2005/04/27 00:33:00 30.263 114.125W 2 5.1 BAJA
USGS Felt map for this earthquake
USGS Felt reports page for Outside US
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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Waveform Inversion
The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
The results of this grid search from 0.5 to 19 km depth are as follow:hp c 0.01 3 lp c 0.02 3
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 230 40 -50 4.73 0.2570 WVFGRD96 1.0 280 75 -25 4.68 0.2242 WVFGRD96 2.0 55 70 -70 4.81 0.2560 WVFGRD96 3.0 55 70 -70 4.84 0.2777 WVFGRD96 4.0 60 75 -75 4.93 0.3163 WVFGRD96 5.0 55 70 -75 4.92 0.3301 WVFGRD96 6.0 55 65 -75 4.91 0.3382 WVFGRD96 7.0 55 65 -75 4.92 0.3396 WVFGRD96 8.0 50 65 -80 4.95 0.3389 WVFGRD96 9.0 280 40 -15 4.96 0.3387 WVFGRD96 10.0 280 45 -15 4.95 0.3358 WVFGRD96 11.0 285 50 -5 4.94 0.3307 WVFGRD96 12.0 285 55 -5 4.94 0.3267 WVFGRD96 13.0 285 60 0 4.94 0.3235 WVFGRD96 14.0 285 60 0 4.94 0.3186 WVFGRD96 15.0 285 65 5 4.94 0.3143 WVFGRD96 16.0 285 70 10 4.94 0.3094 WVFGRD96 17.0 285 70 10 4.94 0.3047 WVFGRD96 18.0 285 70 10 4.94 0.2998 WVFGRD96 19.0 285 70 10 4.94 0.2939The mechanism correspond to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed and because the velocity model used in the predictions may not be perfect. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The bandpass filter used in the processing and for the display was
hp c 0.01 3 lp c 0.02 3
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion GLA 348 316 BAR 319 361 TUC 54 391 MWC 321 574 GSC 336 612 ISA 327 724 DAC 335 741 MNTX 77 850 TPH 343 912 MVU 10 931 MNV 339 979 SAO 319 990 LTX 93 1016 CMB 328 1037 SDCO 42 1149 BMN 347 1162 ELK 355 1167 HOPS 322 1268 HWUT 10 1280 AMTX 65 1285 WDC 329 1375 JCT 85 1376 MOD 338 1405 WVOR 345 1409 AHID 10 1413 BW06 15 1446 HLID 359 1476 YBH 331 1488 HUMO 332 1580 LKWY 11 1621 CBKS 50 1623 BOZ 7 1722 COR 336 1783 MSO 0 1839 HAWA 347 1851 KSU1 54 1876 LAO 18 1947 MIAR 71 1986 NEW 354 2015 UALR 70 2102 LTL 83 2240 CCM 62 2272 MPH 70 2325 FVM 62 2340 OXF 72 2366 PVMO 67 2366 SLM 61 2374 SIUC 64 2436 UTMT 67 2439 PLAL 71 2492 WVT 68 2528 LRAL 76 2584 WCI 63 2699 BLO 61 2701 GOGA 75 2912 ACSO 60 3023 DWPF 86 3177 NHSC 76 3220 MCWV 62 3277 ERPA 57 3312 SSPA 61 3457 CBN 65 3474 SDMD 63 3530 BINY 58 3639 PAL 61 3795 NCB 55 3813 HRV 58 4006
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.
The velocity model used for the search is a modified Utah model .
Digital data were collected, instrument 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.
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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 preferred 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. Each solution is plotted as a vector at a given value of strike and dip with the 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. |
Sta Az(deg) Dist(km) GLA 348 316 BAR 319 361 TUC 54 391 MWC 321 574 GSC 336 612 ISA 327 724 DAC 335 741 MNTX 77 850 TPH 343 912 MVU 10 931 MNV 339 979 SAO 319 990 LTX 93 1016 CMB 328 1037 SDCO 42 1149 BMN 347 1162 ELK 355 1167 HOPS 322 1268 HWUT 10 1280 AMTX 65 1285 WDC 329 1375 JCT 85 1376 MOD 338 1405 WVOR 345 1409 AHID 10 1413 BW06 15 1446 HLID 359 1476 YBH 331 1488 HUMO 332 1580 LKWY 11 1621 CBKS 50 1623 BOZ 7 1722 COR 336 1783 MSO 0 1839 HAWA 347 1851 KSU1 54 1876 LAO 18 1947 MIAR 71 1986 NEW 354 2015 UALR 70 2102 LTL 83 2240 CCM 62 2272 MPH 70 2325 FVM 62 2340 OXF 72 2366 PVMO 67 2366 SLM 61 2374 SIUC 64 2436 UTMT 67 2439 PLAL 71 2492 WVT 68 2528 LRAL 76 2584 WCI 63 2699 BLO 61 2701 GOGA 75 2912 ACSO 60 3023 DWPF 86 3177 NHSC 76 3220 MCWV 62 3277 ERPA 57 3312 SSPA 61 3457 CBN 65 3474 SDMD 63 3530 BINY 58 3639 PAL 61 3795 NCB 55 3813 HRV 58 4006
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 observed and predicted waveforms.
The velocity model used for the waveform fit is a modified Utah model .
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 0.03 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
DUG, HAWA had gains too low. MNV had data dropouts, HAWA seemed to have gains slightly too low.
The following stations did not have a valid response files:
