The ANSS event ID is ak2025xbznhg and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak2025xbznhg/executive.
2025/11/23 20:30:18 59.303 -154.503 143.0 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/11/23 20:30:18.0 59.30 -154.50 143.0 4.1 Alaska
Stations used:
AK.HOM AK.N15K AK.N18K AK.O14K AK.O18K AK.O19K AK.Q19K
AV.ACH AV.RED AV.SPCL AV.STLK II.KDAK
Filtering commands used:
cut o DIST/3.5 -30 o DIST/3.5 +60
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 = 168 km
Plane Strike Dip Rake
NP1 170 80 -35
NP2 267 56 -168
Principal Axes:
Axis Value Plunge Azimuth
T 1.78e+22 16 223
N 0.00e+00 54 336
P -1.78e+22 31 123
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.01e+21
Mxy 1.41e+22
Mxz 8.27e+20
Myy -1.52e+21
Myz -9.88e+21
Mzz -3.49e+21
---###########
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--------------------------############
---------#######-----------------#######
-----############--------------------###
----##############----------------------##
--################------------------------
###################-----------------------
###################-----------------------
##################----------------------
###################----------- -------
##################----------- P ------
#### ###########---------- -----
### T ###########-----------------
# ############--------------
###############-------------
#############---------
##########----
Global CMT Convention Moment Tensor:
R T P
-3.49e+21 8.27e+20 9.88e+21
8.27e+20 5.01e+21 -1.41e+22
9.88e+21 -1.41e+22 -1.52e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20251123203018/index.html
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STK = 170
DIP = 80
RAKE = -35
MW = 4.10
HS = 168.0
The NDK file is 20251123203018.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 -30 o DIST/3.5 +60 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 2.0 260 75 10 3.27 0.3433
WVFGRD96 4.0 85 85 35 3.37 0.3634
WVFGRD96 6.0 85 90 35 3.39 0.3743
WVFGRD96 8.0 260 85 -35 3.43 0.3736
WVFGRD96 10.0 80 35 -5 3.52 0.3745
WVFGRD96 12.0 80 40 -5 3.52 0.3732
WVFGRD96 14.0 85 40 10 3.54 0.3725
WVFGRD96 16.0 85 40 10 3.55 0.3709
WVFGRD96 18.0 95 40 20 3.57 0.3745
WVFGRD96 20.0 95 40 20 3.59 0.3775
WVFGRD96 22.0 95 40 20 3.61 0.3803
WVFGRD96 24.0 90 40 20 3.63 0.3829
WVFGRD96 26.0 90 40 15 3.65 0.3836
WVFGRD96 28.0 90 40 15 3.67 0.3820
WVFGRD96 30.0 90 40 15 3.69 0.3761
WVFGRD96 32.0 90 40 15 3.70 0.3657
WVFGRD96 34.0 90 40 15 3.71 0.3506
WVFGRD96 36.0 90 45 15 3.72 0.3315
WVFGRD96 38.0 90 50 20 3.72 0.3115
WVFGRD96 40.0 95 40 25 3.80 0.2787
WVFGRD96 42.0 100 60 40 3.76 0.2660
WVFGRD96 44.0 100 65 45 3.76 0.2570
WVFGRD96 46.0 100 65 45 3.77 0.2489
