The ANSS event ID is aka2026inasvo and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/aka2026inasvo/executive.
2026/04/30 16:03:15 61.363 -146.682 14.7 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/04/30 16:03:15.0 61.36 -146.68 14.7 3.9 Alaska
Stations used:
AK.BAE AK.DIV AK.EYAK AK.FID AK.GLB AK.GLI AK.HIN AK.KNK
AK.RAG AK.RC01 AK.SAW AK.SCM AV.WAZA
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.06 n 3
lp c 0.20 n 3
Best Fitting Double Couple
Mo = 3.16e+21 dyne-cm
Mw = 3.60
Z = 31 km
Plane Strike Dip Rake
NP1 240 75 -50
NP2 347 42 -157
Principal Axes:
Axis Value Plunge Azimuth
T 3.16e+21 20 301
N 0.00e+00 38 48
P -3.16e+21 45 190
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.92e+20
Mxy -1.51e+21
Mxz 2.08e+21
Myy 2.00e+21
Myz -5.93e+20
Mzz -1.21e+21
#####---------
#############---------
##################----------
#####################---------
########################----------
## ######################-------##
### T ######################-#########
#### #################------##########
####################-----------#########
##################---------------#########
###############------------------#########
#############--------------------#########
##########-----------------------#########
#######-------------------------########
#####---------------------------########
##-------------- ------------#######
--------------- P -----------#######
-------------- -----------######
-------------------------#####
-----------------------#####
------------------####
-------------#
Global CMT Convention Moment Tensor:
R T P
-1.21e+21 2.08e+21 5.93e+20
2.08e+21 -7.92e+20 1.51e+21
5.93e+20 1.51e+21 2.00e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260430160315/index.html
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STK = 240
DIP = 75
RAKE = -50
MW = 3.60
HS = 31.0
The NDK file is 20260430160315.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 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.06 n 3 lp c 0.20 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 210 30 -65 2.71 0.1193
WVFGRD96 2.0 195 35 -90 2.95 0.1533
WVFGRD96 3.0 220 55 -55 2.97 0.1802
WVFGRD96 4.0 240 20 -25 3.05 0.2038
WVFGRD96 5.0 50 80 -50 3.04 0.2227
WVFGRD96 6.0 320 40 -5 3.09 0.2343
WVFGRD96 7.0 320 45 -5 3.13 0.2417
WVFGRD96 8.0 325 45 -5 3.22 0.2438
WVFGRD96 9.0 325 45 -5 3.26 0.2399
WVFGRD96 10.0 325 50 -5 3.29 0.2298
WVFGRD96 11.0 325 55 -5 3.32 0.2166
WVFGRD96 12.0 325 50 0 3.34 0.2025
WVFGRD96 13.0 240 80 40 3.36 0.1892
WVFGRD96 14.0 240 85 45 3.38 0.1860
WVFGRD96 15.0 55 90 -35 3.41 0.1845
WVFGRD96 16.0 240 90 35 3.43 0.1873
WVFGRD96 17.0 235 50 -25 3.48 0.1978
WVFGRD96 18.0 235 55 -25 3.49 0.2162
WVFGRD96 19.0 235 55 -25 3.51 0.2328
WVFGRD96 20.0 235 60 -25 3.52 0.2483
WVFGRD96 21.0 235 60 -30 3.54 0.2690
WVFGRD96 22.0 235 60 -30 3.55 0.2874
WVFGRD96 23.0 245 65 -30 3.55 0.3049
WVFGRD96 24.0 245 70 -35 3.56 0.3205
WVFGRD96 25.0 245 80 -40 3.57 0.3469
WVFGRD96 26.0 245 80 -40 3.58 0.3694
WVFGRD96 27.0 245 80 -40 3.58 0.3825
WVFGRD96 28.0 245 80 -40 3.59 0.3982
WVFGRD96 29.0 245 75 -40 3.58 0.4064
WVFGRD96 30.0 240 75 -50 3.60 0.4117
WVFGRD96 31.0 240 75 -50 3.60 0.4213
WVFGRD96 32.0 240 75 -50 3.60 0.4204
WVFGRD96 33.0 240 75 -50 3.59 0.4176
WVFGRD96 34.0 240 75 -50 3.59 0.4148
WVFGRD96 35.0 230 70 -45 3.60 0.4101
WVFGRD96 36.0 230 70 -40 3.59 0.4056
WVFGRD96 37.0 230 70 -40 3.59 0.4005
WVFGRD96 38.0 230 70 -40 3.60 0.3996
WVFGRD96 39.0 230 70 -40 3.61 0.3965
WVFGRD96 40.0 235 75 -50 3.67 0.3897
WVFGRD96 41.0 235 75 -50 3.69 0.3935
WVFGRD96 42.0 230 70 -45 3.70 0.3918
WVFGRD96 43.0 240 75 -50 3.70 0.3920
WVFGRD96 44.0 240 70 -45 3.71 0.3887
WVFGRD96 45.0 240 70 -45 3.72 0.3885
WVFGRD96 46.0 240 70 -45 3.73 0.3896
WVFGRD96 47.0 240 70 -45 3.73 0.3868
WVFGRD96 48.0 240 70 -45 3.74 0.3854
WVFGRD96 49.0 240 70 -45 3.75 0.3856
WVFGRD96 50.0 240 75 -45 3.75 0.3850
WVFGRD96 51.0 240 75 -45 3.76 0.3839
WVFGRD96 52.0 240 75 -45 3.77 0.3858
WVFGRD96 53.0 240 75 -45 3.77 0.3841
WVFGRD96 54.0 240 75 -45 3.78 0.3832
WVFGRD96 55.0 240 75 -45 3.78 0.3835
WVFGRD96 56.0 240 80 -45 3.79 0.3812
WVFGRD96 57.0 240 80 -45 3.79 0.3795
WVFGRD96 58.0 240 80 -45 3.80 0.3777
WVFGRD96 59.0 240 80 -45 3.80 0.3751
The best solution is
WVFGRD96 31.0 240 75 -50 3.60 0.4213
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 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.06 n 3 lp c 0.20 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