The ANSS event ID is ak02550duu6u and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02550duu6u/executive.
2025/04/19 07:18:55 61.169 -151.468 83.3 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/04/19 07:18:55:0 61.17 -151.47 83.3 4.3 Alaska
Stations used:
AK.BAE AK.BRLK AK.CAPN AK.FIRE AK.GHO AK.HOM AK.KNK AK.L19K
AK.N18K AK.N19K AK.O18K AK.O19K AK.PWL AK.RC01 AK.SAW
AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD 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 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 6.61e+22 dyne-cm
Mw = 4.48
Z = 80 km
Plane Strike Dip Rake
NP1 312 66 141
NP2 60 55 30
Principal Axes:
Axis Value Plunge Azimuth
T 6.61e+22 44 272
N 0.00e+00 45 105
P -6.61e+22 7 8
Moment Tensor: (dyne-cm)
Component Value
Mxx -6.39e+22
Mxy -9.99e+21
Mxz -6.62e+21
Myy 3.28e+22
Myz -3.41e+22
Mzz 3.10e+22
-------- P ---
------------ -------
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##########------------------------
##############----------------------
##################-------------------#
######################---------------###
########################------------####
######## ################---------######
######## T ##################------#######
######## ###################----########
##########################################
#############################---########
##########################-------#######
######################-----------#####
#################----------------###
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Global CMT Convention Moment Tensor:
R T P
3.10e+22 -6.62e+21 3.41e+22
-6.62e+21 -6.39e+22 9.99e+21
3.41e+22 9.99e+21 3.28e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250419071855/index.html
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STK = 60
DIP = 55
RAKE = 30
MW = 4.48
HS = 80.0
The NDK file is 20250419071855.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 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 95 45 -95 3.70 0.1855
WVFGRD96 4.0 315 55 -25 3.69 0.1830
WVFGRD96 6.0 140 75 25 3.73 0.1911
WVFGRD96 8.0 135 45 -30 3.81 0.2030
WVFGRD96 10.0 145 65 25 3.83 0.2050
WVFGRD96 12.0 145 65 25 3.86 0.2073
WVFGRD96 14.0 130 80 20 3.90 0.2065
WVFGRD96 16.0 40 60 10 3.91 0.2058
WVFGRD96 18.0 40 65 5 3.94 0.2112
WVFGRD96 20.0 40 65 5 3.96 0.2164
WVFGRD96 22.0 40 65 5 3.98 0.2222
WVFGRD96 24.0 45 65 10 4.00 0.2287
WVFGRD96 26.0 45 65 10 4.02 0.2358
WVFGRD96 28.0 45 65 10 4.04 0.2443
WVFGRD96 30.0 45 65 10 4.06 0.2535
WVFGRD96 32.0 45 65 10 4.09 0.2641
WVFGRD96 34.0 45 65 10 4.11 0.2762
WVFGRD96 36.0 45 65 0 4.14 0.2923
WVFGRD96 38.0 45 65 0 4.17 0.3163
WVFGRD96 40.0 45 55 5 4.26 0.3466
WVFGRD96 42.0 65 65 35 4.27 0.3637
WVFGRD96 44.0 60 65 30 4.29 0.3865
WVFGRD96 46.0 60 65 30 4.31 0.4067
WVFGRD96 48.0 60 60 25 4.33 0.4238
WVFGRD96 50.0 60 60 30 4.35 0.4354
WVFGRD96 52.0 60 60 30 4.36 0.4469
WVFGRD96 54.0 60 60 25 4.37 0.4578
WVFGRD96 56.0 60 60 25 4.39 0.4682
WVFGRD96 58.0 60 60 25 4.40 0.4773
WVFGRD96 60.0 65 55 30 4.41 0.4866
WVFGRD96 62.0 65 55 30 4.42 0.4943
WVFGRD96 64.0 65 55 30 4.42 0.5011
WVFGRD96 66.0 65 55 30 4.43 0.5080
WVFGRD96 68.0 60 55 30 4.44 0.5140
WVFGRD96 70.0 60 55 30 4.45 0.5189
WVFGRD96 72.0 60 55 30 4.45 0.5229
WVFGRD96 74.0 60 55 30 4.46 0.5264
WVFGRD96 76.0 60 55 30 4.47 0.5280
WVFGRD96 78.0 60 55 30 4.47 0.5295
WVFGRD96 80.0 60 55 30 4.48 0.5304
WVFGRD96 82.0 60 55 30 4.48 0.5293
WVFGRD96 84.0 60 55 30 4.48 0.5287
WVFGRD96 86.0 60 55 30 4.49 0.5269
WVFGRD96 88.0 60 55 30 4.49 0.5243
WVFGRD96 90.0 60 55 30 4.49 0.5218
WVFGRD96 92.0 60 55 30 4.50 0.5175
WVFGRD96 94.0 60 55 30 4.50 0.5143
WVFGRD96 96.0 60 55 30 4.50 0.5095
WVFGRD96 98.0 60 55 30 4.50 0.5059
WVFGRD96 100.0 60 55 30 4.51 0.5005
WVFGRD96 102.0 60 55 30 4.51 0.4964
WVFGRD96 104.0 60 55 30 4.51 0.4904
WVFGRD96 106.0 60 55 30 4.51 0.4861
WVFGRD96 108.0 60 55 30 4.51 0.4808
WVFGRD96 110.0 60 55 30 4.52 0.4765
WVFGRD96 112.0 55 55 25 4.52 0.4717
WVFGRD96 114.0 55 55 25 4.52 0.4682
WVFGRD96 116.0 55 55 25 4.53 0.4644
WVFGRD96 118.0 55 55 25 4.53 0.4607
WVFGRD96 120.0 55 55 25 4.53 0.4579
WVFGRD96 122.0 55 55 25 4.53 0.4541
WVFGRD96 124.0 55 55 25 4.53 0.4515
WVFGRD96 126.0 55 55 25 4.54 0.4480
WVFGRD96 128.0 55 55 25 4.54 0.4454
WVFGRD96 130.0 55 55 25 4.54 0.4413
WVFGRD96 132.0 55 55 25 4.54 0.4361
WVFGRD96 134.0 55 55 25 4.54 0.4339
WVFGRD96 136.0 55 55 25 4.55 0.4298
WVFGRD96 138.0 55 55 25 4.55 0.4271
WVFGRD96 140.0 55 55 25 4.55 0.4235
WVFGRD96 142.0 55 55 25 4.55 0.4168
WVFGRD96 144.0 55 55 25 4.55 0.4086
WVFGRD96 146.0 55 55 25 4.55 0.3934
WVFGRD96 148.0 55 55 25 4.54 0.3716
The best solution is
WVFGRD96 80.0 60 55 30 4.48 0.5304
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 0.03 n 3 lp c 0.08 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