The ANSS event ID is ak024b9eizri and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024b9eizri/executive.
2024/09/01 13:20:21 59.665 -151.522 48.2 4.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/09/01 13:20:21:0 59.67 -151.52 48.2 4.6 Alaska
Stations used:
AK.BAE AK.BRLK AK.CUT AK.DIV AK.FID AK.GLI AK.KLU AK.L22K
AK.N18K AK.O18K AK.O19K AK.P23K AK.PWL AK.SLK AK.SWD AV.ACH
AV.RED AV.STLK II.KDAK
Filtering commands used:
cut o DIST/3.4 -30 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 5.96e+22 dyne-cm
Mw = 4.45
Z = 49 km
Plane Strike Dip Rake
NP1 190 75 -70
NP2 315 25 -142
Principal Axes:
Axis Value Plunge Azimuth
T 5.96e+22 27 264
N 0.00e+00 19 5
P -5.96e+22 56 125
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.89e+21
Mxy 1.37e+22
Mxz 1.36e+22
Myy 3.39e+22
Myz -4.68e+22
Mzz -2.80e+22
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-----####----#########
-##############----#########
###############--------#######
################-----------#######
#################-------------######
#################----------------#####
#################------------------#####
#################-------------------####
##################-------------------#####
#### ##########---------------------####
#### T ##########---------------------####
#### ##########--------- ----------###
###############---------- P ----------##
###############---------- ----------##
##############----------------------##
#############----------------------#
############---------------------#
##########--------------------
#########-------------------
#######---------------
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Global CMT Convention Moment Tensor:
R T P
-2.80e+22 1.36e+22 4.68e+22
1.36e+22 -5.89e+21 -1.37e+22
4.68e+22 -1.37e+22 3.39e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240901132021/index.html
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STK = 190
DIP = 75
RAKE = -70
MW = 4.45
HS = 49.0
The NDK file is 20240901132021.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/3.4 -30 o DIST/3.4 +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 0 45 -90 3.63 0.1622
WVFGRD96 2.0 0 45 -90 3.74 0.2098
WVFGRD96 3.0 175 40 80 3.80 0.2041
WVFGRD96 4.0 5 45 -85 3.82 0.1910
WVFGRD96 5.0 185 65 -30 3.78 0.1853
WVFGRD96 6.0 185 65 -30 3.80 0.1853
WVFGRD96 7.0 175 90 -50 3.79 0.1960
WVFGRD96 8.0 0 80 60 3.86 0.2079
WVFGRD96 9.0 0 80 60 3.87 0.2227
WVFGRD96 10.0 0 80 60 3.88 0.2360
WVFGRD96 11.0 0 80 60 3.89 0.2477
WVFGRD96 12.0 0 80 60 3.90 0.2582
WVFGRD96 13.0 0 80 55 3.91 0.2678
WVFGRD96 14.0 0 80 60 3.92 0.2766
WVFGRD96 15.0 -5 80 55 3.93 0.2846
WVFGRD96 16.0 -5 80 55 3.94 0.2919
WVFGRD96 17.0 -5 80 55 3.95 0.3000
WVFGRD96 18.0 -5 80 60 3.96 0.3075
WVFGRD96 19.0 -5 80 60 3.97 0.3148
WVFGRD96 20.0 -5 80 60 3.98 0.3217
WVFGRD96 21.0 -5 80 60 4.00 0.3283
WVFGRD96 22.0 -5 80 60 4.01 0.3346
WVFGRD96 23.0 -5 80 60 4.02 0.3401
WVFGRD96 24.0 80 35 -10 4.06 0.3446
WVFGRD96 25.0 45 45 -40 4.08 0.3532
WVFGRD96 26.0 45 45 -35 4.09 0.3604
WVFGRD96 27.0 45 45 -35 4.10 0.3671
WVFGRD96 28.0 45 45 -35 4.11 0.3735
WVFGRD96 29.0 205 80 -60 4.13 0.3870
WVFGRD96 30.0 205 80 -60 4.14 0.3995
WVFGRD96 31.0 205 80 -60 4.16 0.4129
WVFGRD96 32.0 205 80 -60 4.17 0.4257
WVFGRD96 33.0 200 80 -60 4.18 0.4386
WVFGRD96 34.0 200 80 -60 4.19 0.4512
WVFGRD96 35.0 200 80 -60 4.20 0.4633
WVFGRD96 36.0 200 80 -60 4.21 0.4742
WVFGRD96 37.0 200 80 -60 4.22 0.4836
WVFGRD96 38.0 200 80 -60 4.23 0.4923
WVFGRD96 39.0 195 75 -60 4.25 0.5017
WVFGRD96 40.0 195 75 -70 4.37 0.5133
WVFGRD96 41.0 195 75 -70 4.38 0.5264
WVFGRD96 42.0 195 75 -70 4.39 0.5366
WVFGRD96 43.0 195 75 -70 4.40 0.5452
WVFGRD96 44.0 195 75 -70 4.41 0.5523
WVFGRD96 45.0 195 75 -70 4.42 0.5580
WVFGRD96 46.0 195 75 -70 4.43 0.5620
WVFGRD96 47.0 195 75 -70 4.43 0.5645
WVFGRD96 48.0 190 75 -70 4.44 0.5661
WVFGRD96 49.0 190 75 -70 4.45 0.5676
WVFGRD96 50.0 190 75 -70 4.45 0.5671
WVFGRD96 51.0 195 80 -65 4.46 0.5668
WVFGRD96 52.0 195 80 -65 4.46 0.5657
WVFGRD96 53.0 195 80 -65 4.47 0.5641
WVFGRD96 54.0 195 80 -65 4.47 0.5623
WVFGRD96 55.0 195 80 -65 4.48 0.5594
WVFGRD96 56.0 195 80 -65 4.48 0.5561
WVFGRD96 57.0 195 80 -65 4.48 0.5522
WVFGRD96 58.0 195 80 -65 4.49 0.5470
WVFGRD96 59.0 195 80 -65 4.49 0.5422
The best solution is
WVFGRD96 49.0 190 75 -70 4.45 0.5676
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/3.4 -30 o DIST/3.4 +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