The ANSS event ID is ak020d3ozvj8 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020d3ozvj8/executive.
2020/10/11 21:55:35 61.688 -150.526 59.4 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/10/11 21:55:35:0 61.69 -150.53 59.4 3.8 Alaska
Stations used:
AK.CUT AK.GHO AK.KNK AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN
AK.SLK AK.SSN AT.PMR AV.RED AV.SPU AV.STLK TA.M22K
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
Best Fitting Double Couple
Mo = 1.14e+22 dyne-cm
Mw = 3.97
Z = 66 km
Plane Strike Dip Rake
NP1 20 80 65
NP2 270 27 157
Principal Axes:
Axis Value Plunge Azimuth
T 1.14e+22 49 263
N 0.00e+00 25 25
P -1.14e+22 31 130
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.45e+21
Mxy 4.75e+21
Mxz 2.52e+21
Myy -7.02e+19
Myz -9.37e+21
Mzz 3.52e+21
--------------
------------------####
---------------------#######
-------###############-#######
-----##################------#####
----####################--------####
---#####################-----------###
--#######################-------------##
-#######################---------------#
-########################----------------#
-######## ############------------------
######### T ###########-------------------
######### ###########-------------------
#####################-------------------
####################--------------------
##################---------- -------
#################---------- P ------
###############----------- -----
############------------------
##########------------------
#####-----------------
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Global CMT Convention Moment Tensor:
R T P
3.52e+21 2.52e+21 9.37e+21
2.52e+21 -3.45e+21 -4.75e+21
9.37e+21 -4.75e+21 -7.02e+19
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201011215535/index.html
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STK = 20
DIP = 80
RAKE = 65
MW = 3.97
HS = 66.0
The NDK file is 20201011215535.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 35 45 -100 3.20 0.1064
WVFGRD96 4.0 260 30 -25 3.22 0.1103
WVFGRD96 6.0 190 85 55 3.25 0.1340
WVFGRD96 8.0 195 80 55 3.35 0.1487
WVFGRD96 10.0 200 70 55 3.39 0.1564
WVFGRD96 12.0 205 65 55 3.43 0.1601
WVFGRD96 14.0 200 70 50 3.45 0.1607
WVFGRD96 16.0 205 75 45 3.50 0.1603
WVFGRD96 18.0 205 75 45 3.52 0.1611
WVFGRD96 20.0 25 40 60 3.54 0.1615
WVFGRD96 22.0 195 65 -35 3.58 0.1622
WVFGRD96 24.0 195 70 -35 3.60 0.1688
WVFGRD96 26.0 195 70 -35 3.62 0.1738
WVFGRD96 28.0 195 75 -35 3.64 0.1772
WVFGRD96 30.0 195 80 -45 3.65 0.1816
WVFGRD96 32.0 20 90 40 3.67 0.1854
WVFGRD96 34.0 20 90 45 3.69 0.1924
WVFGRD96 36.0 20 90 45 3.70 0.1988
WVFGRD96 38.0 25 80 50 3.72 0.2104
WVFGRD96 40.0 25 80 60 3.84 0.2238
WVFGRD96 42.0 25 80 60 3.86 0.2410
WVFGRD96 44.0 25 80 60 3.88 0.2546
WVFGRD96 46.0 25 80 60 3.89 0.2642
WVFGRD96 48.0 25 80 60 3.90 0.2730
WVFGRD96 50.0 30 75 60 3.92 0.2792
WVFGRD96 52.0 25 80 65 3.92 0.2856
WVFGRD96 54.0 25 80 65 3.93 0.2901
WVFGRD96 56.0 25 80 65 3.94 0.2942
WVFGRD96 58.0 25 80 65 3.95 0.2975
WVFGRD96 60.0 25 80 65 3.96 0.2994
WVFGRD96 62.0 25 80 65 3.96 0.3007
WVFGRD96 64.0 20 80 65 3.97 0.3013
WVFGRD96 66.0 20 80 65 3.97 0.3022
WVFGRD96 68.0 20 80 65 3.98 0.3017
WVFGRD96 70.0 20 80 70 3.99 0.3009
WVFGRD96 72.0 20 80 70 3.99 0.2992
WVFGRD96 74.0 20 80 70 3.99 0.2964
WVFGRD96 76.0 15 80 70 4.00 0.2944
WVFGRD96 78.0 15 80 70 4.00 0.2913
WVFGRD96 80.0 15 80 70 4.00 0.2878
WVFGRD96 82.0 15 80 70 4.01 0.2842
WVFGRD96 84.0 15 80 70 4.01 0.2800
WVFGRD96 86.0 10 80 75 4.02 0.2758
WVFGRD96 88.0 10 80 75 4.02 0.2719
WVFGRD96 90.0 10 80 75 4.02 0.2668
WVFGRD96 92.0 15 75 70 4.01 0.2623
WVFGRD96 94.0 15 75 70 4.01 0.2593
WVFGRD96 96.0 15 75 70 4.02 0.2558
WVFGRD96 98.0 15 75 70 4.02 0.2516
The best solution is
WVFGRD96 66.0 20 80 65 3.97 0.3022
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 0.10 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