The ANSS event ID is aka2026ihbayw and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/aka2026ihbayw/executive.
2026/04/28 03:33:31 63.066 -149.833 92.1 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/04/28 03:33:31.0 63.07 -149.83 92.1 4.3 Alaska
Stations used:
AK.BAE AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.HDA AK.I21K
AK.I23K AK.J25K AK.K24K AK.KNK AK.L22K AK.MCK AK.PAX
AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.WAT6 AT.PMR AV.STLK
IM.IL31 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 0.07 n 3
Best Fitting Double Couple
Mo = 2.79e+22 dyne-cm
Mw = 4.23
Z = 102 km
Plane Strike Dip Rake
NP1 344 58 138
NP2 100 55 40
Principal Axes:
Axis Value Plunge Azimuth
T 2.79e+22 51 310
N 0.00e+00 39 134
P -2.79e+22 2 43
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.03e+22
Mxy -1.93e+22
Mxz 8.16e+21
Myy -6.49e+21
Myz -1.10e+22
Mzz 1.68e+22
###-----------
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##############------------ P
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########## ###########--------------
########### T ############--------------
########### #############-------------
-############################-------------
--###########################-------------
----#########################-------------
------########################------------
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Global CMT Convention Moment Tensor:
R T P
1.68e+22 8.16e+21 1.10e+22
8.16e+21 -1.03e+22 1.93e+22
1.10e+22 1.93e+22 -6.49e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260428033331/index.html
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STK = 100
DIP = 55
RAKE = 40
MW = 4.23
HS = 102.0
The NDK file is 20260428033331.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/3.5 -40 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 110 45 -85 3.47 0.3032
WVFGRD96 4.0 140 70 -55 3.52 0.2785
WVFGRD96 6.0 330 90 45 3.53 0.3061
WVFGRD96 8.0 150 90 -50 3.60 0.3321
WVFGRD96 10.0 340 70 40 3.64 0.3542
WVFGRD96 12.0 340 70 45 3.66 0.3686
WVFGRD96 14.0 340 65 40 3.68 0.3769
WVFGRD96 16.0 340 70 40 3.70 0.3803
WVFGRD96 18.0 335 75 45 3.70 0.3793
WVFGRD96 20.0 335 75 45 3.72 0.3754
WVFGRD96 22.0 70 50 30 3.74 0.3716
WVFGRD96 24.0 70 50 30 3.76 0.3702
WVFGRD96 26.0 70 55 25 3.78 0.3711
WVFGRD96 28.0 70 55 25 3.80 0.3719
WVFGRD96 30.0 75 60 30 3.82 0.3722
WVFGRD96 32.0 75 60 25 3.84 0.3711
WVFGRD96 34.0 80 65 20 3.87 0.3678
WVFGRD96 36.0 95 70 30 3.89 0.3667
WVFGRD96 38.0 95 70 30 3.92 0.3767
WVFGRD96 40.0 95 70 40 3.99 0.3762
WVFGRD96 42.0 95 70 35 4.01 0.3862
WVFGRD96 44.0 95 70 35 4.03 0.3929
WVFGRD96 46.0 100 55 35 4.06 0.4045
WVFGRD96 48.0 100 55 35 4.08 0.4254
WVFGRD96 50.0 100 55 35 4.10 0.4495
WVFGRD96 52.0 100 55 35 4.11 0.4742
WVFGRD96 54.0 100 55 35 4.13 0.4970
WVFGRD96 56.0 100 55 35 4.14 0.5178
WVFGRD96 58.0 100 55 35 4.15 0.5354
WVFGRD96 60.0 100 50 30 4.16 0.5537
WVFGRD96 62.0 100 55 35 4.17 0.5791
WVFGRD96 64.0 100 55 35 4.17 0.6046
WVFGRD96 66.0 100 55 35 4.18 0.6283
WVFGRD96 68.0 100 55 35 4.19 0.6504
WVFGRD96 70.0 100 55 35 4.19 0.6696
WVFGRD96 72.0 100 55 40 4.19 0.6878
WVFGRD96 74.0 100 55 40 4.20 0.7033
WVFGRD96 76.0 100 55 40 4.20 0.7181
WVFGRD96 78.0 100 55 40 4.21 0.7303
WVFGRD96 80.0 100 55 40 4.21 0.7408
WVFGRD96 82.0 100 55 40 4.21 0.7500
WVFGRD96 84.0 100 55 40 4.21 0.7576
WVFGRD96 86.0 100 55 40 4.22 0.7633
WVFGRD96 88.0 100 55 40 4.22 0.7679
WVFGRD96 90.0 100 55 40 4.22 0.7726
WVFGRD96 92.0 100 55 40 4.22 0.7758
WVFGRD96 94.0 100 55 40 4.23 0.7786
WVFGRD96 96.0 100 55 40 4.23 0.7802
WVFGRD96 98.0 100 55 40 4.23 0.7812
WVFGRD96 100.0 100 55 40 4.23 0.7816
WVFGRD96 102.0 100 55 40 4.23 0.7817
WVFGRD96 104.0 100 55 40 4.24 0.7807
WVFGRD96 106.0 100 55 40 4.24 0.7799
WVFGRD96 108.0 100 55 40 4.24 0.7784
WVFGRD96 110.0 100 55 40 4.24 0.7769
WVFGRD96 112.0 100 55 40 4.25 0.7754
WVFGRD96 114.0 100 55 40 4.25 0.7726
WVFGRD96 116.0 100 55 40 4.25 0.7699
WVFGRD96 118.0 100 55 40 4.25 0.7673
The best solution is
WVFGRD96 102.0 100 55 40 4.23 0.7817
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 0.07 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