The ANSS event ID is ak021bo5tu9w and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak021bo5tu9w/executive.
2021/09/11 05:58:44 60.294 -151.322 59.5 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/09/11 05:58:44:0 60.29 -151.32 59.5 3.9 Alaska
Stations used:
AK.BRLK AK.CNP AK.DIV AK.FID AK.HOM AK.KLU AK.KNK AK.M19K
AK.M20K AK.N19K AK.O19K AK.PWL AK.Q19K AK.RC01 AK.SKN
AK.SLK AK.SWD AT.PMR AV.ACH AV.ILS AV.RED AV.SPCP II.KDAK
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 2.19e+22 dyne-cm
Mw = 4.16
Z = 96 km
Plane Strike Dip Rake
NP1 340 70 142
NP2 85 55 25
Principal Axes:
Axis Value Plunge Azimuth
T 2.19e+22 41 297
N 0.00e+00 48 136
P -2.19e+22 9 35
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.14e+22
Mxy -1.52e+22
Mxz 2.16e+21
Myy 2.75e+21
Myz -1.16e+22
Mzz 8.69e+21
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######--------------
###########------------ P --
##############---------- ---
#################-----------------
###################-----------------
####### ###########-----------------
######## T ############-----------------
######## #############----------------
#########################-----------------
##########################---------------#
###########################-------------##
-##########################-----------####
--#########################-------######
------#####################---##########
----------#############---############
--------------------------##########
-------------------------#########
-----------------------#######
----------------------######
-------------------###
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Global CMT Convention Moment Tensor:
R T P
8.69e+21 2.16e+21 1.16e+22
2.16e+21 -1.14e+22 1.52e+22
1.16e+22 1.52e+22 2.75e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210911055844/index.html
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STK = 85
DIP = 55
RAKE = 25
MW = 4.16
HS = 96.0
The NDK file is 20210911055844.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 0.10 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 2.0 260 90 0 3.33 0.2376
WVFGRD96 4.0 260 80 0 3.44 0.3010
WVFGRD96 6.0 265 70 20 3.51 0.3200
WVFGRD96 8.0 265 70 20 3.56 0.3275
WVFGRD96 10.0 265 75 20 3.59 0.3276
WVFGRD96 12.0 265 75 20 3.61 0.3282
WVFGRD96 14.0 80 90 20 3.63 0.3301
WVFGRD96 16.0 80 90 20 3.66 0.3334
WVFGRD96 18.0 80 90 20 3.68 0.3363
WVFGRD96 20.0 260 90 -25 3.71 0.3389
WVFGRD96 22.0 85 80 25 3.73 0.3425
WVFGRD96 24.0 85 80 30 3.75 0.3466
WVFGRD96 26.0 85 80 30 3.77 0.3506
WVFGRD96 28.0 85 80 30 3.79 0.3545
WVFGRD96 30.0 85 75 25 3.80 0.3595
WVFGRD96 32.0 85 70 25 3.82 0.3659
WVFGRD96 34.0 85 70 25 3.84 0.3727
WVFGRD96 36.0 85 70 25 3.86 0.3803
WVFGRD96 38.0 85 70 25 3.88 0.3851
WVFGRD96 40.0 85 65 30 3.94 0.3883
WVFGRD96 42.0 85 65 30 3.96 0.3889
WVFGRD96 44.0 85 65 30 3.98 0.3887
WVFGRD96 46.0 85 65 30 3.99 0.3893
WVFGRD96 48.0 85 65 30 4.00 0.3901
WVFGRD96 50.0 85 60 30 4.02 0.3927
WVFGRD96 52.0 85 60 30 4.03 0.3947
WVFGRD96 54.0 85 55 30 4.04 0.3973
WVFGRD96 56.0 85 55 30 4.05 0.4002
WVFGRD96 58.0 85 55 30 4.06 0.4036
WVFGRD96 60.0 85 55 25 4.06 0.4054
WVFGRD96 62.0 85 55 25 4.07 0.4078
WVFGRD96 64.0 85 55 25 4.07 0.4099
WVFGRD96 66.0 85 55 25 4.08 0.4113
WVFGRD96 68.0 85 55 25 4.09 0.4130
WVFGRD96 70.0 85 55 25 4.09 0.4148
WVFGRD96 72.0 85 55 25 4.10 0.4163
WVFGRD96 74.0 85 55 25 4.10 0.4174
WVFGRD96 76.0 85 55 25 4.11 0.4179
WVFGRD96 78.0 85 55 25 4.11 0.4186
WVFGRD96 80.0 85 55 25 4.12 0.4197
WVFGRD96 82.0 85 55 25 4.12 0.4210
WVFGRD96 84.0 85 55 25 4.13 0.4214
WVFGRD96 86.0 85 55 25 4.13 0.4213
WVFGRD96 88.0 85 55 25 4.14 0.4220
WVFGRD96 90.0 85 55 25 4.14 0.4219
WVFGRD96 92.0 85 55 25 4.15 0.4211
WVFGRD96 94.0 85 55 25 4.15 0.4225
WVFGRD96 96.0 85 55 25 4.16 0.4227
WVFGRD96 98.0 85 55 25 4.16 0.4218
The best solution is
WVFGRD96 96.0 85 55 25 4.16 0.4227
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 0.10 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