The ANSS event ID is ak2026alfphj and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak2026alfphj/executive.
2026/01/07 02:37:51 63.079 -150.989 127.0 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/01/07 02:37:51.0 63.08 -150.99 127.0 4.5 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.FIRE AK.GCSA AK.GHO
AK.GLI AK.H24K AK.HDA AK.I23K AK.J19K AK.J20K AK.J25K
AK.KNK AK.L19K AK.L22K AK.MCK AK.NEA2 AK.PAX AK.POKR
AK.RC01 AK.SAW AK.SCM AK.SKN AK.WRH 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 0.08 n 3
Best Fitting Double Couple
Mo = 5.19e+22 dyne-cm
Mw = 4.41
Z = 138 km
Plane Strike Dip Rake
NP1 55 75 30
NP2 316 61 163
Principal Axes:
Axis Value Plunge Azimuth
T 5.19e+22 32 279
N 0.00e+00 57 79
P -5.19e+22 9 183
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.95e+22
Mxy -8.75e+21
Mxz 1.17e+22
Myy 3.65e+22
Myz -2.24e+22
Mzz 1.30e+22
--------------
----------------------
----------------------------
#######-----------------------
#############---------------------
#################-----------------##
#####################-------------####
########################---------#######
#### ###################-----#########
##### T ####################---###########
##### ####################--############
##########################-----###########
########################--------##########
####################------------########
#################----------------#######
#############-------------------######
#######-------------------------####
-------------------------------###
-----------------------------#
----------------------------
-------- -----------
---- P -------
Global CMT Convention Moment Tensor:
R T P
1.30e+22 1.17e+22 2.24e+22
1.17e+22 -4.95e+22 8.75e+21
2.24e+22 8.75e+21 3.65e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260107023751/index.html
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STK = 55
DIP = 75
RAKE = 30
MW = 4.41
HS = 138.0
The NDK file is 20260107023751.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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 60.0 60 75 15 4.24 0.3185
WVFGRD96 62.0 55 70 15 4.25 0.3380
WVFGRD96 64.0 55 70 15 4.26 0.3546
WVFGRD96 66.0 55 70 15 4.27 0.3674
WVFGRD96 68.0 55 70 15 4.28 0.3766
WVFGRD96 70.0 55 70 15 4.29 0.3853
WVFGRD96 72.0 55 70 15 4.30 0.3929
WVFGRD96 74.0 55 70 15 4.30 0.4006
WVFGRD96 76.0 55 70 15 4.31 0.4077
WVFGRD96 78.0 55 70 15 4.31 0.4148
WVFGRD96 80.0 55 70 15 4.32 0.4224
WVFGRD96 82.0 55 70 15 4.32 0.4289
WVFGRD96 84.0 55 70 15 4.33 0.4347
WVFGRD96 86.0 60 70 20 4.33 0.4404
WVFGRD96 88.0 60 70 20 4.33 0.4466
WVFGRD96 90.0 60 70 20 4.34 0.4517
WVFGRD96 92.0 60 70 20 4.34 0.4564
WVFGRD96 94.0 55 75 20 4.35 0.4617
WVFGRD96 96.0 55 75 20 4.35 0.4669
WVFGRD96 98.0 55 75 20 4.36 0.4713
WVFGRD96 100.0 60 75 25 4.36 0.4757
WVFGRD96 102.0 60 75 25 4.37 0.4791
WVFGRD96 104.0 60 75 25 4.37 0.4837
WVFGRD96 106.0 60 75 25 4.37 0.4870
WVFGRD96 108.0 60 75 25 4.38 0.4897
WVFGRD96 110.0 60 75 25 4.38 0.4927
WVFGRD96 112.0 60 75 25 4.38 0.4953
WVFGRD96 114.0 60 75 25 4.39 0.4973
WVFGRD96 116.0 60 75 25 4.39 0.4988
WVFGRD96 118.0 60 75 30 4.39 0.5017
WVFGRD96 120.0 60 75 30 4.40 0.5025
WVFGRD96 122.0 60 75 30 4.40 0.5042
WVFGRD96 124.0 55 75 30 4.40 0.5058
WVFGRD96 126.0 55 75 30 4.40 0.5067
WVFGRD96 128.0 55 75 30 4.40 0.5076
WVFGRD96 130.0 55 75 30 4.41 0.5085
WVFGRD96 132.0 55 75 30 4.41 0.5079
WVFGRD96 134.0 55 75 30 4.41 0.5087
WVFGRD96 136.0 55 75 30 4.41 0.5083
WVFGRD96 138.0 55 75 30 4.41 0.5089
WVFGRD96 140.0 55 75 30 4.42 0.5080
WVFGRD96 142.0 55 75 30 4.42 0.5071
WVFGRD96 144.0 55 75 30 4.42 0.5064
WVFGRD96 146.0 55 75 30 4.42 0.5055
WVFGRD96 148.0 55 75 30 4.42 0.5049
WVFGRD96 150.0 60 70 30 4.42 0.5028
WVFGRD96 152.0 60 70 30 4.42 0.5026
WVFGRD96 154.0 60 70 30 4.43 0.5007
WVFGRD96 156.0 60 70 30 4.43 0.5002
WVFGRD96 158.0 60 70 30 4.43 0.4978
WVFGRD96 160.0 60 70 30 4.43 0.4971
WVFGRD96 162.0 60 70 30 4.43 0.4952
WVFGRD96 164.0 60 70 30 4.44 0.4938
WVFGRD96 166.0 60 70 30 4.44 0.4923
WVFGRD96 168.0 60 70 30 4.44 0.4919
WVFGRD96 170.0 60 70 30 4.44 0.4896
WVFGRD96 172.0 60 70 30 4.44 0.4854
WVFGRD96 174.0 60 70 30 4.44 0.4691
WVFGRD96 176.0 55 75 30 4.44 0.4448
WVFGRD96 178.0 55 75 35 4.43 0.4219
WVFGRD96 180.0 55 75 35 4.43 0.4005
WVFGRD96 182.0 55 75 35 4.42 0.3796
WVFGRD96 184.0 55 75 35 4.41 0.3575
WVFGRD96 186.0 55 75 35 4.41 0.3391
WVFGRD96 188.0 50 80 30 4.41 0.3195
The best solution is
WVFGRD96 138.0 55 75 30 4.41 0.5089
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