The ANSS event ID is ak02177xtoen and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02177xtoen/executive.
2021/06/06 17:55:16 62.467 -148.243 42.0 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/06/06 17:55:16:0 62.47 -148.24 42.0 4.0 Alaska
Stations used:
AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.GHO AK.GLB AK.GLI
AK.HDA AK.J25K AK.K24K AK.KLU AK.KNK AK.L20K AK.MCK AK.PAX
AK.POKR AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN
AK.SLK AK.TRF AT.PMR AV.SPCP IM.IL31
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 = 1.66e+22 dyne-cm
Mw = 4.08
Z = 54 km
Plane Strike Dip Rake
NP1 44 47 -105
NP2 245 45 -75
Principal Axes:
Axis Value Plunge Azimuth
T 1.66e+22 1 144
N 0.00e+00 11 54
P -1.66e+22 79 240
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.08e+22
Mxy -8.09e+21
Mxz 1.28e+21
Myy 5.19e+21
Myz 2.75e+21
Mzz -1.60e+22
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#######----------------------------#######
#####------------- -------------########
####-------------- P -------------########
###--------------- -----------##########
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#################### T
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Global CMT Convention Moment Tensor:
R T P
-1.60e+22 1.28e+21 -2.75e+21
1.28e+21 1.08e+22 8.09e+21
-2.75e+21 8.09e+21 5.19e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210606175516/index.html
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STK = 245
DIP = 45
RAKE = -75
MW = 4.08
HS = 54.0
The NDK file is 20210606175516.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 2.0 235 45 85 3.27 0.2291
WVFGRD96 4.0 185 45 -10 3.28 0.2391
WVFGRD96 6.0 40 65 75 3.38 0.2755
WVFGRD96 8.0 40 65 75 3.47 0.2983
WVFGRD96 10.0 30 70 60 3.49 0.2906
WVFGRD96 12.0 270 45 -40 3.51 0.2912
WVFGRD96 14.0 270 55 -35 3.54 0.2993
WVFGRD96 16.0 270 55 -30 3.57 0.3121
WVFGRD96 18.0 275 60 -25 3.60 0.3225
WVFGRD96 20.0 275 65 -20 3.63 0.3316
WVFGRD96 22.0 275 65 -20 3.65 0.3428
WVFGRD96 24.0 275 70 -20 3.67 0.3552
WVFGRD96 26.0 275 70 -20 3.69 0.3641
WVFGRD96 28.0 275 70 -20 3.71 0.3773
WVFGRD96 30.0 275 75 -25 3.73 0.3985
WVFGRD96 32.0 275 70 -25 3.75 0.4201
WVFGRD96 34.0 275 70 -25 3.77 0.4369
WVFGRD96 36.0 275 60 -20 3.80 0.4567
WVFGRD96 38.0 275 45 -25 3.85 0.4760
WVFGRD96 40.0 265 40 -40 3.95 0.5151
WVFGRD96 42.0 265 40 -40 3.98 0.5258
WVFGRD96 44.0 255 35 -55 4.02 0.5385
WVFGRD96 46.0 255 40 -55 4.03 0.5539
WVFGRD96 48.0 245 40 -70 4.05 0.5654
WVFGRD96 50.0 245 40 -70 4.06 0.5743
WVFGRD96 52.0 245 40 -70 4.07 0.5777
WVFGRD96 54.0 245 45 -75 4.08 0.5797
WVFGRD96 56.0 245 45 -75 4.08 0.5776
WVFGRD96 58.0 250 45 -70 4.08 0.5724
WVFGRD96 60.0 250 45 -70 4.08 0.5684
WVFGRD96 62.0 260 50 -55 4.08 0.5637
WVFGRD96 64.0 255 50 -60 4.09 0.5604
WVFGRD96 66.0 265 55 -50 4.09 0.5576
WVFGRD96 68.0 265 55 -50 4.09 0.5539
WVFGRD96 70.0 270 60 -45 4.09 0.5523
WVFGRD96 72.0 270 60 -45 4.09 0.5511
WVFGRD96 74.0 270 60 -45 4.09 0.5478
WVFGRD96 76.0 265 60 -50 4.10 0.5434
WVFGRD96 78.0 265 60 -50 4.10 0.5407
WVFGRD96 80.0 265 60 -50 4.10 0.5371
WVFGRD96 82.0 265 60 -50 4.10 0.5332
WVFGRD96 84.0 275 70 -35 4.10 0.5315
WVFGRD96 86.0 280 75 -25 4.09 0.5302
WVFGRD96 88.0 280 75 -25 4.09 0.5292
WVFGRD96 90.0 280 75 -25 4.09 0.5282
WVFGRD96 92.0 280 75 -25 4.10 0.5265
WVFGRD96 94.0 280 75 -25 4.10 0.5250
WVFGRD96 96.0 280 80 -25 4.10 0.5240
WVFGRD96 98.0 280 80 -25 4.10 0.5224
The best solution is
WVFGRD96 54.0 245 45 -75 4.08 0.5797
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