The ANSS event ID is aka2026qoynpe and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/aka2026qoynpe/executive.
2026/08/22 07:42:40 60.911 -149.666 31.3 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/08/22 07:42:40.0 60.91 -149.67 31.3 3.9 Alaska
Stations used:
AK.BAE AK.CAST AK.CNP AK.CUT AK.FID AK.FIRE AK.GHO AK.GLB
AK.GLI AK.KNK AK.L22K AK.MCAR AK.N18K AK.P23K AK.PPLA
AK.RAG AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AT.PMR
AV.SPCL
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 0.10 n 3
Best Fitting Double Couple
Mo = 1.30e+22 dyne-cm
Mw = 4.01
Z = 51 km
Plane Strike Dip Rake
NP1 2 50 -113
NP2 215 45 -65
Principal Axes:
Axis Value Plunge Azimuth
T 1.30e+22 3 108
N 0.00e+00 17 17
P -1.30e+22 72 206
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.26e+20
Mxy -4.22e+21
Mxz 3.19e+21
Myy 1.16e+22
Myz 2.23e+21
Mzz -1.18e+22
#########-----
###############-------
################----########
##############--------########
#############------------#########
############--------------##########
###########-----------------##########
###########------------------###########
##########--------------------##########
##########---------------------###########
#########----------------------###########
########-----------------------###########
########--------- -----------###########
######---------- P -----------########
######---------- ----------######### T
#####-----------------------#########
####----------------------##########
###---------------------##########
##-------------------#########
#------------------#########
--------------########
--------######
Global CMT Convention Moment Tensor:
R T P
-1.18e+22 3.19e+21 -2.23e+21
3.19e+21 2.26e+20 4.22e+21
-2.23e+21 4.22e+21 1.16e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260822074240/index.html
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STK = 215
DIP = 45
RAKE = -65
MW = 4.01
HS = 51.0
The NDK file is 20260822074240.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 15 40 85 3.15 0.1366
WVFGRD96 2.0 15 40 90 3.30 0.1796
WVFGRD96 3.0 145 30 0 3.31 0.1685
WVFGRD96 4.0 140 30 -5 3.34 0.2060
WVFGRD96 5.0 140 30 -5 3.37 0.2306
WVFGRD96 6.0 220 80 -65 3.40 0.2513
WVFGRD96 7.0 220 80 -60 3.41 0.2711
WVFGRD96 8.0 220 80 -65 3.49 0.2875
WVFGRD96 9.0 220 85 -60 3.51 0.3052
WVFGRD96 10.0 45 90 60 3.53 0.3186
WVFGRD96 11.0 45 85 60 3.55 0.3309
WVFGRD96 12.0 45 85 60 3.57 0.3402
WVFGRD96 13.0 45 85 60 3.58 0.3466
WVFGRD96 14.0 45 80 60 3.60 0.3516
WVFGRD96 15.0 45 80 60 3.61 0.3555
WVFGRD96 16.0 45 80 60 3.63 0.3581
WVFGRD96 17.0 45 80 60 3.64 0.3595
WVFGRD96 18.0 45 80 60 3.66 0.3594
WVFGRD96 19.0 45 80 60 3.67 0.3588
WVFGRD96 20.0 50 75 60 3.68 0.3578
WVFGRD96 21.0 50 75 60 3.70 0.3558
WVFGRD96 22.0 50 75 60 3.71 0.3540
WVFGRD96 23.0 50 75 60 3.72 0.3507
WVFGRD96 24.0 50 80 60 3.73 0.3478
WVFGRD96 25.0 50 80 60 3.74 0.3447
WVFGRD96 26.0 50 80 60 3.75 0.3398
WVFGRD96 27.0 55 80 60 3.76 0.3354
WVFGRD96 28.0 50 85 55 3.76 0.3311
WVFGRD96 29.0 50 85 55 3.77 0.3292
WVFGRD96 30.0 225 80 -50 3.76 0.3301
WVFGRD96 31.0 225 75 -50 3.77 0.3411
WVFGRD96 32.0 225 70 -50 3.77 0.3535
WVFGRD96 33.0 225 70 -50 3.78 0.3671
WVFGRD96 34.0 225 65 -50 3.79 0.3798
WVFGRD96 35.0 225 65 -50 3.80 0.3938
WVFGRD96 36.0 225 65 -50 3.81 0.4066
WVFGRD96 37.0 225 60 -50 3.82 0.4181
WVFGRD96 38.0 225 60 -50 3.83 0.4314
WVFGRD96 39.0 230 60 -50 3.85 0.4448
WVFGRD96 40.0 225 60 -60 3.93 0.4467
WVFGRD96 41.0 225 55 -55 3.94 0.4569
WVFGRD96 42.0 220 55 -60 3.95 0.4640
WVFGRD96 43.0 220 50 -60 3.97 0.4687
WVFGRD96 44.0 220 50 -60 3.97 0.4733
WVFGRD96 45.0 215 50 -65 3.98 0.4736
WVFGRD96 46.0 215 50 -65 3.99 0.4758
WVFGRD96 47.0 215 50 -65 4.00 0.4758
WVFGRD96 48.0 215 45 -65 4.00 0.4752
WVFGRD96 49.0 215 45 -65 4.01 0.4758
WVFGRD96 50.0 215 45 -65 4.01 0.4745
WVFGRD96 51.0 215 45 -65 4.01 0.4763
WVFGRD96 52.0 215 45 -65 4.02 0.4737
WVFGRD96 53.0 215 45 -65 4.02 0.4737
WVFGRD96 54.0 215 45 -65 4.02 0.4724
WVFGRD96 55.0 220 45 -65 4.03 0.4694
WVFGRD96 56.0 220 45 -65 4.03 0.4697
WVFGRD96 57.0 220 45 -65 4.03 0.4663
WVFGRD96 58.0 215 40 -70 4.04 0.4655
WVFGRD96 59.0 215 40 -70 4.04 0.4644
The best solution is
WVFGRD96 51.0 215 45 -65 4.01 0.4763
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 0.10 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