The ANSS event ID is ak02539pgcbe and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02539pgcbe/executive.
2025/03/12 17:29:50 68.213 -168.059 38.5 5.0 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/03/12 17:29:50:0 68.21 -168.06 38.5 5.0 Alaska
Stations used:
AK.ANM AK.F15K AK.RDOG AK.TNA
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 0.06 n 3
Best Fitting Double Couple
Mo = 1.68e+23 dyne-cm
Mw = 4.75
Z = 14 km
Plane Strike Dip Rake
NP1 122 81 160
NP2 215 70 10
Principal Axes:
Axis Value Plunge Azimuth
T 1.68e+23 21 77
N 0.00e+00 68 278
P -1.68e+23 7 170
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.52e+23
Mxy 6.19e+22
Mxz 3.35e+22
Myy 1.33e+23
Myz 5.07e+22
Mzz 1.87e+22
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---------------------#########
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#-------------------################
####---------------###################
#######------------#####################
#########--------################## ##
#############----################### T ###
#################################### ###
##############----########################
#############--------#####################
###########------------#################
###########----------------#############
#########---------------------########
#######--------------------------###
######----------------------------
###---------------------------
##--------------------------
------------- ------
--------- P --
Global CMT Convention Moment Tensor:
R T P
1.87e+22 3.35e+22 -5.07e+22
3.35e+22 -1.52e+23 -6.19e+22
-5.07e+22 -6.19e+22 1.33e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250312172950/index.html
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STK = 215
DIP = 70
RAKE = 10
MW = 4.75
HS = 14.0
The NDK file is 20250312172950.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 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 210 70 -15 4.38 0.4206
WVFGRD96 2.0 25 70 -30 4.48 0.4881
WVFGRD96 3.0 25 65 -30 4.54 0.5229
WVFGRD96 4.0 30 75 -25 4.56 0.5315
WVFGRD96 5.0 30 75 -25 4.58 0.5335
WVFGRD96 6.0 30 80 -25 4.60 0.5413
WVFGRD96 7.0 30 85 -25 4.61 0.5525
WVFGRD96 8.0 30 85 -25 4.65 0.5723
WVFGRD96 9.0 210 90 25 4.66 0.5766
WVFGRD96 10.0 210 90 25 4.67 0.5861
WVFGRD96 11.0 30 90 -25 4.68 0.5916
WVFGRD96 12.0 30 90 -20 4.70 0.5937
WVFGRD96 13.0 215 65 10 4.74 0.6101
WVFGRD96 14.0 215 70 10 4.75 0.6118
WVFGRD96 15.0 215 70 10 4.76 0.6100
WVFGRD96 16.0 215 70 10 4.77 0.6039
WVFGRD96 17.0 210 75 10 4.75 0.5955
WVFGRD96 18.0 210 75 10 4.76 0.5886
WVFGRD96 19.0 210 75 10 4.77 0.5793
WVFGRD96 20.0 210 75 5 4.78 0.5687
WVFGRD96 21.0 210 75 5 4.78 0.5569
WVFGRD96 22.0 210 75 5 4.79 0.5444
WVFGRD96 23.0 210 80 5 4.80 0.5315
WVFGRD96 24.0 210 80 5 4.80 0.5181
WVFGRD96 25.0 210 80 5 4.80 0.5049
WVFGRD96 26.0 210 80 5 4.81 0.4916
WVFGRD96 27.0 210 80 5 4.81 0.4782
WVFGRD96 28.0 210 80 5 4.82 0.4651
WVFGRD96 29.0 210 80 5 4.82 0.4519
WVFGRD96 30.0 210 85 5 4.83 0.4396
WVFGRD96 31.0 210 85 5 4.84 0.4278
WVFGRD96 32.0 210 85 5 4.85 0.4159
WVFGRD96 33.0 210 90 5 4.85 0.4049
WVFGRD96 34.0 30 90 -5 4.87 0.3946
WVFGRD96 35.0 30 90 -5 4.88 0.3846
WVFGRD96 36.0 30 90 -5 4.89 0.3749
WVFGRD96 37.0 30 90 -5 4.91 0.3647
WVFGRD96 38.0 210 90 5 4.93 0.3538
WVFGRD96 39.0 25 75 10 4.90 0.3444
WVFGRD96 40.0 25 65 -10 4.95 0.3329
WVFGRD96 41.0 30 70 10 4.96 0.3217
WVFGRD96 42.0 30 70 10 4.97 0.3112
WVFGRD96 43.0 30 70 10 4.97 0.3002
WVFGRD96 44.0 30 70 10 4.98 0.2889
WVFGRD96 45.0 30 70 10 4.98 0.2772
WVFGRD96 46.0 30 70 10 4.99 0.2654
WVFGRD96 47.0 30 75 10 4.99 0.2536
WVFGRD96 48.0 75 60 -20 4.91 0.2576
WVFGRD96 49.0 75 60 -20 4.92 0.2601
WVFGRD96 50.0 75 60 -20 4.93 0.2626
WVFGRD96 51.0 80 65 -20 4.94 0.2654
WVFGRD96 52.0 80 65 -15 4.95 0.2683
WVFGRD96 53.0 80 65 -15 4.96 0.2711
WVFGRD96 54.0 80 65 -15 4.97 0.2736
WVFGRD96 55.0 80 65 -15 4.98 0.2759
WVFGRD96 56.0 80 65 -15 4.99 0.2778
WVFGRD96 57.0 80 65 -15 5.00 0.2811
WVFGRD96 58.0 85 70 -15 5.01 0.2843
WVFGRD96 59.0 85 70 -15 5.02 0.2878
The best solution is
WVFGRD96 14.0 215 70 10 4.75 0.6118
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 0.06 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