The ANSS event ID is ak0232axzttx and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0232axzttx/executive.
2023/02/19 15:32:54 63.281 -150.553 131.3 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/02/19 15:32:54:0 63.28 -150.55 131.3 3.8 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GCSA AK.GHO AK.HARP
AK.HDA AK.I21K AK.J25K AK.K24K AK.KNK AK.KTH AK.L22K AK.MCK
AK.MLY AK.NEA2 AK.POKR AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM
AK.WAT6 AK.WRH AT.PMR IU.COLA
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
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.40e+22 dyne-cm
Mw = 4.03
Z = 136 km
Plane Strike Dip Rake
NP1 210 55 -75
NP2 5 38 -110
Principal Axes:
Axis Value Plunge Azimuth
T 1.40e+22 9 289
N 0.00e+00 12 21
P -1.40e+22 75 164
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.05e+20
Mxy -4.01e+21
Mxz 4.10e+21
Myy 1.21e+22
Myz -2.96e+21
Mzz -1.27e+22
###########---
#################--###
#################-----######
################--------######
###############------------#######
###############--------------#######
############-----------------#######
T ###########------------------########
#########---------------------#######
############----------------------########
############----------------------########
###########-----------------------########
##########----------- ----------########
#########----------- P ---------########
########------------ ---------########
#######-----------------------########
######----------------------########
#####---------------------########
###--------------------#######
###-----------------########
---------------#######
--------######
Global CMT Convention Moment Tensor:
R T P
-1.27e+22 4.10e+21 2.96e+21
4.10e+21 6.05e+20 4.01e+21
2.96e+21 4.01e+21 1.21e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230219153254/index.html
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STK = 210
DIP = 55
RAKE = -75
MW = 4.03
HS = 136.0
The NDK file is 20230219153254.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 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 5 50 95 3.27 0.3325
WVFGRD96 4.0 160 80 65 3.36 0.3030
WVFGRD96 6.0 160 80 65 3.36 0.3673
WVFGRD96 8.0 155 85 65 3.41 0.3873
WVFGRD96 10.0 330 90 -60 3.40 0.4002
WVFGRD96 12.0 325 85 -55 3.40 0.4087
WVFGRD96 14.0 325 80 -55 3.42 0.4153
WVFGRD96 16.0 325 80 -55 3.43 0.4185
WVFGRD96 18.0 325 75 -55 3.46 0.4202
WVFGRD96 20.0 330 70 -55 3.48 0.4206
WVFGRD96 22.0 330 70 -55 3.50 0.4175
WVFGRD96 24.0 330 70 -55 3.52 0.4130
WVFGRD96 26.0 330 70 -55 3.54 0.4068
WVFGRD96 28.0 325 70 -55 3.55 0.3983
WVFGRD96 30.0 325 70 -55 3.57 0.3885
WVFGRD96 32.0 150 55 40 3.58 0.3746
WVFGRD96 34.0 155 50 50 3.60 0.3762
WVFGRD96 36.0 160 50 55 3.62 0.3744
WVFGRD96 38.0 100 50 55 3.64 0.3690
WVFGRD96 40.0 165 45 60 3.75 0.3542
WVFGRD96 42.0 165 40 65 3.76 0.3481
WVFGRD96 44.0 100 50 65 3.76 0.3404
WVFGRD96 46.0 85 50 45 3.77 0.3297
WVFGRD96 48.0 85 50 45 3.78 0.3238
WVFGRD96 50.0 85 50 45 3.79 0.3181
WVFGRD96 52.0 55 60 -20 3.80 0.3135
WVFGRD96 54.0 55 60 -20 3.82 0.3230
WVFGRD96 56.0 55 60 -20 3.83 0.3317
WVFGRD96 58.0 50 60 -30 3.86 0.3398
WVFGRD96 60.0 50 60 -30 3.87 0.3490
WVFGRD96 62.0 50 60 -30 3.88 0.3557
WVFGRD96 64.0 50 60 -30 3.89 0.3607
WVFGRD96 66.0 215 60 -65 3.85 0.3700
WVFGRD96 68.0 215 60 -65 3.86 0.3928
WVFGRD96 70.0 220 60 -60 3.87 0.4163
WVFGRD96 72.0 215 55 -65 3.88 0.4438
WVFGRD96 74.0 215 55 -65 3.89 0.4676
WVFGRD96 76.0 210 55 -70 3.90 0.4899
WVFGRD96 78.0 210 55 -70 3.91 0.5090
WVFGRD96 80.0 210 55 -70 3.92 0.5281
WVFGRD96 82.0 210 55 -70 3.92 0.5462
WVFGRD96 84.0 210 55 -70 3.93 0.5631
WVFGRD96 86.0 210 55 -70 3.93 0.5789
WVFGRD96 88.0 210 55 -70 3.94 0.5935
WVFGRD96 90.0 210 55 -70 3.94 0.6067
WVFGRD96 92.0 210 55 -70 3.95 0.6193
WVFGRD96 94.0 210 55 -70 3.95 0.6308
WVFGRD96 96.0 210 55 -70 3.96 0.6410
WVFGRD96 98.0 210 55 -75 3.96 0.6501
WVFGRD96 100.0 210 55 -75 3.97 0.6583
WVFGRD96 102.0 210 55 -75 3.97 0.6656
WVFGRD96 104.0 210 55 -75 3.97 0.6721
WVFGRD96 106.0 210 55 -75 3.98 0.6789
WVFGRD96 108.0 210 55 -75 3.98 0.6851
WVFGRD96 110.0 210 55 -75 3.99 0.6900
WVFGRD96 112.0 210 55 -75 3.99 0.6946
WVFGRD96 114.0 210 55 -75 3.99 0.6993
WVFGRD96 116.0 210 55 -75 4.00 0.7036
WVFGRD96 118.0 210 55 -75 4.00 0.7070
WVFGRD96 120.0 210 55 -75 4.00 0.7106
WVFGRD96 122.0 210 55 -75 4.01 0.7128
WVFGRD96 124.0 210 55 -75 4.01 0.7156
WVFGRD96 126.0 210 55 -75 4.01 0.7179
WVFGRD96 128.0 210 55 -75 4.02 0.7185
WVFGRD96 130.0 210 55 -75 4.02 0.7202
WVFGRD96 132.0 210 55 -75 4.02 0.7209
WVFGRD96 134.0 210 55 -75 4.02 0.7207
WVFGRD96 136.0 210 55 -75 4.03 0.7212
WVFGRD96 138.0 215 60 -75 4.03 0.7211
WVFGRD96 140.0 215 60 -75 4.04 0.7197
WVFGRD96 142.0 215 60 -75 4.04 0.7194
WVFGRD96 144.0 215 60 -75 4.04 0.7178
WVFGRD96 146.0 215 60 -75 4.04 0.7160
WVFGRD96 148.0 215 60 -75 4.04 0.7146
WVFGRD96 150.0 215 60 -75 4.05 0.7122
WVFGRD96 152.0 215 60 -75 4.05 0.7105
WVFGRD96 154.0 215 60 -75 4.05 0.7078
WVFGRD96 156.0 215 60 -75 4.05 0.7056
WVFGRD96 158.0 215 60 -75 4.05 0.7028
The best solution is
WVFGRD96 136.0 210 55 -75 4.03 0.7212
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 br c 0.12 0.25 n 4 p 2
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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