SAC Publications Report No. SAC 95-03

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Characterization of Ground Motions During the Northridge Earthquake of January 17, 1994, by Paul Somerville, Robert Graves, and Chandan Saikia, Woodward-Clyde Federal Services, Pasadena, California, December, 1995.

The purpose of this report is to characterize ground motions generated by the January 17, 1994 Northridge Earthquake by (1) developing sets of ground motion time histories for particular sites of steel moment resisting frame buildings that were to be the subject of detailed analytical studies (as part of SAC Phase I Task 3) and (2) developing ground motion contour maps to be used in conjunction with regional and individual building damage assessment efforts (undertaken in SAC Phase I Task 2).

The report begins with a review of the earthquake source and strong motion characteristics of the Northridge earthquake. It includes a qualitative description of rupture directivity effects and their geographical distribution around the fault, and describes the rupture model that was used in the development of both the ground motion simulations and the contour mapping. This section also notes that while the peak accelerations produced by the Northridge earthquake are higher than would be predicted by current attenuation relationships for thrust earthquakes, they are not anomalous, and may be more representative of ground motions close to these events.

Section 3 describes the methods that were used to develop the ground motion inputs. These procedures were developed by the authors in previous investigations funded by the U.S. Geological Survey (USGS) and the National Science Foundation (NSF) as part of the National Earthquake Hazards Reduction Program (NEHRP), and were validated by comparing simulated ground motions with recorded ground motions from the Northridge earthquake, as well as a large number of other events. These validated procedures formed the basis for generating broadband time histories for the Northridge earthquake at the building sites in the SAC Phase I Task 3 analyses that did not have strong-motion recordings. A series of nine such time histories were provided for each of these sites, as described in Section 4. Basin effects created by the Los Angeles Basin were considered in the development of simulated time histories for buildings sited in the basin.

The procedures used in the generation of contour maps are also described in Section 3. Average alluvial site conditions were considered in the development of these maps. Site-specific effects are not considered directly. Empirical attenuation relations were used as the base estimate for all points in a grid used to map the contours. These values were then modified using smoothed residuals of nearby strong motion recordings from the attenuation relations for the appropriate site category, either soil or rock.

Section 4 provides the ground motion inputs that were provided to the SAC Phase I Task 2 and 3 investigators. Contour maps of peak acceleration, peak velocity and response spectral velocity throughout the region strongly shaken by the Northridge earthquake are provided. Response spectral acceleration contour plots at periods of 0.2, 0.5, 1.0, and 3.0 seconds for 5% damping were developed. Because of the differences in the ground motion intensities, separate maps were provided for the north, east and average of the two horizontal components. Locations of strong motion recording stations used in the development of the contours are also noted.

The large number of ground motion time histories provided to the SAC Phase I Task 3 investigators is also included in Section 4. The first series of records includes site specific records from the Northridge earthquake. This series includes a set of the closest representative recorded ground motions for each site and a suite of nine simulated time histories. The second series of motions consists of "benchmark" records from the Northridge earthquake, which were used by all of the SAC Phase I Task 3 participants to compare across the various buildings. The Northridge benchmark recordings include University of Southern California (USC) Canoga Park, California Division of Mines and Geology (CDMG) Sylmar/Olive View Hospital and a profile of simulated records that crosses the top center of the fault and extends 20 km on either side at 2 km increments. A series of time histories from other events were also provided. The El Centro record from the 1940 Imperial Valley earthquake (Magnitude 7, strike-slip mechanism) is included in the set, since it has been used in many previous analytical investigations. Larger events, such as the record from the 1978 Tabas, Iran earthquake (Magnitude 7.4, thrust mechanism), the Lucerne record from the 1992 Landers, California earthquake (Magnitude 7.3, strike-slip mechanism), and a profile of simulated records for a large (Magnitude 7. 1) Elysian Park thrust earthquake are also provided. Finally, a probabilistic response spectrum for an earthquake with a 10% chance of exceedance in 50 years (475 year return period) for an alluvial site in the northern San Fernando Valley was developed, and recorded time histories of the Northridge earthquake that matched this spectrum were provided.

The authors drew upon many sources in the development of the ground motion inputs and the collection of ground motion records. Data from the U.S. Geological Survey, the California Strong Motion Instrumentation Program (CSMIP), the Southern California Earthquake Center (SCEC), the National Science Foundation, and other sources were used. The authors and the SAC Steel Project gratefully acknowledge these contributions to this report.

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