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Recent seismicity and ground uplift in the area are described. A comparison with other areas in the Cascades is made and the possibility of the Long Valley Magma Chamber as a source for eruptions is discussed. -P.N.Chroston
Because California is seismically active, planners and decisionmakers must try to anticipate earthquake hazards there and, where possible, to reduce the hazards. Geologic and seismologic information provides the basis for the necessary plans and actions. Two examples of how such information is used are presented. The first involves assessing the impact of a major earthquake on critical facilities in southern California, and the second involves strengthening or removing unsafe masonry buildings in the Los Angeles area. -from Authors
Springs discharging from the Snake Plain aquifer contribute approximately 6,000 cubic feet per second (170 cubic metres per second) to flow in the Snake River between Milner and King Hill. Before irrigation began on the Snake River Plain north and east of the springs, total spring discharge was about 4,200 cubic feet per second (120 cubic meters per second). Increasing amounts of irrigated acreage from the early 1900's to the mid-1940's contributed more irrigation-return water to the aquifer resulting in increased discharge at the springs. Maximum discharge of about 6,800 cubic feet per second (190 cubic metres per second) occurred during the late 1940's and early 1950's. Increased use of pumped ground water for irrigation and changing irrigation practices have since resulted in a decline in spring discharge.
Direct-current resistivity soundings and exploratory drilling suggest that the basalt of the Snake River Group is relatively thin in the area along the Snake River that is topographically suitable for pumping large quantities of ground water in exchange for surface water. The formations underlying the Snake River Group appear to have low permeability and probably would not yield large amounts of water. Previous studies have indicated that the southern edge of the Snake Plain aquifer extended to the Snake River. Data presented in this report implies that, in general, the southern boundary should, in fact, be several miles north of the river.
The results of drilling test holes to depths of approximately 1,000 feet in the Mud Lake region show that a large part of the region is underlain by both sedimentary deposits and basalt flows. At some locations, predominantly sedimentary deposits were penetrated; at others, basalt flows predominated. The so-called Mud Lake-Market Lake barrier denotes a change in geology. From the vicinity of the barrier area, as described by Stearns, Crandall, and Steward (1938, p. 111), up the water-table gradient for at least a few tens of miles, the saturated geologic section consists predominantly of beds of sediments that are intercalated with numerous basalt flows. Downgradient from the barrier, sedimentary deposits are not common and practically all the water-bearing formations are basalt, at least to the depths explored so far. Thus, the barrier is a transition zone from a sedimentary-basaltic sequence to a basaltic sequence. The sedimentary-basaltic sequence forms a complex hydrologic system in which water occurs under water-table conditions in the upper few tens of feet of saturated material and under artesian conditions in the deeper material in the southwest part of the region. The well data indicate that southwest of the barrier, artesian pressures are not significant. Southwest of the barrier, few sedimentary deposits occur in the basalt section and, as described by Mundorff, Crosthwaite, and Kilburn (1964). ground water occurs in a manner typical of the Snake Plain aquifer. In several wells, artesian pressures are higher in the deeper formations than in the shallower ones, but the reverse was found in a few wells. The available data are not adequate to describe the water-bearing characteristics of the artesian aquifer nor the effects that pumping in one zone would have on adjacent zones. The water-table aquifer yields large quantities of water to irrigation wells.
The Little Lost River basin, an elongated, northwest trending structurally formed intermontane valley, drains an area of about 900 square miles into a closed depression near the northwestern edge of the Snake River Plain. Runoff from snowmelt and rainfall on the Lost River Range on the west and the Lemhi Range on the east maintains the flow of the Little Lost River, and recharges the ground-water reservoir, Both mountain ranges are complexly faulted and are underlain by a variety of rocks, dominantly limestone, of Paleozoic age The principal aquifers are highly transmissive alluvial fill in the middle and upper valley and alluvial fill interfingered with basalt in the southernmost part of the valley.
