2007/01/09 15:49:35 59.37N 136.87W 15 5.7 British Columbia, Canada
USGS Felt map for this earthquake
USGS Felt reports page for Canada
|
|
|
|
STK = 275
DIP = 60
RAKE = 30
MW = 5.59
HS = 13
The surface-wave spectral amplitude solution is preferred since its Mw is closer to teleseismic estimates. The wavefrom inversion is similar except that the moment is about 0.18 units smaller.
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.
|
|
|
|
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 0.01 n 3 lp c 0.10 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 255 80 -35 5.25 0.5165
WVFGRD96 1.0 250 75 -40 5.27 0.5156
WVFGRD96 2.0 245 70 -55 5.36 0.4995
WVFGRD96 3.0 250 80 -65 5.42 0.4975
WVFGRD96 4.0 265 45 0 5.26 0.5321
WVFGRD96 5.0 265 45 5 5.27 0.5726
WVFGRD96 6.0 265 45 5 5.28 0.6080
WVFGRD96 7.0 265 50 10 5.30 0.6402
WVFGRD96 8.0 265 50 10 5.31 0.6675
WVFGRD96 9.0 265 50 10 5.32 0.6889
WVFGRD96 10.0 270 45 25 5.35 0.7074
WVFGRD96 11.0 270 45 25 5.36 0.7222
WVFGRD96 12.0 270 45 25 5.38 0.7333
WVFGRD96 13.0 270 45 25 5.39 0.7407
WVFGRD96 14.0 270 45 25 5.40 0.7448
WVFGRD96 15.0 270 45 25 5.41 0.7455
WVFGRD96 16.0 275 45 30 5.41 0.7434
WVFGRD96 17.0 275 45 30 5.42 0.7397
WVFGRD96 18.0 275 45 30 5.43 0.7335
WVFGRD96 19.0 275 45 30 5.44 0.7250
The best solution is
WVFGRD96 15.0 270 45 25 5.41 0.7455
The mechanism correspond to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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 n 3 lp c 0.10 n 3
|
|
|
|
| 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.
|
|
|
The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 168.88
DIP= 64.34
RAKE= 146.31
OR
STK= 274.98
DIP= 60.00
RAKE= 30.00
DEPTH = 013.0 km
Mw = 5.59
Best Fit 0.8015 - 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 SKAG 91 98 iP_D PNL 280 134 iP_D WHY 42 179 iP_D WRAK 139 441 iP_C EYAK 288 499 eP_X CRAG 150 506 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.
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.
|
|
|
|
| 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. |
The distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km) PNL 280 134 WRAK 139 441 CRAG 150 506 DAWY 347 541 PAX 314 596 EGAK 342 629 COLA 322 822 BPAW 311 895 PPLA 302 897 BMBC 106 952 BBB 142 986 INK 8 1000 COLD 328 1083 YKW3 65 1256 YKW1 65 1262 LLLB 129 1380 A05A 134 1516 B04A 139 1549 EDM 105 1600 TNA 306 1708 JCC 151 2268 RES 29 2405 FCC 73 2416 GASB 149 2427 LAO 111 2476 HOPS 150 2486 BW06 122 2643 HWUT 127 2656 ILON 44 2767 ULM 93 2786 RSSD 113 2803 EYMN 93 3196 MVCO 128 3210 PFO 143 3252 SDCO 123 3298 SNQN 70 3324 BAR 144 3342 CBKS 114 3487 FRB 52 3494 ANMO 127 3520 SCIA 103 3571 TUC 136 3595 KSU1 110 3626 GLMI 92 3820 WMOK 118 3907 SLM 104 4012 SFJD 41 4066 AAM 94 4074 SCHQ 66 4086 SADO 87 4135 SIUC 104 4147 BLO 100 4172 MIAR 112 4193 USIN 102 4211 UALR 110 4238 JCT 122 4260 WCI 101 4265 UTMT 105 4286 ALLY 91 4326 MPH 107 4336 WVT 104 4362 NATX 116 4393 LONY 83 4404 OXF 107 4419 FRNY 82 4444 PLAL 106 4450 NCB 84 4476 BINY 87 4518 SSPA 90 4537 ACCN 84 4554 LBNH 82 4584 KVTX 122 4634 BLA 96 4682 SDMD 91 4699 LTL 112 4709 BRNJ 88 4722 PAL 87 4733 CPNY 87 4748 CUNY 87 4760 CBN 92 4770 GOGA 102 4843 BRAL 108 4863 DRLN 66 4975 CNNC 96 4996 NHSC 99 5054 DWPF 105 5436
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 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 0.01 n 3 lp c 0.10 n 3
|
|
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
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 CUS 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.
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
