2007/06/14 21:57:56 45.13N 120.95W 23 3.9 Oregon
USGS Felt map for this earthquake
USGS Felt reports page for Pacific Northwest US
SLU Moment Tensor Solution 2007/06/14 21:57:56 45.13N 120.95W 23 3.9 Oregon Best Fitting Double Couple Mo = 2.95e+21 dyne-cm Mw = 3.58 Z = 18 km Plane Strike Dip Rake NP1 255 85 20 NP2 163 70 175 Principal Axes: Axis Value Plunge Azimuth T 2.95e+21 18 121 N 0.00e+00 69 268 P -2.95e+21 10 27 Moment Tensor: (dyne-cm) Component Value Mxx -1.54e+21 Mxy -2.35e+21 Mxz -8.98e+20 Myy 1.37e+21 Myz 4.91e+20 Mzz 1.75e+20 #------------- ####-------------- P - #######-------------- ---- ########---------------------- ##########------------------------ ###########------------------------- ############-------------------------- #############--------------------------- ##############---------------------##### ###############------------############### ###############-----###################### #############---########################## #######----------######################### ##--------------######################## -----------------################# ### -----------------################ T ## -----------------############### # -----------------################# ----------------############## ----------------############ ---------------####### ------------## Harvard Convention Moment Tensor: R T F 1.75e+20 -8.98e+20 -4.91e+20 -8.98e+20 -1.54e+21 2.35e+21 -4.91e+20 2.35e+21 1.37e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070614215757/index.html |
University of Washington First Motion Solution |
University of Washington Automated moment Tensor |
STK = 255 DIP = 85 RAKE = 20 MW = 3.58 HS = 18
The solutions from the two techniques are in agreement
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
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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:
hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.2 n 4 p 2The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 35 40 -90 3.07 0.1598 WVFGRD96 1.0 260 75 0 2.98 0.1759 WVFGRD96 2.0 255 75 5 3.15 0.2838 WVFGRD96 3.0 255 80 20 3.24 0.3574 WVFGRD96 4.0 255 80 25 3.31 0.4205 WVFGRD96 5.0 75 90 -35 3.38 0.4800 WVFGRD96 6.0 75 90 -35 3.42 0.5372 WVFGRD96 7.0 255 85 35 3.44 0.5820 WVFGRD96 8.0 75 90 -35 3.48 0.6126 WVFGRD96 9.0 255 85 35 3.49 0.6356 WVFGRD96 10.0 75 90 -30 3.50 0.6500 WVFGRD96 11.0 255 85 30 3.51 0.6609 WVFGRD96 12.0 75 90 -30 3.53 0.6681 WVFGRD96 13.0 255 85 25 3.53 0.6737 WVFGRD96 14.0 75 90 -25 3.54 0.6758 WVFGRD96 15.0 255 85 25 3.55 0.6795 WVFGRD96 16.0 75 90 -20 3.57 0.6791 WVFGRD96 17.0 75 90 -20 3.58 0.6784 WVFGRD96 18.0 255 85 20 3.58 0.6795 WVFGRD96 19.0 75 90 -20 3.61 0.6740 WVFGRD96 20.0 75 90 -20 3.62 0.6704 WVFGRD96 21.0 75 90 -25 3.65 0.6656 WVFGRD96 22.0 255 85 25 3.65 0.6669 WVFGRD96 23.0 255 85 25 3.66 0.6610 WVFGRD96 24.0 255 85 25 3.67 0.6534 WVFGRD96 25.0 255 85 25 3.68 0.6458 WVFGRD96 26.0 255 80 20 3.68 0.6364 WVFGRD96 27.0 255 80 20 3.69 0.6257 WVFGRD96 28.0 255 80 20 3.70 0.6157 WVFGRD96 29.0 255 80 20 3.71 0.6026
The best solution is
WVFGRD96 18.0 255 85 20 3.58 0.6795
The 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 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.2 n 4 p 2
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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. |
The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES STK= 68.28 DIP= 85.29 RAKE= -20.07 OR STK= 159.99 DIP= 70.00 RAKE= -174.99 DEPTH = 15.0 km Mw = 3.58 Best Fit 0.9089 - P-T axis plot gives solutions with FIT greater than FIT90
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion G06A 64 27 iP_D G05A 294 31 iP_C H05A 202 58 iP_D H06A 132 66 iP_C F06A 10 72 iP_D F05A 335 93 iP_C G07A 81 102 iP_C H04A 244 110 iP_D I05A 193 110 iP_D F07A 43 117 iP_D G04A 275 120 iP_C I06A 156 144 iP_C F04A 308 145 iP_C E06A 359 157 iP_D I07A 135 164 iP_C E05A 339 171 eP_- HAWA 38 178 iP_D E07A 28 180 iP_D G03A 277 184 eP_+ F08A 66 185 iP_D I04A 219 189 iP_D H03A 256 192 eP_X H08A 110 193 eP_+
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. |
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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled. |
Sta Az(deg) Dist(km) G07A 81 102 H04A 244 110 I05A 193 110 F07A 43 117 G04A 275 120 I06A 156 144 F04A 308 145 PIN 178 147 E06A 359 157 I07A 135 164 E05A 339 171 HAWA 38 178 E07A 28 180 G03A 277 184 F08A 66 185 I04A 219 189 H03A 256 192 LON 340 192 H08A 110 193 J05A 186 206 E08A 44 211 I03A 236 225 J04A 204 229 I08A 125 233 J07A 146 235 D07A 18 241 D05A 341 242 F09A 74 247 G09A 85 250 I02A 242 261 D04A 328 263 H09A 100 265 K06A 168 265 D08A 36 266 E09A 54 267 K05A 179 267 J08A 134 279 I09A 116 286 J02A 228 287 K04A 193 287 C05A 349 291 C07A 13 293 D09A 43 296 G10A 85 301 K07A 153 304 F10A 71 305 C06A 1 310 J09A 127 323 C04A 332 327 L04A 193 337 NLWA 319 337 H10A 99 338 L05A 178 343 WVOR 147 353 D10A 52 356 B05A 346 359 G11A 84 369 L07A 159 369 K09A 135 374 B07A 9 376 B06A 354 379 B08A 18 380 F11A 76 384 E11A 68 393 H11A 95 393 NEW 39 455 MSO 68 575 BW06 102 950 TPNV 155 990 DGMT 68 1329
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:
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 n 3 lp c 0.10 n 3 br c 0.12 0.2 n 4 p 2
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 digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: