USGS Felt map for this earthquake
USGS Felt reports page for California
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
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NODAL PLANES
STK= 307.15
DIP= 55.61
RAKE= 96.94
OR
STK= 114.99
DIP= 35.00
RAKE= 79.99
DEPTH = 6.0 km
Mw = 6.54 km
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Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.
Digital data were collected, intreument response removed and traces converted
to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms. The figure
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. The Each solution is plotted as a vector at a given value of strike and dip witht he angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. A nearly vertical strike-slip fault striking at 75 or 165 degrees is preferred. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled. |
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) P-first motion CMB 14 266 DAC 78 322 HOPS 335 404 TPH 52 434 TPNV 71 457 PFO 117 485 WDC 347 555 BMN 32 625 MOD 6 691 WVOR 15 776 MVU 66 848 WUAZ 89 881 CTU 53 987 HVU 44 988 COR 350 1003 TUC 109 1024 HLID 31 1043 HWUT 49 1056 AHID 45 1164 BW06 48 1266 YMR 38 1315 ANMO 89 1332 LKWY 40 1339 BOZ 33 1361 OCWA 350 1361 MSO 24 1371 SDCO 76 1410 NEW 12 1432 RSSD 52 1727 LTX 108 1781 WMOK 86 2030 JCT 100 2066 HKT 98 2443 ULM 44 2594 UALR 84 2610 CCM 76 2663 FVM 76 2735 SLM 74 2747 JFWS 64 2761 GNAR 80 2796 PVMO 79 2818 MPH 82 2820 SIUC 76 2844 OXF 83 2880 UTMT 79 2893 USIN 75 2977 WVT 79 2988 PLAL 82 2989 BLO 72 3065 WCI 74 3094 LRAL 86 3135 DIV 336 3321 ACSO 70 3360 GOGA 83 3442 PMR 334 3485 BLA 75 3622 MCWV 70 3633 MCK 337 3643 COLA 340 3700 DWPF 91 3818 CBN 73 3868 BINY 66 3930 UNV 314 4026 NCB 62 4059 PAL 67 4116 HNH 63 4216 LBNH 62 4240 HRV 64 4289 BRYW 65 4299 WES 65 4310 RES 10 4558 SCHQ 45 4611 PAYG 135 5141
Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the osberved and predicted waveforms.
The fits to the waveforms with the given mechanism are show below:
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This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:
hp c 0.01 3 lp c 0.025 3
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The focal mechanism nodal planes are well developed. However since the P-wave first motion data are of poor quality and the local velocity model is poorly known, we cannot resolve whther this is a thrust or normal faulting event.
Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.
Dr. Harley Benz, USGS, provided the USGS USNSN digital data.
The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the
theoretical predictions as a function of period for the mechanism given above. The CUS earth model
was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.
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Here we tabulate the reasons for not using certain digital data sets
