The ANSS event ID is ak0255ihrlkl and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0255ihrlkl/executive.
2025/04/30 02:14:40 60.244 -152.665 104.6 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/04/30 02:14:40:0 60.24 -152.66 104.6 3.9 Alaska
Stations used:
AK.BRLK AK.GHO AK.HOM AK.L19K AK.O18K AK.O19K AK.RC01
AK.SKN AK.SSN AK.SWD AT.PMR 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 = 1.97e+22 dyne-cm
Mw = 4.13
Z = 120 km
Plane Strike Dip Rake
NP1 288 54 127
NP2 55 50 50
Principal Axes:
Axis Value Plunge Azimuth
T 1.97e+22 60 258
N 0.00e+00 29 83
P -1.97e+22 2 352
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.91e+22
Mxy 3.67e+21
Mxz -2.53e+21
Myy 4.23e+21
Myz -8.18e+21
Mzz 1.49e+22
--- P --------
------- ------------
----------------------------
------------------------------
----------------------------------
-----------------------------------#
---####################-------------##
############################---------###
###############################-----####
###################################-######
############ ####################-######
############ T ###################----####
############ ##################------###
##############################---------#
############################------------
#########################-------------
#####################---------------
###############-------------------
------------------------------
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Global CMT Convention Moment Tensor:
R T P
1.49e+22 -2.53e+21 8.18e+21
-2.53e+21 -1.91e+22 -3.67e+21
8.18e+21 -3.67e+21 4.23e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250430021440/index.html
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STK = 55
DIP = 50
RAKE = 50
MW = 4.13
HS = 120.0
The NDK file is 20250430021440.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 90 45 -80 3.41 0.3092
WVFGRD96 4.0 135 65 20 3.40 0.2896
WVFGRD96 6.0 135 65 30 3.46 0.3147
WVFGRD96 8.0 135 60 35 3.53 0.3213
WVFGRD96 10.0 310 70 40 3.55 0.3348
WVFGRD96 12.0 310 65 40 3.58 0.3437
WVFGRD96 14.0 310 65 40 3.61 0.3435
WVFGRD96 16.0 310 65 40 3.63 0.3368
WVFGRD96 18.0 230 60 35 3.65 0.3335
WVFGRD96 20.0 225 60 30 3.66 0.3390
WVFGRD96 22.0 225 60 30 3.69 0.3459
WVFGRD96 24.0 225 60 30 3.71 0.3520
WVFGRD96 26.0 225 60 30 3.73 0.3584
WVFGRD96 28.0 225 60 30 3.75 0.3628
WVFGRD96 30.0 225 65 30 3.77 0.3647
WVFGRD96 32.0 225 70 25 3.79 0.3730
WVFGRD96 34.0 220 70 25 3.81 0.3809
WVFGRD96 36.0 220 70 25 3.83 0.3840
WVFGRD96 38.0 220 70 20 3.86 0.3852
WVFGRD96 40.0 225 65 35 3.93 0.3829
WVFGRD96 42.0 225 55 35 3.96 0.3790
WVFGRD96 44.0 225 55 35 3.98 0.3736
WVFGRD96 46.0 220 60 25 3.98 0.3677
WVFGRD96 48.0 220 60 25 3.99 0.3628
WVFGRD96 50.0 220 60 25 4.01 0.3575
WVFGRD96 52.0 20 60 -25 4.00 0.3590
WVFGRD96 54.0 20 60 -25 4.01 0.3609
WVFGRD96 56.0 35 50 35 4.02 0.3702
WVFGRD96 58.0 35 50 35 4.03 0.3849
WVFGRD96 60.0 35 50 35 4.03 0.3980
WVFGRD96 62.0 40 45 35 4.04 0.4099
WVFGRD96 64.0 40 45 35 4.04 0.4216
WVFGRD96 66.0 45 45 35 4.04 0.4316
WVFGRD96 68.0 45 45 35 4.04 0.4430
WVFGRD96 70.0 45 45 35 4.05 0.4524
WVFGRD96 72.0 45 45 35 4.05 0.4602
WVFGRD96 74.0 45 45 35 4.05 0.4675
WVFGRD96 76.0 45 45 35 4.06 0.4735
WVFGRD96 78.0 50 40 40 4.06 0.4782
WVFGRD96 80.0 50 45 40 4.06 0.4836
WVFGRD96 82.0 50 45 40 4.07 0.4891
WVFGRD96 84.0 50 45 40 4.07 0.4939
WVFGRD96 86.0 55 45 45 4.08 0.4984
WVFGRD96 88.0 55 45 50 4.09 0.5032
WVFGRD96 90.0 55 40 50 4.09 0.5081
WVFGRD96 92.0 55 40 50 4.09 0.5128
WVFGRD96 94.0 55 45 50 4.09 0.5161
WVFGRD96 96.0 55 45 50 4.10 0.5193
WVFGRD96 98.0 55 45 50 4.10 0.5227
WVFGRD96 100.0 55 45 50 4.10 0.5253
WVFGRD96 102.0 55 45 50 4.10 0.5273
WVFGRD96 104.0 55 45 50 4.11 0.5292
WVFGRD96 106.0 55 45 50 4.11 0.5319
WVFGRD96 108.0 55 45 50 4.11 0.5353
WVFGRD96 110.0 55 50 50 4.12 0.5377
WVFGRD96 112.0 55 50 50 4.12 0.5396
WVFGRD96 114.0 55 50 50 4.13 0.5431
WVFGRD96 116.0 55 50 50 4.13 0.5453
WVFGRD96 118.0 55 50 50 4.13 0.5455
WVFGRD96 120.0 55 50 50 4.13 0.5460
WVFGRD96 122.0 60 50 50 4.14 0.5459
WVFGRD96 124.0 55 50 50 4.14 0.5445
WVFGRD96 126.0 55 50 50 4.14 0.5457
WVFGRD96 128.0 60 45 50 4.14 0.5450
WVFGRD96 130.0 60 45 50 4.15 0.5442
WVFGRD96 132.0 60 45 50 4.15 0.5435
WVFGRD96 134.0 60 45 50 4.15 0.5428
WVFGRD96 136.0 60 45 50 4.15 0.5428
WVFGRD96 138.0 60 45 50 4.16 0.5420
WVFGRD96 140.0 60 45 50 4.16 0.5408
WVFGRD96 142.0 60 45 50 4.16 0.5403
WVFGRD96 144.0 60 50 50 4.17 0.5385
WVFGRD96 146.0 60 45 50 4.17 0.5379
WVFGRD96 148.0 60 45 50 4.17 0.5365
The best solution is
WVFGRD96 120.0 55 50 50 4.13 0.5460
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