The ANSS event ID is aka2026qtpptp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/aka2026qtpptp/executive.
2026/08/24 20:13:35 58.966 -154.314 125.5 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2026/08/24 20:13:35.0 58.97 -154.31 125.5 3.9 Alaska
Stations used:
AK.CNP AK.N18K AK.O18K AK.O19K AK.P17K AK.Q19K AV.ACH
AV.RED AV.SPCL AV.STLK II.KDAK
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 0.10 n 3
Best Fitting Double Couple
Mo = 9.89e+21 dyne-cm
Mw = 3.93
Z = 132 km
Plane Strike Dip Rake
NP1 280 60 85
NP2 110 30 99
Principal Axes:
Axis Value Plunge Azimuth
T 9.89e+21 74 177
N 0.00e+00 4 282
P -9.89e+21 15 14
Moment Tensor: (dyne-cm)
Component Value
Mxx -8.02e+21
Mxy -2.16e+21
Mxz -4.92e+21
Myy -5.12e+20
Myz -4.31e+20
Mzz 8.53e+21
---------- -
-------------- P -----
----------------- --------
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-----#####################--------------
-##############################---------
--#################################-------
--###################################-----
---####################################---
----################ ##################-
----############### T ##################
-----############## ##################
------################################
-------############################-
--------#######################---
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Global CMT Convention Moment Tensor:
R T P
8.53e+21 -4.92e+21 4.31e+20
-4.92e+21 -8.02e+21 2.16e+21
4.31e+20 2.16e+21 -5.12e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260824201335/index.html
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STK = 280
DIP = 60
RAKE = 85
MW = 3.93
HS = 132.0
The NDK file is 20260824201335.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.
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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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 50.0 110 50 -60 3.74 0.2195
WVFGRD96 52.0 255 75 35 3.80 0.2194
WVFGRD96 54.0 255 75 35 3.80 0.2197
WVFGRD96 56.0 260 70 45 3.80 0.2286
WVFGRD96 58.0 260 70 45 3.81 0.2400
WVFGRD96 60.0 260 75 45 3.82 0.2502
WVFGRD96 62.0 260 75 45 3.83 0.2601
WVFGRD96 64.0 255 65 35 3.83 0.2712
WVFGRD96 66.0 255 65 35 3.84 0.2814
WVFGRD96 68.0 255 65 35 3.84 0.2910
WVFGRD96 70.0 255 65 35 3.85 0.2994
WVFGRD96 72.0 260 65 45 3.84 0.3092
WVFGRD96 74.0 260 65 50 3.84 0.3295
WVFGRD96 76.0 265 60 60 3.83 0.3514
WVFGRD96 78.0 270 60 65 3.84 0.3742
WVFGRD96 80.0 270 60 65 3.85 0.3915
WVFGRD96 82.0 270 60 65 3.85 0.4028
WVFGRD96 84.0 270 60 65 3.86 0.4133
WVFGRD96 86.0 275 60 70 3.86 0.4234
WVFGRD96 88.0 275 60 70 3.87 0.4338
WVFGRD96 90.0 275 60 70 3.87 0.4431
WVFGRD96 92.0 280 60 75 3.88 0.4524
WVFGRD96 94.0 280 60 80 3.88 0.4614
WVFGRD96 96.0 280 60 80 3.88 0.4695
WVFGRD96 98.0 280 60 80 3.89 0.4774
WVFGRD96 100.0 285 60 85 3.89 0.4842
WVFGRD96 102.0 285 60 85 3.90 0.4907
WVFGRD96 104.0 285 60 85 3.90 0.4962
WVFGRD96 106.0 285 60 85 3.90 0.5011
WVFGRD96 108.0 285 60 85 3.91 0.5064
WVFGRD96 110.0 285 60 85 3.91 0.5111
WVFGRD96 112.0 285 60 85 3.91 0.5150
WVFGRD96 114.0 285 60 85 3.91 0.5183
WVFGRD96 116.0 285 60 85 3.92 0.5206
WVFGRD96 118.0 285 60 85 3.92 0.5229
WVFGRD96 120.0 285 60 85 3.92 0.5258
WVFGRD96 122.0 285 60 85 3.92 0.5287
WVFGRD96 124.0 285 60 85 3.92 0.5301
WVFGRD96 126.0 285 60 85 3.93 0.5307
WVFGRD96 128.0 285 60 85 3.93 0.5320
WVFGRD96 130.0 280 60 85 3.93 0.5334
WVFGRD96 132.0 280 60 85 3.93 0.5336
WVFGRD96 134.0 280 60 85 3.93 0.5335
WVFGRD96 136.0 280 60 85 3.93 0.5334
WVFGRD96 138.0 280 60 85 3.93 0.5333
WVFGRD96 140.0 280 60 85 3.94 0.5320
WVFGRD96 142.0 280 60 85 3.94 0.5306
WVFGRD96 144.0 280 60 85 3.94 0.5294
WVFGRD96 146.0 280 60 85 3.94 0.5282
WVFGRD96 148.0 105 30 95 3.94 0.5260
The best solution is
WVFGRD96 132.0 280 60 85 3.93 0.5336
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 0.10 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