The ANSS event ID is ak025ao0swgd and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak025ao0swgd/executive.
2025/08/20 20:35:26 64.713 -149.238 12.5 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/08/20 20:35:26:0 64.71 -149.24 12.5 4.1 Alaska
Stations used:
AK.BAE AK.BPAW AK.CAST AK.CCB AK.COLD AK.DHY AK.DIV AK.DOT
AK.E24K AK.E25K AK.F20K AK.F21K AK.FYU AK.G19K AK.GCSA
AK.GHO AK.GLI AK.H22K AK.H24K AK.HARP AK.HDA AK.I21K
AK.I23K AK.I27K AK.J19K AK.J20K AK.J25K AK.K24K AK.KLU
AK.KNK AK.L19K AK.L22K AK.L26K AK.M20K AK.M26K AK.MCK
AK.MLY AK.NEA2 AK.PAX AK.POKR AK.PPD AK.PPLA AK.PWL AK.RC01
AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.TOLK AK.WAT6
AK.WRH AT.PMR AT.TTA AV.SPCL AV.STLK AV.WAZA IM.IL31
IU.COLA
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.78e+22 dyne-cm
Mw = 4.10
Z = 19 km
Plane Strike Dip Rake
NP1 304 86 -135
NP2 210 45 -5
Principal Axes:
Axis Value Plunge Azimuth
T 1.78e+22 27 68
N 0.00e+00 45 307
P -1.78e+22 33 177
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.05e+22
Mxy 5.59e+21
Mxz 1.08e+22
Myy 1.20e+22
Myz 6.26e+21
Mzz -1.55e+21
--------------
----------------######
---------------#############
-------------#################
------------######################
###---------########################
#######----##################### ###
###########-##################### T ####
##########-----################## ####
##########--------########################
#########------------#####################
#########---------------##################
########-------------------###############
#######---------------------############
#######------------------------#########
######--------------------------######
#####-----------------------------##
####------------ ---------------
###----------- P -------------
##----------- ------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
-1.55e+21 1.08e+22 -6.26e+21
1.08e+22 -1.05e+22 -5.59e+21
-6.26e+21 -5.59e+21 1.20e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250820203526/index.html
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STK = 210
DIP = 45
RAKE = -5
MW = 4.10
HS = 19.0
The NDK file is 20250820203526.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/3.3 -40 o DIST/3.3 +60 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 1.0 335 45 90 3.52 0.1771
WVFGRD96 2.0 335 45 90 3.69 0.2572
WVFGRD96 3.0 305 70 30 3.65 0.2202
WVFGRD96 4.0 120 80 -40 3.70 0.2324
WVFGRD96 5.0 120 80 -45 3.74 0.2604
WVFGRD96 6.0 215 40 10 3.77 0.2932
WVFGRD96 7.0 215 45 15 3.80 0.3346
WVFGRD96 8.0 215 40 10 3.88 0.3691
WVFGRD96 9.0 215 45 15 3.91 0.4097
WVFGRD96 10.0 215 45 15 3.93 0.4465
WVFGRD96 11.0 215 45 15 3.96 0.4789
WVFGRD96 12.0 215 45 15 3.98 0.5065
WVFGRD96 13.0 215 45 15 4.00 0.5293
WVFGRD96 14.0 215 45 10 4.02 0.5479
WVFGRD96 15.0 215 45 10 4.04 0.5624
WVFGRD96 16.0 210 45 -5 4.06 0.5738
WVFGRD96 17.0 210 45 -5 4.07 0.5816
WVFGRD96 18.0 210 45 -5 4.09 0.5861
WVFGRD96 19.0 210 45 -5 4.10 0.5873
WVFGRD96 20.0 210 45 -5 4.11 0.5854
WVFGRD96 21.0 205 40 -10 4.13 0.5818
WVFGRD96 22.0 205 40 -10 4.14 0.5759
WVFGRD96 23.0 205 40 -10 4.15 0.5676
WVFGRD96 24.0 205 40 -10 4.16 0.5573
WVFGRD96 25.0 205 40 -10 4.17 0.5452
WVFGRD96 26.0 205 40 -10 4.17 0.5313
WVFGRD96 27.0 205 40 -10 4.18 0.5161
WVFGRD96 28.0 205 40 -10 4.18 0.4993
WVFGRD96 29.0 200 40 -15 4.19 0.4814
The best solution is
WVFGRD96 19.0 210 45 -5 4.10 0.5873
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/3.3 -40 o DIST/3.3 +60 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