Estimated Time: Three forty-five minute class periods

5. The student will use computers and/or graphing calculators to produce tables, graphs, and spreadsheet calculations.
6. The student will read a technical selection and interpret it appropriately.

At least — inner core, outer core, mantle, lithosphere- crust and upper mantle

At least — plate tectonics, sea floor spreading, faulting, earthquakes, and volcanoes

By James P. Reger

Note: Charts and graphs omitted here. See web site for complete article.

INTRODUCTION

Earthquakes can be among the most devastating and terrifying of natural hazards. Although floods, tornadoes and hurricanes account for much greater annual loss in the United States, severe earthquakes pose the largest risk in terms of sudden loss of life and property. Many interrelated factors determine the extent of loss of property and life from an earthquake. Each of the following should be prefaced with "all other factors being equal. . . ."

• Amount of seismic energy released: The greater the vibrational energy, the greater the chance for destruction.
• Duration of shaking: This is one of the most important parameters of ground motion for causing damage.
• Depth of focus, or hypocenter: The shallower the focus (the point of an earthquake's origin within the earth), usually the greater the potential for destructive shock waves reaching the earth's surface. Even stronger events of much greater depth typically produce only moderate shaking at ground level.
• Distance from epicenter: The potential for damage tends to be greatest near the epicenter (the point on the ground directly above the focus), and decreases away from it.
• Geologic setting: A wide range of foundation materials exhibits a similarly wide range of responses to seismic vibrations. For example, in soft unconsolidated material, earthquake vibrations last longer and develop greater amplitudes, which produce more ground shaking, than in areas underlain by hard bedrock. Likewise, areas having active faults are at greater risk.
• Geographic and topographic setting: This characteristic relates more to secondary effects of earthquakes than to primary effects such as ground shaking, ground rupture, and local uplift and subsidence. Secondary effects include land- slides (generally in hilly or mountainous areas), seismic sea waves, or tsunamis (pretty much restricted to oceans and coastal areas), and fires (from ruptured gas lines and downed utility lines).
• Population and building density: In general, risk increases as population and building density increase. Types of buildings: Wooden frame structures tend to respond to earthquakes better than do more rigid brick or masonry buildings. Taller buildings are more vulnerable than one- or two-story buildings when located on soft, unconsolidated sediments, but taller buildings tend to be the more stable when on a hard bedrock foundation.
• Time of day: Experience shows there are fewer casualties if an earthquake occurs in late evening or early morning because most people are at home and awake and thus in a good position to respond properly.

Although earthquakes have been the objects of study and superstition for many centuries, the modern science of seismology gained impetus after the famous San Francisco earthquake of 1906. Since then, geologists have learned much more about the structure and composition of the Earth's interior and, more recently, have made progress in earthquake forecasting and in hazard and risk mitigation.

Most earthquakes occur when great stresses building up within the Earth are suddenly released. This sudden release of this stored energy causes movement of the earth's crust along fractures, called faults, and generates shock waves. These shock waves, or seismic waves, radiate in all directions from the focus, much as ripples radiate outward in two dimensions when a pebble is dropped into a pond.

The two basic types of seismic waves are body waves, or primary waves, which travel through the interior of the earth, and surface waves, which travel along the earth's surface and are believed to be responsible for most earthquake damage.

There are two types of body waves: P waves, or primary waves, and S waves, or secondary waves. The faster moving P waves are compressional waves, and the slower S waves are shear waves. Compressional waves involve a "push-pull" vibration of Earth material in the same direction as the P waves are moving. In contrast, shear waves "shake" material at right angles to their path. Differences in P- and S-wave characteristics have provided much information about the structure and composition of the earth's interior.

Although most earthquakes are associated with movement along faults, they can also be triggered by volcanic activity, by large landslides, and by some types of human activity. However, in areas not known for frequent earthquakes, pinpointing the cause of the rare tremor can be very difficult.

The theory of plate tectonics explains most earthquake occurrences. Ninety percent or more of all earthquakes occur along boundaries between large, slowly moving slabs, or plates, of the earth's crust and upper mantle, collectively called the lithosphere.

Most earthquakes are shallow (0-40 miles to the focus), occurring in the lithosphere. The mechanism for most very shallow earthquakes probably involves fracturing of brittle rock in the crust or relief of internal stresses due to frictional resistance locking opposite sides of a fault.

Very little is known about the causes of earthquakes in the eastern United States. In general, there is no clear association among seismicity, geologic structure, and surface displacement, in contrast to a common association in the western U.S.

