The ANSS event ID is uw62050041 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/uw62050041/executive.
2024/09/26 11:05:20 48.574 -123.250 52.0 4.o BC, Canada
USGS/SLU Moment Tensor Solution
ENS 2024/09/26 11:05:20:0 48.57 -123.25 52.0 4.0 BC, Canada
Stations used:
CN.CLRS CN.PGC CN.PTRF CN.QEPB CN.SYMB CN.VDEB CN.VGZ
PQ.ALBHB PQ.DAOB UW.BHAM UW.BHW UW.CHIMA UW.DONK UW.DOSE
UW.EQUIL UW.GUEM UW.HILL UW.LOPEZ UW.LRIV UW.LUMI UW.MBW2
UW.MULN UW.OLGA UW.OSQM UW.PASS UW.RNWY UW.SAXON UW.TURTL
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 8.61e+21 dyne-cm
Mw = 3.89
Z = 53 km
Plane Strike Dip Rake
NP1 340 75 -80
NP2 126 18 -123
Principal Axes:
Axis Value Plunge Azimuth
T 8.61e+21 29 62
N 0.00e+00 10 157
P -8.61e+21 59 264
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.42e+21
Mxy 2.47e+21
Mxz 2.15e+21
Myy 2.82e+21
Myz 7.03e+21
Mzz -4.24e+21
##############
------################
----------##################
-------------#################
----------------##################
------------------##################
#-------------------########### ####
#---------------------########## T #####
#---------------------########## #####
##----------------------##################
##---------- ----------#################
##---------- P ----------#################
###--------- -----------################
###----------------------###############
###-----------------------##############
###----------------------#############
####---------------------###########
#####-------------------##########
#####-----------------########
#######--------------#####--
#########-------------
##############
Global CMT Convention Moment Tensor:
R T P
-4.24e+21 2.15e+21 -7.03e+21
2.15e+21 1.42e+21 -2.47e+21
-7.03e+21 -2.47e+21 2.82e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240926110520/index.html
|
STK = 340
DIP = 75
RAKE = -80
MW = 3.89
HS = 53.0
The NDK file is 20240926110520.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.
 |
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.
|
|
|
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 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 140 70 15 2.97 0.1854
WVFGRD96 2.0 330 45 95 3.20 0.3187
WVFGRD96 3.0 320 80 5 3.23 0.3243
WVFGRD96 4.0 190 75 35 3.26 0.3342
WVFGRD96 5.0 190 70 50 3.32 0.3850
WVFGRD96 6.0 190 75 55 3.33 0.4248
WVFGRD96 7.0 195 70 55 3.34 0.4505
WVFGRD96 8.0 195 70 60 3.41 0.4658
WVFGRD96 9.0 190 75 55 3.39 0.4763
WVFGRD96 10.0 345 65 -55 3.43 0.4960
WVFGRD96 11.0 345 60 -60 3.46 0.5120
WVFGRD96 12.0 345 60 -60 3.47 0.5236
WVFGRD96 13.0 345 65 -55 3.46 0.5301
WVFGRD96 14.0 350 55 -55 3.51 0.5369
WVFGRD96 15.0 350 55 -55 3.52 0.5438
WVFGRD96 16.0 350 55 -55 3.53 0.5479
WVFGRD96 17.0 350 55 -55 3.53 0.5494
WVFGRD96 18.0 350 55 -55 3.54 0.5487
WVFGRD96 19.0 355 60 -50 3.54 0.5460
WVFGRD96 20.0 350 60 -50 3.55 0.5436
WVFGRD96 21.0 180 85 45 3.52 0.5462
WVFGRD96 22.0 180 85 50 3.52 0.5511
WVFGRD96 23.0 350 90 -50 3.53 0.5556
WVFGRD96 24.0 350 90 -50 3.54 0.5607
WVFGRD96 25.0 345 90 -55 3.55 0.5653
WVFGRD96 26.0 345 90 -55 3.56 0.5703
WVFGRD96 27.0 170 85 55 3.58 0.5778
WVFGRD96 28.0 345 90 -55 3.59 0.5792
WVFGRD96 29.0 170 85 55 3.60 0.5841
WVFGRD96 30.0 345 90 -60 3.61 0.5839
WVFGRD96 31.0 170 85 60 3.62 0.5928
WVFGRD96 32.0 345 90 -60 3.63 0.5974
WVFGRD96 33.0 345 90 -60 3.64 0.6038
WVFGRD96 34.0 165 90 60 3.65 0.6101
WVFGRD96 35.0 165 90 65 3.65 0.6162
WVFGRD96 36.0 340 85 -65 3.66 0.6267
WVFGRD96 37.0 345 85 -65 3.66 0.6329
WVFGRD96 38.0 340 80 -65 3.67 0.6392
WVFGRD96 39.0 340 80 -65 3.67 0.6456
WVFGRD96 40.0 340 80 -75 3.81 0.6526
WVFGRD96 41.0 340 80 -75 3.82 0.6668
WVFGRD96 42.0 340 80 -75 3.82 0.6787
WVFGRD96 43.0 340 80 -75 3.83 0.6887
WVFGRD96 44.0 340 80 -75 3.84 0.6969
WVFGRD96 45.0 340 80 -75 3.85 0.7038
WVFGRD96 46.0 335 75 -75 3.85 0.7104
WVFGRD96 47.0 335 75 -75 3.85 0.7150
WVFGRD96 48.0 335 75 -75 3.86 0.7192
WVFGRD96 49.0 335 75 -80 3.87 0.7228
WVFGRD96 50.0 335 75 -80 3.87 0.7254
WVFGRD96 51.0 335 75 -80 3.88 0.7269
WVFGRD96 52.0 335 75 -80 3.88 0.7280
WVFGRD96 53.0 340 75 -80 3.89 0.7290
WVFGRD96 54.0 340 75 -80 3.89 0.7287
WVFGRD96 55.0 340 75 -80 3.89 0.7279
WVFGRD96 56.0 340 75 -80 3.90 0.7271
WVFGRD96 57.0 335 70 -80 3.90 0.7263
WVFGRD96 58.0 330 70 -85 3.90 0.7261
WVFGRD96 59.0 135 20 -110 3.91 0.7258
WVFGRD96 60.0 330 70 -85 3.91 0.7241
WVFGRD96 61.0 330 70 -90 3.92 0.7226
WVFGRD96 62.0 145 20 -95 3.92 0.7221
WVFGRD96 63.0 330 70 -90 3.92 0.7203
WVFGRD96 64.0 150 20 -95 3.93 0.7188
WVFGRD96 65.0 150 20 -95 3.93 0.7166
WVFGRD96 66.0 155 20 -90 3.93 0.7150
WVFGRD96 67.0 155 20 -90 3.94 0.7128
WVFGRD96 68.0 160 20 -85 3.94 0.7107
WVFGRD96 69.0 335 70 -90 3.94 0.7070
The best solution is
WVFGRD96 53.0 340 75 -80 3.89 0.7290
The mechanism corresponding 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 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 0.10 n 3 br c 0.12 0.25 n 4 p 2
|
| 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. |
|
| 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