The ANSS event ID is us7000ishe and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us7000ishe/executive.
2022/11/26 03:50:16 49.271 -126.092 33.4 4.9 BC, Canada
USGS/SLU Moment Tensor Solution
ENS 2022/11/26 03:50:16:0 49.27 -126.09 33.4 4.9 BC, Canada
Stations used:
C8.BCOV CN.BPEB CN.CBB CN.CLRS CN.FHBB CN.GDR CN.MGRB
CN.NLLB CN.NTKA CN.OZB CN.PABB CN.PACB CN.PGC CN.PHC
CN.PTRF CN.SYMB CN.TAHB CN.TXDB CN.VDEB CN.VGZ CN.WOSB
CN.WSLR PQ.ALBHB UW.BHAM UW.CROWN UW.DONK UW.HDW UW.LRIV
UW.MULN UW.OHOH UW.OTR UW.RNWY UW.SAXON UW.SLDQ UW.SNAG
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 1.46e+23 dyne-cm
Mw = 4.71
Z = 33 km
Plane Strike Dip Rake
NP1 323 81 -160
NP2 230 70 -10
Principal Axes:
Axis Value Plunge Azimuth
T 1.46e+23 7 95
N 0.00e+00 68 347
P -1.46e+23 21 188
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.24e+23
Mxy -3.15e+22
Mxz 4.66e+22
Myy 1.40e+23
Myz 2.52e+22
Mzz -1.63e+22
--------------
----------------------
###-------------------------
######------------------------
##########-----------------#######
#############-----------############
###############-------################
##################--####################
##################-#####################
################------####################
##############---------################
#############------------############## T
###########---------------#############
#########-----------------##############
#######--------------------#############
#####----------------------###########
###------------------------#########
#--------------------------#######
----------- ------------####
---------- P -------------##
------- ------------
--------------
Global CMT Convention Moment Tensor:
R T P
-1.63e+22 4.66e+22 -2.52e+22
4.66e+22 -1.24e+23 3.15e+22
-2.52e+22 3.15e+22 1.40e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221126035016/index.html
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STK = 230
DIP = 70
RAKE = -10
MW = 4.71
HS = 33.0
The NDK file is 20221126035016.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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:
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 320 75 -15 4.05 0.2179
WVFGRD96 2.0 320 75 -20 4.17 0.2872
WVFGRD96 3.0 320 75 -15 4.22 0.3173
WVFGRD96 4.0 320 80 -15 4.25 0.3345
WVFGRD96 5.0 230 80 -20 4.29 0.3446
WVFGRD96 6.0 230 80 -20 4.32 0.3669
WVFGRD96 7.0 230 80 -20 4.35 0.3886
WVFGRD96 8.0 230 80 -20 4.39 0.4121
WVFGRD96 9.0 230 80 -20 4.41 0.4274
WVFGRD96 10.0 230 85 -25 4.42 0.4444
WVFGRD96 11.0 235 90 -25 4.44 0.4624
WVFGRD96 12.0 55 90 20 4.46 0.4804
WVFGRD96 13.0 235 90 -20 4.48 0.4976
WVFGRD96 14.0 230 70 -15 4.50 0.5138
WVFGRD96 15.0 230 70 -15 4.52 0.5340
WVFGRD96 16.0 230 70 -15 4.53 0.5540
WVFGRD96 17.0 230 70 -15 4.54 0.5743
WVFGRD96 18.0 230 70 -10 4.56 0.5934
WVFGRD96 19.0 230 70 -10 4.57 0.6116
WVFGRD96 20.0 230 70 -10 4.59 0.6289
WVFGRD96 21.0 230 70 -10 4.60 0.6445
WVFGRD96 22.0 230 70 -10 4.61 0.6601
WVFGRD96 23.0 230 70 -10 4.62 0.6749
WVFGRD96 24.0 230 70 -10 4.63 0.6887
WVFGRD96 25.0 230 70 -10 4.64 0.7018
WVFGRD96 26.0 230 70 -10 4.65 0.7136
WVFGRD96 27.0 230 70 -10 4.66 0.7233
WVFGRD96 28.0 230 70 -10 4.67 0.7318
WVFGRD96 29.0 230 70 -10 4.68 0.7381
WVFGRD96 30.0 230 70 -10 4.69 0.7421
WVFGRD96 31.0 230 70 -10 4.70 0.7455
WVFGRD96 32.0 230 70 -10 4.71 0.7465
WVFGRD96 33.0 230 70 -10 4.71 0.7467
WVFGRD96 34.0 230 70 -10 4.72 0.7451
WVFGRD96 35.0 230 70 -10 4.73 0.7428
WVFGRD96 36.0 230 70 -10 4.74 0.7401
WVFGRD96 37.0 230 70 -10 4.75 0.7379
WVFGRD96 38.0 230 70 -10 4.77 0.7368
WVFGRD96 39.0 230 70 -10 4.78 0.7371
WVFGRD96 40.0 230 65 -15 4.82 0.7313
WVFGRD96 41.0 230 65 -15 4.83 0.7338
WVFGRD96 42.0 230 65 -15 4.84 0.7344
WVFGRD96 43.0 230 70 -15 4.85 0.7331
WVFGRD96 44.0 230 70 -15 4.85 0.7314
WVFGRD96 45.0 230 70 -15 4.86 0.7288
WVFGRD96 46.0 230 70 -15 4.87 0.7252
WVFGRD96 47.0 230 70 -15 4.87 0.7210
WVFGRD96 48.0 230 70 -10 4.88 0.7169
WVFGRD96 49.0 230 70 -10 4.89 0.7129
WVFGRD96 50.0 230 70 -10 4.89 0.7077
WVFGRD96 51.0 225 65 -10 4.90 0.7037
WVFGRD96 52.0 225 65 -10 4.90 0.6992
WVFGRD96 53.0 225 65 -10 4.91 0.6952
WVFGRD96 54.0 225 65 -10 4.91 0.6902
WVFGRD96 55.0 225 65 -10 4.92 0.6852
WVFGRD96 56.0 225 65 -10 4.92 0.6805
WVFGRD96 57.0 225 65 -10 4.92 0.6753
WVFGRD96 58.0 225 65 -10 4.93 0.6701
WVFGRD96 59.0 225 65 -10 4.93 0.6646
The best solution is
WVFGRD96 33.0 230 70 -10 4.71 0.7467
The mechanism corresponding 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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3
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| Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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 check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01
Model after 8 iterations
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
1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00
6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00
13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00
19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00
0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00