The ANSS event ID is ak023e0o5q4j and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023e0o5q4j/executive.
2023/11/01 15:53:07 61.873 -148.040 8.9 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/11/01 15:53:07:0 61.87 -148.04 8.9 3.7 Alaska
Stations used:
AK.BAE AK.CUT AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO AK.GLI
AK.KLU AK.KNK AK.PAX AK.PWL AK.RC01 AK.SAW AK.SCM AK.WAT6
AT.PMR
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
Best Fitting Double Couple
Mo = 6.76e+21 dyne-cm
Mw = 3.82
Z = 42 km
Plane Strike Dip Rake
NP1 74 65 -95
NP2 265 25 -80
Principal Axes:
Axis Value Plunge Azimuth
T 6.76e+21 20 167
N 0.00e+00 4 76
P -6.76e+21 69 335
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.98e+21
Mxy -9.31e+20
Mxz -4.17e+21
Myy 1.25e+20
Myz 1.43e+21
Mzz -5.10e+21
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#############---############
######-------------------#####
#####-------------------------####
####-----------------------------###
###---------------------------------##
###------------- -------------------##
#--------------- P --------------------#
##--------------- -------------------##-
#------------------------------------#####
----------------------------------########
-------------------------------###########
--------------------------##############
##------------------####################
######################################
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##################################
################# ##########
################ T #########
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Global CMT Convention Moment Tensor:
R T P
-5.10e+21 -4.17e+21 -1.43e+21
-4.17e+21 4.98e+21 9.31e+20
-1.43e+21 9.31e+20 1.25e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231101155307/index.html
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STK = 265
DIP = 25
RAKE = -80
MW = 3.82
HS = 42.0
The NDK file is 20231101155307.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 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 105 40 85 3.00 0.1820
WVFGRD96 2.0 105 40 85 3.15 0.2410
WVFGRD96 3.0 85 40 -85 3.21 0.2065
WVFGRD96 4.0 225 65 -55 3.17 0.1951
WVFGRD96 5.0 235 80 80 3.24 0.2182
WVFGRD96 6.0 235 85 75 3.24 0.2409
WVFGRD96 7.0 235 85 75 3.24 0.2598
WVFGRD96 8.0 235 85 80 3.33 0.2763
WVFGRD96 9.0 235 85 80 3.34 0.2940
WVFGRD96 10.0 55 90 -75 3.35 0.3108
WVFGRD96 11.0 55 90 -75 3.37 0.3267
WVFGRD96 12.0 65 70 -75 3.39 0.3470
WVFGRD96 13.0 65 70 -75 3.41 0.3652
WVFGRD96 14.0 65 70 -75 3.42 0.3812
WVFGRD96 15.0 65 70 -75 3.43 0.3945
WVFGRD96 16.0 65 70 -75 3.45 0.4058
WVFGRD96 17.0 65 70 -80 3.46 0.4156
WVFGRD96 18.0 70 75 -80 3.47 0.4266
WVFGRD96 19.0 70 75 -80 3.48 0.4373
WVFGRD96 20.0 70 75 -80 3.49 0.4472
WVFGRD96 21.0 70 75 -85 3.52 0.4566
WVFGRD96 22.0 230 15 -110 3.53 0.4663
WVFGRD96 23.0 70 75 -85 3.54 0.4756
WVFGRD96 24.0 70 75 -90 3.55 0.4850
WVFGRD96 25.0 250 15 -90 3.57 0.4958
WVFGRD96 26.0 255 15 -85 3.58 0.5068
WVFGRD96 27.0 255 15 -85 3.59 0.5170
WVFGRD96 28.0 255 15 -85 3.60 0.5269
WVFGRD96 29.0 260 20 -75 3.61 0.5377
WVFGRD96 30.0 260 20 -80 3.62 0.5488
WVFGRD96 31.0 260 20 -80 3.63 0.5597
WVFGRD96 32.0 255 20 -90 3.63 0.5707
WVFGRD96 33.0 260 20 -85 3.64 0.5826
WVFGRD96 34.0 260 20 -85 3.65 0.5921
WVFGRD96 35.0 260 20 -85 3.65 0.5973
WVFGRD96 36.0 260 25 -85 3.66 0.6061
WVFGRD96 37.0 265 25 -80 3.67 0.6120
WVFGRD96 38.0 260 25 -85 3.67 0.6147
WVFGRD96 39.0 260 25 -85 3.68 0.6142
WVFGRD96 40.0 265 25 -80 3.80 0.6106
WVFGRD96 41.0 265 25 -80 3.81 0.6145
WVFGRD96 42.0 265 25 -80 3.82 0.6151
WVFGRD96 43.0 265 25 -80 3.82 0.6127
WVFGRD96 44.0 265 25 -80 3.83 0.6093
WVFGRD96 45.0 265 25 -80 3.83 0.6032
WVFGRD96 46.0 265 25 -80 3.84 0.5971
WVFGRD96 47.0 265 25 -80 3.84 0.5903
WVFGRD96 48.0 270 30 -75 3.85 0.5839
WVFGRD96 49.0 270 30 -75 3.85 0.5774
WVFGRD96 50.0 270 30 -75 3.85 0.5698
WVFGRD96 51.0 270 30 -70 3.86 0.5622
WVFGRD96 52.0 270 30 -70 3.86 0.5536
WVFGRD96 53.0 270 30 -70 3.86 0.5449
WVFGRD96 54.0 270 30 -70 3.86 0.5362
WVFGRD96 55.0 265 30 -75 3.86 0.5271
WVFGRD96 56.0 270 30 -70 3.86 0.5189
WVFGRD96 57.0 265 30 -75 3.86 0.5100
WVFGRD96 58.0 265 30 -75 3.86 0.5025
WVFGRD96 59.0 265 30 -75 3.86 0.4944
The best solution is
WVFGRD96 42.0 265 25 -80 3.82 0.6151
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
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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