The ANSS event ID is ak023btef8mo and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023btef8mo/executive.
2023/09/14 22:35:05 59.314 -153.501 103.7 4.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/09/14 22:35:05:0 59.31 -153.50 103.7 4.4 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AK.M19K AK.M20K AK.N18K AK.N19K
AK.O18K AK.P17K AK.Q19K AK.SLK AV.ACH AV.PLK3 AV.SPCP
II.KDAK
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 = 3.55e+22 dyne-cm
Mw = 4.30
Z = 106 km
Plane Strike Dip Rake
NP1 60 80 85
NP2 267 11 116
Principal Axes:
Axis Value Plunge Azimuth
T 3.55e+22 55 324
N 0.00e+00 5 61
P -3.55e+22 35 154
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.17e+22
Mxy 3.71e+21
Mxz 2.85e+22
Myy -3.85e+20
Myz -1.71e+22
Mzz 1.21e+22
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---######################---
--##########################--
--##############################--
-##################################-
-############ ###################--#
-############# T #################-----#
############## ###############--------
-#############################-----------#
###########################---------------
########################------------------
#####################---------------------
#################-----------------------
#############---------------------------
########------------------------------
#--------------------- -----------
--------------------- P ----------
------------------- --------
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Global CMT Convention Moment Tensor:
R T P
1.21e+22 2.85e+22 1.71e+22
2.85e+22 -1.17e+22 -3.71e+21
1.71e+22 -3.71e+21 -3.85e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230914223505/index.html
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STK = 60
DIP = 80
RAKE = 85
MW = 4.30
HS = 106.0
The NDK file is 20230914223505.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 2.0 150 90 0 3.27 0.2391
WVFGRD96 4.0 155 75 20 3.38 0.2658
WVFGRD96 6.0 150 70 -15 3.44 0.2874
WVFGRD96 8.0 150 65 -15 3.51 0.3087
WVFGRD96 10.0 160 60 25 3.56 0.3197
WVFGRD96 12.0 155 65 20 3.59 0.3297
WVFGRD96 14.0 155 65 20 3.62 0.3344
WVFGRD96 16.0 60 75 -15 3.65 0.3475
WVFGRD96 18.0 60 75 -10 3.68 0.3606
WVFGRD96 20.0 60 75 -10 3.71 0.3735
WVFGRD96 22.0 60 75 -15 3.74 0.3863
WVFGRD96 24.0 60 75 -15 3.76 0.3977
WVFGRD96 26.0 60 75 -15 3.78 0.4073
WVFGRD96 28.0 60 75 -15 3.80 0.4183
WVFGRD96 30.0 60 80 -15 3.82 0.4276
WVFGRD96 32.0 60 80 -15 3.84 0.4357
WVFGRD96 34.0 60 80 -15 3.86 0.4404
WVFGRD96 36.0 60 80 -15 3.89 0.4470
WVFGRD96 38.0 60 85 -10 3.92 0.4523
WVFGRD96 40.0 245 75 20 3.98 0.4648
WVFGRD96 42.0 245 75 20 4.00 0.4700
WVFGRD96 44.0 245 75 20 4.02 0.4724
WVFGRD96 46.0 245 75 20 4.03 0.4729
WVFGRD96 48.0 245 75 15 4.05 0.4734
WVFGRD96 50.0 65 85 35 4.08 0.4854
WVFGRD96 52.0 65 85 35 4.09 0.4958
WVFGRD96 54.0 65 85 35 4.10 0.5061
WVFGRD96 56.0 65 85 40 4.11 0.5174
WVFGRD96 58.0 65 85 40 4.12 0.5266
WVFGRD96 60.0 65 80 45 4.13 0.5359
WVFGRD96 62.0 65 80 45 4.14 0.5475
WVFGRD96 64.0 70 80 50 4.15 0.5547
WVFGRD96 66.0 75 75 60 4.17 0.5685
WVFGRD96 68.0 75 75 60 4.18 0.5790
WVFGRD96 70.0 75 75 60 4.18 0.5908
WVFGRD96 72.0 75 75 65 4.19 0.5985
WVFGRD96 74.0 75 75 65 4.20 0.6056
WVFGRD96 76.0 75 75 65 4.20 0.6128
WVFGRD96 78.0 75 75 65 4.20 0.6183
WVFGRD96 80.0 70 75 70 4.22 0.6219
WVFGRD96 82.0 70 75 75 4.24 0.6283
WVFGRD96 84.0 70 75 75 4.24 0.6354
WVFGRD96 86.0 70 75 75 4.24 0.6407
WVFGRD96 88.0 65 80 80 4.26 0.6481
WVFGRD96 90.0 65 80 80 4.26 0.6563
WVFGRD96 92.0 65 80 80 4.26 0.6631
WVFGRD96 94.0 65 80 85 4.28 0.6690
WVFGRD96 96.0 65 80 85 4.28 0.6743
WVFGRD96 98.0 65 80 85 4.28 0.6792
WVFGRD96 100.0 60 80 85 4.30 0.6838
WVFGRD96 102.0 60 80 85 4.30 0.6871
WVFGRD96 104.0 60 80 85 4.30 0.6894
WVFGRD96 106.0 60 80 85 4.30 0.6900
WVFGRD96 108.0 60 80 85 4.30 0.6897
WVFGRD96 110.0 60 85 90 4.30 0.6898
WVFGRD96 112.0 60 85 90 4.30 0.6896
WVFGRD96 114.0 60 85 90 4.30 0.6884
WVFGRD96 116.0 60 85 90 4.30 0.6879
WVFGRD96 118.0 60 85 90 4.30 0.6864
WVFGRD96 120.0 60 85 90 4.30 0.6836
WVFGRD96 122.0 130 -5 -20 4.28 0.6695
WVFGRD96 124.0 60 85 90 4.30 0.6739
WVFGRD96 126.0 80 -5 -70 4.29 0.6716
WVFGRD96 128.0 80 -5 -70 4.29 0.6681
WVFGRD96 130.0 90 -5 -60 4.29 0.6627
WVFGRD96 132.0 80 -5 -70 4.29 0.6561
WVFGRD96 134.0 90 -5 -60 4.28 0.6517
WVFGRD96 136.0 95 -5 -60 4.27 0.6478
WVFGRD96 138.0 80 -5 -75 4.28 0.6422
WVFGRD96 140.0 90 -5 -65 4.27 0.6367
WVFGRD96 142.0 235 5 80 4.28 0.6320
WVFGRD96 144.0 85 -5 -70 4.27 0.6284
WVFGRD96 146.0 85 -5 -70 4.27 0.6211
WVFGRD96 148.0 245 5 90 4.27 0.6159
The best solution is
WVFGRD96 106.0 60 80 85 4.30 0.6900
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