The ANSS event ID is ak0222cjwd3a and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0222cjwd3a/executive.
2022/02/20 12:58:15 61.785 -151.807 111.2 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2022/02/20 12:58:15:0 61.78 -151.81 111.2 4.3 Alaska
Stations used:
AK.CNP AK.CUT AK.DHY AK.FIRE AK.GHO AK.L22K AK.O18K AK.O19K
AK.RC01 AK.RND AK.SKN AK.SLK AK.SSN AK.SWD AV.SPCP AV.STLK
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 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 3.20e+22 dyne-cm
Mw = 4.27
Z = 110 km
Plane Strike Dip Rake
NP1 55 80 25
NP2 320 65 169
Principal Axes:
Axis Value Plunge Azimuth
T 3.20e+22 25 280
N 0.00e+00 63 75
P -3.20e+22 10 186
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.99e+22
Mxy -7.59e+21
Mxz 7.52e+21
Myy 2.53e+22
Myz -1.14e+22
Mzz 4.62e+21
--------------
----------------------
##--------------------------
########----------------------
#############---------------------
#################-----------------##
####################-------------#####
#######################---------########
### ##################------##########
#### T ####################--#############
#### ####################-##############
########################-----#############
#####################---------############
##################------------##########
###############----------------#########
##########---------------------#######
######-------------------------#####
------------------------------####
----------------------------##
---------------------------#
-------- -----------
---- P -------
Global CMT Convention Moment Tensor:
R T P
4.62e+21 7.52e+21 1.14e+22
7.52e+21 -2.99e+22 7.59e+21
1.14e+22 7.59e+21 2.53e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220220125815/index.html
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STK = 55
DIP = 80
RAKE = 25
MW = 4.27
HS = 110.0
The NDK file is 20220220125815.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 0.08 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 50 80 -15 3.48 0.3326
WVFGRD96 4.0 50 75 -15 3.56 0.3864
WVFGRD96 6.0 230 70 -20 3.62 0.4042
WVFGRD96 8.0 230 70 -20 3.66 0.4160
WVFGRD96 10.0 230 70 -15 3.68 0.4216
WVFGRD96 12.0 235 75 -5 3.71 0.4248
WVFGRD96 14.0 235 75 -5 3.74 0.4251
WVFGRD96 16.0 235 75 -5 3.76 0.4266
WVFGRD96 18.0 235 80 15 3.78 0.4283
WVFGRD96 20.0 235 75 0 3.80 0.4320
WVFGRD96 22.0 235 75 0 3.82 0.4377
WVFGRD96 24.0 240 70 15 3.87 0.4440
WVFGRD96 26.0 240 75 15 3.89 0.4512
WVFGRD96 28.0 240 75 15 3.91 0.4592
WVFGRD96 30.0 240 75 10 3.93 0.4655
WVFGRD96 32.0 240 75 10 3.95 0.4733
WVFGRD96 34.0 240 75 5 3.98 0.4769
WVFGRD96 36.0 240 75 5 4.00 0.4827
WVFGRD96 38.0 240 75 5 4.03 0.4843
WVFGRD96 40.0 240 70 5 4.08 0.4844
WVFGRD96 42.0 240 75 5 4.09 0.4825
WVFGRD96 44.0 235 70 -10 4.10 0.4838
WVFGRD96 46.0 235 70 -10 4.12 0.4922
WVFGRD96 48.0 235 75 -10 4.13 0.5024
WVFGRD96 50.0 235 75 -15 4.15 0.5131
WVFGRD96 52.0 235 75 -15 4.17 0.5250
WVFGRD96 54.0 235 80 -15 4.17 0.5363
WVFGRD96 56.0 235 80 -15 4.18 0.5454
WVFGRD96 58.0 230 75 -25 4.19 0.5546
WVFGRD96 60.0 235 80 -15 4.20 0.5597
WVFGRD96 62.0 235 85 -15 4.19 0.5643
WVFGRD96 64.0 235 85 -15 4.20 0.5700
WVFGRD96 66.0 235 85 -15 4.21 0.5740
WVFGRD96 68.0 235 85 -15 4.22 0.5787
WVFGRD96 70.0 235 85 -15 4.22 0.5823
WVFGRD96 72.0 235 85 -15 4.23 0.5859
WVFGRD96 74.0 235 85 -15 4.24 0.5885
WVFGRD96 76.0 235 85 -15 4.24 0.5911
WVFGRD96 78.0 235 90 -25 4.23 0.5934
WVFGRD96 80.0 235 90 -25 4.24 0.5967
WVFGRD96 82.0 235 90 -25 4.24 0.5987
WVFGRD96 84.0 235 90 -25 4.24 0.6009
WVFGRD96 86.0 55 85 25 4.24 0.6042
WVFGRD96 88.0 55 85 25 4.24 0.6063
WVFGRD96 90.0 55 85 25 4.25 0.6084
WVFGRD96 92.0 55 85 25 4.25 0.6098
WVFGRD96 94.0 55 85 25 4.25 0.6112
WVFGRD96 96.0 55 85 25 4.26 0.6120
WVFGRD96 98.0 55 85 25 4.26 0.6124
WVFGRD96 100.0 55 85 25 4.26 0.6129
WVFGRD96 102.0 55 80 25 4.26 0.6128
WVFGRD96 104.0 55 80 25 4.26 0.6127
WVFGRD96 106.0 55 80 25 4.26 0.6129
WVFGRD96 108.0 55 80 25 4.27 0.6133
WVFGRD96 110.0 55 80 25 4.27 0.6141
WVFGRD96 112.0 55 80 25 4.28 0.6138
WVFGRD96 114.0 55 80 25 4.28 0.6130
WVFGRD96 116.0 55 80 25 4.28 0.6120
WVFGRD96 118.0 55 80 25 4.29 0.6111
WVFGRD96 120.0 55 80 25 4.29 0.6115
WVFGRD96 122.0 55 80 25 4.29 0.6109
WVFGRD96 124.0 55 80 25 4.30 0.6091
WVFGRD96 126.0 55 75 30 4.29 0.6068
WVFGRD96 128.0 55 75 30 4.29 0.6077
WVFGRD96 130.0 55 75 30 4.30 0.6068
WVFGRD96 132.0 55 75 30 4.30 0.6047
WVFGRD96 134.0 55 75 30 4.30 0.6033
WVFGRD96 136.0 55 75 30 4.30 0.6031
WVFGRD96 138.0 55 75 30 4.31 0.6012
WVFGRD96 140.0 55 75 30 4.31 0.5994
WVFGRD96 142.0 55 75 30 4.31 0.5987
WVFGRD96 144.0 55 75 30 4.32 0.5971
WVFGRD96 146.0 55 75 30 4.32 0.5955
WVFGRD96 148.0 55 75 30 4.32 0.5945
The best solution is
WVFGRD96 110.0 55 80 25 4.27 0.6141
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 0.08 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