Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number 1434-HQ-98GR00023 (PI-s: M. Todorovska and V.W. Lee), in cooperation with the Strong Motion Programs of the USGS and of the Los Angeles Department of Water and Power (DWP), and coordinated with the data reduction of the Strong Motion Instrumentation Program of the California Division of Mines and Geology (CDMG). 

Web address of this document: http://www.usc.edu/dept/civil_eng/Earthquake_eng/North_M5/Repeatability_of_Site_Effects.html

Strong Motion Data of M_L=6.4 Northridge, California, Earthquake and of Five M>5 Aftershocks Between 17 January and 20 March 1994 Recorded at Strong Motion Stations

Data was gathered and organized in a small database of records of five M>5 aftershocks of the Northridge, California, earthquake (17 January 1994, ML=6.4) recorded at strong motion stations of four networks deployed in the Los Angeles metropolitan area (USC, USGS, CDMG and DWP) between 17 January and 20 March, 1994. The contributing events include the M=5.9 aftershock which occurred one minute following the main shock, and the M=5.2 aftershock of 20 March 1994 which triggered the largest number of stations. For completeness and for convenience to users of these data, this reports also includes the records of the main event.  The database described includes 276 aftershock records and 211 records of the main event.  The majority of the aftershock records were digitized and processed as part of this project (LeAuto and LeBatch software packages of Lee and Trifunac were used).  The CDMG and part of the DWP processed data was provided by these agencies.  The tables, maps and data files of records digitized and processed for this project can be accessed via the internet (see hot link below).  These data will be useful for studies of wave propagation in complex geologic environment, and of the effects of the local geology and near surface soils on the amplitudes, spectral content and duration of strong ground motion.  These studies will contribute to better understanding of the degree to which the local site conditions affect the amplitudes of ground shaking, compared to other factors such as radiation pattern and directivity of the earthquake source, propagation path effects and three dimensional focussing of waves reflected from the deep geologic structure.  The ultimate benefit will be more realistic assessment of hazards from strong earthquake shaking, leading to safer and optimal construction.   

Reference:  

Todorovska, M.I, M.D. Trifunac, V.W. Lee, C.D. Stephens, K.A. Fogleman, C. Davis and R. Tognazzini (1999). “The ML=6.4 Northridge, California, Earthquake  and Five M>5 Aftershocks between 17 January and 20 March 1994 - Summary of processed strong motion data,”  Report No. 99-01, Dept. of Civil Engrg, U. of So. California, Los Angeles, CA (in press).  

Web address of document http://www.usc.edu/dept/civil_eng/Earthquake_eng/North_M5/   

Reoccurrence of Site Specific Response

In a research paper, the period and amplitude variations of local peaks in the Fourier amplitude spectra of free-field strong ground motion recorded at five stations in San Fernando Valley of metropolitan Los Angeles, California (USC No. 03, 06, 53, 55 and 56), are described, searching for peaks that reoccur during different earthquakes. The data suggest that some local peaks reoccur (about 50% of the time), during shaking by small local earthquakes (peak ground velocities, vmax < 10 to 20 cm/s). During large strong motion amplitudes (vmax > 20 cm/s),  these peaks are shifted towards longer periods (by nonlinear response of soils) or disappear. The data also suggest that densification and settlement of soil, minutes and hours following the strong shaking, may contribute towards fluctuations in the effective stiffness of the shallow surface layers.  

Reference:  

Trifunac, M.D., T.Y. Hao and M.I. Todorovska (1999a). “On reoccurrence of site specific response,” Soil Dynamics & Earthquake Engrg, (in press).