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 us6000c97v and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us6000c97v/executive.
2020/10/18 16:46:37 62.255 -124.413 2.2 4 Canada, NWT
USGS/SLU Moment Tensor Solution
ENS 2020/10/18 16:46:37:0 62.26 -124.41 2.2 4.0 Canada, NWT
Stations used:
1E.MONT8 AK.BESE AK.CHX AK.GRNC AK.I26K AK.I27K AK.JIS
AK.K27K AK.L26K AK.M26K AK.M27K AK.MCAR AK.PIN AK.PNL
AK.R32K AK.RKAV AK.S31K AK.S32K AK.SAMH AK.TABL AK.U33K
AK.V35K AT.SIT AT.SKAG CN.BMTB CN.BRWY CN.BVCY CN.DAWY
CN.DLBC CN.FNSB CN.FSJB CN.HYT CN.NAB2 CN.NAHA CN.NBC1
CN.NBC8 CN.PLBC CN.RUBB CN.WHY CN.YUK2 CN.YUK3 CN.YUK6
CN.YUK7 EO.FSJ2 NY.MMPY NY.WGLY NY.WTLY RV.SNUFA TA.E29M
TA.F30M TA.F31M TA.H29M TA.I28M TA.I29M TA.J29N TA.J30M
TA.K29M TA.L27K TA.L29M TA.M29M TA.M31M TA.N30M TA.N31M
TA.N32M TA.O28M TA.O29M TA.P29M TA.P30M TA.P32M TA.P33M
TA.Q32M TA.R31K TA.R33M TA.S34M TA.T33K TA.T35M US.WRAK
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 0.06 n 3
Best Fitting Double Couple
Mo = 1.14e+22 dyne-cm
Mw = 3.97
Z = 4 km
Plane Strike Dip Rake
NP1 310 70 80
NP2 157 22 116
Principal Axes:
Axis Value Plunge Azimuth
T 1.14e+22 64 204
N 0.00e+00 9 313
P -1.14e+22 24 48
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.39e+21
Mxy -3.86e+21
Mxz -6.99e+21
Myy -4.79e+21
Myz -4.99e+21
Mzz 7.18e+21
--------------
#---------------------
##--------------------------
##----------------------------
---####-------------------- ----
---#########---------------- P -----
---#############------------- ------
----################--------------------
---###################------------------
----######################----------------
----########################--------------
----#########################-------------
-----##########################-----------
----############ ############---------
-----########### T ##############-------
-----########## ###############-----
-----############################---
-----############################-
-----#########################
------######################
-------###############
---------####-
Global CMT Convention Moment Tensor:
R T P
7.18e+21 -6.99e+21 4.99e+21
-6.99e+21 -2.39e+21 3.86e+21
4.99e+21 3.86e+21 -4.79e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201018164637/index.html
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STK = 310
DIP = 70
RAKE = 80
MW = 3.97
HS = 4.0
The NDK file is 20201018164637.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 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 120 15 60 4.04 0.6473
WVFGRD96 2.0 305 75 80 4.01 0.6471
WVFGRD96 3.0 305 75 80 3.99 0.6625
WVFGRD96 4.0 310 70 80 3.97 0.6698
WVFGRD96 5.0 310 70 80 3.96 0.6694
WVFGRD96 6.0 315 65 85 3.96 0.6612
WVFGRD96 7.0 310 65 85 3.95 0.6503
WVFGRD96 8.0 310 65 85 3.94 0.6368
WVFGRD96 9.0 310 65 85 3.92 0.6186
WVFGRD96 10.0 310 65 85 3.95 0.6124
WVFGRD96 11.0 305 70 75 3.92 0.5935
WVFGRD96 12.0 235 40 -75 4.00 0.5860
WVFGRD96 13.0 315 50 -80 3.97 0.5951
WVFGRD96 14.0 120 40 -100 3.97 0.6039
WVFGRD96 15.0 315 50 -80 3.96 0.6058
WVFGRD96 16.0 315 50 -85 3.96 0.6039
WVFGRD96 17.0 320 50 -75 3.96 0.5990
WVFGRD96 18.0 320 50 -75 3.96 0.5923
WVFGRD96 19.0 320 50 -75 3.97 0.5837
WVFGRD96 20.0 325 55 -70 3.99 0.5735
WVFGRD96 21.0 325 55 -70 3.99 0.5660
WVFGRD96 22.0 325 55 -70 3.99 0.5569
WVFGRD96 23.0 325 55 -70 3.99 0.5468
WVFGRD96 24.0 325 55 -70 3.99 0.5361
WVFGRD96 25.0 325 55 -65 4.00 0.5249
WVFGRD96 26.0 330 55 -65 4.00 0.5132
WVFGRD96 27.0 330 55 -60 4.00 0.5016
WVFGRD96 28.0 330 55 -60 4.00 0.4896
WVFGRD96 29.0 330 55 -60 4.01 0.4772
The best solution is
WVFGRD96 4.0 310 70 80 3.97 0.6698
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 0.06 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