The ANSS event ID is aka2026gioyfa and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/aka2026gioyfa/executive.
2026/03/31 15:44:28 59.765 -152.359 81.4 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/03/31 15:44:28.0 59.76 -152.36 81.4 4.1 Alaska
Stations used:
AK.BRLK AK.CAPN AK.FIRE AK.GHO AK.N18K AK.O18K AK.O19K
AK.RC01 AK.SKN AK.SLK AK.SSN AT.PMR AV.ACH AV.RED AV.SPCL
AV.STLK
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
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.78e+22 dyne-cm
Mw = 4.10
Z = 98 km
Plane Strike Dip Rake
NP1 310 76 159
NP2 45 70 15
Principal Axes:
Axis Value Plunge Azimuth
T 1.78e+22 24 266
N 0.00e+00 65 97
P -1.78e+22 4 358
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.76e+22
Mxy 1.48e+21
Mxz -1.66e+21
Myy 1.47e+22
Myz -6.65e+21
Mzz 2.96e+21
----- P ------
--------- ----------
----------------------------
------------------------------
#####--------------------------###
##########---------------------#####
##############------------------######
#################---------------########
####################----------##########
#######################-------############
#### ##################----#############
#### T ###################################
#### ##################----#############
#######################-------##########
#####################-----------########
#################---------------######
##############------------------####
##########----------------------##
###---------------------------
----------------------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
2.96e+21 -1.66e+21 6.65e+21
-1.66e+21 -1.76e+22 -1.48e+21
6.65e+21 -1.48e+21 1.47e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260331154428/index.html
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STK = 45
DIP = 70
RAKE = 15
MW = 4.10
HS = 98.0
The NDK file is 20260331154428.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 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 215 85 0 3.31 0.3668
WVFGRD96 4.0 215 80 -5 3.38 0.4097
WVFGRD96 6.0 40 75 0 3.44 0.4272
WVFGRD96 8.0 40 75 5 3.48 0.4445
WVFGRD96 10.0 40 80 0 3.51 0.4514
WVFGRD96 12.0 40 80 0 3.53 0.4504
WVFGRD96 14.0 35 80 -10 3.54 0.4464
WVFGRD96 16.0 215 90 10 3.56 0.4392
WVFGRD96 18.0 35 80 -10 3.58 0.4338
WVFGRD96 20.0 35 80 -15 3.60 0.4291
WVFGRD96 22.0 35 80 -15 3.62 0.4230
WVFGRD96 24.0 35 80 -15 3.63 0.4162
WVFGRD96 26.0 35 80 -15 3.65 0.4078
WVFGRD96 28.0 35 80 -20 3.67 0.4023
WVFGRD96 30.0 35 80 -20 3.69 0.3981
WVFGRD96 32.0 35 80 -20 3.71 0.3950
WVFGRD96 34.0 35 85 -20 3.73 0.3939
WVFGRD96 36.0 35 85 -20 3.75 0.3954
WVFGRD96 38.0 220 90 15 3.79 0.3956
WVFGRD96 40.0 35 80 -25 3.85 0.4106
WVFGRD96 42.0 35 80 -20 3.86 0.4164
WVFGRD96 44.0 35 80 -20 3.88 0.4206
WVFGRD96 46.0 35 80 -15 3.90 0.4254
WVFGRD96 48.0 35 80 -15 3.91 0.4302
WVFGRD96 50.0 35 80 -15 3.93 0.4356
WVFGRD96 52.0 40 80 -10 3.95 0.4430
WVFGRD96 54.0 40 80 -10 3.97 0.4525
WVFGRD96 56.0 40 80 -10 3.98 0.4616
WVFGRD96 58.0 40 80 -10 3.99 0.4712
WVFGRD96 60.0 40 75 0 4.00 0.4811
WVFGRD96 62.0 40 70 0 4.01 0.4938
WVFGRD96 64.0 40 70 0 4.02 0.5065
WVFGRD96 66.0 40 70 0 4.03 0.5182
WVFGRD96 68.0 40 70 0 4.04 0.5285
WVFGRD96 70.0 40 70 5 4.04 0.5389
WVFGRD96 72.0 40 70 5 4.05 0.5485
WVFGRD96 74.0 40 70 5 4.05 0.5565
WVFGRD96 76.0 40 75 10 4.05 0.5651
WVFGRD96 78.0 40 75 10 4.06 0.5713
WVFGRD96 80.0 40 75 10 4.06 0.5781
WVFGRD96 82.0 45 70 10 4.08 0.5833
WVFGRD96 84.0 45 70 10 4.09 0.5877
WVFGRD96 86.0 45 70 10 4.09 0.5918
WVFGRD96 88.0 45 70 10 4.09 0.5952
WVFGRD96 90.0 45 70 10 4.10 0.5972
WVFGRD96 92.0 45 70 15 4.09 0.5989
WVFGRD96 94.0 45 70 15 4.10 0.6010
WVFGRD96 96.0 45 70 15 4.10 0.6021
WVFGRD96 98.0 45 70 15 4.10 0.6026
WVFGRD96 100.0 45 70 15 4.10 0.6025
WVFGRD96 102.0 45 70 15 4.11 0.6025
WVFGRD96 104.0 45 70 15 4.11 0.6018
WVFGRD96 106.0 45 70 15 4.11 0.6016
WVFGRD96 108.0 45 70 15 4.11 0.6008
WVFGRD96 110.0 45 70 15 4.11 0.5995
WVFGRD96 112.0 45 70 15 4.11 0.5984
WVFGRD96 114.0 45 70 15 4.12 0.5970
WVFGRD96 116.0 45 70 15 4.12 0.5953
WVFGRD96 118.0 45 70 15 4.12 0.5941
WVFGRD96 120.0 45 70 15 4.12 0.5922
WVFGRD96 122.0 45 70 15 4.12 0.5902
WVFGRD96 124.0 45 70 15 4.12 0.5886
WVFGRD96 126.0 45 70 15 4.12 0.5870
WVFGRD96 128.0 40 75 15 4.11 0.5854
The best solution is
WVFGRD96 98.0 45 70 15 4.10 0.6026
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 br c 0.12 0.25 n 4 p 2
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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