Summary

Acknowledgments

Acknowledgment of Data Sources

Contact

Naming Conventions

Database Summary-Contributing Events and Strong Motion Stations

Recording Stations

Strong Motion Records

Digitization and Data Processing Procedures

Data Format

References on Strong Motion Data of the 1994 Northridge Earthquake Sequence

References on Strong Motion Data Processing

This Web document presents tables, maps and links to data files of processed data of five M>5 aftershocks of the Northridge, California, earthquake (17 January 1994, M_L=6.4, M_S=6.7) 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 convenience to users, the records of the main event are also summarized. This database includes 276 aftershock records and 211 records of the main event.

This database of Northridge aftershock accelerograms is unique.  The temporary digital arrays installed for a limited period of time following the main shock, mostly in San Fernando Valley, Santa Monica and at selected locations in Los Angeles, recorded hundreds of aftershocks and resulted in thousands of recordings but typically of magnitude M< 3.  The largest aftershocks were recorded almost exclusively by the permanent strong motion stations.

The USC, USGS and part of the DWP aftershock records were digitized and processed at the USC Strong Motion Data Processing Laboratory, using the LeAuto and LeBatch software packages of Lee and Trifunac. The CDMG data and part of the DWP processed data was made available by these agencies.  The DWP main shock data differs from previous releases (all analogue records in this release have been reprocessed using the LeAuto and LeBatch software). The new versions are longer (the full recorded length was digitized) and in many cases include the first aftershock (M=5.9, ~60 s following the main event) which was recorded on the same film, continuously following the main event. This release also includes some reprocessed USGS main event records, at stations which recorded continuously the main event and the first aftershock.

This database 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.

Most of the contents of this document have been extracted from the following report:

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).

The creation of this Web site  was supported by the U.S. Geological Survey, Department of the Interior, under USGS award number 1434-HQ-98GR00023, as part of the project entitled: "Repeatability of Site Effects-: Northridge Aftershock Data Recorded at Strong Motion Stations"  (PI-s: M. Todorovska and V.W. Lee; <home page>).  The digitization and processing of records of the five M>5 Northridge aftershocks, at USC, USGS and DWP Power Systems stations, and gathering of processed data at CDMG and DWP Water Systems stations was also supported by the USGS as part of the same project.  The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

The processed CDMG and DWP Water Systems aftershock records were made available by V. Graizer of California Division of Mines and Geology, and C. Davis of Los Angeles Department of Water and Power (CDMG data Graizer et al., 1998; and the DWP Water Systems data, SEEC, 1995). The film records at USGS stations were provided by C.D. Stephens and K.A Fogleman of the USGS Earthquake Hazards Team in the Menlo Park office, and at DWP Power Systems records-by R. Tognazzini and C. Davis. The film records at the USC stations are from the archives of the Los Angeles Strong Motion Network.

The new version of the challenging to process (due to many stalls in the film motion)  Rinaldi record of the main event  was digitized free of charge by Prof. Trifunac of USC before this project became active (Trifunac et al, 1998).   All of the reprocessed USGS main shock records include the M=5.9 aftershock. The main event portion was digitized at USC free of charge.  The DWP Water system records (main event and aftershocks) were processed for the DWP commercially (SEEC, 1995); the Power Systems records were processed or reprocessed for this project

The financial support from the USGS, the efforts of the agencies operating the four networks, and the help from the mentioned individuals who made the raw and processed data available for this project are gratefully acknowledged.

The processed data in this database are due to the efforts both of those that maintain the strong motion stations and of those that processed the data.  End Users of data digitized and processed by USC under this project are kindly requested to acknowledge in publications both the agencies/institutions operating the networks and processing the data.   The CDMG has specified their own guidelines for acknowledgment.

Some data files in this release may be modified in the future.  To insure that the integrity of the data is maintained,  and that the end users get the most recent version, it is recommended that Secondary Data Distributors first contact the representatives of the four agencies contributing the data.   Unauthorized redistribution of these data is discouraged.  The USC group is not responsible for the contents of records it has processed but which have been distributed without authorization.  Secondary Data Distributors are also requested to advice end users to acknowledge appropriately the Data Sources (see the preceding paragraph).

Information Services Providers are advised that appropriate reference to this data release is the URL address of this web document

The FTP directories where the data files are stored provide no supporting information and should be accessed by first time users only via this web page.  Any references to the FTP directories without reference to this web site is inappropriate.

The data files are residing on and can be downloaded from servers of the agencies which operate the networks.  CDMG also has a service for distribution on floppy disks.

