Joshua Tree Aftershock Data

Summary

Introduction

The Ml 6.1 Joshua Tree earthquake of April 23rd, 1992 04:50 GMT occurred at 33.961°N, 116.318°W approximately 8 km northeast of the southern San Andreas fault and about 20 km south of the Pinto Mountain fault [More information]. The primary aftershock activity occurred in a band extending from 33°N 50.0', -116°W 16.0' to 34°N 15.0', -116°W 19.0'. Southern California Earthquake Center (SCEC) personnel responded within 24 hours by installing portable instruments to record the aftershock sequence. Over 6000 aftershocks were recorded between April 25 - May 31 by up to 11 stations. 3174 were also recorded and located by the Southern California Seismic Network (SCSN) and associated with the portable records. These data have had all known timing errors corrected, both P and S-phases picked, and are available from the SCEC Data Management Center at Caltech and in SEED format from the IRIS Data Management Center at the University of Washington.

Field deployment

Within the first 48 hours following the main shock, SCEC personnel installed six portable stations, with the maximum of eleven simultaneously recording stations operating within the first week. Five of the twelve instruments were deployed within the Joshua Tree National Monument area, with the remaining seven located in the vicinity of Desert Hot Springs. The station locations are shown in Figure 1. Over 3 Gb of raw data were acquired during the 40 day deployment. A time line showing the recording status of each station throughout the deployment is shown in Figure 2, indicating an excellent level of data recovery.

The equipment consisted of Refraction Technology (Reftek) 72A-02 16-bit dataloggers recording either Mark Products L-22 velocity sensors, Guralp CMG3t broadband velocity sensors, Kinemetrics FBA-23 accelerometers or both L22 and FBA sensors. The total system response for each instrument parameter configuration has been calculated and can be used to correct for instrument characteristics. Figure 3 shows the normalized velocity and acceleration response for four of the instrumentation configurations.

Station locations were determined by a differential GPS survey providing accuracy to 1 meter (Scott et al., 1994).

Data Processing

Data from the field were copied to 8 mm Exabyte tape with a Reftek field Exabyte drive. The data were then read on a Sun computer where it was converted to SEGY format and the datalogger state of health logs were generated. All known timing errors were corrected utilizing software from the PASSCAL Instrument Center at Lamont Doherty Earth Observatory (PIC). Table 1 shows the time shifts applied to each station in order to correct for those errors.

A time based event association was then done with software from the PIC which grouped all triggers that occurred within a specified time window. All SEGY data were then converted to CSS 3.0 and both P and S phases were picked by hand. An association with the SCSN catalogue was then done utilizing software from the University of Colorado which associates predicted arrivals with the actual phase picks. The data were then converted to the SCEC database format and shipped to Caltech for integration into the CUSP system. The data was also converted into SEED format and delivered to the IRIS DMC in Washington state.

A problem was discovered within the SCEC database that affecled the sensor channel polarities for some stations. Data that were retrieved prior to February 8, 1995 may be affected by this bug and should be replaced with the data currently on-line at Caltech.

Data

The data set consists of 3174 events, all of which are associated with an SCSN catalogue location. The distribution of aftershocks is shown on the map in Figure 1. The magnitude distribution is shown in the upper part of Figure 4, indicating that the majority of events fall between MILS and Ml 3.5. The lower part of Figure 4 is a histogram showing the number of events recorded versus the number of stations recording each event. Travel time residuals were calculated for each station while the event association was done. This gives us a qualitative view of the accuracy of the data timing. Figure 5 shows the travel time residuals for both P and S-phase associations for each station. The residuals are essentially zero for each station which indicates the high quality of data timing in this set. Figure 6 shows two examples of typical events recorded by the portable stations. The upper event in the figure is one of the larger aftershocks (Ml 4.74) which was recorded by nine of the stations.

Acknowledgments

Several people contributed to the data acquisition, processing and preparation of this product. Personnel from the California Institute of Technology, the University of California at San Diego, the University of California at Santa Barbara, and the University of Southern California participated in the work. Aaron Martin and Craig Nicholson from UCSB played key roles in the field deployment and initial data processing, and Dave Johnson from Caltech also contributed greatly to the field deployment. Katrin Hafner from Caltech, and Dan Shaw from UCSD played important roles in finishing the data processing. Software support was provided by personnel at the PASSCAL Instrument Center at Lamont Doherty Earth Observatory and the JSPC at the University of Colorado at Boulder. Funding from SCEC fund #40140A provided support for the data processing.

