Modern methods of predicting earthquakes recognize that quakes, far fr

Modern methods of predicting earthquakes recognize that quakes, far from being geologic anomalies, are part of the periodic accumulation and discharge of seismic energy. As continents receive the horizontal thrust of seafloor plates, crustal strains develop. Accumulation of strain can take anywhere from 100 years in certain coastal locations to over a millennium in some inland regions before a critical point is reached and a rupture occurs. In both areas, the buildup of strain is accompanied by long- and short-range precursory phenomena that are crucial to earthquake prediction. Quakes along active faults—like those along the Pacific coasts—are usually frequent; scientists designate such areas as quake-prone. However, when the time interval between quakes is great, as in inland regions, locating active faults is only a beginning. Geological scars of past subsidence, cracks, and offsets are useful in determining potential quake locations, as are seismicity gaps, areas where no small quakes have been recorded. Seismologists may also consult the historical record. Primary sources range from eyewitness accounts of ancient quakes to recent official documentation of quake-related damage.

Once the perimeters of a quake-prone zone are established, a network of base stations can monitor precursory phenomena. Stations must extend over a wide area yet be placed at measured intervals to obtain precise readings. Changes in geochemical readings (electric currents, radon concentrations) and in groundwater levels, as well as the occurrence of microearthquakes, are valuable precursors. Crustal movements—tilting, rising, and expansion or contraction of the ground’s surface—can be read through triangulation and leveling surveys taken over the course of decades. Theoretically, if an area’s critical strain—the magnitude of strain necessary to produce a rupture—is known, subtracting the measured accumulated crustal strain from the critical strain will indicate a time frame for an impending quake.

Violent tilting and foreshocks are among phenomena classified as short-term precursors. Many are still being identified as new quakes occur. Such precursors are valuable since their appearance can permit prediction of a quake to within hours of the primary rupture. Here, too, historical documents are useful. Seismologists recognized the liquefaction of sand as a precursor after a 1964 quake in Japan.

1. According to the passage, a major difference between coastal regions and inland regions is that in coastal regions

A) crustal strain does not occur.
B) earthquakes are less numerous.
C) critical points are reached more quickly.
D) precursory phenomena are seldom observed.
E) seafloor plate action is less powerful.

2. The primary purpose of the passage is to

A) clarify the way in which earthquakes develop in inland locations.
B) show that earthquakes are a result of the normal accumulation and discharge of seismic energy.
C) discuss the accumulation of crustal strain in coastal regions.
D) argue that precursory phenomena should be disregarded in attempts at quake prediction.
E) describe methods of earthquake prediction and explain the importance of precursory phenomena.

3. The primary function of the third paragraph is to

A) explain the relationship between accumulated and critical strain.
B) describe the use of precise intervals in establishing networks of base stations.
C) summarize the differences between earthquakes in coastal and inland regions.
D) outline some of the methods used by seismologists to predict earthquakes.
E) suggest that critical strain is not spread evenly along most major fault lines.

4. According to the passage, knowledge of an area’s critical strain can help seismologists

A) estimate the date of a future earthquake.
B) calculate the severity of an initial rupture.
C) measure the seismic force along a fault.
D) revise the distances between base stations.
E) predict the rate of future crustal movement.