2004/11/16 18:21:28 42.00N 120.45W 9 3.8 Oregon
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western 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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NODAL PLANES
STK= 149.99
DIP= 74.99
RAKE= -130.00
OR
STK= 42.85
DIP= 42.27
RAKE= -22.64
DEPTH = 012.0 km
Mw = 3.95
Best Fit 0.8641 - 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 WDC 229 237 eP_X BMN 122 322 eP_+ CMB 179 441 eP_X TPH 147 516 eP_X
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) YBH 262 190 WDC 229 237 ORV 198 286 BMN 122 322 JCC 247 327 COR 322 369 HOPS 214 402 CMB 179 441 TPH 147 516 HLID 69 523 SAO 189 588 TPNV 146 667 OCWA 336 704 HWUT 90 739 NEW 19 743 MSO 42 745 AHID 81 774 MVU 116 800 BOZ 57 816 SHB 344 885 BW06 81 900 WALA 31 934 LLLB 354 963 LAO 60 1246 BBB 336 1271 ISCO 96 1272 EDM 21 1355 SDCO 105 1361 TUC 138 1374 DGMT 55 1461 MOBC 329 1510 CBKS 94 1789 ULM 56 2095
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 0.05 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
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:
