USGS Felt map for this earthquake
USGS Felt reports page for Western Mountain Region
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= 174.99
DIP= 54.99
RAKE= -95.00
OR
STK= 3.66
DIP= 35.31
RAKE= -82.91
DEPTH = 4.0 km
Mw = 4.39
Best Fit 0.8648 - 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 SDCO 329 119 e- ANMO 216 255 e- ISCO 348 344 i- CBKS 62 499 e+
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) SDCO 330 119 ANMO 216 255 ISCO 349 334 CBKS 63 500 WMOK 111 593 WUAZ 258 607 MVU 288 678 TUC 229 741 BW06 330 775 HWUT 314 788 RSSD 4 813 AHID 323 851 HVU 311 879 TPH 281 1106 HLID 316 1107 PFO 255 1117 HKT 130 1130 DAC 271 1145 BMN 294 1151 UALR 98 1153 CCM 79 1208 FVM 80 1282 SLM 77 1302 WVOR 302 1338 PVMO 88 1354 MPH 94 1357 CMB 280 1383 SIUC 81 1388 JFWS 57 1416 UTMT 88 1430 MOD 298 1446 WVT 88 1522 PLAL 93 1523 USIN 80 1524 WDC 291 1594 HOPS 284 1620 NEW 326 1620 BLO 75 1627 WCI 79 1642 ULM 23 1655 LRAL 99 1679 OCWA 314 1996 MCWV 74 2202 SSPA 72 2372 DWPF 107 2397 BINY 68 2537 PAL 71 2706 LBNH 63 2884 HRV 67 2903 SCHQ 43 3500
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 0.10 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
The following stations did not have a valid response files: AAM ACCN ACSO BEKR BGU BOZ CBKS CPCT DWPF FFD FRNY GNW HAWA HBAR HUMO ISCO JLU KSU1 LON LUPA MGP MIAR MPU MVL MYNC NCB NHSC NIKO NLU OXF REDW SAO SDMD SPUT SRU SSPA SWET TCUT WCN YBH
The following stations had an incorrect response file: JCT and BLA.
The following stations were not used because of questions about instrument function: LKWY and GOGA
