The ANSS event ID is ak0237yfl15l and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0237yfl15l/executive.
2023/06/22 19:50:19 62.187 -149.438 31.1 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/06/22 19:50:19:0 62.19 -149.44 31.1 3.7 Alaska
Stations used:
AK.CUT AK.DIV AK.GHO AK.GLI AK.KLU AK.L22K AK.PWL AK.RC01
AK.RND AK.SKN AK.SLK AK.SWD 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 = 7.00e+21 dyne-cm
Mw = 3.83
Z = 60 km
Plane Strike Dip Rake
NP1 250 70 -80
NP2 43 22 -116
Principal Axes:
Axis Value Plunge Azimuth
T 7.00e+21 24 332
N 0.00e+00 9 67
P -7.00e+21 64 176
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.18e+21
Mxy -2.30e+21
Mxz 5.10e+21
Myy 1.25e+21
Myz -1.42e+21
Mzz -4.43e+21
##############
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##### ####################
###### T #####################
######## ######################-
###################################-
####################################--
########################--------------##
##################--------------------##
##############-------------------------###
###########----------------------------###
########------------------------------####
######--------------------------------####
###---------------- --------------####
#------------------ P -------------#####
------------------ ------------#####
-------------------------------#####
----------------------------######
------------------------######
#-------------------########
###---------##########
##############
Global CMT Convention Moment Tensor:
R T P
-4.43e+21 5.10e+21 1.42e+21
5.10e+21 3.18e+21 2.30e+21
1.42e+21 2.30e+21 1.25e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230622195019/index.html
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STK = 250
DIP = 70
RAKE = -80
MW = 3.83
HS = 60.0
The NDK file is 20230622195019.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 2.0 45 45 85 3.06 0.2863
WVFGRD96 4.0 270 60 -55 3.12 0.1944
WVFGRD96 6.0 135 35 25 3.14 0.2415
WVFGRD96 8.0 145 20 25 3.19 0.2792
WVFGRD96 10.0 145 20 25 3.21 0.3138
WVFGRD96 12.0 145 20 25 3.23 0.3383
WVFGRD96 14.0 130 40 35 3.31 0.3600
WVFGRD96 16.0 130 40 40 3.34 0.3776
WVFGRD96 18.0 130 40 40 3.37 0.3920
WVFGRD96 20.0 130 40 45 3.40 0.4013
WVFGRD96 22.0 120 35 -10 3.35 0.4112
WVFGRD96 24.0 125 35 -5 3.38 0.4230
WVFGRD96 26.0 125 35 -10 3.39 0.4334
WVFGRD96 28.0 120 30 -20 3.41 0.4438
WVFGRD96 30.0 110 30 -25 3.44 0.4577
WVFGRD96 32.0 105 30 -35 3.46 0.4771
WVFGRD96 34.0 105 35 -35 3.49 0.4992
WVFGRD96 36.0 100 35 -45 3.52 0.5201
WVFGRD96 38.0 100 40 -45 3.56 0.5476
WVFGRD96 40.0 90 35 -60 3.67 0.5827
WVFGRD96 42.0 90 35 -60 3.70 0.6044
WVFGRD96 44.0 80 30 -70 3.71 0.6179
WVFGRD96 46.0 245 60 -90 3.73 0.6356
WVFGRD96 48.0 250 65 -80 3.75 0.6532
WVFGRD96 50.0 250 65 -80 3.77 0.6727
WVFGRD96 52.0 250 65 -80 3.78 0.6880
WVFGRD96 54.0 250 65 -80 3.80 0.6987
WVFGRD96 56.0 250 65 -80 3.81 0.7038
WVFGRD96 58.0 250 70 -80 3.82 0.7092
WVFGRD96 60.0 250 70 -80 3.83 0.7131
WVFGRD96 62.0 250 70 -80 3.84 0.7127
WVFGRD96 64.0 250 70 -80 3.84 0.7083
WVFGRD96 66.0 255 75 -75 3.86 0.7000
WVFGRD96 68.0 255 75 -75 3.87 0.6958
WVFGRD96 70.0 255 75 -75 3.87 0.6887
WVFGRD96 72.0 250 75 -80 3.87 0.6786
WVFGRD96 74.0 250 75 -80 3.88 0.6649
WVFGRD96 76.0 250 75 -80 3.88 0.6513
WVFGRD96 78.0 255 80 -75 3.90 0.6392
WVFGRD96 80.0 255 80 -75 3.90 0.6272
WVFGRD96 82.0 255 80 -75 3.90 0.6144
WVFGRD96 84.0 255 80 -75 3.90 0.5987
WVFGRD96 86.0 255 80 -70 3.90 0.5837
WVFGRD96 88.0 255 80 -70 3.90 0.5664
WVFGRD96 90.0 255 80 -70 3.90 0.5505
WVFGRD96 92.0 255 85 -75 3.92 0.5373
WVFGRD96 94.0 255 85 -75 3.92 0.5246
WVFGRD96 96.0 255 85 -75 3.92 0.5120
WVFGRD96 98.0 255 85 -75 3.91 0.4984
The best solution is
WVFGRD96 60.0 250 70 -80 3.83 0.7131
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