2008/01/30 10:07:07 62.433 -137.052 10.0 4.4 Yukon, Canada
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2008/01/30 10:07:07 62.433 -137.052 10.0 4.4 Yukon, Canada
Best Fitting Double Couple
Mo = 3.43e+22 dyne-cm
Mw = 4.29
Z = 12 km
Plane Strike Dip Rake
NP1 302 56 113
NP2 85 40 60
Principal Axes:
Axis Value Plunge Azimuth
T 3.43e+22 69 263
N 0.00e+00 19 109
P -3.43e+22 9 16
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.09e+22
Mxy -8.31e+21
Mxz -6.28e+21
Myy 1.69e+21
Myz -1.26e+22
Mzz 2.92e+22
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############## T ##################------#
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Harvard Convention
Moment Tensor:
R T F
2.92e+22 -6.28e+21 1.26e+22
-6.28e+21 -3.09e+22 8.31e+21
1.26e+22 8.31e+21 1.69e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080130100707/index.html
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STK = 85
DIP = 40
RAKE = 60
MW = 4.29
HS = 12
The waveform inversion solution is preferred. The surface-wave amplitude spectrum result is simliar.
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 0.25 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 80 85 -10 4.09 0.5057
WVFGRD96 1.0 80 80 -10 4.11 0.5075
WVFGRD96 2.0 80 75 -20 4.15 0.4834
WVFGRD96 3.0 260 90 45 4.20 0.4727
WVFGRD96 4.0 80 90 -45 4.20 0.4873
WVFGRD96 5.0 260 90 45 4.20 0.5037
WVFGRD96 6.0 75 85 -45 4.19 0.5186
WVFGRD96 7.0 120 30 80 4.33 0.5457
WVFGRD96 8.0 115 35 75 4.33 0.5716
WVFGRD96 9.0 110 35 70 4.31 0.5827
WVFGRD96 10.0 115 35 75 4.34 0.5861
WVFGRD96 11.0 85 40 60 4.30 0.5888
WVFGRD96 12.0 85 40 60 4.29 0.5914
WVFGRD96 13.0 85 40 60 4.29 0.5897
WVFGRD96 14.0 85 40 55 4.29 0.5863
WVFGRD96 15.0 80 40 50 4.29 0.5816
WVFGRD96 16.0 80 40 50 4.30 0.5755
WVFGRD96 17.0 80 40 50 4.30 0.5683
WVFGRD96 18.0 80 40 50 4.31 0.5606
WVFGRD96 19.0 75 45 45 4.31 0.5518
WVFGRD96 20.0 75 40 45 4.34 0.5413
WVFGRD96 21.0 75 40 40 4.35 0.5328
WVFGRD96 22.0 75 40 40 4.36 0.5232
WVFGRD96 23.0 75 40 40 4.36 0.5122
WVFGRD96 24.0 75 40 40 4.37 0.5029
WVFGRD96 25.0 70 40 35 4.38 0.4910
WVFGRD96 26.0 70 40 35 4.38 0.4788
WVFGRD96 27.0 70 40 35 4.39 0.4678
WVFGRD96 28.0 70 40 35 4.40 0.4545
WVFGRD96 29.0 70 40 35 4.40 0.4427
The best solution is
WVFGRD96 12.0 85 40 60 4.29 0.5914
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 0.25 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= 329.30
DIP= 58.23
RAKE= 137.57
OR
STK= 84.99
DIP= 55.00
RAKE= 40.00
DEPTH = 12.0 km
Mw = 4.35
Best Fit 0.8116 - 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 DAWY 328 217 -12345 WHY 149 229 -12345 EGAK 324 331 -12345 PNL 203 333 -12345 SKAG 164 344 -12345 DOT 294 379 -12345 PAX 282 435 -12345 EYAK 249 508 -12345 DLBC 136 589 -12345 COLA 302 599 -12345 MCK 289 617 -12345 PMR 267 639 -12345
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.
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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. 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) DAWY 328 217 WHY 149 229 EGAK 324 331 PNL 203 333 SKAG 164 344 DOT 294 379 PAX 282 435 BESE 163 446 EYAK 250 508 DLBC 136 589 SAW 268 592 COLA 302 599 SIT 170 607 MCK 289 617 PMR 267 639 WRAK 156 721 BPAW 291 722 COLD 316 819 FNBB 111 861 CTLN 69 1068 BMBC 123 1105 COWN 65 1301 EDM 116 1726 PGC 145 1750 PNT 136 1810 NLWA 148 1867 WALA 126 2059 HAWA 140 2102 ARVN 74 2232 FCC 80 2344 WVOR 143 2533 LAO 118 2610 LOHW 128 2706 SNOW 129 2714 ELK 139 2816 SMCO 128 3284 JFWS 104 3714 KSU1 115 3749 HDIL 105 3971 WMOK 121 4065 AAM 98 4102
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:
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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 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
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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 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 CUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
MODEL.01 CUS Model with Q from simple gamma values ISOTROPIC KGS FLAT EARTH 1-D CONSTANT VELOCITY LINE08 LINE09 LINE10 LINE11 H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Wed Jan 30 10:35:17 CST 2008
