The ANSS event ID is us6000lhe2 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us6000lhe2/executive.
2023/10/22 20:13:59 59.336 -152.937 81.1 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/10/22 20:13:59:0 59.34 -152.94 81.1 3.8 Alaska
Stations used:
AK.BMR AK.BRLK AK.BRSE AK.HOM AK.M19K AK.N18K AK.N19K
AK.O18K AK.O19K AK.O20K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +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 = 1.14e+22 dyne-cm
Mw = 3.97
Z = 86 km
Plane Strike Dip Rake
NP1 70 70 45
NP2 321 48 153
Principal Axes:
Axis Value Plunge Azimuth
T 1.14e+22 45 295
N 0.00e+00 42 89
P -1.14e+22 13 191
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.40e+21
Mxy -4.12e+21
Mxz 4.84e+21
Myy 4.24e+21
Myz -4.68e+21
Mzz 5.16e+21
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----------------------
#########-------------------
##############----------------
###################---------------
#######################-------------
##########################------------
######## #################-----------#
######## T ###################-------###
######### ####################----######
#################################-########
###############################---########
############################------########
######################------------######
################------------------######
---------------------------------#####
--------------------------------####
-------------------------------###
-----------------------------#
--------- ---------------#
------ P -------------
-- ---------
Global CMT Convention Moment Tensor:
R T P
5.16e+21 4.84e+21 4.68e+21
4.84e+21 -9.40e+21 4.12e+21
4.68e+21 4.12e+21 4.24e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231022201359/index.html
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STK = 70
DIP = 70
RAKE = 45
MW = 3.97
HS = 86.0
The NDK file is 20231022201359.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/3.3 -40 o DIST/3.3 +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 2.0 250 65 30 3.28 0.4355
WVFGRD96 4.0 245 55 5 3.35 0.4926
WVFGRD96 6.0 240 70 -20 3.38 0.5218
WVFGRD96 8.0 240 65 -20 3.43 0.5351
WVFGRD96 10.0 240 70 -20 3.45 0.5407
WVFGRD96 12.0 60 70 -30 3.48 0.5468
WVFGRD96 14.0 60 70 -30 3.51 0.5527
WVFGRD96 16.0 60 70 -25 3.52 0.5585
WVFGRD96 18.0 60 70 -25 3.54 0.5639
WVFGRD96 20.0 60 70 -20 3.56 0.5692
WVFGRD96 22.0 60 70 -20 3.58 0.5735
WVFGRD96 24.0 60 70 -20 3.60 0.5771
WVFGRD96 26.0 60 70 -15 3.62 0.5798
WVFGRD96 28.0 60 70 -15 3.64 0.5824
WVFGRD96 30.0 60 65 -10 3.66 0.5886
WVFGRD96 32.0 60 65 -10 3.68 0.5981
WVFGRD96 34.0 60 65 -10 3.70 0.6084
WVFGRD96 36.0 60 70 -10 3.72 0.6199
WVFGRD96 38.0 60 70 -10 3.75 0.6288
WVFGRD96 40.0 55 65 5 3.80 0.6375
WVFGRD96 42.0 55 65 5 3.83 0.6412
WVFGRD96 44.0 55 65 5 3.84 0.6422
WVFGRD96 46.0 55 65 5 3.86 0.6429
WVFGRD96 48.0 55 65 10 3.87 0.6459
WVFGRD96 50.0 55 65 10 3.89 0.6505
WVFGRD96 52.0 60 65 20 3.90 0.6559
WVFGRD96 54.0 60 65 20 3.91 0.6628
WVFGRD96 56.0 60 65 20 3.92 0.6688
WVFGRD96 58.0 60 65 20 3.92 0.6745
WVFGRD96 60.0 60 65 20 3.93 0.6801
WVFGRD96 62.0 60 65 20 3.94 0.6831
WVFGRD96 64.0 60 65 20 3.94 0.6861
WVFGRD96 66.0 65 65 30 3.95 0.6901
WVFGRD96 68.0 65 65 30 3.95 0.6930
WVFGRD96 70.0 65 65 30 3.96 0.6957
WVFGRD96 72.0 65 65 30 3.96 0.6968
WVFGRD96 74.0 65 65 30 3.96 0.6987
WVFGRD96 76.0 65 65 30 3.97 0.6991
WVFGRD96 78.0 65 70 35 3.96 0.7001
WVFGRD96 80.0 65 70 35 3.96 0.7013
WVFGRD96 82.0 65 70 40 3.97 0.7015
WVFGRD96 84.0 70 70 45 3.97 0.7012
WVFGRD96 86.0 70 70 45 3.97 0.7016
WVFGRD96 88.0 70 70 45 3.98 0.7013
WVFGRD96 90.0 70 70 45 3.98 0.7005
WVFGRD96 92.0 70 70 45 3.98 0.6985
WVFGRD96 94.0 70 70 45 3.98 0.6957
WVFGRD96 96.0 70 75 50 3.98 0.6961
WVFGRD96 98.0 70 75 50 3.98 0.6961
WVFGRD96 100.0 70 75 50 3.98 0.6955
WVFGRD96 102.0 70 75 50 3.98 0.6936
WVFGRD96 104.0 70 75 50 3.98 0.6915
WVFGRD96 106.0 70 75 50 3.98 0.6914
WVFGRD96 108.0 70 75 50 3.99 0.6897
WVFGRD96 110.0 70 75 50 3.99 0.6867
WVFGRD96 112.0 70 75 50 3.99 0.6859
WVFGRD96 114.0 70 80 55 3.99 0.6849
WVFGRD96 116.0 70 80 55 3.99 0.6825
WVFGRD96 118.0 70 80 55 3.99 0.6815
WVFGRD96 120.0 70 80 55 3.99 0.6805
WVFGRD96 122.0 70 80 55 3.99 0.6773
WVFGRD96 124.0 70 80 55 3.99 0.6773
WVFGRD96 126.0 75 80 60 4.00 0.6758
WVFGRD96 128.0 75 80 60 4.00 0.6735
WVFGRD96 130.0 75 80 60 4.00 0.6734
WVFGRD96 132.0 75 80 60 4.01 0.6703
WVFGRD96 134.0 75 80 60 4.01 0.6693
WVFGRD96 136.0 75 85 65 4.02 0.6678
WVFGRD96 138.0 75 85 65 4.02 0.6664
WVFGRD96 140.0 75 85 65 4.02 0.6663
WVFGRD96 142.0 75 85 65 4.02 0.6633
WVFGRD96 144.0 75 85 65 4.02 0.6632
WVFGRD96 146.0 75 85 65 4.02 0.6599
WVFGRD96 148.0 70 85 55 4.01 0.6482
The best solution is
WVFGRD96 86.0 70 70 45 3.97 0.7016
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/3.3 -40 o DIST/3.3 +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