Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is us6000fwbp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us6000fwbp/executive.
2021/10/21 03:23:21 52.571 -115.222 17.0 4.2 Alberta, Canada
USGS/SLU Moment Tensor Solution
ENS 2021/10/21 03:23:21:0 52.57 -115.22 17.0 4.2 Alberta, Canada
Stations used:
1E.MONT1 1E.MONT9 CN.EDM CN.FSJB CN.HOPB CN.LLLB CN.PNT
CN.VDEB CN.WPB CN.WSLR MB.GBMT MB.JTMT MB.LDM RE.GRCO2
RV.BDMTA RV.BELVA RV.BRLDA RV.FOXCA RV.HSPGA RV.PKSKA
RV.STPRA RV.SWHSA RV.TONYA RV.WTMTA RV.YELLA TD.TD002
TD.TD008 TD.TD009 TD.TD022 US.EGMT US.MSO US.NEW UW.CBS
UW.DAVN UW.DY2 UW.EPH2 UW.ETW UW.HOPR UW.NEL UW.OMAK
UW.PASS UW.RPW2 UW.RVSD UW.SAW UW.SHUK UW.SLF UW.TWISP
UW.WOLL XL.MG01 XL.MG10
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 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.02e+22 dyne-cm
Mw = 3.94
Z = 14 km
Plane Strike Dip Rake
NP1 320 50 85
NP2 148 40 96
Principal Axes:
Axis Value Plunge Azimuth
T 1.02e+22 84 195
N 0.00e+00 4 323
P -1.02e+22 5 54
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.48e+21
Mxy -4.82e+21
Mxz -1.58e+21
Myy -6.56e+21
Myz -9.88e+20
Mzz 1.00e+22
--------------
----------------------
--#####---------------------
--###########----------------
---################------------ P
----##################---------- -
-----####################-------------
------######################------------
------#######################-----------
-------########################-----------
-------#########################----------
--------########### ###########---------
---------########## T ############--------
--------########## #############------
---------#########################------
----------#######################-----
----------######################----
-----------####################---
-----------##################-
--------------#############-
----------------######
--------------
Global CMT Convention Moment Tensor:
R T P
1.00e+22 -1.58e+21 9.88e+20
-1.58e+21 -3.48e+21 4.82e+21
9.88e+20 4.82e+21 -6.56e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20211021032321/index.html
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STK = 320
DIP = 50
RAKE = 85
MW = 3.94
HS = 14.0
The NDK file is 20211021032321.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 0.08 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 1.0 285 35 -90 3.77 0.4899
WVFGRD96 2.0 285 45 -85 3.80 0.4353
WVFGRD96 3.0 125 80 -75 3.91 0.4605
WVFGRD96 4.0 130 85 -75 3.89 0.4955
WVFGRD96 5.0 125 80 -75 3.87 0.5196
WVFGRD96 6.0 335 20 -60 3.89 0.5361
WVFGRD96 7.0 340 25 -55 3.89 0.5550
WVFGRD96 8.0 320 60 85 3.92 0.5800
WVFGRD96 9.0 320 55 85 3.94 0.6201
WVFGRD96 10.0 320 55 85 3.95 0.6380
WVFGRD96 11.0 320 50 85 3.96 0.6637
WVFGRD96 12.0 320 50 85 3.95 0.6775
WVFGRD96 13.0 320 50 85 3.94 0.6840
WVFGRD96 14.0 320 50 85 3.94 0.6852
WVFGRD96 15.0 320 50 85 3.93 0.6831
WVFGRD96 16.0 320 50 85 3.93 0.6786
WVFGRD96 17.0 145 40 90 3.94 0.6713
WVFGRD96 18.0 145 40 90 3.94 0.6633
WVFGRD96 19.0 145 40 90 3.94 0.6540
WVFGRD96 20.0 145 40 90 3.96 0.6467
WVFGRD96 21.0 145 40 90 3.96 0.6371
WVFGRD96 22.0 145 40 90 3.96 0.6265
WVFGRD96 23.0 325 50 95 3.97 0.6148
WVFGRD96 24.0 325 50 95 3.97 0.6032
WVFGRD96 25.0 325 50 95 3.97 0.5904
WVFGRD96 26.0 325 50 95 3.98 0.5765
WVFGRD96 27.0 135 40 80 3.98 0.5622
WVFGRD96 28.0 130 40 75 3.99 0.5470
WVFGRD96 29.0 130 40 75 3.99 0.5313
The best solution is
WVFGRD96 14.0 320 50 85 3.94 0.6852
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 0.08 n 3 br c 0.12 0.25 n 4 p 2
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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 CUS.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 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