The ANSS event ID is ak0218n94m2m and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0218n94m2m/executive.
2021/07/07 20:31:25 60.903 -146.339 6.5 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/07/07 20:31:25:0 60.90 -146.34 6.5 3.7 Alaska
Stations used:
AK.BMR AK.BRLK AK.CNP AK.DHY AK.DIV AK.EYAK AK.GHO AK.GLB
AK.GLI AK.HIN AK.K24K AK.KLU AK.KNK AK.M26K AK.M27K AK.MCAR
AK.MCK AK.PPLA AK.PWL AK.RAG AK.RC01 AK.RND AK.SAW AK.SCM
AK.VRDI AT.PMR AV.STLK
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 0.07 n 3
Best Fitting Double Couple
Mo = 5.50e+21 dyne-cm
Mw = 3.76
Z = 27 km
Plane Strike Dip Rake
NP1 230 70 -40
NP2 336 53 -155
Principal Axes:
Axis Value Plunge Azimuth
T 5.50e+21 11 287
N 0.00e+00 46 28
P -5.50e+21 42 187
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.56e+21
Mxy -1.80e+21
Mxz 3.00e+21
Myy 4.83e+21
Myz -6.36e+20
Mzz -2.27e+21
--------------
#######---------------
#############---------------
################--------------
####################-------#######
#######################-############
######################---#############
#################-------#############
T ###############----------############
# ############--------------############
##############-----------------###########
#############------------------###########
###########---------------------##########
#########----------------------#########
#######------------------------#########
#####-------------------------########
###------------ -----------#######
#------------- P -----------######
------------ -----------####
------------------------####
--------------------##
--------------
Global CMT Convention Moment Tensor:
R T P
-2.27e+21 3.00e+21 6.36e+20
3.00e+21 -2.56e+21 1.80e+21
6.36e+20 1.80e+21 4.83e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210707203125/index.html
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STK = 230
DIP = 70
RAKE = -40
MW = 3.76
HS = 27.0
The NDK file is 20210707203125.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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 195 45 90 3.25 0.2689
WVFGRD96 2.0 195 45 90 3.37 0.3532
WVFGRD96 3.0 10 45 80 3.43 0.3381
WVFGRD96 4.0 235 70 -25 3.38 0.3342
WVFGRD96 5.0 235 70 -30 3.41 0.3529
WVFGRD96 6.0 240 65 35 3.45 0.3735
WVFGRD96 7.0 240 65 30 3.47 0.4007
WVFGRD96 8.0 245 60 40 3.54 0.4228
WVFGRD96 9.0 65 75 40 3.54 0.4439
WVFGRD96 10.0 70 70 45 3.56 0.4678
WVFGRD96 11.0 240 90 -40 3.57 0.4890
WVFGRD96 12.0 235 80 -40 3.59 0.5155
WVFGRD96 13.0 235 80 -40 3.60 0.5431
WVFGRD96 14.0 235 75 -40 3.62 0.5712
WVFGRD96 15.0 235 75 -40 3.63 0.5966
WVFGRD96 16.0 230 70 -45 3.64 0.6198
WVFGRD96 17.0 230 70 -40 3.66 0.6404
WVFGRD96 18.0 230 70 -40 3.67 0.6585
WVFGRD96 19.0 230 70 -40 3.68 0.6739
WVFGRD96 20.0 230 70 -40 3.69 0.6871
WVFGRD96 21.0 230 70 -40 3.71 0.6974
WVFGRD96 22.0 230 70 -40 3.72 0.7064
WVFGRD96 23.0 230 70 -40 3.73 0.7138
WVFGRD96 24.0 230 70 -40 3.73 0.7197
WVFGRD96 25.0 230 70 -40 3.74 0.7232
WVFGRD96 26.0 230 70 -40 3.75 0.7259
WVFGRD96 27.0 230 70 -40 3.76 0.7261
WVFGRD96 28.0 230 70 -40 3.77 0.7254
WVFGRD96 29.0 230 65 -40 3.77 0.7225
WVFGRD96 30.0 230 65 -40 3.78 0.7189
WVFGRD96 31.0 230 65 -40 3.79 0.7127
WVFGRD96 32.0 230 65 -40 3.79 0.7056
WVFGRD96 33.0 230 65 -40 3.80 0.6954
WVFGRD96 34.0 230 65 -40 3.80 0.6840
WVFGRD96 35.0 230 70 -40 3.81 0.6723
WVFGRD96 36.0 235 70 -35 3.82 0.6607
WVFGRD96 37.0 230 70 -35 3.83 0.6499
WVFGRD96 38.0 230 70 -35 3.84 0.6397
WVFGRD96 39.0 230 70 -35 3.85 0.6314
WVFGRD96 40.0 230 70 -45 3.92 0.6131
WVFGRD96 41.0 230 70 -45 3.93 0.6112
WVFGRD96 42.0 230 70 -45 3.93 0.6066
WVFGRD96 43.0 230 70 -45 3.94 0.6006
WVFGRD96 44.0 230 70 -40 3.95 0.5933
WVFGRD96 45.0 230 70 -40 3.95 0.5861
WVFGRD96 46.0 230 70 -40 3.96 0.5777
WVFGRD96 47.0 230 70 -40 3.96 0.5695
WVFGRD96 48.0 235 70 -35 3.96 0.5619
WVFGRD96 49.0 235 70 -35 3.97 0.5552
The best solution is
WVFGRD96 27.0 230 70 -40 3.76 0.7261
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 0.07 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