The URL addresses of the  web sites of the agencies operating the four networks contributing to this database, and the URL addresses of thedirectories with the data are:
 

There is a one-to-one correspondence between records and data files, except for the first aftershock. This event occurred about 60 seconds following the main event, and was recorded continuously with the main event at some USGS, DWP and USC stations. Since for some studies it may be of interest to use the continuous record of both events, at those stations, the entire recorded signal (following the trigger from the main event) was digitized as one continuous record, and is stored in a file corresponding to the main event. These long records are listed at the end of Table 3.2.

The contributing events have been assigned an event number (001, 009, 142, 151 and 392 for the aftershocks and 000 or "main" for the main event).  There are four groups of  recording stations, named by the contribution networks and distinguished by different symbols: circle (USC), square (USGS), triangle (CDMG) and diamond (DWP).   For the purpose of this project, each station is assigned a station number, unique within the group.  The network code and the station number uniquely determine the station.  For the USC group, the station code coincides with the standard USC station number.

The data files processed by the USC group have been named consistently, as follows.  There are separate directories for each event, and the file name is derived from the station code.  The first two letters (v1, v2 or v3) refer to the type of data (v1: uncorrected acceleration; v2: corrected acceleration, velocity and displacement; v3: Fourier and Response Spectra) .  For example, v10022.usg refers to v1 data at station USGS no. 22 (similar convention applies for the dwp records), and v25611.usc refers to v2 data at station USC no. 56.      The CDMG uses its own naming convention (by letters, derived from the station name).

The USC, USGS and DWP data have exactly the same format (the current version of the USC format, i.e. the standard output from the LeBatch software).  The format of the CDMG files is practically identical to the USC format. (The LeBatch software and the current software used by the CDMG both derive from the late 1970's version of the Lee and Trifunac data processing software, and the changes in the format have since been minor).

All stations
Event Code Date/Time (GMT) M Depth km 000 1994/01/17 12 30 6.7 18.4 001 1994/01/17 12 31 5.9   6.0 009 1994/01/17 12 40 5.2   6.0   Event Code Date/Time (GMT) M Depth km 142 1994/01/17 23 33 5.6   9.8 151 1994/01/18 00 43 5.2 11.3 392 1994/03/20 21 20 5.2 13.0
Figure 2.1  Map of NW part of the Los Angeles metropolitan area showing the epicenters of the Northridge main event and of the five M>5 aftershocks between 17 January and 20 March 1994, and the locations of strong motion stations which recorded any of these events.  Different symbols are used to distinguish stations of the four networks: circles-USC, squares-USGS, triangles-CDMG and diamonds-DWP.

Figure 2.2 shows the USC stations only, identified by a two digit code, unique for the USC set only. Table 2.2 shows a list of these stations. The station code is in the first column. The second column shows the site address, the fifth and sixth columns show the station latitude and longitude. The seventh and eight columns show the USC station number (for this set only, the station code is equal to the USC station number) and the instrument type and serial number. The last column of symbols "x" indicates that a particular event was recorded. Figure 2.3 and Table 2.3 show the same but for the USGS stations, Figure 2.4 and Table 2.4 - for the USGS stations, and Figure 2.5 and Table 2.5 show the same but for the DWP stations.

Note: All tables can be opened as Word documents in rtf format.
 

List	Map	FTP Explanation
Table 2.1	Figure 2.1	All stations which recorded the Northridge main event and the five M>5 aftershocks.
Table 2.2	Figure 2.2	USC stations which recorded the Northridge main event and the five M>5 aftershocks.
Table 2.3	Figure 2.3	UGS stations which recorded the Northridge main event and the five M>5 aftershocks.
Table 2.4	Figure 2.4	CDMG stations which recorded the Northridge main event and the five M>5 aftershocks.
Table 2.5	Figure 2.5	DWP stations which recorded the Northridge main event and the five M>5 aftershocks.

