The ANSS event ID is ak024bwl81gj and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024bwl81gj/executive.
2024/09/15 15:17:03 60.603 -146.917 19.8 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/09/15 15:17:03:0 60.60 -146.92 19.8 4.3 Alaska
Stations used:
AK.BAE AK.BERG AK.BMR AK.BPAW AK.BRLK AK.CAST AK.DHY AK.DIV
AK.DOT AK.EYAK AK.GHO AK.GLB AK.HDA AK.HIN AK.K24K AK.KLU
AK.KNK AK.L22K AK.L26K AK.LOGN AK.M26K AK.MCAR AK.MCK
AK.MESA AK.P23K AK.PAX AK.PWL AK.RAG AK.RC01 AK.RIDG AK.RND
AK.SAW AK.SCRK AK.SLK AK.SWD AK.VRDI AK.WAT6 AK.WRH AK.YAH
CN.BVCY
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 0.07 n 3
Best Fitting Double Couple
Mo = 2.69e+22 dyne-cm
Mw = 4.22
Z = 29 km
Plane Strike Dip Rake
NP1 338 48 -109
NP2 185 45 -70
Principal Axes:
Axis Value Plunge Azimuth
T 2.69e+22 2 81
N 0.00e+00 14 351
P -2.69e+22 76 178
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.38e+20
Mxy 4.21e+21
Mxz 6.48e+21
Myy 2.62e+22
Myz 5.67e+20
Mzz -2.53e+22
------########
########-#############
#########------#############
########----------############
#########-------------############
#########---------------############
#########-----------------############
##########------------------############
#########--------------------##########
##########---------------------######### T
##########----------------------########
##########----------------------##########
##########---------- ----------#########
#########---------- P ----------########
#########---------- ----------########
#########----------------------#######
########----------------------######
########--------------------######
#######-------------------####
#######-----------------####
#####---------------##
###-----------
Global CMT Convention Moment Tensor:
R T P
-2.53e+22 6.48e+21 -5.67e+20
6.48e+21 -9.38e+20 -4.21e+21
-5.67e+20 -4.21e+21 2.62e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240915151703/index.html
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STK = 185
DIP = 45
RAKE = -70
MW = 4.22
HS = 29.0
The NDK file is 20240915151703.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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 350 45 90 3.72 0.2865
WVFGRD96 2.0 165 45 90 3.85 0.3792
WVFGRD96 3.0 170 45 90 3.90 0.3606
WVFGRD96 4.0 165 40 80 3.89 0.2803
WVFGRD96 5.0 25 75 -40 3.82 0.2530
WVFGRD96 6.0 40 60 25 3.85 0.2731
WVFGRD96 7.0 220 70 35 3.87 0.2961
WVFGRD96 8.0 215 90 50 3.92 0.3107
WVFGRD96 9.0 220 80 45 3.94 0.3338
WVFGRD96 10.0 220 80 45 3.95 0.3562
WVFGRD96 11.0 225 60 35 3.99 0.3780
WVFGRD96 12.0 190 25 -65 4.01 0.4032
WVFGRD96 13.0 185 25 -70 4.03 0.4335
WVFGRD96 14.0 -15 65 -100 4.04 0.4610
WVFGRD96 15.0 170 30 -90 4.06 0.4877
WVFGRD96 16.0 195 40 -50 4.08 0.5148
WVFGRD96 17.0 195 45 -50 4.10 0.5443
WVFGRD96 18.0 195 45 -50 4.11 0.5719
WVFGRD96 19.0 190 45 -60 4.12 0.5976
WVFGRD96 20.0 190 45 -60 4.14 0.6207
WVFGRD96 21.0 190 45 -60 4.15 0.6384
WVFGRD96 22.0 190 45 -60 4.16 0.6561
WVFGRD96 23.0 190 45 -60 4.17 0.6709
WVFGRD96 24.0 190 45 -60 4.18 0.6829
WVFGRD96 25.0 190 45 -60 4.19 0.6926
WVFGRD96 26.0 185 45 -65 4.20 0.7008
WVFGRD96 27.0 185 45 -70 4.20 0.7084
WVFGRD96 28.0 185 45 -70 4.21 0.7140
WVFGRD96 29.0 185 45 -70 4.22 0.7167
WVFGRD96 30.0 185 45 -70 4.23 0.7163
WVFGRD96 31.0 185 45 -70 4.24 0.7123
WVFGRD96 32.0 185 45 -70 4.24 0.7045
WVFGRD96 33.0 185 45 -70 4.25 0.6930
WVFGRD96 34.0 185 45 -70 4.25 0.6782
WVFGRD96 35.0 185 45 -70 4.26 0.6608
WVFGRD96 36.0 185 45 -70 4.27 0.6413
WVFGRD96 37.0 185 45 -70 4.27 0.6218
WVFGRD96 38.0 185 50 -65 4.29 0.6040
WVFGRD96 39.0 185 50 -65 4.30 0.5879
WVFGRD96 40.0 -5 40 -85 4.37 0.5652
WVFGRD96 41.0 350 40 -90 4.38 0.5662
WVFGRD96 42.0 170 50 -90 4.39 0.5628
WVFGRD96 43.0 170 50 -90 4.40 0.5563
WVFGRD96 44.0 170 50 -90 4.41 0.5469
WVFGRD96 45.0 170 50 -90 4.41 0.5353
WVFGRD96 46.0 15 45 -60 4.40 0.5267
WVFGRD96 47.0 15 45 -60 4.41 0.5180
WVFGRD96 48.0 15 45 -60 4.41 0.5084
WVFGRD96 49.0 15 45 -60 4.42 0.4979
WVFGRD96 50.0 15 45 -60 4.42 0.4881
WVFGRD96 51.0 15 45 -60 4.42 0.4775
WVFGRD96 52.0 15 45 -55 4.43 0.4671
WVFGRD96 53.0 15 45 -55 4.43 0.4576
WVFGRD96 54.0 15 45 -55 4.43 0.4476
WVFGRD96 55.0 20 50 -50 4.43 0.4382
WVFGRD96 56.0 25 50 -45 4.43 0.4298
WVFGRD96 57.0 25 50 -45 4.43 0.4213
WVFGRD96 58.0 25 50 -45 4.44 0.4125
WVFGRD96 59.0 20 60 -55 4.43 0.4065
The best solution is
WVFGRD96 29.0 185 45 -70 4.22 0.7167
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 0.07 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