The ANSS event ID is ak023298h9go and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023298h9go/executive.
2023/02/18 12:00:25 62.865 -148.197 66.6 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/02/18 12:00:25:0 62.87 -148.20 66.6 3.8 Alaska
Stations used:
AK.CUT AK.DHY AK.GHO AK.HDA AK.KLU AK.KTH AK.L22K AK.MCK
AK.PAX AK.RND AK.SCM AK.SKN AK.WAT6 AK.WRH AT.PMR 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 = 1.10e+22 dyne-cm
Mw = 3.96
Z = 78 km
Plane Strike Dip Rake
NP1 251 77 -128
NP2 145 40 -20
Principal Axes:
Axis Value Plunge Azimuth
T 1.10e+22 23 9
N 0.00e+00 37 260
P -1.10e+22 44 123
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.44e+21
Mxy 4.00e+21
Mxz 6.84e+21
Myy -3.75e+21
Myz -3.99e+21
Mzz -3.69e+21
##############
############ #######
-############## T ##########
--############## ###########
---###############################
----################################
----##################################
-----###########################--------
-----###################----------------
-------############-----------------------
-------#######----------------------------
--------##--------------------------------
-------#----------------------------------
---#####-------------------- ---------
-########------------------- P ---------
##########----------------- --------
##########--------------------------
###########-----------------------
############------------------
##############--------------
######################
##############
Global CMT Convention Moment Tensor:
R T P
-3.69e+21 6.84e+21 3.99e+21
6.84e+21 7.44e+21 -4.00e+21
3.99e+21 -4.00e+21 -3.75e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230218120025/index.html
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STK = 145
DIP = 40
RAKE = -20
MW = 3.96
HS = 78.0
The NDK file is 20230218120025.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 10 50 65 3.28 0.3817
WVFGRD96 4.0 165 40 0 3.33 0.4226
WVFGRD96 6.0 160 40 -5 3.36 0.4779
WVFGRD96 8.0 165 40 10 3.40 0.4986
WVFGRD96 10.0 165 45 10 3.41 0.5109
WVFGRD96 12.0 165 50 10 3.42 0.5198
WVFGRD96 14.0 165 50 10 3.44 0.5275
WVFGRD96 16.0 160 50 -5 3.46 0.5342
WVFGRD96 18.0 160 55 0 3.48 0.5417
WVFGRD96 20.0 155 55 -20 3.52 0.5492
WVFGRD96 22.0 160 50 -5 3.53 0.5563
WVFGRD96 24.0 160 50 -5 3.55 0.5644
WVFGRD96 26.0 160 50 -5 3.57 0.5722
WVFGRD96 28.0 160 55 -5 3.59 0.5800
WVFGRD96 30.0 160 55 -5 3.61 0.5853
WVFGRD96 32.0 160 55 -5 3.62 0.5907
WVFGRD96 34.0 160 55 -5 3.64 0.5940
WVFGRD96 36.0 160 55 -10 3.67 0.5959
WVFGRD96 38.0 160 60 -10 3.69 0.6007
WVFGRD96 40.0 160 45 -5 3.77 0.6024
WVFGRD96 42.0 160 50 -5 3.78 0.6043
WVFGRD96 44.0 160 50 -5 3.80 0.6057
WVFGRD96 46.0 160 50 -5 3.81 0.6071
WVFGRD96 48.0 160 50 -5 3.82 0.6083
WVFGRD96 50.0 160 50 -5 3.84 0.6101
WVFGRD96 52.0 155 45 -10 3.86 0.6129
WVFGRD96 54.0 140 45 -25 3.88 0.6170
WVFGRD96 56.0 140 45 -25 3.89 0.6260
WVFGRD96 58.0 140 45 -25 3.90 0.6339
WVFGRD96 60.0 140 45 -25 3.90 0.6426
WVFGRD96 62.0 140 45 -25 3.91 0.6508
WVFGRD96 64.0 140 40 -25 3.92 0.6562
WVFGRD96 66.0 140 40 -25 3.93 0.6639
WVFGRD96 68.0 140 40 -25 3.94 0.6681
WVFGRD96 70.0 140 40 -25 3.94 0.6726
WVFGRD96 72.0 140 40 -20 3.95 0.6750
WVFGRD96 74.0 145 40 -20 3.95 0.6771
WVFGRD96 76.0 145 40 -20 3.96 0.6781
WVFGRD96 78.0 145 40 -20 3.96 0.6782
WVFGRD96 80.0 145 40 -20 3.97 0.6781
WVFGRD96 82.0 145 40 -20 3.97 0.6761
WVFGRD96 84.0 140 40 -25 3.97 0.6733
WVFGRD96 86.0 140 40 -25 3.97 0.6705
WVFGRD96 88.0 140 40 -25 3.98 0.6669
WVFGRD96 90.0 140 40 -25 3.98 0.6637
WVFGRD96 92.0 140 40 -30 3.98 0.6597
WVFGRD96 94.0 140 45 -30 3.98 0.6559
WVFGRD96 96.0 140 45 -30 3.98 0.6525
WVFGRD96 98.0 140 45 -30 3.98 0.6488
WVFGRD96 100.0 140 45 -35 3.99 0.6455
WVFGRD96 102.0 140 45 -35 3.99 0.6424
WVFGRD96 104.0 140 45 -35 3.99 0.6390
WVFGRD96 106.0 140 45 -35 4.00 0.6348
WVFGRD96 108.0 140 45 -40 4.00 0.6314
WVFGRD96 110.0 140 45 -40 4.01 0.6281
WVFGRD96 112.0 140 45 -40 4.01 0.6242
WVFGRD96 114.0 135 45 -45 4.01 0.6206
WVFGRD96 116.0 135 45 -45 4.02 0.6174
WVFGRD96 118.0 135 45 -50 4.03 0.6147
WVFGRD96 120.0 135 45 -50 4.03 0.6104
WVFGRD96 122.0 135 45 -50 4.03 0.5999
WVFGRD96 124.0 135 45 -50 4.03 0.5848
WVFGRD96 126.0 135 45 -50 4.03 0.5689
WVFGRD96 128.0 135 45 -50 4.03 0.5511
WVFGRD96 130.0 130 45 -60 4.05 0.5383
WVFGRD96 132.0 130 45 -60 4.05 0.5324
WVFGRD96 134.0 135 50 -55 4.04 0.5261
WVFGRD96 136.0 135 50 -55 4.04 0.5171
WVFGRD96 138.0 135 50 -60 4.06 0.5043
WVFGRD96 140.0 135 50 -60 4.06 0.4895
WVFGRD96 142.0 125 45 -65 4.06 0.4747
WVFGRD96 144.0 125 45 -70 4.07 0.4580
WVFGRD96 146.0 125 45 -70 4.07 0.4371
WVFGRD96 148.0 125 45 -70 4.07 0.4202
The best solution is
WVFGRD96 78.0 145 40 -20 3.96 0.6782
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