The ANSS event ID is aka2026sxksxl and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/aka2026sxksxl/executive.
2026/09/24 01:27:20 62.039 -149.756 26.8 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/09/24 01:27:20.0 62.04 -149.76 26.8 3.8 Alaska
Stations used:
AK.BAE AK.GHO AK.GLI AK.KNK AK.L22K AK.PAX AK.PPLA AK.RC01
AK.RND AK.SAW AK.SCM AK.SKN AT.PMR AV.SPCL
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 6.31e+21 dyne-cm
Mw = 3.80
Z = 60 km
Plane Strike Dip Rake
NP1 235 75 -60
NP2 349 33 -152
Principal Axes:
Axis Value Plunge Azimuth
T 6.31e+21 24 302
N 0.00e+00 29 46
P -6.31e+21 51 179
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.03e+21
Mxy -2.33e+21
Mxz 4.34e+21
Myy 3.76e+21
Myz -2.05e+21
Mzz -2.73e+21
#######-------
###############-------
####################--------
#######################-------
###########################-------
### ##############################
#### T ##################------#######
##### ###############----------#######
####################-------------#######
##################-----------------#######
################-------------------#######
##############---------------------#######
###########------------------------#######
#########-------------------------######
#######---------------------------######
####-------------- -----------######
##--------------- P -----------#####
---------------- ----------#####
--------------------------####
-----------------------#####
-------------------###
------------##
Global CMT Convention Moment Tensor:
R T P
-2.73e+21 4.34e+21 2.05e+21
4.34e+21 -1.03e+21 2.33e+21
2.05e+21 2.33e+21 3.76e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260924012720/index.html
|
STK = 235
DIP = 75
RAKE = -60
MW = 3.80
HS = 60.0
The NDK file is 20260924012720.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.
Map showing station locations used for computing the ML's. No distinction is made whether the vertical (Z) or horizontal (H) components were used.
 |
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.
|
|
|
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 -40 o DIST/3.3 +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 1.0 205 45 85 2.91 0.2009
WVFGRD96 2.0 210 45 95 3.04 0.2558
WVFGRD96 3.0 20 45 80 3.10 0.2358
WVFGRD96 4.0 330 45 -30 3.10 0.2213
WVFGRD96 5.0 330 45 -30 3.12 0.2481
WVFGRD96 6.0 330 50 -35 3.15 0.2675
WVFGRD96 7.0 130 40 15 3.10 0.2862
WVFGRD96 8.0 135 30 20 3.18 0.3017
WVFGRD96 9.0 130 35 20 3.19 0.3194
WVFGRD96 10.0 135 35 25 3.21 0.3339
WVFGRD96 11.0 90 50 45 3.30 0.3480
WVFGRD96 12.0 90 50 45 3.32 0.3636
WVFGRD96 13.0 90 50 45 3.33 0.3768
WVFGRD96 14.0 90 50 45 3.35 0.3891
WVFGRD96 15.0 90 50 45 3.36 0.3995
WVFGRD96 16.0 90 50 45 3.37 0.4085
WVFGRD96 17.0 85 55 40 3.37 0.4162
WVFGRD96 18.0 85 60 45 3.38 0.4228
WVFGRD96 19.0 85 60 45 3.39 0.4291
WVFGRD96 20.0 85 60 45 3.40 0.4344
WVFGRD96 21.0 85 60 45 3.42 0.4389
WVFGRD96 22.0 85 60 45 3.43 0.4418
WVFGRD96 23.0 85 60 45 3.44 0.4437
WVFGRD96 24.0 85 60 45 3.45 0.4445
WVFGRD96 25.0 85 65 45 3.45 0.4458
WVFGRD96 26.0 80 70 40 3.44 0.4467
WVFGRD96 27.0 80 70 40 3.45 0.4483
WVFGRD96 28.0 80 70 40 3.46 0.4491
WVFGRD96 29.0 80 70 40 3.47 0.4490
WVFGRD96 30.0 245 75 -40 3.46 0.4561
WVFGRD96 31.0 245 75 -40 3.47 0.4646
WVFGRD96 32.0 245 75 -40 3.48 0.4725
WVFGRD96 33.0 240 70 -45 3.49 0.4798
WVFGRD96 34.0 240 70 -45 3.50 0.4891
WVFGRD96 35.0 240 70 -45 3.52 0.4979
WVFGRD96 36.0 240 70 -45 3.53 0.5067
WVFGRD96 37.0 240 70 -45 3.54 0.5155
WVFGRD96 38.0 240 70 -45 3.56 0.5246
WVFGRD96 39.0 240 65 -45 3.58 0.5342
WVFGRD96 40.0 240 70 -55 3.65 0.5319
WVFGRD96 41.0 240 75 -55 3.66 0.5427
WVFGRD96 42.0 240 75 -55 3.67 0.5537
WVFGRD96 43.0 240 75 -55 3.68 0.5649
WVFGRD96 44.0 235 70 -60 3.70 0.5763
WVFGRD96 45.0 235 70 -60 3.71 0.5872
WVFGRD96 46.0 235 70 -60 3.72 0.5971
WVFGRD96 47.0 235 70 -60 3.73 0.6062
WVFGRD96 48.0 235 70 -60 3.74 0.6150
WVFGRD96 49.0 235 70 -60 3.74 0.6228
WVFGRD96 50.0 235 70 -60 3.75 0.6293
WVFGRD96 51.0 235 70 -60 3.76 0.6342
WVFGRD96 52.0 235 70 -60 3.76 0.6399
WVFGRD96 53.0 235 70 -60 3.77 0.6428
WVFGRD96 54.0 235 70 -60 3.78 0.6450
WVFGRD96 55.0 235 75 -60 3.77 0.6470
WVFGRD96 56.0 235 75 -60 3.78 0.6501
WVFGRD96 57.0 235 75 -60 3.78 0.6525
WVFGRD96 58.0 235 75 -60 3.79 0.6536
WVFGRD96 59.0 235 75 -60 3.79 0.6547
WVFGRD96 60.0 235 75 -60 3.80 0.6547
WVFGRD96 61.0 235 75 -60 3.80 0.6534
WVFGRD96 62.0 235 75 -60 3.80 0.6526
WVFGRD96 63.0 235 75 -60 3.81 0.6505
WVFGRD96 64.0 240 80 -55 3.81 0.6486
WVFGRD96 65.0 235 80 -60 3.81 0.6470
WVFGRD96 66.0 235 80 -60 3.82 0.6476
WVFGRD96 67.0 235 80 -60 3.82 0.6468
WVFGRD96 68.0 235 80 -60 3.83 0.6451
WVFGRD96 69.0 235 80 -60 3.83 0.6436
WVFGRD96 70.0 235 80 -60 3.83 0.6415
WVFGRD96 71.0 235 80 -60 3.84 0.6393
WVFGRD96 72.0 235 80 -60 3.84 0.6363
WVFGRD96 73.0 235 80 -60 3.84 0.6328
WVFGRD96 74.0 235 80 -60 3.84 0.6294
WVFGRD96 75.0 235 80 -60 3.84 0.6255
WVFGRD96 76.0 235 85 -60 3.86 0.6233
WVFGRD96 77.0 235 85 -60 3.86 0.6207
WVFGRD96 78.0 235 85 -60 3.86 0.6179
WVFGRD96 79.0 235 85 -60 3.86 0.6144
The best solution is
WVFGRD96 60.0 235 75 -60 3.80 0.6547
The mechanism corresponding to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3
|
| 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. |
|
| 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