Strong Motion Records
 

List	Map	Explanation	FTP link to data files or Email to contact party
Table 3.1	Figure 3.1	Stations that recorded the Northridge main event (000).	<USC>	<USGS>	<DWP>	<CDMG>
Table 3.2	Figure 3.2	Stations that recorded Northridge aftershock 001.	<USC>	<USGS>	<DWP>	<CDMG>
Table 3.3	Figure 3.3	Stations that recorded Northridge aftershock 009.	<USC>	<USGS>	<DWP>	<CDMG>
Table 3.4	Figure 3.4	Stations that recorded Northridge aftershock 142.	<USC>	<USGS>	<DWP>	<CDMG>
Table 3.5	Figure 3.5	Stations that recorded Northridge aftershock 151.	<USC>	<USGS>	<DWP>	<CDMG>
Table 3.6	Figure 3.6	Stations that recorded Northridge aftershock 392.	<USC>	<USGS>	<DWP>	<CDMG>

While in Section 2 the stations were grouped by agency operating the network, in this section they are grouped by events (main and five aftershocks). Figure 3.1 shows by a dark symbol the stations that recorded the main event, and Table 3.1 show lists of the USC, USGS, CDMG and DWP stations that contributed the data. The epicentral distance and length of digitized record are also shown. The record length for USGS stations is the one for the records originally released. For the DWP records, the length is for the records redigitized for this project (Power Systems) or digitized for DWP commercially (SEEC, 1995), except for station DWP 07 which has a digital instrument. Figures 3.2 – 3.6 and Tables 3.2 – 3.6 show the same as Figure 3.1 and Table 3.1 but for the five aftershocks. In Table 3.2, corresponding to event 001, the stations which recorded this event continuously with the main event are separated (listed at the bottom). The listed record length is the one for the entire recorded (main event+aftershocks), but the listed epicentral distance and peak values are for event 001.

To identify a station from these maps, one needs to identify first the agency (network) from the shape of the symbol and then go to the map of stations for that agency and find out the two digit station code, and finally, look up the information from the corresponding table.

Most of the USC aftershock records were identified from the binary time code of the instrument internal clock impressed on the film during recording, and the confidence of identification is high. Trigger times were not available for the aftershock records at USGS and DWP stations. These records were identified by visual comparison with records, which were positively identified, at nearby stations. The aftershocks recorded at CDMG stations were identified by their staff.

The strong motion data, digitized for this project (records at USC, USGS and DWP stations), was recorded by SMA-1 accelerographs recording onto a 70 mm film, or by a central recording system(CR-1) with distributed FBA sensors and recording on a wide film. The software package LeAuto (Lee and Trifunac, 1990; Trifunac et al., 1999) for automatic digitization of the accelerograms (by a using PC and a flat-bed scanner) was used at 600 dpi optical resolution. This package has the (unique) capability of digitizing wide film records simultaneously, and a new algorithm for determining the beginning of the traces accurately. These two features enable accurate relative timing of the traces, which is very important in identification studies of site response and in studies which require combination of recordings at multiple channels (e.g., to calculate the motion with respect to a rotated coordinate system, for analysis of propagation velocities and for studies of coherence of ground motion). The difficulty in determining accurately the beginnings of the traces is due to the fact that the beginning of the traces (1) do not project onto same vertical line on the film, and (2) are weak because the lamp temperature and the light intensity have not reaches their maximum. The two-pulse-per-second code was digitized and used to scale the time, where available.  In general, the agency provided sensitivity constants were used to scale the acceleration.   For the DWP data recorded by SMA-1 accelerographs, sensitivity constants from recent  calibration tests performed for the DWP by SEEC Inc. were used.  The recent version of the software package LeBatch (Lee and Trifunac (1990) was used for processing of the raw digitized data (scaling the digitized time series in time and acceleration units, correction for the transducer transfer-function, filtering of discretization and long period noise, derivation of the time series of velocity and displacement, and computation of Fourier and Response Spectra). The corrected time series were filtered by Ormsby filters (the filter frequencies are specified in the data files), and are available at equally spaced points in time, with Dt = 0.01 s.  The High frequency cut-off is usually at 25-27 Hz for the data processed by USC.  The low frequency cut-off depends on the signal to noise ratio an was chosen separately for each recorded component of motion.  The file format and filters of the LeBatch software differ from those of the USGS (visit their web site at http://nsmp.wr.usgs.gov/   for information on their standard data processing software).

The LeBatch software package has the capability for correction of accelerograms recorded by an SMA-1 accelerograph  for transducer misalignment and cross-axis sensitivity (Todorovska, 1998; Todorovska et al., 1998).   This procedure requires detailed calibration to estimate the angular transducer sensitivity and misalignment angles, and has been performed for the USC and DWP stations.  The errors in the data due to these imperfections of the recording instrument are significant  for large accelerations, like those near the source of the Northridge earthquake.  For compatibility with processed USGS and CDMG data, correcting for such errors was not performed for the data in this release, but is possible for most of the USC and DWP data.

