The ANSS event ID is ak024c4o9vb2 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024c4o9vb2/executive.
2024/09/20 04:02:41 63.166 -150.538 119.1 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/09/20 04:02:41:0 63.17 -150.54 119.1 3.9 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.GHO AK.H24K AK.HDA AK.J19K
AK.J20K AK.J25K AK.K24K AK.KNK AK.L19K AK.L22K AK.MCK
AK.MLY AK.PAX AK.POKR AK.PPD AK.PWL AK.RC01 AK.RND AK.SAW
AK.SCM AK.SLK AK.WAT6 AK.WRH AV.STLK IM.IL31 IU.COLA
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 0.10 n 3
Best Fitting Double Couple
Mo = 1.26e+22 dyne-cm
Mw = 4.00
Z = 124 km
Plane Strike Dip Rake
NP1 45 75 45
NP2 300 47 159
Principal Axes:
Axis Value Plunge Azimuth
T 1.26e+22 42 273
N 0.00e+00 43 60
P -1.26e+22 17 167
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.08e+22
Mxy 2.23e+21
Mxz 3.82e+21
Myy 6.37e+21
Myz -7.08e+21
Mzz 4.45e+21
--------------
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----------------------------
-----------------------------#
--##############---------------###
######################---------#####
##########################-----#######
#############################-##########
#############################--#########
####### ###################-----########
####### T #################--------#######
####### ###############------------#####
########################-------------#####
#####################----------------###
###################-------------------##
###############----------------------#
############------------------------
########--------------------------
###---------------------------
----------------- --------
-------------- P -----
---------- -
Global CMT Convention Moment Tensor:
R T P
4.45e+21 3.82e+21 7.08e+21
3.82e+21 -1.08e+22 -2.23e+21
7.08e+21 -2.23e+21 6.37e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240920040241/index.html
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STK = 45
DIP = 75
RAKE = 45
MW = 4.00
HS = 124.0
The NDK file is 20240920040241.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 300 50 -60 3.11 0.1598
WVFGRD96 4.0 330 50 5 3.12 0.1641
WVFGRD96 6.0 155 60 15 3.18 0.1851
WVFGRD96 8.0 155 55 15 3.26 0.2041
WVFGRD96 10.0 155 60 20 3.31 0.2146
WVFGRD96 12.0 155 60 20 3.34 0.2177
WVFGRD96 14.0 60 90 35 3.37 0.2171
WVFGRD96 16.0 240 90 -35 3.41 0.2224
WVFGRD96 18.0 60 85 35 3.44 0.2258
WVFGRD96 20.0 60 85 30 3.47 0.2300
WVFGRD96 22.0 60 85 35 3.50 0.2367
WVFGRD96 24.0 60 85 35 3.52 0.2448
WVFGRD96 26.0 60 90 35 3.55 0.2527
WVFGRD96 28.0 60 90 35 3.57 0.2608
WVFGRD96 30.0 235 85 -30 3.58 0.2716
WVFGRD96 32.0 235 85 -30 3.60 0.2830
WVFGRD96 34.0 235 85 -30 3.62 0.2903
WVFGRD96 36.0 55 90 30 3.64 0.2942
WVFGRD96 38.0 55 90 25 3.66 0.2985
WVFGRD96 40.0 50 80 15 3.71 0.3035
WVFGRD96 42.0 50 80 15 3.74 0.3086
WVFGRD96 44.0 55 90 30 3.77 0.3134
WVFGRD96 46.0 55 85 30 3.79 0.3210
WVFGRD96 48.0 55 85 30 3.80 0.3294
WVFGRD96 50.0 55 85 30 3.81 0.3378
WVFGRD96 52.0 60 75 35 3.83 0.3478
WVFGRD96 54.0 60 70 35 3.85 0.3579
WVFGRD96 56.0 60 70 35 3.86 0.3692
WVFGRD96 58.0 60 65 35 3.88 0.3837
WVFGRD96 60.0 60 65 35 3.88 0.3969
WVFGRD96 62.0 60 65 35 3.89 0.4089
WVFGRD96 64.0 55 65 30 3.91 0.4216
WVFGRD96 66.0 55 65 30 3.91 0.4335
WVFGRD96 68.0 55 65 30 3.92 0.4428
WVFGRD96 70.0 55 65 30 3.92 0.4516
WVFGRD96 72.0 55 65 30 3.93 0.4613
WVFGRD96 74.0 55 65 30 3.93 0.4710
WVFGRD96 76.0 55 65 30 3.94 0.4793
WVFGRD96 78.0 55 65 30 3.94 0.4865
WVFGRD96 80.0 55 65 30 3.94 0.4932
WVFGRD96 82.0 55 65 30 3.95 0.4997
WVFGRD96 84.0 55 65 30 3.95 0.5048
WVFGRD96 86.0 55 65 30 3.95 0.5086
WVFGRD96 88.0 50 70 40 3.95 0.5124
WVFGRD96 90.0 50 70 40 3.96 0.5169
WVFGRD96 92.0 50 70 40 3.96 0.5222
WVFGRD96 94.0 50 70 40 3.96 0.5263
WVFGRD96 96.0 50 70 45 3.96 0.5302
WVFGRD96 98.0 50 70 45 3.97 0.5340
WVFGRD96 100.0 50 70 45 3.97 0.5364
WVFGRD96 102.0 50 70 45 3.97 0.5401
WVFGRD96 104.0 50 70 45 3.97 0.5435
WVFGRD96 106.0 45 75 45 3.98 0.5458
WVFGRD96 108.0 45 75 45 3.98 0.5474
WVFGRD96 110.0 45 75 45 3.98 0.5487
WVFGRD96 112.0 45 75 45 3.99 0.5494
WVFGRD96 114.0 45 75 45 3.99 0.5511
WVFGRD96 116.0 45 75 45 3.99 0.5528
WVFGRD96 118.0 45 75 45 3.99 0.5530
WVFGRD96 120.0 45 75 45 3.99 0.5524
WVFGRD96 122.0 45 75 45 3.99 0.5533
WVFGRD96 124.0 45 75 45 4.00 0.5536
WVFGRD96 126.0 45 75 45 4.00 0.5529
WVFGRD96 128.0 45 75 45 4.00 0.5524
WVFGRD96 130.0 45 75 45 4.00 0.5521
WVFGRD96 132.0 45 75 45 4.00 0.5510
WVFGRD96 134.0 50 70 45 4.00 0.5491
WVFGRD96 136.0 50 70 45 4.00 0.5492
WVFGRD96 138.0 50 70 45 4.00 0.5483
WVFGRD96 140.0 50 70 45 4.00 0.5463
WVFGRD96 142.0 50 70 45 4.00 0.5460
WVFGRD96 144.0 50 70 45 4.01 0.5435
WVFGRD96 146.0 50 70 45 4.01 0.5426
WVFGRD96 148.0 50 70 45 4.01 0.5408
The best solution is
WVFGRD96 124.0 45 75 45 4.00 0.5536
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 0.10 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