The ANSS event ID is ak023dttcwm3 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023dttcwm3/executive.
2023/10/28 01:00:04 63.485 -150.100 137.6 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/10/28 01:00:04:0 63.49 -150.10 137.6 4.0 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.H24K AK.HDA
AK.I23K AK.J20K AK.K20K AK.KNK AK.L22K AK.MCK AK.MLY
AK.NEA2 AK.PAX AK.POKR AK.PPLA AK.SAW AK.SCM AK.WAT6 AK.WRH
AT.PMR IM.IL31 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 0.10 n 3
Best Fitting Double Couple
Mo = 1.30e+22 dyne-cm
Mw = 4.01
Z = 134 km
Plane Strike Dip Rake
NP1 35 80 70
NP2 279 22 153
Principal Axes:
Axis Value Plunge Azimuth
T 1.30e+22 51 282
N 0.00e+00 20 39
P -1.30e+22 32 142
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.50e+21
Mxy 3.47e+21
Mxz 5.97e+21
Myy 1.31e+21
Myz -9.87e+21
Mzz 4.19e+21
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----------------------
-------############-------##
----####################--####
---########################--#####
--#########################-----####
-##########################--------###
-##########################----------###
######### ##############------------##
########## T #############--------------##
########## ############----------------#
#######################------------------#
######################-------------------#
####################--------------------
##################----------------------
################----------------------
#############------------- -------
###########-------------- P ------
#######---------------- ----
####------------------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
4.19e+21 5.97e+21 9.87e+21
5.97e+21 -5.50e+21 -3.47e+21
9.87e+21 -3.47e+21 1.31e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231028010004/index.html
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STK = 35
DIP = 80
RAKE = 70
MW = 4.01
HS = 134.0
The NDK file is 20231028010004.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 2.0 250 35 -95 3.12 0.1753
WVFGRD96 4.0 295 40 -20 3.14 0.1973
WVFGRD96 6.0 295 45 -20 3.18 0.2292
WVFGRD96 8.0 290 45 -30 3.29 0.2614
WVFGRD96 10.0 295 50 -20 3.32 0.2771
WVFGRD96 12.0 305 60 20 3.35 0.2879
WVFGRD96 14.0 305 60 15 3.38 0.2919
WVFGRD96 16.0 305 55 15 3.42 0.2914
WVFGRD96 18.0 305 55 10 3.44 0.2859
WVFGRD96 20.0 300 50 -5 3.46 0.2787
WVFGRD96 22.0 300 50 -5 3.49 0.2681
WVFGRD96 24.0 210 80 45 3.54 0.2688
WVFGRD96 26.0 210 80 50 3.56 0.2688
WVFGRD96 28.0 210 80 50 3.58 0.2672
WVFGRD96 30.0 210 80 50 3.60 0.2612
WVFGRD96 32.0 210 80 45 3.60 0.2518
WVFGRD96 34.0 210 85 50 3.62 0.2413
WVFGRD96 36.0 210 80 45 3.62 0.2323
WVFGRD96 38.0 210 75 45 3.63 0.2284
WVFGRD96 40.0 210 75 50 3.72 0.2331
WVFGRD96 42.0 210 70 50 3.74 0.2313
WVFGRD96 44.0 210 70 45 3.75 0.2266
WVFGRD96 46.0 210 70 45 3.77 0.2232
WVFGRD96 48.0 210 70 45 3.78 0.2201
WVFGRD96 50.0 210 70 45 3.79 0.2163
WVFGRD96 52.0 30 65 20 3.79 0.2174
WVFGRD96 54.0 30 75 30 3.80 0.2258
WVFGRD96 56.0 30 80 35 3.82 0.2396
WVFGRD96 58.0 30 80 35 3.83 0.2540
WVFGRD96 60.0 30 80 35 3.84 0.2670
WVFGRD96 62.0 30 80 40 3.86 0.2799
WVFGRD96 64.0 30 70 35 3.87 0.2942
WVFGRD96 66.0 30 70 35 3.88 0.3134
WVFGRD96 68.0 30 70 35 3.89 0.3302
WVFGRD96 70.0 30 70 45 3.90 0.3516
WVFGRD96 72.0 25 75 50 3.92 0.3742
WVFGRD96 74.0 30 75 50 3.93 0.3997
WVFGRD96 76.0 30 75 55 3.94 0.4292
WVFGRD96 78.0 30 75 55 3.95 0.4577
WVFGRD96 80.0 30 75 55 3.96 0.4822
WVFGRD96 82.0 30 80 55 3.97 0.5024
WVFGRD96 84.0 30 80 55 3.97 0.5173
WVFGRD96 86.0 30 80 55 3.97 0.5303
WVFGRD96 88.0 30 80 55 3.98 0.5399
WVFGRD96 90.0 30 80 65 3.99 0.5497
WVFGRD96 92.0 30 80 65 3.99 0.5612
WVFGRD96 94.0 30 80 65 3.99 0.5712
WVFGRD96 96.0 30 80 65 3.99 0.5797
WVFGRD96 98.0 30 80 65 4.00 0.5881
WVFGRD96 100.0 30 80 65 4.00 0.5956
WVFGRD96 102.0 30 80 65 4.00 0.6024
WVFGRD96 104.0 35 80 65 4.00 0.6078
WVFGRD96 106.0 30 80 65 4.00 0.6122
WVFGRD96 108.0 35 80 65 4.00 0.6180
WVFGRD96 110.0 30 80 65 4.00 0.6215
WVFGRD96 112.0 35 80 70 4.00 0.6263
WVFGRD96 114.0 35 80 70 4.00 0.6298
WVFGRD96 116.0 35 80 70 4.00 0.6319
WVFGRD96 118.0 35 80 70 4.00 0.6355
WVFGRD96 120.0 35 80 70 4.01 0.6373
WVFGRD96 122.0 35 80 70 4.01 0.6395
WVFGRD96 124.0 35 80 70 4.01 0.6405
WVFGRD96 126.0 35 80 70 4.01 0.6412
WVFGRD96 128.0 35 80 70 4.01 0.6423
WVFGRD96 130.0 35 80 70 4.01 0.6429
WVFGRD96 132.0 35 80 70 4.01 0.6427
WVFGRD96 134.0 35 80 70 4.01 0.6433
WVFGRD96 136.0 35 80 70 4.01 0.6428
WVFGRD96 138.0 35 80 70 4.01 0.6421
WVFGRD96 140.0 35 80 70 4.01 0.6406
WVFGRD96 142.0 35 80 70 4.01 0.6398
WVFGRD96 144.0 35 80 70 4.01 0.6383
WVFGRD96 146.0 35 80 70 4.01 0.6368
WVFGRD96 148.0 35 80 70 4.01 0.6349
The best solution is
WVFGRD96 134.0 35 80 70 4.01 0.6433
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