The mid-Atlantic and central Appalachian region, including Maryland, is characterized by a moderate amount of low-level earthquake activity, but their cause or causes are largely a matter of speculation. In Maryland, for example, there are numerous faults, but none is known or suspected to be active. Because of the relatively low seismic energy release, this region has received relatively little attention from earthquake seismologists (Bollinger, 1969). In the Atlantic Coastal Plain, it is now thought that earthquakes may be associated with nearly vertical faults that formed during the opening of the present Atlantic Ocean during the Triassic period about 220 million years ago (Hanks, 1985). Such faults would occur in the "basement" bedrock, and not in the overlying, younger Coastal Plain sediments themselves. Recent evidence suggests that earthquakes in the Valley and Ridge Province and in the Piedmont Province occur at shallow depths (usually less than 15 miles) in the Precambrian crystalline basement rocks (Wheeler and Bollinger, 1984). The geologic structure that may be responsible for earthquake activity in these areas is a nearly horizontal fault that formed during continental collision and closing of a protoAtlantic Ocean during late Paleozoic time approximately 300 million years ago. It is also possible that some earthquakes in the Piedmont are in some way related to igneous dikes that were intruded into surrounding bedrock during the Triassic and Jurassic periods (roughly 200-175 million years ago).

The vibrations produced by earthquakes are detected and recorded by instruments called seismographs. The time of occurrence, the duration of shaking, the locations of the epicenter and focus, and estimates of the energy released can be obtained from data from seismographs set up around the world.

There are no operational seismograph stations in Maryland at the present time, but there are 13 stations in bordering states. Some of the nearest stations are in Newark, Delaware; Millersville, Pennsylvania; State College, Pennsylvania; Morgantown, West Virginia; and Blacksburg, Virginia.

Measurement of the severity of an earthquake can be expressed in several ways, the two most common being intensity and magnitude. The intensity, reported on the Modified Mercalli Intensity (MMI) Scale, is a subjective measure in terms of eyewitness accounts (Table 1). Intensities are ranked on a 12-level scale and range from barely perceptible (I) to total destruction (XII). The lower intensities are described in terms of people's reactions and sensations, whereas the higher intensities relate chiefly to observable structural damage.

Magnitude is an objective measure of earthquake severity and is closely related to the amount of seismic energy released at the focus of an earthquake. It is based on the amplitude of seismic waves as recorded on standardized seismographs. The standard for magnitude measures is the Richter Scale, an open-ended scale expressed in whole numbers and decimal fractions. The Richter Scale is logarithmic, meaning that an earthquake of magnitude 5.0 has 10 times the wave amplitude of a magnitude 4.0 and 100 times the ground vibration amplitude of a magnitude 3.0 event. As a first approximation, each whole number increment on the Richter Scale corresponds to a release of about 31 times more seismic, or vibrational, energy. Actually, there are several different methods of determining Richter magnitude. One uses surface waves, another body waves, and so on. However, the differences in results are slight.

Although the Richter scale has no upper limit, the greatest magnitude on record is 8.9 for earthquakes that occurred off the northwest coast of South America in 1906 (magnitude estimated) and off the east coast of Honshu, Japan in 1933. By comparison, the famous San Francisco earthquake of 1906 had an estimated magnitude of about 8.3 and an MMI of X.

A comparison of the Modified Mercalli and the Richter Scales is shown in Table 2. It is important to realize that these relationships are only generalizations and can vary for any given earthquake depending upon local geologic conditions. As a general rule of thumb, damage is slight at the magnitude 4.5 level, becomes moderate at about 5.5, and above 6.5 or so can range from considerable to nearly total (Bollinger et al., 1989). This relation may not apply to earthquakes in Maryland, if recent events are any indication. A small tremor in January, 1990, west of Baltimore was assigned an Modified Mercalli Intensity V near the epicenter, but registered only a 2.5 to 2.6 magnitude on the Richter scale.

Microsoft Excel spreadsheet program

Shake, Rattle and Fall Into the Ocean?

Goddard Space Flight Center Earth and Space Sciences Education Project (GESSEP)

Marilyn Tupis: Principal Investigator

Jeff Cottingham, Cheryl Overington, Connie Lenhart

Krayer, W. Montgomery County Public Schools.

Reger., J. Earthquakes in Maryland. Maryland Geological Survey.

USGS. Earthquake Search

USGS. Latest Quake Information.

USGS. Earthquake Information.