The ANSS event ID is ak022c9tj4z7 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak022c9tj4z7/executive.
2022/09/24 15:18:54 61.492 -145.589 41.4 4.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2022/09/24 15:18:54:0 61.49 -145.59 41.4 4.8 Alaska
Stations used:
AK.BARN AK.BERG AK.BMR AK.BRLK AK.CRQ AK.DHY AK.DIV AK.EYAK
AK.FID AK.GHO AK.GLB AK.GLI AK.HIN AK.K24K AK.KAI AK.KLU
AK.KNK AK.L26K AK.M26K AK.M27K AK.MCAR AK.P23K AK.PAX
AK.PWL AK.RAG AK.RC01 AK.RND AK.SAW AK.SCM AK.SLK AK.SUCK
AK.TABL AK.TGL AK.VRDI AK.WRH AT.MENT AT.PMR AV.SPCP
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 = 1.51e+23 dyne-cm
Mw = 4.72
Z = 42 km
Plane Strike Dip Rake
NP1 55 85 30
NP2 322 60 174
Principal Axes:
Axis Value Plunge Azimuth
T 1.51e+23 24 283
N 0.00e+00 60 64
P -1.51e+23 17 185
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.32e+23
Mxy -3.85e+22
Mxz 5.45e+22
Myy 1.18e+23
Myz -5.21e+22
Mzz 1.31e+22
--------------
----------------------
#####-----------------------
##########--------------------
###############-------------------
###################--------------###
######################----------######
########################-------#########
### ####################--############
#### T ####################-##############
#### ##################-----############
######################---------###########
###################-------------##########
###############----------------#########
#############-------------------########
#########----------------------#######
####---------------------------#####
------------------------------####
----------------------------##
----------- -------------#
-------- P -----------
---- -------
Global CMT Convention Moment Tensor:
R T P
1.31e+22 5.45e+22 5.21e+22
5.45e+22 -1.32e+23 3.85e+22
5.21e+22 3.85e+22 1.18e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220924151854/index.html
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STK = 55
DIP = 85
RAKE = 30
MW = 4.72
HS = 42.0
The NDK file is 20220924151854.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 1.0 135 70 -20 3.89 0.1924
WVFGRD96 2.0 130 65 -30 4.03 0.2529
WVFGRD96 3.0 320 75 -5 4.05 0.2748
WVFGRD96 4.0 235 85 15 4.11 0.3028
WVFGRD96 5.0 235 80 25 4.15 0.3252
WVFGRD96 6.0 235 80 25 4.18 0.3443
WVFGRD96 7.0 235 80 20 4.21 0.3596
WVFGRD96 8.0 235 80 25 4.25 0.3739
WVFGRD96 9.0 235 80 25 4.27 0.3783
WVFGRD96 10.0 230 75 -25 4.29 0.3810
WVFGRD96 11.0 230 80 -25 4.30 0.3908
WVFGRD96 12.0 230 80 -25 4.32 0.3990
WVFGRD96 13.0 235 85 -25 4.34 0.4048
WVFGRD96 14.0 235 85 -25 4.35 0.4097
WVFGRD96 15.0 235 85 -25 4.36 0.4136
WVFGRD96 16.0 235 85 -25 4.38 0.4172
WVFGRD96 17.0 235 85 -25 4.39 0.4202
WVFGRD96 18.0 235 85 -25 4.40 0.4225
WVFGRD96 19.0 235 85 -25 4.41 0.4249
WVFGRD96 20.0 230 80 -25 4.42 0.4277
WVFGRD96 21.0 230 85 -30 4.43 0.4308
WVFGRD96 22.0 230 85 -30 4.44 0.4351
WVFGRD96 23.0 230 85 -30 4.45 0.4402
WVFGRD96 24.0 50 90 30 4.46 0.4455
WVFGRD96 25.0 230 90 -30 4.48 0.4520
WVFGRD96 26.0 50 90 30 4.49 0.4592
WVFGRD96 27.0 50 85 30 4.50 0.4693
WVFGRD96 28.0 230 90 -30 4.51 0.4788
WVFGRD96 29.0 230 90 -30 4.52 0.4885
WVFGRD96 30.0 50 85 30 4.54 0.4994
WVFGRD96 31.0 50 85 30 4.55 0.5100
WVFGRD96 32.0 50 85 30 4.56 0.5203
WVFGRD96 33.0 230 90 -25 4.57 0.5254
WVFGRD96 34.0 50 85 25 4.58 0.5361
WVFGRD96 35.0 230 90 -25 4.59 0.5392
WVFGRD96 36.0 55 85 25 4.61 0.5492
WVFGRD96 37.0 230 90 -25 4.62 0.5500
WVFGRD96 38.0 230 90 -25 4.63 0.5537
WVFGRD96 39.0 230 90 -20 4.65 0.5580
WVFGRD96 40.0 55 85 35 4.71 0.5665
WVFGRD96 41.0 55 85 30 4.71 0.5692
WVFGRD96 42.0 55 85 30 4.72 0.5703
WVFGRD96 43.0 230 90 -30 4.73 0.5665
WVFGRD96 44.0 230 90 -30 4.74 0.5661
WVFGRD96 45.0 55 85 30 4.75 0.5688
WVFGRD96 46.0 230 90 -30 4.75 0.5626
WVFGRD96 47.0 230 90 -30 4.76 0.5603
WVFGRD96 48.0 55 85 25 4.76 0.5626
WVFGRD96 49.0 230 90 -30 4.77 0.5560
WVFGRD96 50.0 235 90 -25 4.78 0.5531
WVFGRD96 51.0 55 85 25 4.78 0.5551
WVFGRD96 52.0 55 85 25 4.78 0.5525
WVFGRD96 53.0 55 85 25 4.79 0.5496
WVFGRD96 54.0 55 85 25 4.79 0.5463
WVFGRD96 55.0 55 85 25 4.80 0.5428
WVFGRD96 56.0 230 90 -25 4.80 0.5355
WVFGRD96 57.0 230 90 -25 4.80 0.5325
WVFGRD96 58.0 55 80 25 4.80 0.5339
WVFGRD96 59.0 230 90 -25 4.81 0.5267
The best solution is
WVFGRD96 42.0 55 85 30 4.72 0.5703
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