USGS Felt map for this earthquake
USGS Felt reports page for California
UC Berkeley moment tensor solution
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= 104.99
DIP= 54.99
RAKE= 69.99
OR
STK= 317.40
DIP= 39.68
RAKE= 116.04
DEPTH = 5.0 km
Mw = 4.62
Best Fit 0.8414 - P-T axis plot gives solutions with FIT greater than FIT90
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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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| Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the rpeferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here. |
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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) First motion Sta Az Dist First motion CMB 14 267 i+
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) CMB 14 267 DAC 78 323 HOPS 335 405 TPH 52 435 PFO 117 485 WDC 347 556 BMN 32 626 MOD 6 692 WVOR 15 777 MVU 66 849 WUAZ 88 881 HVU 44 989 COR 350 1004 TUC 109 1024 HLID 31 1044 HWUT 49 1057 AHID 45 1165 BW06 48 1266 ANMO 89 1332 OCWA 350 1362 SDCO 76 1410 NEW 12 1433 RSSD 52 1728 WMOK 86 2030 HKT 98 2443 ULM 44 2595 CCM 76 2663 JFWS 64 2761 SIUC 76 2844 WVT 79 2988 PLAL 82 2989 WCI 74 3094 LRAL 85 3135 MCWV 70 3634 CBN 73 3868 BINY 66 3930 PAL 67 4116 LBNH 62 4241 HRV 64 4290 SCHQ 45 4612
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 0.02 3 lp c 0.05 3
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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 modified Utah model 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
We did not ahve response files for the following stations: AAM KSU1 ACCN LON BEKR LUPA BGU MIAR MPU CBKS MVL CPCT NEN DAWY NLU DLBC PGC FCC PVMO REDW GNW SAO HAWA SPUT HUMO SRU INK SSPA ISCO SWET JLU TCUT KBO KEBM WCN KHMM YBH KRMB YKW3
The data from LKWY, GOGA CBN and JCT were not used because of odd spectral amplitudes. This may indicate instrument or response file problems.
