The ANSS event ID is ak021anrw3oz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak021anrw3oz/executive.
2021/08/20 05:39:28 61.538 -141.320 11.3 3.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/08/20 05:39:28:0 61.54 -141.32 11.3 3.4 Alaska
Stations used:
AK.BARN AK.BERG AK.CRQ AK.DIV AK.DOT AK.EYAK AK.GLB AK.GRNC
AK.HDA AK.I26K AK.ISLE AK.J25K AK.J26L AK.K24K AK.KLU
AK.L26K AK.LOGN AK.M26K AK.M27K AK.RIDG AK.TABL AK.TGL
AK.VRDI AK.WAX AK.YAH AT.MENT AT.SKAG CN.DAWY CN.HYT
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
Best Fitting Double Couple
Mo = 2.75e+21 dyne-cm
Mw = 3.56
Z = 15 km
Plane Strike Dip Rake
NP1 275 80 70
NP2 159 22 153
Principal Axes:
Axis Value Plunge Azimuth
T 2.75e+21 51 162
N 0.00e+00 20 279
P -2.75e+21 32 22
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.17e+20
Mxy -9.90e+20
Mxz -2.44e+21
Myy -1.68e+20
Myz -4.90e+19
Mzz 8.85e+20
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#---------------------
##---------------- -------
##----------------- P --------
###------------------ ----------
###---------------------------------
###-----------------------------------
####------------------------------------
###-------------------------------------
####---###########------------------------
----#############################---------
----###################################---
-----#####################################
----####################################
-----###################################
-----################ ##############
-----############### T #############
-----############## ############
-----#########################
------######################
------################
--------######
Global CMT Convention Moment Tensor:
R T P
8.85e+20 -2.44e+21 4.90e+19
-2.44e+21 -7.17e+20 9.90e+20
4.90e+19 9.90e+20 -1.68e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210820053928/index.html
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STK = 275
DIP = 80
RAKE = 70
MW = 3.56
HS = 15.0
The NDK file is 20210820053928.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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 275 45 90 3.27 0.4105
WVFGRD96 2.0 275 45 90 3.38 0.4852
WVFGRD96 3.0 270 60 85 3.43 0.3540
WVFGRD96 4.0 90 85 70 3.43 0.4220
WVFGRD96 5.0 90 85 70 3.43 0.4920
WVFGRD96 6.0 95 85 65 3.44 0.5423
WVFGRD96 7.0 95 85 65 3.44 0.5739
WVFGRD96 8.0 95 85 65 3.52 0.5926
WVFGRD96 9.0 95 85 65 3.52 0.6076
WVFGRD96 10.0 275 80 70 3.52 0.6262
WVFGRD96 11.0 275 80 70 3.53 0.6450
WVFGRD96 12.0 275 80 70 3.54 0.6575
WVFGRD96 13.0 275 80 70 3.54 0.6654
WVFGRD96 14.0 275 80 70 3.55 0.6692
WVFGRD96 15.0 275 80 70 3.56 0.6695
WVFGRD96 16.0 275 80 70 3.57 0.6671
WVFGRD96 17.0 275 80 70 3.58 0.6617
WVFGRD96 18.0 275 80 70 3.59 0.6536
WVFGRD96 19.0 5 30 -5 3.62 0.6440
WVFGRD96 20.0 5 30 -5 3.63 0.6350
WVFGRD96 21.0 10 30 0 3.65 0.6244
WVFGRD96 22.0 10 30 0 3.66 0.6124
WVFGRD96 23.0 10 30 0 3.67 0.5988
WVFGRD96 24.0 10 30 0 3.67 0.5843
WVFGRD96 25.0 10 30 0 3.68 0.5686
WVFGRD96 26.0 10 30 0 3.69 0.5520
WVFGRD96 27.0 10 30 0 3.70 0.5343
WVFGRD96 28.0 10 30 0 3.70 0.5154
WVFGRD96 29.0 95 70 -50 3.73 0.5037
The best solution is
WVFGRD96 15.0 275 80 70 3.56 0.6695
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
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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