References

Table 1: Time shifts that were applied to the data in order to correct for known timing errors
Station Shift Applied (ms) Comments
AQUA -70 – -331, linearly increasing Linear shift applied to correct for known clock drift from 92116:0092119:18
BRCC 12 – 131, linearly increasing Linearly increasing shift applied to correct for known clock drift from 92116:00 – 92128:23
COVF No shifts applied No corrections necessary.
DWSR 234 Static shift to correct for Omega clock jerk from 92119:23 - 92120:00
EDCl No shifts applied No corrections necessary.
EDCY No shifts applied No corrections necessary.
EDOM 11435,10000 – 10029, 1 – 55 linearly increasing, -175 Shifts applied to correct for known clock drifts:
11435 from 92116:00 – 92121:11
10000 &ndash 10029 linearly increasing from 92121:11 – 92125:17
1 &ndash 55 linearly increasing from 92125:20 – 92133:30 -175 static shift between 92140:0092140:19
HDVL 23000,6400 Static shift to correct for Omega clock jerks: 23000 from 92116:03 – 92133:23 6400 from 92133:23 – 92140:23
KEYV 14 Static shift to correct for Omega clock jerk from 92119:22 – 92119:23
LMVR -5000 Static shift to correct for Omega clock jerk from 92116:00 – 92116:02
SDCE No shifts applied No corrections necessary.
UCVF -4000,-113 Static shifts applied to correct for Omega clock jerks: -4000 from 92116:18 – 92117:03 -113 from 92118:01- 92118:02

Station Map

Station map

Station List

(table version)

sta   ondate   offdate   lat   lon   elev   staname   statype   refsta   dnorth   deast   
AQUA   1992116   1992155   33.932   -116.3802   0.458   Colorado River Aqueduct, Desert Hot Springs, Calif   ss   -   0   0   
BRCJ   1992116   1992155   34.069   -116.3923   1.233   Black Rock Campground Joshua, Joshua Tree National   ss   -   0   0   
COVF   1992119   1992145   34.0302   -116.3482   1.638   Covington Flats, Joshua Tree National Monument, Ca   ss   -   0   0   
DWSR   1992119   1992151   33.7658   -116.5461   0.162   Desert Water South Reservoir, Palm Springs. Calif.   ss   -   0   0   
EDC1   1992119   1992147   33.9166   -116.3266   0.48   East Deception Canyon 1, Sky Valley, Calif.   ss   -   0   0   
EDCY   1992124   1992145   33.9038   -116.3364   0.377   East Deception Canyon, Sky Valley, Calif.   ss   -   0   0   
EDOM   1992116   1992147   33.8696   -116.4298   0.454   Edom Hill, Indio Hills, Calif.   ss   -   0   0   
HDVL   1992116   1992147   34.0098   -116.1685   1.261   Hidden Valley, Joshua Tree National Monument, Cali   ss   -   0   0   
KEYV   1992119   1992141   33.9262   -116.1816   1.515   Key's View, Joshua Tree National Monument, Calif.   ss   -   0   0   
LMVR   1992116   1992119   33.9929   -116.5241   0.406   Little Morongo Valley Ranch, Desert Hot Springs, C   ss   -   0   0   
SDCE   1992116   1992155   33.9347   -116.4043   0.416   Smith Desert Country Estate, Desert Hot Springs, C   ss   -   0   0   
UCVF   1992116   1992155   34.0108   -116.3057   1.441   Upper Covington Flats, Joshua Tree National Monume   ss   -   0   0   

Total: 12

Channel List

(table version)

