The ANSS event ID is ak0232cfl6rx and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0232cfl6rx/executive.
2023/02/20 05:34:16 58.932 -154.518 130.5 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/02/20 05:34:16:0 58.93 -154.52 130.5 4.3 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AK.N18K AK.N19K AK.O18K AK.O19K
AK.Q19K AK.SLK AK.SWD AV.ACH AV.P19K AV.PLBL AV.PLK3 AV.RED
AV.STLK II.KDAK
Filtering commands used:
cut o DIST/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 6.38e+22 dyne-cm
Mw = 4.47
Z = 142 km
Plane Strike Dip Rake
NP1 319 58 138
NP2 75 55 40
Principal Axes:
Axis Value Plunge Azimuth
T 6.38e+22 51 285
N 0.00e+00 39 109
P -6.38e+22 2 18
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.60e+22
Mxy -2.50e+22
Mxz 6.30e+21
Myy 1.74e+22
Myz -3.07e+22
Mzz 3.86e+22
------------ P
---------------- ---
##--------------------------
##########--------------------
###############-------------------
###################-----------------
######################----------------
#########################---------------
######### ###############-------------
########## T ################-----------##
########## #################---------###
###############################------#####
################################---#######
###############################-########
-###########################-----#######
----##################----------######
--------------------------------####
-------------------------------###
-----------------------------#
----------------------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
3.86e+22 6.30e+21 3.07e+22
6.30e+21 -5.60e+22 2.50e+22
3.07e+22 2.50e+22 1.74e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230220053416/index.html
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STK = 75
DIP = 55
RAKE = 40
MW = 4.47
HS = 142.0
The NDK file is 20230220053416.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 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 100 45 -85 3.65 0.3162
WVFGRD96 4.0 305 85 -55 3.66 0.2504
WVFGRD96 6.0 130 90 50 3.69 0.3092
WVFGRD96 8.0 130 90 50 3.76 0.3416
WVFGRD96 10.0 130 85 45 3.79 0.3596
WVFGRD96 12.0 220 40 10 3.82 0.3711
WVFGRD96 14.0 220 45 10 3.85 0.3789
WVFGRD96 16.0 220 45 10 3.87 0.3823
WVFGRD96 18.0 225 45 15 3.90 0.3819
WVFGRD96 20.0 225 45 15 3.92 0.3781
WVFGRD96 22.0 225 45 15 3.94 0.3722
WVFGRD96 24.0 235 45 30 3.96 0.3655
WVFGRD96 26.0 235 45 30 3.98 0.3599
WVFGRD96 28.0 240 50 35 4.00 0.3520
WVFGRD96 30.0 240 50 35 4.01 0.3399
WVFGRD96 32.0 240 55 40 4.02 0.3230
WVFGRD96 34.0 240 55 40 4.03 0.3034
WVFGRD96 36.0 240 55 40 4.03 0.2820
WVFGRD96 38.0 215 50 0 4.05 0.2676
WVFGRD96 40.0 120 45 -60 4.14 0.2765
WVFGRD96 42.0 120 45 -60 4.17 0.2823
WVFGRD96 44.0 120 45 -60 4.19 0.2839
WVFGRD96 46.0 120 45 -65 4.20 0.2827
WVFGRD96 48.0 120 45 -65 4.22 0.2810
WVFGRD96 50.0 120 45 -65 4.23 0.2769
WVFGRD96 52.0 125 45 -60 4.23 0.2702
WVFGRD96 54.0 245 50 45 4.23 0.2642
WVFGRD96 56.0 250 50 50 4.24 0.2684
WVFGRD96 58.0 60 60 30 4.24 0.2726
WVFGRD96 60.0 60 60 30 4.25 0.2874
WVFGRD96 62.0 60 65 45 4.28 0.3113
WVFGRD96 64.0 65 60 45 4.29 0.3388
WVFGRD96 66.0 65 60 40 4.31 0.3662
WVFGRD96 68.0 65 55 40 4.32 0.3944
WVFGRD96 70.0 65 55 40 4.34 0.4213
WVFGRD96 72.0 65 55 40 4.35 0.4456
WVFGRD96 74.0 65 55 35 4.36 0.4672
WVFGRD96 76.0 65 55 35 4.37 0.4892
WVFGRD96 78.0 65 55 35 4.38 0.5108
WVFGRD96 80.0 65 55 40 4.38 0.5307
WVFGRD96 82.0 65 55 40 4.39 0.5487
WVFGRD96 84.0 70 55 45 4.39 0.5640
WVFGRD96 86.0 70 55 45 4.40 0.5801
WVFGRD96 88.0 70 55 45 4.40 0.5942
WVFGRD96 90.0 70 55 45 4.41 0.6067
WVFGRD96 92.0 70 55 45 4.41 0.6171
WVFGRD96 94.0 70 55 45 4.41 0.6266
WVFGRD96 96.0 80 50 40 4.42 0.6368
WVFGRD96 98.0 80 50 40 4.43 0.6465
WVFGRD96 100.0 80 50 40 4.43 0.6549
WVFGRD96 102.0 80 50 40 4.43 0.6631
WVFGRD96 104.0 80 50 40 4.44 0.6704
WVFGRD96 106.0 80 50 40 4.44 0.6765
WVFGRD96 108.0 80 50 40 4.44 0.6816
WVFGRD96 110.0 80 50 40 4.44 0.6874
WVFGRD96 112.0 80 50 40 4.45 0.6920
WVFGRD96 114.0 80 50 40 4.45 0.6965
WVFGRD96 116.0 75 55 40 4.45 0.7005
WVFGRD96 118.0 75 55 40 4.45 0.7046
WVFGRD96 120.0 75 55 40 4.45 0.7075
WVFGRD96 122.0 75 55 40 4.45 0.7100
WVFGRD96 124.0 75 55 40 4.46 0.7126
WVFGRD96 126.0 75 55 40 4.46 0.7153
WVFGRD96 128.0 75 55 40 4.46 0.7172
WVFGRD96 130.0 75 55 40 4.46 0.7184
WVFGRD96 132.0 75 55 40 4.46 0.7193
WVFGRD96 134.0 75 55 40 4.46 0.7213
WVFGRD96 136.0 75 55 40 4.47 0.7223
WVFGRD96 138.0 75 55 40 4.47 0.7214
WVFGRD96 140.0 75 55 40 4.47 0.7222
WVFGRD96 142.0 75 55 40 4.47 0.7227
WVFGRD96 144.0 80 55 45 4.47 0.7222
WVFGRD96 146.0 80 55 45 4.47 0.7213
WVFGRD96 148.0 80 55 45 4.47 0.7211
WVFGRD96 150.0 80 55 40 4.48 0.7206
WVFGRD96 152.0 80 55 40 4.48 0.7190
WVFGRD96 154.0 80 55 40 4.48 0.7186
WVFGRD96 156.0 80 55 40 4.49 0.7163
WVFGRD96 158.0 80 55 40 4.49 0.7146
The best solution is
WVFGRD96 142.0 75 55 40 4.47 0.7227
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 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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