The ANSS event ID is ak2025yhikfe and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak2025yhikfe/executive.
2025/12/10 22:17:16 68.257 -156.275 36.3 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/12/10 22:17:16.0 68.26 -156.27 36.3 4.2 Alaska
Stations used:
AK.COLD AK.E19K AK.E21K AK.E25K AK.G19K AK.GCSA AK.H17K
AK.H22K AK.I21K AK.MLY AK.RDOG AK.TOLK
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 0.10 n 3
Best Fitting Double Couple
Mo = 1.91e+22 dyne-cm
Mw = 4.12
Z = 11 km
Plane Strike Dip Rake
NP1 294 66 -116
NP2 165 35 -45
Principal Axes:
Axis Value Plunge Azimuth
T 1.91e+22 17 43
N 0.00e+00 24 306
P -1.91e+22 60 165
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.71e+21
Mxy 9.86e+21
Mxz 1.19e+22
Myy 7.95e+21
Myz 1.59e+21
Mzz -1.27e+22
-#############
---###################
----########################
----###################### #
-----####################### T ###
-----######################## ####
######--------########################
######--------------####################
######------------------################
#######----------------------#############
#######-------------------------##########
#######---------------------------########
########----------------------------######
#######------------- -------------####
########------------ P ---------------##
########----------- ----------------
########----------------------------
########--------------------------
########----------------------
#########-------------------
########--------------
#########-----
Global CMT Convention Moment Tensor:
R T P
-1.27e+22 1.19e+22 -1.59e+21
1.19e+22 4.71e+21 -9.86e+21
-1.59e+21 -9.86e+21 7.95e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20251210221716/index.html
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STK = 165
DIP = 35
RAKE = -45
MW = 4.12
HS = 11.0
The NDK file is 20251210221716.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 +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 1.0 310 50 85 3.73 0.3091
WVFGRD96 2.0 315 55 90 3.87 0.3718
WVFGRD96 3.0 320 85 -65 3.95 0.3562
WVFGRD96 4.0 320 85 -65 3.96 0.4431
WVFGRD96 5.0 315 80 -65 3.97 0.5113
WVFGRD96 6.0 180 25 -25 3.98 0.5634
WVFGRD96 7.0 165 25 -45 4.00 0.6036
WVFGRD96 8.0 165 25 -45 4.08 0.6426
WVFGRD96 9.0 165 30 -45 4.10 0.6689
WVFGRD96 10.0 165 30 -45 4.11 0.6850
WVFGRD96 11.0 165 35 -45 4.12 0.6900
WVFGRD96 12.0 170 35 -40 4.13 0.6891
WVFGRD96 13.0 170 35 -40 4.14 0.6812
WVFGRD96 14.0 170 35 -40 4.15 0.6681
WVFGRD96 15.0 175 35 -35 4.16 0.6522
WVFGRD96 16.0 170 40 -35 4.18 0.6349
WVFGRD96 17.0 170 40 -35 4.18 0.6175
WVFGRD96 18.0 340 55 -50 4.20 0.6043
WVFGRD96 19.0 340 55 -50 4.21 0.5857
WVFGRD96 20.0 340 55 -45 4.22 0.5646
WVFGRD96 21.0 340 60 -50 4.23 0.5442
WVFGRD96 22.0 160 45 -45 4.23 0.5265
WVFGRD96 23.0 160 45 -45 4.23 0.5062
WVFGRD96 24.0 165 45 -40 4.24 0.4848
WVFGRD96 25.0 160 45 -40 4.24 0.4651
WVFGRD96 26.0 160 45 -40 4.25 0.4452
WVFGRD96 27.0 160 45 -40 4.25 0.4240
WVFGRD96 28.0 165 45 -40 4.25 0.4079
WVFGRD96 29.0 165 45 -40 4.25 0.3962
WVFGRD96 30.0 160 50 -45 4.25 0.3872
WVFGRD96 31.0 160 50 -45 4.26 0.3859
WVFGRD96 32.0 160 45 -45 4.27 0.3814
WVFGRD96 33.0 160 45 -45 4.27 0.3751
WVFGRD96 34.0 110 55 55 4.28 0.3823
WVFGRD96 35.0 105 55 50 4.29 0.3906
WVFGRD96 36.0 105 55 50 4.30 0.4003
WVFGRD96 37.0 105 55 50 4.31 0.4066
WVFGRD96 38.0 105 55 50 4.32 0.4117
WVFGRD96 39.0 105 55 55 4.34 0.4163
WVFGRD96 40.0 115 50 65 4.41 0.4391
WVFGRD96 41.0 110 55 60 4.42 0.4317
WVFGRD96 42.0 110 55 60 4.43 0.4243
WVFGRD96 43.0 110 55 60 4.44 0.4171
WVFGRD96 44.0 110 55 60 4.45 0.4126
WVFGRD96 45.0 110 55 60 4.46 0.4074
WVFGRD96 46.0 110 55 60 4.46 0.4054
WVFGRD96 47.0 105 55 55 4.47 0.4051
WVFGRD96 48.0 105 55 55 4.47 0.4068
WVFGRD96 49.0 105 55 55 4.48 0.4069
WVFGRD96 50.0 105 55 55 4.48 0.4073
WVFGRD96 51.0 105 55 55 4.48 0.4068
WVFGRD96 52.0 110 55 60 4.49 0.4060
WVFGRD96 53.0 115 50 65 4.49 0.4071
WVFGRD96 54.0 115 50 65 4.49 0.4058
WVFGRD96 55.0 115 50 65 4.50 0.4069
WVFGRD96 56.0 115 50 65 4.50 0.4050
WVFGRD96 57.0 135 80 -65 4.52 0.4084
WVFGRD96 58.0 135 80 -65 4.52 0.4115
WVFGRD96 59.0 135 80 -65 4.53 0.4151
The best solution is
WVFGRD96 11.0 165 35 -45 4.12 0.6900
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 +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