful for identifying the depth of bedrock under unconsolidated sediment. Gravity surveys can also locate contacts between two distinct densities of rock at depth. Magnetic surveys identify magnetic contrasts in rocks. This is useful for mapping faults, folds, and rocks with more magnetic minerals. Both surveys can be conducted on the ground for high-resolution data. They can also be run from an airplane for lower resolution and greater coverage. These surveys are generally faster and cheaper to acquire than other data types. However, interpreting the data requires more training and is seldom used independently.
The Washington Geological Survey maintains a state-wide database of many types of subsurface data. Survey geologists use this data to develop and produce resource maps, to help constrain the subsurface interpretation of geologic maps, to better understand faults and earthquakes, and to produce 3D models of the geology. The subsurface data also contain the locations and information for water wells, geotechnical borings, oil and gas wells, and geothermal wells. The centralization of these data represents a significant effort that benefits the entire geologic and geophysical community in Washington.
Geologic maps are important data sources for many types of work. Certain types of rock are used for construction materials, and a geologic map shows where they are located at the surface. Other types of rock might contain valuable minerals, and a geologic map can be used as a preliminary tool for deciding where to drill or prospect.
Other types of maps used in the study of plate tectonics include: earthquake epicenter maps, volcanic activity maps, ocean floor age maps, plate movement direction maps, and many others. Much of the work done by geologists involves the production of a map.
The map below shows the type of data that helped geologists understand where and why most earthquakes occur. Study the global distribution and depth of large earthquakes that occurred between 1975 and 1995. Then click on the map to compare the locations of earthquakes with plate boundaries. Compare the location of earthquakes and plate boundaries. Then answer the questions that follow.
Geologists use many types of evidence to locate the boundaries between plates. Most of these indicators are signs of stress, which develop as the two plates interact. The clearest indicators are patterns in the locations of earthquakes and volcanoes. From examining a map of earthquake and volcanic activity, it is very apparent that these events do not occur randomly. Both tend to occur in linear belts which mark the plate boundaries. Each type of plate boundary has a somewhat different pattern of earthquakes and/or volcanoes. At converging plate boundaries, two situations are possible. First, both volcanoes and earthquakes form where one plate sinks under the other. This process, called subduction, takes place because one plate is denser than the other. The denser plate, which invariably has oceanic crust on its top, does the sinking. Earthquakes occur along this plate as it sinks and is pulled into the upper mantle. Second, only earthquakes occur when two plates collide (obduct), building a mountain range. This situation is common when two plates with continental crust on top converge. The density of continental crust is too low for it to subduct; it is like wood floating on water. Instead, the two plates have a head on collision - building a mountain range. The Himalaya Mountains in Asia formed this way, from a collision between the Indian and Asian Plates. At transform plate boundaries, the two plates slide by each other. This generates little volcanic activity (there is no \"gap\" between the plates) or mountain building. Earthquakes, however, are common. At diverging plate boundaries, earthquakes occurs as the plates pull away from each other. Volcanoes also form between the plates, as magma rises upward from the underlying mantle. We rarely see these volcanoes erupt, as most of them are on the ocean floor. Transform plate boundaries commonly have only earthquakes.
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. 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. TOP 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. TOP 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. TOP 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. TOP 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 TOP 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. TOP 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. TOP 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. TOP 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: County Offices of Emergency Management see your local phone book's blue-pages for number. TOP 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. TOP 59ce067264