The ANSS event ID is ak02393mlqi5 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02393mlqi5/executive.
2023/07/17 10:40:53 61.442 -149.476 40.5 3.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/07/17 10:40:53:0 61.44 -149.48 40.5 3.4 Alaska
Stations used:
AK.FIRE AK.GHO AK.GLI AK.KNK AK.PWL AK.RC01 AK.SAW AK.SCM
AT.PMR
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 = 3.27e+21 dyne-cm
Mw = 3.61
Z = 40 km
Plane Strike Dip Rake
NP1 143 80 170
NP2 235 80 10
Principal Axes:
Axis Value Plunge Azimuth
T 3.27e+21 14 99
N 0.00e+00 76 280
P -3.27e+21 0 189
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.11e+21
Mxy -9.94e+20
Mxz -1.16e+20
Myy 2.92e+21
Myz 7.65e+20
Mzz 1.94e+20
--------------
----------------------
##--------------------------
####--------------------------
######----------------------------
########----------------------######
##########------------------##########
#############------------###############
##############--------##################
################-----#####################
#################-########################
###############----################## ##
#############-------################# T ##
##########----------################ #
########--------------##################
#####------------------###############
##---------------------#############
------------------------##########
------------------------######
-------------------------###
------ -------------
-- P ---------
Global CMT Convention Moment Tensor:
R T P
1.94e+20 -1.16e+20 -7.65e+20
-1.16e+20 -3.11e+21 9.94e+20
-7.65e+20 9.94e+20 2.92e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230717104053/index.html
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STK = 235
DIP = 80
RAKE = 10
MW = 3.61
HS = 40.0
The NDK file is 20230717104053.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 1.0 330 85 5 2.84 0.2233
WVFGRD96 2.0 55 90 -5 2.97 0.3243
WVFGRD96 3.0 55 90 -5 3.03 0.3746
WVFGRD96 4.0 55 75 0 3.08 0.4150
WVFGRD96 5.0 55 75 0 3.12 0.4498
WVFGRD96 6.0 55 75 5 3.16 0.4800
WVFGRD96 7.0 55 80 0 3.19 0.5078
WVFGRD96 8.0 230 70 -10 3.23 0.5301
WVFGRD96 9.0 235 75 0 3.25 0.5479
WVFGRD96 10.0 235 75 0 3.28 0.5622
WVFGRD96 11.0 235 75 0 3.29 0.5731
WVFGRD96 12.0 240 70 25 3.34 0.5836
WVFGRD96 13.0 240 70 30 3.37 0.5943
WVFGRD96 14.0 240 70 25 3.38 0.6027
WVFGRD96 15.0 240 70 25 3.39 0.6091
WVFGRD96 16.0 240 70 25 3.40 0.6137
WVFGRD96 17.0 240 75 30 3.42 0.6179
WVFGRD96 18.0 235 75 20 3.40 0.6225
WVFGRD96 19.0 235 75 20 3.41 0.6279
WVFGRD96 20.0 235 75 20 3.42 0.6335
WVFGRD96 21.0 235 80 25 3.44 0.6385
WVFGRD96 22.0 235 80 25 3.45 0.6440
WVFGRD96 23.0 235 80 25 3.46 0.6495
WVFGRD96 24.0 235 80 25 3.47 0.6560
WVFGRD96 25.0 235 80 20 3.46 0.6621
WVFGRD96 26.0 235 80 20 3.47 0.6677
WVFGRD96 27.0 235 80 20 3.48 0.6732
WVFGRD96 28.0 235 80 20 3.49 0.6781
WVFGRD96 29.0 235 80 15 3.49 0.6836
WVFGRD96 30.0 235 80 15 3.49 0.6890
WVFGRD96 31.0 235 80 15 3.50 0.6933
WVFGRD96 32.0 235 80 10 3.50 0.6977
WVFGRD96 33.0 235 80 10 3.51 0.7019
WVFGRD96 34.0 235 80 10 3.52 0.7034
WVFGRD96 35.0 235 80 10 3.53 0.7057
WVFGRD96 36.0 235 85 10 3.54 0.7073
WVFGRD96 37.0 235 85 10 3.56 0.7079
WVFGRD96 38.0 235 85 10 3.57 0.7108
WVFGRD96 39.0 235 85 10 3.59 0.7111
WVFGRD96 40.0 235 80 10 3.61 0.7147
WVFGRD96 41.0 235 80 10 3.62 0.7137
WVFGRD96 42.0 235 80 10 3.63 0.7134
WVFGRD96 43.0 235 80 10 3.64 0.7130
WVFGRD96 44.0 235 80 10 3.65 0.7115
WVFGRD96 45.0 235 80 10 3.66 0.7121
WVFGRD96 46.0 235 80 10 3.67 0.7107
WVFGRD96 47.0 235 80 10 3.68 0.7092
WVFGRD96 48.0 235 80 10 3.68 0.7085
WVFGRD96 49.0 235 80 10 3.69 0.7062
WVFGRD96 50.0 235 85 10 3.70 0.7062
WVFGRD96 51.0 235 85 10 3.70 0.7040
WVFGRD96 52.0 235 85 10 3.71 0.7047
WVFGRD96 53.0 235 85 10 3.71 0.7030
WVFGRD96 54.0 235 85 10 3.72 0.7023
WVFGRD96 55.0 235 85 10 3.73 0.7001
WVFGRD96 56.0 235 85 10 3.73 0.7006
WVFGRD96 57.0 235 85 10 3.74 0.6976
WVFGRD96 58.0 55 90 -10 3.74 0.6969
WVFGRD96 59.0 55 90 -10 3.75 0.6939
The best solution is
WVFGRD96 40.0 235 80 10 3.61 0.7147
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