The ANSS event ID is ak0258go7zsl and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0258go7zsl/executive.
2025/07/03 22:40:12 61.851 -151.765 99.1 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/07/03 22:40:12.0 61.85 -151.76 99.1 4.1 Alaska
Stations used:
AK.BAE AK.BPAW AK.BRLK AK.CAST AK.DHY AK.DIV AK.FID AK.FIRE
AK.GHO AK.HIN AK.J20K AK.KLU AK.KNK AK.L19K AK.L22K AK.M20K
AK.MCK AK.N18K AK.N19K AK.O19K AK.P23K AK.PPLA AK.PWL
AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SWD AK.WAT6
AT.PMR AV.RED AV.SPCL AV.STLK
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.04 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.51e+22 dyne-cm
Mw = 4.20
Z = 106 km
Plane Strike Dip Rake
NP1 90 55 65
NP2 309 42 121
Principal Axes:
Axis Value Plunge Azimuth
T 2.51e+22 69 305
N 0.00e+00 20 105
P -2.51e+22 7 198
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.14e+22
Mxy -8.70e+21
Mxz 7.79e+21
Myy -7.60e+14
Myz -6.09e+21
Mzz 2.14e+22
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-#############----------------
####################--------------
########################------------
############################----------
##############################----------
############## ###############--------
############### T ################--------
############### #################-------
-###################################-----#
---##################################--###
----####################################
--------########################-----###
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-----------------------------------#
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---- ---------------
P -----------
Global CMT Convention Moment Tensor:
R T P
2.14e+22 7.79e+21 6.09e+21
7.79e+21 -2.14e+22 8.70e+21
6.09e+21 8.70e+21 -7.60e+14
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250703224012/index.html
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STK = 90
DIP = 55
RAKE = 65
MW = 4.20
HS = 106.0
The NDK file is 20250703224012.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 -40 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 55 45 90 3.31 0.1568
WVFGRD96 4.0 160 30 -5 3.37 0.1647
WVFGRD96 6.0 160 30 0 3.41 0.2126
WVFGRD96 8.0 155 30 -5 3.50 0.2415
WVFGRD96 10.0 170 35 25 3.55 0.2625
WVFGRD96 12.0 210 40 55 3.58 0.2806
WVFGRD96 14.0 210 40 55 3.61 0.2850
WVFGRD96 16.0 210 45 55 3.64 0.2859
WVFGRD96 18.0 215 45 55 3.66 0.2824
WVFGRD96 20.0 225 45 60 3.68 0.2731
WVFGRD96 22.0 225 50 55 3.70 0.2580
WVFGRD96 24.0 225 50 55 3.71 0.2394
WVFGRD96 26.0 230 50 55 3.71 0.2169
WVFGRD96 28.0 60 90 -35 3.76 0.2088
WVFGRD96 30.0 245 80 30 3.78 0.2163
WVFGRD96 32.0 245 75 30 3.80 0.2187
WVFGRD96 34.0 245 75 30 3.81 0.2193
WVFGRD96 36.0 245 75 30 3.83 0.2164
WVFGRD96 38.0 245 85 15 3.86 0.2147
WVFGRD96 40.0 65 85 -20 3.92 0.2341
WVFGRD96 42.0 65 85 -20 3.95 0.2432
WVFGRD96 44.0 245 75 -10 3.98 0.2532
WVFGRD96 46.0 245 75 -10 4.01 0.2681
WVFGRD96 48.0 245 75 -10 4.03 0.2803
WVFGRD96 50.0 245 70 -15 4.06 0.2928
WVFGRD96 52.0 245 70 -15 4.08 0.3048
WVFGRD96 54.0 245 75 -25 4.08 0.3117
WVFGRD96 56.0 245 75 -25 4.10 0.3171
WVFGRD96 58.0 245 75 -25 4.11 0.3214
WVFGRD96 60.0 95 50 60 4.06 0.3368
WVFGRD96 62.0 95 50 60 4.07 0.3591
WVFGRD96 64.0 95 50 60 4.09 0.3811
WVFGRD96 66.0 95 50 60 4.10 0.4015
WVFGRD96 68.0 95 50 60 4.11 0.4210
WVFGRD96 70.0 95 50 60 4.12 0.4388
WVFGRD96 72.0 95 50 60 4.12 0.4560
WVFGRD96 74.0 95 50 60 4.13 0.4702
WVFGRD96 76.0 95 50 60 4.14 0.4833
WVFGRD96 78.0 95 50 60 4.14 0.4940
WVFGRD96 80.0 95 50 60 4.15 0.5048
WVFGRD96 82.0 95 50 60 4.15 0.5145
WVFGRD96 84.0 95 50 60 4.16 0.5228
WVFGRD96 86.0 90 55 60 4.17 0.5303
WVFGRD96 88.0 90 55 60 4.17 0.5365
WVFGRD96 90.0 90 55 60 4.17 0.5415
WVFGRD96 92.0 90 55 60 4.18 0.5466
WVFGRD96 94.0 90 55 60 4.18 0.5504
WVFGRD96 96.0 90 55 60 4.18 0.5540
WVFGRD96 98.0 90 55 60 4.19 0.5564
WVFGRD96 100.0 90 55 60 4.19 0.5579
WVFGRD96 102.0 90 55 65 4.19 0.5589
WVFGRD96 104.0 90 55 65 4.19 0.5607
WVFGRD96 106.0 90 55 65 4.20 0.5617
WVFGRD96 108.0 90 55 65 4.20 0.5613
WVFGRD96 110.0 90 55 65 4.20 0.5611
WVFGRD96 112.0 90 55 65 4.20 0.5600
WVFGRD96 114.0 90 55 65 4.21 0.5590
WVFGRD96 116.0 90 55 65 4.21 0.5576
WVFGRD96 118.0 90 55 65 4.21 0.5548
WVFGRD96 120.0 90 55 65 4.21 0.5518
WVFGRD96 122.0 90 55 65 4.22 0.5498
WVFGRD96 124.0 90 55 65 4.22 0.5469
WVFGRD96 126.0 90 55 65 4.22 0.5430
WVFGRD96 128.0 90 55 65 4.22 0.5407
WVFGRD96 130.0 95 50 65 4.22 0.5367
WVFGRD96 132.0 95 50 65 4.22 0.5332
WVFGRD96 134.0 95 50 65 4.22 0.5308
WVFGRD96 136.0 95 50 65 4.22 0.5275
WVFGRD96 138.0 95 50 65 4.22 0.5246
WVFGRD96 140.0 95 50 65 4.23 0.5213
WVFGRD96 142.0 95 50 65 4.23 0.5184
WVFGRD96 144.0 95 50 65 4.23 0.5165
WVFGRD96 146.0 95 50 65 4.23 0.5131
WVFGRD96 148.0 95 50 65 4.24 0.5104
The best solution is
WVFGRD96 106.0 90 55 65 4.20 0.5617
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 0.04 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