The automatic digitization of accelerograms is an estimation process in which a signal is reconstructed from a gray-scale bitmap.  This process  is non unique and has limitations inherent to the hardware and to the algorithms currently in use.  One such limitation is filtering of high frequencies, which is due to the varying optical density of the bit map image and the fact that the automatic tracing algorithm works with a predefined threshold level, common for the entire digitized trace segment.   This filtering effect can be reduced at the expense of significant operator involvement, by running specialized software routines with dynamic selection of optimum threshold levels locally.  This  increases the cost of processing per record.  Such digitization was a must for the large amplitude records of the Northridge main event, and was also necessary for some segments of the aftershock data.  The overall quality of the digitized data, however, critically depends on the experience of the operator.  Due to possible effects on results of studies using strong motion data, users should be aware of various details of the processing.   A recommended reading is the following recent publication, made available for a limited time on the internet:

Trifunac, M.D., V.W. Lee and M.I. Todorovska (1999). `Selected common problems in automatic digitization of accelerograms', Soil Dynamics & Earthquake Engrg, 18, 519-530.

For help in reading the data and conversion to other user defined formats,  Fortarn programs V1READ.for, V2READ.for and V3READ.for have been provided.  ASCII files of these programs can be downloaded from subdirectory \TOOLS of the USC FTP data (ftp://cwis.usc.edu/pub/todorovs/North_aft_M5/tools).  These sample programs can be modified to rewrite the data in any desired format.

1. Bardet, J.P. and C.A. Davis (1996). " Engineering observations of ground motion at the Van Norman complex after the 1994 Northridge earthquake," Bull. Seism. Soc. Amer., 86(1B), S333-S349.
2. Graizer, V.M., T.Q. Cao and A.F. Shakal (1998). "Processed CSMIP strong motion records from aftershocks of the Northridge, California earthquake of January 17 1994," Report No. OSMS 98-01, California Dept. of Conservation Division of Mines and Geology, Sacramento, CA.
3. Porcella, R.L., E.C. Etheredge, R.P. Maley and A.V. Acosta (1994). "Accelerograms recorded at USGS National Strong-Motion Network station during the M_s = 6.6 Northridge, California earthquake of January 17, 1994." Open File Report 94-141, Dept. of the Interior, U.S. Geological Survey.
4. Shakal, A., M. Huang, R. Darragh, T. Cao, R. Sherburne, P. Malhotra, C. Cramer, R. Sydnor, V. Graizer, G. Maldonado, C. Petersen, and J. Wampole (1994). "CSMIP strong motion records from the Northridge, California, earthquake of 17 January 1994." Report No. OSMS 94-07, Calif. Dept. of Conservation, Division of Mines and Geology, Sacramento, California.
5. Structural and Earthquake Engineering Consultants (1995). "Processed strong motion records of the 1994 Northridge earthquake and its aftershocks at Water Systems stations of Los Angeles Department of Water and Power," unpublished report prepared by for the Los Angeles Department of Water and Power, Arcadia, CA.
6. Todorovska, M.I. (1997). `Strong motion recordings of the 1994 Northridge, California, earthquake at stations of the Los Angeles Strong Motion Array', Proc. Northridge Earthquake Research Conf., 20-22 August, 1997, Los Angeles, CUREe, Richmond, CA, 1998, Vol. II, pp. II.413-420.
7. Todorovska, M.I, M.D. Trifunac, V.W. Lee, C.D. Stephens, K.A. Fogleman, C. Davis and R. Tognazzini (1999). "The M_L=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.
8. Trifunac, M.D., M.I. Todorovska & V.W. Lee (1998). `The Rinaldi strong motion accelerogram of the Northridge, California, earthquake of 17 January, 1994', Earthquake Spectra, 14(1), 225-239.

1. Lee, V.W. and M.D. Trifunac (1990). "Automatic digitization and processing of accelerograms using PC," Report No. 90-03, Dept. of Civil Engrg, U. of So. California, Los Angeles, CA.
2. Todorovska, M.I. (1998). `Cross-axis sensitivity of accelerographs with pendulum like transducers: mathematical model and the inverse problem,' Earthquake Engrg & Struct. Dynamics, 27, 1031-1051.
3. Todorovska, M.I., E.I. Novikova, M.D. Trifunac & S.S. Ivanovic (1998). `Advanced sensitivity calibration of  the Los Angeles Strong Motion Array,' Earthquake Engrg & Struct. Dynamics, 27, 1053-1068.
4. Trifunac, M.D., V.W. Lee and M.I. Todorovska (1999b). `Selected common problems in automatic digitization of accelerograms', Soil Dynamics & Earthquake Engrg, 18, 519-530.