We have taken advantage of the recent increase in strong-motion data at close distances to derive new attenuation relations for peak horizontal acceleration and velocity. This new analysis uses a magnitude-independent shape, based on geometrical spreading and anelastic attenuation, for the attenuation curve. An innovation in technique is introduced that decouples the determination of the distance dependence of the data from the magnitude dependence. The resulting equations are log A =-1.02 + 0.249M-log r-0.00255r + 0.26P r_-(d2-1-7.32) 1/2 5.0_
The Whittier Narrows earthquake sequence (local magnitude, M(L) = 5.9), which caused over $358-million damage, indicates that assessments of earthquake hazards in the Los Angeles metropolitan area may be underestimated. The sequence ruptured a previously unidentified thrust fault that may be part of a large system of thrust faults that extends across the entire east-west length of the northern margin of the Los Angeles basin. Peak horizontal accelerations from the main shock, which were measured at ground level and in structures, were as high as 0.6g (where g is the acceleration of gravity at sea level) within 50 kilometers of the epicenter. The distribution of the modified Mercalli intensity VII reflects a broad north-south elongated zone of damage that is approximately centered on the main shock epicenter.
PREFACEThe National Strong-Motion Program (NSMP) designs, develops, and operates a nation-wide earthquake instrumentation network in order to record potentially damaging ground motion and to monitor the structural response of buildings, bridges, and dams in seismically prone regions.The Program also disseminates resultant data and information about recordings, sites, and structures to the international earthquake engineering research and design community. The network consists of approximately 1,000 accelerographs located in 39 states and Puerto Rico, and includes instrumentation at more than 600 ground sites, 56 dams, 5 bridges, and more than 60 large buildings.
In the early 1970's the U.S. Geological Survey (USGS) began a program accumulating geotechnical and seismic data to be used in developing methods for estimating specific ground-response characteristics during strong local earthquakes. Recent site studies in the Los Angeles and San Francisco Bay regions indicate significant correlations exist between shear-wave velocity and various physical properties of the near-surface materials; these studies further suggest that these correlations can be used to define seismicallydistinct map units. The selection of specific locations for investigation has been based on sites where ground-motion data were recorded during earthquakes (and nuclear explosions), the availability of detailed geologic maps, and the distribution of intensity data from selected earthquakes. This study involved investigations at most of the Imperial Valley, California accelerograph stations, and was done because of the unique strongmotion data set recorded during the magnitude 6.5 earthquake of October 15, 1979. The project included the following investigations: (1) electronic conepenetrometer soundings at nine stations; (2) drilling, sampling, and logging of 22 borings to depths of from about 30 to 244 m; (3) downhole P-and S-wave velocity surveys at 22 stations; (4) high-amplitude resonant column tests of undisturbed samples from several stations; and (5) numerous gamma, S-P, and resistivity logs and call per and temperature measurements at selected stations. This study is just one part of an ongoing USGS program to compile geotechnical data at selected locations in various regions and to use these data to make detailed comparisons of the geologic and seismic characteristics that will provide a means for quantitatively estimating strong ground motion at a given site and facilitate the development of seismic zonation techniques applicable to other regions. i v This report will focus on results of the electronic cone-penetration tests and, in particular, the downhole velocity surveys. Results of other investigations undertaken as part of this project will be discussed briefly and a reference regarding the availability of additional information or a report wil 1 be given.
This report to preliminary and has not been reviewed for conformity with U. 8. Geological Survey editorial standards Cor with the North American Stratigraphic Code). Any use of trade, product, or firm names to for descriptive purposes only
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