The ANSS event ID is ak2026clpiur and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak2026clpiur/executive.
2026/02/04 15:24:48 61.495 -151.529 82.3 4.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/02/04 15:24:48.0 61.49 -151.53 82.3 4.4 Alaska
Stations used:
AK.CAPN AK.CAST AK.CUT AK.DHY AK.FIRE AK.GHO AK.J20K
AK.L22K AK.MCK AK.N18K AK.O18K AK.O19K AK.PPLA AK.PWL
AK.RC01 AK.RND AK.SAW AK.SKN AK.SLK AK.SWD AK.WAT6 AT.TTA
AV.SPCL AV.STLK
Filtering commands used:
cut o DIST/3.8 -40 o DIST/3.8 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 7.33e+22 dyne-cm
Mw = 4.51
Z = 100 km
Plane Strike Dip Rake
NP1 302 57 130
NP2 65 50 45
Principal Axes:
Axis Value Plunge Azimuth
T 7.33e+22 57 269
N 0.00e+00 33 98
P -7.33e+22 4 5
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.23e+22
Mxy -5.97e+21
Mxz -5.92e+21
Myy 2.13e+22
Myz -3.40e+22
Mzz 5.10e+22
------- P ----
----------- --------
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----###---------------------------
################--------------------
#####################-----------------
#########################-------------##
############################----------##
###############################-------####
########### ###################----#####
########### T ####################--######
########### ####################--######
###############################-----####
#############################-------####
#########################-----------##
--##################---------------#
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Global CMT Convention Moment Tensor:
R T P
5.10e+22 -5.92e+21 3.40e+22
-5.92e+21 -7.23e+22 5.97e+21
3.40e+22 5.97e+21 2.13e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260204152448/index.html
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STK = 65
DIP = 50
RAKE = 45
MW = 4.51
HS = 100.0
The NDK file is 20260204152448.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/3.8 -40 o DIST/3.8 +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 2.0 105 45 -75 3.75 0.1675
WVFGRD96 4.0 310 35 -35 3.82 0.1707
WVFGRD96 6.0 155 40 20 3.83 0.1923
WVFGRD96 8.0 60 80 50 3.91 0.2162
WVFGRD96 10.0 60 80 50 3.94 0.2367
WVFGRD96 12.0 60 75 45 3.97 0.2483
WVFGRD96 14.0 60 75 45 3.99 0.2522
WVFGRD96 16.0 60 80 40 4.01 0.2520
WVFGRD96 18.0 60 80 40 4.03 0.2495
WVFGRD96 20.0 60 80 35 4.05 0.2457
WVFGRD96 22.0 225 70 -30 4.06 0.2448
WVFGRD96 24.0 230 75 -20 4.08 0.2453
WVFGRD96 26.0 230 75 -20 4.10 0.2482
WVFGRD96 28.0 230 75 -15 4.12 0.2526
WVFGRD96 30.0 230 75 -10 4.14 0.2585
WVFGRD96 32.0 230 75 -10 4.16 0.2650
WVFGRD96 34.0 50 75 -10 4.20 0.2729
WVFGRD96 36.0 50 80 -10 4.22 0.2795
WVFGRD96 38.0 50 75 -10 4.25 0.2888
WVFGRD96 40.0 50 75 -15 4.31 0.3079
WVFGRD96 42.0 50 75 -15 4.33 0.3133
WVFGRD96 44.0 50 75 -15 4.35 0.3166
WVFGRD96 46.0 50 75 -15 4.37 0.3198
WVFGRD96 48.0 50 75 -15 4.38 0.3236
WVFGRD96 50.0 50 75 -15 4.40 0.3266
WVFGRD96 52.0 50 70 -15 4.41 0.3301
WVFGRD96 54.0 50 70 -15 4.42 0.3337
WVFGRD96 56.0 50 70 -15 4.43 0.3361
WVFGRD96 58.0 50 70 -15 4.43 0.3373
WVFGRD96 60.0 65 55 40 4.45 0.3468
WVFGRD96 62.0 65 55 45 4.45 0.3617
WVFGRD96 64.0 65 55 45 4.46 0.3747
WVFGRD96 66.0 65 55 45 4.47 0.3870
WVFGRD96 68.0 65 55 45 4.47 0.3979
WVFGRD96 70.0 65 55 45 4.48 0.4075
WVFGRD96 72.0 65 55 45 4.48 0.4154
WVFGRD96 74.0 65 55 45 4.48 0.4226
WVFGRD96 76.0 65 55 45 4.49 0.4290
WVFGRD96 78.0 65 55 45 4.49 0.4336
WVFGRD96 80.0 65 55 45 4.49 0.4385
WVFGRD96 82.0 65 55 45 4.49 0.4422
WVFGRD96 84.0 65 55 45 4.50 0.4447
WVFGRD96 86.0 65 55 45 4.50 0.4472
WVFGRD96 88.0 65 55 45 4.50 0.4492
WVFGRD96 90.0 65 55 45 4.50 0.4508
WVFGRD96 92.0 65 50 45 4.50 0.4516
WVFGRD96 94.0 65 50 45 4.50 0.4526
WVFGRD96 96.0 65 50 45 4.50 0.4543
WVFGRD96 98.0 65 50 45 4.51 0.4551
WVFGRD96 100.0 65 50 45 4.51 0.4559
WVFGRD96 102.0 65 50 45 4.51 0.4552
WVFGRD96 104.0 65 50 50 4.51 0.4551
WVFGRD96 106.0 65 50 50 4.52 0.4556
WVFGRD96 108.0 65 50 50 4.52 0.4556
WVFGRD96 110.0 65 50 50 4.52 0.4542
WVFGRD96 112.0 65 50 50 4.52 0.4538
WVFGRD96 114.0 65 50 50 4.53 0.4533
WVFGRD96 116.0 65 50 50 4.53 0.4517
WVFGRD96 118.0 65 50 50 4.53 0.4501
WVFGRD96 120.0 65 50 50 4.53 0.4496
WVFGRD96 122.0 70 45 50 4.53 0.4473
WVFGRD96 124.0 70 45 50 4.54 0.4463
WVFGRD96 126.0 70 45 50 4.54 0.4449
WVFGRD96 128.0 70 45 50 4.54 0.4430
WVFGRD96 130.0 70 45 55 4.55 0.4417
WVFGRD96 132.0 65 45 50 4.55 0.4402
WVFGRD96 134.0 65 45 50 4.55 0.4388
WVFGRD96 136.0 65 45 50 4.55 0.4383
WVFGRD96 138.0 65 45 50 4.55 0.4362
WVFGRD96 140.0 65 45 50 4.56 0.4307
WVFGRD96 142.0 65 45 50 4.55 0.4191
WVFGRD96 144.0 65 45 50 4.55 0.4049
WVFGRD96 146.0 65 45 50 4.55 0.3881
WVFGRD96 148.0 70 40 50 4.55 0.3707
The best solution is
WVFGRD96 100.0 65 50 45 4.51 0.4559
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/3.8 -40 o DIST/3.8 +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