Joshua Tree Aftershock Data

Summary

Introduction

The Ml 6.1 Joshua Tree earthquake of April 23rd, 1992 05:50 GMT occurred at 33°N 57.7', 116°W 19.2' approximately 8 km northeast of the southern San Andreas fault and about 20 km south of the Pinto Mountain fault. 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

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

Total: 12

Channel List

(plain text 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: 2006-10-26 (299) 22:59:09 UTC]