The ANSS event ID is ak0242hlzvp5 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0242hlzvp5/executive.
2024/02/23 18:43:56 62.808 -149.415 67.5 4.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/02/23 18:43:56:0 62.81 -149.41 67.5 4.6 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DOT AK.FID AK.GHO AK.GLI
AK.HDA AK.HIN AK.K24K AK.KLU AK.KNK AK.L22K AK.MCAR AK.MCK
AK.NEA2 AK.PAX AK.PPD AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW
AK.SCM AK.SLK AK.TGL AK.TRF AK.WRH AT.PMR
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 = 5.37e+22 dyne-cm
Mw = 4.42
Z = 76 km
Plane Strike Dip Rake
NP1 44 76 154
NP2 140 65 15
Principal Axes:
Axis Value Plunge Azimuth
T 5.37e+22 28 360
N 0.00e+00 61 198
P -5.37e+22 8 94
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.19e+22
Mxy 2.92e+21
Mxz 2.25e+22
Myy -5.25e+22
Myz -7.25e+21
Mzz 1.06e+22
##############
######### ##########
-########### T #############
--########### #############-
----##########################----
-----#########################------
-------#######################--------
---------#####################----------
---------###################------------
-----------#################--------------
------------##############-------------
--------------###########-------------- P
---------------########----------------
----------------####--------------------
-----------------#----------------------
---------------###--------------------
------------#######-----------------
--------#############-------------
---###################--------
##########################--
######################
##############
Global CMT Convention Moment Tensor:
R T P
1.06e+22 2.25e+22 7.25e+21
2.25e+22 4.19e+22 -2.92e+21
7.25e+21 -2.92e+21 -5.25e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240223184356/index.html
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STK = 140
DIP = 65
RAKE = 15
MW = 4.42
HS = 76.0
The NDK file is 20240223184356.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 50 80 -10 3.54 0.2686
WVFGRD96 4.0 230 90 15 3.63 0.3085
WVFGRD96 6.0 50 75 -15 3.70 0.3244
WVFGRD96 8.0 325 70 15 3.77 0.3620
WVFGRD96 10.0 140 80 -10 3.82 0.3844
WVFGRD96 12.0 140 80 -10 3.85 0.4057
WVFGRD96 14.0 140 90 10 3.89 0.4223
WVFGRD96 16.0 320 85 -10 3.92 0.4399
WVFGRD96 18.0 140 90 5 3.95 0.4564
WVFGRD96 20.0 140 85 -5 3.97 0.4732
WVFGRD96 22.0 140 80 -5 3.99 0.4903
WVFGRD96 24.0 140 80 -5 4.01 0.5062
WVFGRD96 26.0 140 75 5 4.04 0.5226
WVFGRD96 28.0 140 75 10 4.06 0.5398
WVFGRD96 30.0 140 75 10 4.08 0.5569
WVFGRD96 32.0 140 75 10 4.10 0.5681
WVFGRD96 34.0 140 75 5 4.11 0.5806
WVFGRD96 36.0 140 75 10 4.14 0.5977
WVFGRD96 38.0 140 75 10 4.17 0.6122
WVFGRD96 40.0 140 70 15 4.22 0.6295
WVFGRD96 42.0 140 70 15 4.25 0.6376
WVFGRD96 44.0 140 70 15 4.26 0.6467
WVFGRD96 46.0 140 70 15 4.28 0.6554
WVFGRD96 48.0 140 70 15 4.30 0.6650
WVFGRD96 50.0 140 70 15 4.31 0.6738
WVFGRD96 52.0 140 70 15 4.32 0.6831
WVFGRD96 54.0 140 70 15 4.33 0.6910
WVFGRD96 56.0 140 70 15 4.34 0.6999
WVFGRD96 58.0 140 70 15 4.35 0.7071
WVFGRD96 60.0 140 70 15 4.36 0.7140
WVFGRD96 62.0 140 70 15 4.37 0.7202
WVFGRD96 64.0 140 70 15 4.38 0.7249
WVFGRD96 66.0 140 65 15 4.39 0.7277
WVFGRD96 68.0 140 65 15 4.39 0.7299
WVFGRD96 70.0 140 65 15 4.40 0.7323
WVFGRD96 72.0 140 65 15 4.41 0.7340
WVFGRD96 74.0 140 65 15 4.41 0.7345
WVFGRD96 76.0 140 65 15 4.42 0.7345
WVFGRD96 78.0 140 65 15 4.42 0.7329
WVFGRD96 80.0 140 65 15 4.43 0.7311
WVFGRD96 82.0 140 65 15 4.43 0.7300
WVFGRD96 84.0 140 65 15 4.44 0.7282
WVFGRD96 86.0 140 65 15 4.44 0.7246
WVFGRD96 88.0 140 65 15 4.44 0.7214
WVFGRD96 90.0 140 65 15 4.45 0.7196
WVFGRD96 92.0 140 65 15 4.45 0.7156
WVFGRD96 94.0 140 65 15 4.46 0.7126
WVFGRD96 96.0 140 65 15 4.46 0.7093
WVFGRD96 98.0 140 65 15 4.47 0.7047
The best solution is
WVFGRD96 76.0 140 65 15 4.42 0.7345
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