The ANSS event ID is aka2026sbqvff and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/aka2026sbqvff/executive.
2026/09/12 06:38:26 62.832 -149.050 76.8 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/09/12 06:38:26.0 62.83 -149.05 76.8 3.9 Alaska
Stations used:
AK.BAE AK.CAST AK.CUT AK.DHY AK.FID AK.GHO AK.GLI AK.KNK
AK.L22K AK.M20K AK.MCK AK.PAX AK.RC01 AK.RIDG AK.RND AK.SAW
AK.SCM AK.SKN AK.SLK AT.PMR IU.COLA
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 8.04e+21 dyne-cm
Mw = 3.87
Z = 84 km
Plane Strike Dip Rake
NP1 90 70 25
NP2 351 67 158
Principal Axes:
Axis Value Plunge Azimuth
T 8.04e+21 32 311
N 0.00e+00 58 126
P -8.04e+21 2 220
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.18e+21
Mxy -6.84e+21
Mxz 2.60e+21
Myy -5.98e+14
Myz -2.49e+21
Mzz 2.18e+21
#####---------
##########------------
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#################-------------
##### ############--------------
###### T ############---------------
####### #############---------------
#########################---------------
#########################---------------
###########################---------------
###########################---------------
--#########################-------------##
------#####################---------######
-------------#############-#############
--------------------------##############
-------------------------#############
------------------------############
-----------------------###########
------------------#########
P -----------------#########
----------------######
-----------###
Global CMT Convention Moment Tensor:
R T P
2.18e+21 2.60e+21 2.49e+21
2.60e+21 -2.18e+21 6.84e+21
2.49e+21 6.84e+21 -5.98e+14
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260912063826/index.html
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STK = 90
DIP = 70
RAKE = 25
MW = 3.87
HS = 84.0
The NDK file is 20260912063826.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.
Map showing station locations used for computing the ML's. No distinction is made whether the vertical (Z) or horizontal (H) components were used.
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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/3.5 -40 o DIST/3.5 +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 2.0 145 50 -65 3.09 0.2719
WVFGRD96 4.0 165 65 -35 3.10 0.2941
WVFGRD96 6.0 0 70 30 3.15 0.3256
WVFGRD96 8.0 180 70 35 3.22 0.3483
WVFGRD96 10.0 255 55 -30 3.28 0.3612
WVFGRD96 12.0 255 55 -25 3.31 0.3841
WVFGRD96 14.0 260 55 -20 3.34 0.4030
WVFGRD96 16.0 260 55 -20 3.37 0.4220
WVFGRD96 18.0 260 55 -20 3.40 0.4401
WVFGRD96 20.0 260 55 -20 3.43 0.4579
WVFGRD96 22.0 260 55 -20 3.46 0.4764
WVFGRD96 24.0 260 55 -20 3.48 0.4924
WVFGRD96 26.0 260 55 -15 3.50 0.5055
WVFGRD96 28.0 260 55 -15 3.51 0.5161
WVFGRD96 30.0 260 55 -15 3.53 0.5224
WVFGRD96 32.0 260 60 -15 3.54 0.5240
WVFGRD96 34.0 260 60 -15 3.55 0.5218
WVFGRD96 36.0 260 65 -15 3.56 0.5192
WVFGRD96 38.0 260 70 -15 3.58 0.5192
WVFGRD96 40.0 260 65 -20 3.64 0.5234
WVFGRD96 42.0 265 85 -30 3.68 0.5262
WVFGRD96 44.0 265 90 -35 3.71 0.5353
WVFGRD96 46.0 265 90 -35 3.73 0.5453
WVFGRD96 48.0 90 75 35 3.75 0.5555
WVFGRD96 50.0 90 75 35 3.77 0.5697
WVFGRD96 52.0 90 75 35 3.78 0.5854
WVFGRD96 54.0 90 70 35 3.80 0.6001
WVFGRD96 56.0 90 70 35 3.81 0.6161
WVFGRD96 58.0 90 70 35 3.82 0.6298
WVFGRD96 60.0 90 70 35 3.83 0.6425
WVFGRD96 62.0 90 70 35 3.83 0.6517
WVFGRD96 64.0 90 70 30 3.83 0.6630
WVFGRD96 66.0 90 70 30 3.84 0.6711
WVFGRD96 68.0 90 70 30 3.85 0.6776
WVFGRD96 70.0 90 70 30 3.85 0.6825
WVFGRD96 72.0 90 70 25 3.85 0.6874
WVFGRD96 74.0 90 70 25 3.86 0.6912
WVFGRD96 76.0 90 70 25 3.86 0.6942
WVFGRD96 78.0 90 70 25 3.86 0.6960
WVFGRD96 80.0 90 70 25 3.87 0.6968
WVFGRD96 82.0 90 70 25 3.87 0.6966
WVFGRD96 84.0 90 70 25 3.87 0.6969
WVFGRD96 86.0 90 70 25 3.88 0.6958
WVFGRD96 88.0 90 70 20 3.88 0.6951
WVFGRD96 90.0 90 70 20 3.88 0.6940
WVFGRD96 92.0 90 70 20 3.88 0.6921
WVFGRD96 94.0 90 70 20 3.89 0.6901
WVFGRD96 96.0 90 75 20 3.89 0.6873
WVFGRD96 98.0 90 75 20 3.89 0.6859
The best solution is
WVFGRD96 84.0 90 70 25 3.87 0.6969
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/3.5 -40 o DIST/3.5 +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