The ANSS event ID is ak0242e66il5 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0242e66il5/executive.
2024/02/21 10:47:30 60.370 -153.061 137.9 4.0 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/02/21 10:47:30:0 60.37 -153.06 137.9 4.0 Alaska
Stations used:
AK.CAST AK.FIRE AK.J19K AK.L17K AK.L19K AK.L20K AK.N18K
AK.O19K AK.P17K AK.PWL AK.RC01 AK.SAW AK.SLK AT.PMR
Filtering commands used:
cut o DIST/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 2.60e+22 dyne-cm
Mw = 4.21
Z = 114 km
Plane Strike Dip Rake
NP1 60 70 45
NP2 311 48 153
Principal Axes:
Axis Value Plunge Azimuth
T 2.60e+22 45 285
N 0.00e+00 42 79
P -2.60e+22 13 181
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.38e+22
Mxy -3.52e+21
Mxz 9.05e+21
Myy 1.20e+22
Myz -1.25e+22
Mzz 1.18e+22
--------------
----------------------
----------------------------
###########-------------------
##################----------------
######################--------------
#########################-----------##
############################--------####
######## ###################----######
######### T ####################-#########
######### ###################---########
#############################------#######
##########################----------######
######################-------------#####
###################-----------------####
#############----------------------###
#####-----------------------------##
---------------------------------#
------------------------------
------------ -------------
--------- P ----------
----- ------
Global CMT Convention Moment Tensor:
R T P
1.18e+22 9.05e+21 1.25e+22
9.05e+21 -2.38e+22 3.52e+21
1.25e+22 3.52e+21 1.20e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240221104730/index.html
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STK = 60
DIP = 70
RAKE = 45
MW = 4.21
HS = 114.0
The NDK file is 20240221104730.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/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 330 50 -30 3.42 0.3588
WVFGRD96 4.0 330 75 -25 3.40 0.3764
WVFGRD96 6.0 335 85 -25 3.43 0.3927
WVFGRD96 8.0 340 30 15 3.59 0.4031
WVFGRD96 10.0 335 35 5 3.59 0.4078
WVFGRD96 12.0 335 35 5 3.62 0.4092
WVFGRD96 14.0 335 90 -20 3.54 0.4098
WVFGRD96 16.0 340 35 10 3.66 0.4068
WVFGRD96 18.0 340 35 10 3.69 0.4007
WVFGRD96 20.0 345 30 10 3.73 0.3924
WVFGRD96 22.0 175 -5 0 3.84 0.3932
WVFGRD96 24.0 265 90 -85 3.86 0.3958
WVFGRD96 26.0 85 90 85 3.89 0.3953
WVFGRD96 28.0 85 90 85 3.91 0.3889
WVFGRD96 30.0 235 0 60 3.91 0.3707
WVFGRD96 32.0 220 -5 50 3.93 0.3511
WVFGRD96 34.0 155 0 -20 3.93 0.3239
WVFGRD96 36.0 125 10 -50 3.91 0.2992
WVFGRD96 38.0 260 75 60 3.87 0.2889
WVFGRD96 40.0 265 70 70 4.00 0.2947
WVFGRD96 42.0 265 70 70 4.03 0.3047
WVFGRD96 44.0 265 75 65 4.05 0.3122
WVFGRD96 46.0 60 80 -30 3.96 0.3207
WVFGRD96 48.0 60 80 -40 4.00 0.3351
WVFGRD96 50.0 60 80 -35 4.01 0.3476
WVFGRD96 52.0 60 80 -35 4.03 0.3588
WVFGRD96 54.0 60 85 -25 4.03 0.3693
WVFGRD96 56.0 60 85 -10 4.03 0.3814
WVFGRD96 58.0 240 90 0 4.04 0.3936
WVFGRD96 60.0 240 90 -15 4.06 0.4079
WVFGRD96 62.0 60 90 15 4.07 0.4224
WVFGRD96 64.0 70 65 45 4.17 0.4374
WVFGRD96 66.0 70 65 45 4.18 0.4500
WVFGRD96 68.0 65 65 45 4.20 0.4621
WVFGRD96 70.0 65 65 45 4.21 0.4723
WVFGRD96 72.0 65 65 40 4.19 0.4806
WVFGRD96 74.0 70 70 55 4.22 0.4952
WVFGRD96 76.0 70 70 55 4.22 0.5084
WVFGRD96 78.0 65 70 55 4.24 0.5205
WVFGRD96 80.0 65 70 55 4.24 0.5299
WVFGRD96 82.0 65 70 55 4.24 0.5365
WVFGRD96 84.0 65 70 55 4.24 0.5426
WVFGRD96 86.0 65 70 55 4.23 0.5471
WVFGRD96 88.0 65 70 55 4.23 0.5508
WVFGRD96 90.0 65 70 50 4.21 0.5537
WVFGRD96 92.0 65 70 50 4.21 0.5569
WVFGRD96 94.0 60 70 50 4.24 0.5601
WVFGRD96 96.0 60 70 50 4.24 0.5623
WVFGRD96 98.0 60 70 50 4.23 0.5655
WVFGRD96 100.0 60 70 50 4.23 0.5664
WVFGRD96 102.0 60 70 50 4.23 0.5678
WVFGRD96 104.0 60 70 50 4.23 0.5691
WVFGRD96 106.0 60 70 50 4.23 0.5697
WVFGRD96 108.0 60 70 45 4.21 0.5690
WVFGRD96 110.0 60 70 45 4.21 0.5698
WVFGRD96 112.0 60 70 45 4.21 0.5707
WVFGRD96 114.0 60 70 45 4.21 0.5707
WVFGRD96 116.0 60 70 45 4.21 0.5699
WVFGRD96 118.0 60 70 45 4.21 0.5688
WVFGRD96 120.0 60 70 45 4.21 0.5685
WVFGRD96 122.0 60 70 45 4.21 0.5682
WVFGRD96 124.0 60 70 45 4.21 0.5673
WVFGRD96 126.0 60 70 45 4.21 0.5663
WVFGRD96 128.0 60 70 40 4.20 0.5655
WVFGRD96 130.0 60 70 40 4.20 0.5644
WVFGRD96 132.0 60 70 40 4.20 0.5634
WVFGRD96 134.0 60 70 40 4.20 0.5619
WVFGRD96 136.0 60 70 40 4.20 0.5605
WVFGRD96 138.0 60 70 40 4.20 0.5591
WVFGRD96 140.0 60 70 40 4.20 0.5580
WVFGRD96 142.0 60 70 40 4.20 0.5564
WVFGRD96 144.0 60 70 40 4.20 0.5549
WVFGRD96 146.0 60 70 40 4.20 0.5533
WVFGRD96 148.0 60 70 40 4.20 0.5519
The best solution is
WVFGRD96 114.0 60 70 45 4.21 0.5707
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/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2
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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