sta   chan   ondate   chanid   offdate   ctype   edepth   hang   vang   descrip   
AQUA   EHE   1992116   64   1992155   n   -0   90   90   -   
AQUA   EHN   1992116   65   1992155   n   -0   0   90   -   
AQUA   EHZ   1992116   66   1992155   n   -0   0   180   -   
AQUA   ELE   1992116   67   1992155   n   -0   90   90   -   
AQUA   ELN   1992116   68   1992155   n   -0   0   0   -   
AQUA   ELZ   1992116   69   1992155   n   -0   0   90   -   
BRCJ   EHE   1992116   70   1992155   n   -0   90   90   -   
BRCJ   EHN   1992116   71   1992155   n   -0   0   90   -   
BRCJ   EHZ   1992116   72   1992155   n   -0   0   180   -   
BRCJ   ELE   1992116   73   1992155   n   -0   90   90   -   
BRCJ   ELN   1992116   74   1992155   n   -0   0   0   -   
BRCJ   ELZ   1992116   75   1992155   n   -0   0   90   -   
COVF   EHE   1992119   76   1992145   n   -0   90   90   -   
COVF   EHN   1992119   77   1992145   n   -0   0   90   -   
COVF   EHZ   1992119   78   1992145   n   -0   0   180   -   
DWSR   EHE   1992119   79   1992151   n   -0   90   90   -   
DWSR   EHN   1992119   80   1992151   n   -0   0   90   -   
DWSR   EHZ   1992119   81   1992151   n   -0   0   180   -   
DWSR   ELE   1992119   82   1992151   n   -0   90   90   -   
DWSR   ELN   1992119   83   1992151   n   -0   0   0   -   
DWSR   ELZ   1992119   84   1992151   n   -0   0   90   -   
EDC1   EHE   1992119   85   1992147   n   -0   90   90   -   
EDC1   EHN   1992119   86   1992147   n   -0   0   90   -   
EDC1   EHZ   1992119   87   1992147   n   -0   0   180   -   
EDCY   EHE   1992124   88   1992145   n   -0   90   90   -   
EDCY   EHN   1992124   89   1992145   n   -0   0   90   -   
EDCY   EHZ   1992124   90   1992145   n   -0   0   180   -   
EDOM   ELE   1992116   91   1992147   n   -0   90   90   -   
EDOM   ELN   1992116   92   1992147   n   -0   0   90   -   
EDOM   ELZ   1992116   93   1992147   n   -0   0   0   -   
EDOM   HHE   1992116   94   1992147   n   -0   90   90   -   
EDOM   HHN   1992116   95   1992147   n   -0   0   90   -   
EDOM   HHZ   1992116   96   1992147   n   -0   0   0   -   
HDVL   ELE   1992116   97   1992147   n   -0   90   90   -   
HDVL   ELN   1992116   98   1992147   n   -0   0   90   -   
HDVL   ELZ   1992116   99   1992147   n   -0   0   0   -   
HDVL   HHE   1992116   100   1992147   n   -0   90   90   -   
HDVL   HHN   1992116   101   1992147   n   -0   0   90   -   
HDVL   HHZ   1992116   102   1992147   n   -0   0   0   -   
KEYV   EHE   1992119   103   1992141   n   -0   90   90   -   
KEYV   EHN   1992119   104   1992141   n   -0   0   90   -   
KEYV   EHZ   1992119   105   1992141   n   -0   0   180   -   
LMVR   EHE   1992116   106   1992119   n   -0   90   90   -   
LMVR   EHN   1992116   107   1992119   n   -0   0   90   -   
LMVR   EHZ   1992116   108   1992119   n   -0   0   180   -   
LMVR   ELE   1992116   109   1992119   n   -0   90   90   -   
LMVR   ELN   1992116   110   1992119   n   -0   0   0   -   
LMVR   ELZ   1992116   111   1992119   n   -0   0   90   -   
SDCE   EHE   1992116   112   1992155   n   -0   90   90   -   
SDCE   EHN   1992116   113   1992155   n   -0   0   90   -   
SDCE   EHZ   1992116   114   1992155   n   -0   0   180   -   
SDCE   ELE   1992116   115   1992155   n   -0   90   90   -   
SDCE   ELN   1992116   116   1992155   n   -0   0   0   -   
SDCE   ELZ   1992116   117   1992155   n   -0   0   90   -   
UCVF   EHE   1992116   118   1992155   n   -0   90   90   -   
UCVF   EHN   1992116   119   1992155   n   -0   0   90   -   
UCVF   EHZ   1992116   120   1992155   n   -0   0   180   -   
UCVF   ELE   1992116   121   1992155   n   -0   90   90   -   
UCVF   ELN   1992116   122   1992155   n   -0   0   0   -   
UCVF   ELZ   1992116   123   1992155   n   -0   0   90   -   

Total: 60

URL: http://eqinfo.ucsd.edu/deployments/portable_aftershock_studies/joshua_tree.php [Last updated: 2015-10-22 (295) 22:24:32 UTC]