The study area comprises about 1,600 square miles (4,100 square kilometers) in west-central Idaho and includes the entire Weiser River basin and small areas both west and south of Weiser outside the basin. The basin is sparsely populated and the economy is chiefly agricultural.
Virginia is a State of considerable seismic activity, although the earthquakes are rarely strong. Thirty-five shocks, intensity MM V or greater (Modified Mercalli Scale), are listed with epicenters within its borders. The locations of several of the older events are not precise; thus, the above count i subject to alteration. A detailed study of Virginia earthquakes by G. A. Bollinger and M. G. Hopper of the Virginia Polytechnic Institute and State University listed 137 shocks (71 from 1774 to 1899, 66 from 1900 to 1970). Many of these were felt with intensities below MM V.
Seven earthquakes of intensity V or greater on the Modified Mercalli Scale (MM) are known to have originated within Vermont. Many additional shocks centered in other New England States and Canada have been strongly felt in Vermont.
Seventeen earthquakes, intensity V or greater, have centered in Texas since 1882, when the first shock was reported. The strongest earthquake, a maximum intensity VIII, was in western Texas in 1931 and was felt over 1 165 000 km 2 . Three shocks in the Panhandle region in 1925, 1936, and 1943 were widely felt.
The western part of the State was shaken strongly by the New Madrid, Mo., earthquakes of 1811-12 and by earthquakes in 1843 and 1895. The area has also experienced minor shocks. Additional activity has occurred in the eastern part of the State, near the North Carolina border. Forty shocks of intensity V (Modified Mercalli scale) or greater have been cataloged as occurring within the State. Many other earthquakes centered in bordering States have affected points in Tennessee. The following summary covers only hose shocks of intensity VI or greater.
Twelve earthquakes of intensity V or greater (Modified Mercalli scale) have centered within the borders of South Dakota. All the shocks were rather localized, except that of 1911 which was felt over an area of approximately 100,000 km 2 . Some earthquakes from neighboring States were felt strongly in South Dakota.
An estimated $23 million damage was caused by one of the great earthquakes in United States history in 1886. Charleston, S.C, and nearby cities suffered most of the damage, although points as far as 160 km away were strongly shaken. Many of the 20 earthquakes of intensity V or greater (Modified Mercalli scale) that centered within South Carolina occurred near Charleston. A 1924 shock in the western part of the State was felt over 145,000 km 2 . Several earthquakes outside the State borders were felt strongly in South Carolina.
Only three shocks (intensity V or greater, Modified Moercalli Scale) have centered in Rhode Island, although several earthquakes in New England and the St.Lawerence Valley have been felt in the State.
Record of early earthquakes in Northeastern United States provide limited information on effects in pennsylvania until 1737, 55 years after the first permanent settlement was established. A very severe earthquake that centered in the St.Lawrence River region in 1663 may have been felt in Pennsylvania, but historical accounts are not definite. Likewise, a damaging shock at Newbury, Mass., in 1727 probably affected towns in Pennsylvania. A strong earthquake on December 18, 1737, toppled chimneys at New York City and was reported felt at Boston, Mass., Philadelphia, Pa. and New Castle, Del. Other shocks with origins outside the State were felt in 1758, 1783, and 1791. Since 1800, when two earthquakes (March 17 and November 29) were reported as "severe" at Philadelphia, 16 tremors of intensity V or greater (Modified Mercalli Scale) have originated within the State. On November 11 and 14, 1840, sever earthquakes at Philadelphia were accompnaied by a great and unusual swell on the Delaware River.
Although situated between two States (California and Washington) that have has many violent earthquakes, Oregon is noticeably less active seismically. the greatest damage experienced resulted from a major shock near Olympia, Wash., in 1949. During the short history record available (since 1841), 34 earthquakes of intensity V, Modified Mercalli Scale, or greater have centered within Oregon or near its borders. Only 13 of the earthquakes had an intensity above V, and many of the shocks were local. However, a 1936 earthquake in the eastern Oregon-Washington region caused extensive damage and was felt over an area of 272,000 square kilometers.