The ANSS event ID is ak020f7rgkgu and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020f7rgkgu/executive.
2020/11/26 21:12:10 62.010 -150.014 42.1 4.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/11/26 21:12:10:0 62.01 -150.01 42.1 4.6 Alaska
Stations used:
AK.CUT AK.DIV AK.FID AK.FIRE AK.GHO AK.GLI AK.KLU AK.KNK
AK.MCK AK.PAX AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN
AK.SLK AK.SSN AK.WRH AT.PMR AV.RED AV.SPU AV.STLK TA.M22K
TA.O22K
Filtering commands used:
cut o DIST/3.3 -50 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.04e+23 dyne-cm
Mw = 4.61
Z = 53 km
Plane Strike Dip Rake
NP1 345 55 -60
NP2 120 45 -126
Principal Axes:
Axis Value Plunge Azimuth
T 1.04e+23 6 54
N 0.00e+00 24 147
P -1.04e+23 65 312
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.68e+22
Mxy 5.78e+22
Mxz -2.07e+22
Myy 5.74e+22
Myz 3.73e+22
Mzz -8.42e+22
---###########
----------############
---------------#############
------------------###########
---------------------########## T
-----------------------######### #
#------------------------#############
##------------ ----------#############
###----------- P -----------############
####----------- -----------#############
#####-------------------------############
######------------------------############
#######-----------------------############
########----------------------##########
##########--------------------##########
###########------------------#########
#############--------------#########
################----------######--
########################------
######################------
##################----
#############-
Global CMT Convention Moment Tensor:
R T P
-8.42e+22 -2.07e+22 -3.73e+22
-2.07e+22 2.68e+22 -5.78e+22
-3.73e+22 -5.78e+22 5.74e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201126211210/index.html
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STK = 345
DIP = 55
RAKE = -60
MW = 4.61
HS = 53.0
The NDK file is 20201126211210.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 -50 o DIST/3.3 +40 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 1.0 255 45 -75 3.73 0.2020
WVFGRD96 2.0 250 45 -85 3.88 0.2587
WVFGRD96 3.0 95 85 -50 3.88 0.2434
WVFGRD96 4.0 300 70 60 3.94 0.2748
WVFGRD96 5.0 0 40 -25 3.95 0.3059
WVFGRD96 6.0 0 40 -25 3.97 0.3304
WVFGRD96 7.0 0 45 -25 3.99 0.3478
WVFGRD96 8.0 0 40 -30 4.06 0.3571
WVFGRD96 9.0 0 40 -30 4.08 0.3684
WVFGRD96 10.0 5 45 -25 4.09 0.3739
WVFGRD96 11.0 20 45 25 4.11 0.3783
WVFGRD96 12.0 20 45 25 4.12 0.3802
WVFGRD96 13.0 20 45 25 4.14 0.3792
WVFGRD96 14.0 205 45 35 4.15 0.3823
WVFGRD96 15.0 205 50 35 4.17 0.3863
WVFGRD96 16.0 205 50 35 4.18 0.3886
WVFGRD96 17.0 205 50 35 4.19 0.3898
WVFGRD96 18.0 210 50 35 4.21 0.3913
WVFGRD96 19.0 205 55 35 4.22 0.3924
WVFGRD96 20.0 205 55 30 4.23 0.3923
WVFGRD96 21.0 205 60 40 4.24 0.3945
WVFGRD96 22.0 205 60 35 4.25 0.3953
WVFGRD96 23.0 210 60 40 4.26 0.3981
WVFGRD96 24.0 0 60 -35 4.27 0.4014
WVFGRD96 25.0 0 60 -35 4.28 0.4057
WVFGRD96 26.0 0 60 -35 4.29 0.4106
WVFGRD96 27.0 0 60 -35 4.30 0.4141
WVFGRD96 28.0 0 65 -35 4.31 0.4211
WVFGRD96 29.0 0 60 -30 4.32 0.4322
WVFGRD96 30.0 0 60 -30 4.33 0.4440
WVFGRD96 31.0 0 60 -30 4.34 0.4561
WVFGRD96 32.0 0 60 -35 4.35 0.4663
WVFGRD96 33.0 0 60 -35 4.36 0.4759
WVFGRD96 34.0 -5 60 -40 4.37 0.4853
WVFGRD96 35.0 -5 60 -40 4.38 0.4954
WVFGRD96 36.0 -5 60 -40 4.38 0.5043
WVFGRD96 37.0 -5 60 -45 4.39 0.5121
WVFGRD96 38.0 -5 60 -45 4.40 0.5191
WVFGRD96 39.0 355 60 -45 4.42 0.5254
WVFGRD96 40.0 350 60 -55 4.50 0.5322
WVFGRD96 41.0 345 55 -55 4.52 0.5435
WVFGRD96 42.0 345 55 -60 4.53 0.5539
WVFGRD96 43.0 345 55 -60 4.54 0.5612
WVFGRD96 44.0 345 55 -60 4.55 0.5674
WVFGRD96 45.0 345 55 -60 4.56 0.5751
WVFGRD96 46.0 345 55 -60 4.57 0.5818
WVFGRD96 47.0 345 55 -60 4.58 0.5877
WVFGRD96 48.0 345 55 -60 4.58 0.5932
WVFGRD96 49.0 345 55 -60 4.59 0.5974
WVFGRD96 50.0 345 55 -60 4.60 0.6005
WVFGRD96 51.0 345 55 -60 4.60 0.6018
WVFGRD96 52.0 345 55 -60 4.60 0.6038
WVFGRD96 53.0 345 55 -60 4.61 0.6040
WVFGRD96 54.0 345 55 -60 4.61 0.6039
WVFGRD96 55.0 345 55 -60 4.62 0.6035
WVFGRD96 56.0 345 55 -60 4.62 0.6027
WVFGRD96 57.0 345 55 -60 4.62 0.6011
WVFGRD96 58.0 345 55 -60 4.62 0.6001
WVFGRD96 59.0 345 55 -60 4.63 0.5975
The best solution is
WVFGRD96 53.0 345 55 -60 4.61 0.6040
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 -50 o DIST/3.3 +40 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