WVFGRD96 48.0 110 55 55 3.81 0.2412
WVFGRD96 50.0 115 55 65 3.82 0.2354
WVFGRD96 52.0 110 60 60 3.82 0.2310
WVFGRD96 54.0 110 60 60 3.83 0.2264
WVFGRD96 56.0 110 60 60 3.84 0.2216
WVFGRD96 58.0 115 55 65 3.86 0.2189
WVFGRD96 60.0 110 60 60 3.86 0.2155
WVFGRD96 62.0 110 60 60 3.86 0.2145
WVFGRD96 64.0 195 50 15 3.83 0.2244
WVFGRD96 66.0 195 45 20 3.86 0.2405
WVFGRD96 68.0 195 45 20 3.88 0.2575
WVFGRD96 70.0 190 50 15 3.89 0.2754
WVFGRD96 72.0 175 65 -5 3.88 0.3019
WVFGRD96 74.0 175 65 -5 3.90 0.3393
WVFGRD96 76.0 175 65 -5 3.92 0.3769
WVFGRD96 78.0 175 70 -25 3.92 0.4143
WVFGRD96 80.0 175 70 -30 3.94 0.4549
WVFGRD96 82.0 175 75 -30 3.95 0.4938
WVFGRD96 84.0 175 75 -30 3.96 0.5284
WVFGRD96 86.0 175 75 -35 3.97 0.5567
WVFGRD96 88.0 175 75 -35 3.98 0.5790
WVFGRD96 90.0 175 75 -35 3.99 0.5986
WVFGRD96 92.0 -5 90 25 3.98 0.6142
WVFGRD96 94.0 175 80 -30 3.99 0.6336
WVFGRD96 96.0 -5 90 25 3.99 0.6339
WVFGRD96 98.0 -5 90 25 4.00 0.6379
WVFGRD96 100.0 175 80 -30 4.01 0.6524
WVFGRD96 102.0 175 80 -30 4.01 0.6563
WVFGRD96 104.0 175 80 -30 4.02 0.6604
WVFGRD96 106.0 175 80 -30 4.02 0.6643
WVFGRD96 108.0 175 80 -30 4.02 0.6675
WVFGRD96 110.0 175 80 -30 4.03 0.6707
WVFGRD96 112.0 175 80 -30 4.03 0.6748
WVFGRD96 114.0 175 80 -30 4.03 0.6779
WVFGRD96 116.0 175 80 -30 4.04 0.6815
WVFGRD96 118.0 175 80 -30 4.04 0.6841
WVFGRD96 120.0 175 80 -30 4.04 0.6859
WVFGRD96 122.0 175 80 -30 4.05 0.6895
WVFGRD96 124.0 175 80 -30 4.05 0.6918
WVFGRD96 126.0 175 80 -30 4.05 0.6941
WVFGRD96 128.0 175 80 -30 4.06 0.6955
WVFGRD96 130.0 175 80 -30 4.06 0.6982
WVFGRD96 132.0 175 80 -30 4.06 0.7002
WVFGRD96 134.0 175 80 -30 4.07 0.7008
WVFGRD96 136.0 175 80 -30 4.07 0.7026
WVFGRD96 138.0 175 80 -30 4.07 0.7039
WVFGRD96 140.0 175 80 -30 4.08 0.7048
WVFGRD96 142.0 175 80 -30 4.08 0.7055
WVFGRD96 144.0 175 80 -30 4.08 0.7068
WVFGRD96 146.0 175 80 -30 4.08 0.7065
WVFGRD96 148.0 175 80 -30 4.09 0.7077
WVFGRD96 150.0 175 80 -30 4.09 0.7076
WVFGRD96 152.0 175 80 -30 4.09 0.7074
WVFGRD96 154.0 175 80 -30 4.10 0.7076
WVFGRD96 156.0 170 80 -35 4.09 0.7087
WVFGRD96 158.0 170 80 -35 4.09 0.7086
WVFGRD96 160.0 170 80 -35 4.09 0.7104
WVFGRD96 162.0 170 80 -35 4.09 0.7095
WVFGRD96 164.0 170 80 -35 4.10 0.7112
WVFGRD96 166.0 170 80 -35 4.10 0.7107
WVFGRD96 168.0 170 80 -35 4.10 0.7113
WVFGRD96 170.0 170 80 -35 4.11 0.7111
WVFGRD96 172.0 170 80 -35 4.11 0.7107
WVFGRD96 174.0 170 80 -35 4.11 0.7105
WVFGRD96 176.0 170 80 -35 4.11 0.7098
WVFGRD96 178.0 170 80 -35 4.11 0.7095
The best solution is
WVFGRD96 168.0 170 80 -35 4.10 0.7113
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 -30 o DIST/3.5 +60 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