USGS Felt map for this earthquake
USGS Felt reports page for Pacific Northwest 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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UCB Moment Tensor
Inversion method: complete waveform Stations used: HUMO MOD YBH WDC JCC Berkeley Moment Tensor Solution Best Fitting Double-Couple: Mo = 7.96E+22 Dyne-cm Mw = 4.57 Z = 24. km Plane Strike Rake Dip NP1 159 8 87 NP2 69 177 82 Principal Axes: Axis Value Plunge Azimuth T 7.960 8 24 N 0.000 81 179 P -7.960 4 294 Event Date/Time: August 19, 2004 at 06:06:03 UTC Event ID: uw08190606 Moment Tensor: Scale = 10**22 Dyne-cm Component Value Mxx 5.252 Mxy 5.811 Mxz 0.780 Myy -5.368 Myz 0.881 Mzz 0.116 ####### --############## ------############# T ### --------############# ##### -----------###################### -------------###################### ------------####################### P -------------######################- --------------###################--- ------------------###############-------- -------------------##########------------ --------------------#####---------------- --------------------#-------------------- ----------------#####-------------------- ---------############------------------ -#####################----------------- #####################---------------- #####################-------------- #####################------------ ####################--------- ###################------ #################-- #######NODAL PLANES STK= 154.99 DIP= 64.99 RAKE= 74.99 OR STK= 7.38 DIP= 28.91 RAKE= 119.04 DEPTH = 23.0 km Mw = 4.70 Best Fit 0.8539 - P-T axis plot gives solutions with FIT greater than FIT90
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion GNW 19 343 i+ OCWA 2 344 i+ HAWA 61 420 i+
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) LON 39 303 GNW 19 343 OCWA 2 344 HAWA 61 420 PGC 8 449 WDC 162 476 WVOR 116 521 CBB 353 603 WSLR 9 617 PNT 33 629 NEW 52 682 LLLB 14 686 HLID 95 801 CMB 155 806 MSO 70 842 MNV 141 861 BBB 343 883 TPH 139 941 SPU 107 1036 DUG 114 1067 YFT 86 1070 AHID 97 1084 REDW 93 1088 HWUT 104 1090 DAC 146 1091 NOQ 110 1093 ISA 152 1115 CTU 109 1119 TCU 106 1124 NLU 113 1135 JLU 109 1146 MPU 112 1164 BW06 95 1206 EDM 36 1244 SRU 114 1298 LAO 74 1425 WUAZ 128 1496 DGMT 68 1595 ISCO 103 1631 SDCO 110 1749 YKW3 14 2090 CBKS 99 2136 WMOK 108 2437 FCC 42 2578 INK 352 2690 MIAR 103 2847 FVM 93 2910 UALR 101 2930 PVMO 95 3047 MPH 98 3099 USIN 91 3130 OXF 99 3174 LRAL 99 3451 ERPA 79 3560 NCB 73 3935 NHSC 93 3984 ACCN 74 3999 PAL 78 4077
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 0.02 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:
