The ANSS event ID is ak02239qtmiy and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02239qtmiy/executive.
2022/03/12 19:59:48 60.790 -152.082 94.7 4.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2022/03/12 19:59:48:0 60.79 -152.08 94.7 4.8 Alaska
Stations used:
AK.BPAW AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.FIRE
AK.GHO AK.HIN AK.HOM AK.KNK AK.KTH AK.L18K AK.L19K AK.L20K
AK.L22K AK.M16K AK.M20K AK.MCK AK.N18K AK.N19K AK.O18K
AK.O19K AK.P17K AK.P23K AK.Q19K AK.R18K AK.RC01 AK.RND
AK.SAW AK.SKN AK.SLK AK.SSN AK.SWD AK.TRF AT.PMR AV.ILS
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 0.07 n 3
Best Fitting Double Couple
Mo = 2.63e+23 dyne-cm
Mw = 4.88
Z = 106 km
Plane Strike Dip Rake
NP1 303 83 135
NP2 40 45 10
Principal Axes:
Axis Value Plunge Azimuth
T 2.63e+23 36 251
N 0.00e+00 44 116
P -2.63e+23 24 360
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.99e+23
Mxy 5.43e+22
Mxz -1.40e+23
Myy 1.54e+23
Myz -1.18e+23
Mzz 4.57e+22
--------------
--------- ----------
------------ P ------------#
------------- -------------#
--------------------------------##
###------------------------------###
########--------------------------####
#############----------------------#####
################------------------######
####################---------------#######
#######################-----------########
##########################--------########
####### ##################-----#########
###### T #####################-#########
###### ####################---########
###########################------#####
########################----------##
####################--------------
###############---------------
##########------------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
4.57e+22 -1.40e+23 1.18e+23
-1.40e+23 -1.99e+23 -5.43e+22
1.18e+23 -5.43e+22 1.54e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220312195948/index.html
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STK = 40
DIP = 45
RAKE = 10
MW = 4.88
HS = 106.0
The NDK file is 20220312195948.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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 115 65 -10 3.99 0.2053
WVFGRD96 4.0 115 75 -10 4.05 0.2257
WVFGRD96 6.0 115 80 -20 4.11 0.2263
WVFGRD96 8.0 120 80 25 4.17 0.2296
WVFGRD96 10.0 115 85 25 4.19 0.2281
WVFGRD96 12.0 115 90 25 4.21 0.2251
WVFGRD96 14.0 115 80 20 4.23 0.2203
WVFGRD96 16.0 215 70 10 4.26 0.2234
WVFGRD96 18.0 215 75 15 4.29 0.2295
WVFGRD96 20.0 215 75 15 4.31 0.2352
WVFGRD96 22.0 215 75 15 4.33 0.2399
WVFGRD96 24.0 215 75 15 4.35 0.2441
WVFGRD96 26.0 220 75 20 4.38 0.2480
WVFGRD96 28.0 215 75 15 4.39 0.2515
WVFGRD96 30.0 215 75 15 4.41 0.2545
WVFGRD96 32.0 215 75 15 4.43 0.2564
WVFGRD96 34.0 35 80 -15 4.46 0.2595
WVFGRD96 36.0 30 80 -10 4.48 0.2679
WVFGRD96 38.0 30 80 -10 4.51 0.2798
WVFGRD96 40.0 30 75 -10 4.56 0.2960
WVFGRD96 42.0 30 80 -10 4.59 0.3089
WVFGRD96 44.0 30 75 -10 4.62 0.3218
WVFGRD96 46.0 30 75 -10 4.64 0.3352
WVFGRD96 48.0 30 75 -10 4.66 0.3490
WVFGRD96 50.0 25 70 -10 4.67 0.3628
WVFGRD96 52.0 25 70 -10 4.68 0.3790
WVFGRD96 54.0 25 70 -15 4.70 0.3960
WVFGRD96 56.0 25 70 -10 4.71 0.4165
WVFGRD96 58.0 25 65 -10 4.73 0.4336
WVFGRD96 60.0 30 65 -5 4.74 0.4466
WVFGRD96 62.0 30 60 0 4.75 0.4605
WVFGRD96 64.0 30 60 0 4.76 0.4730
WVFGRD96 66.0 30 60 0 4.77 0.4828
WVFGRD96 68.0 30 60 5 4.77 0.4906
WVFGRD96 70.0 35 55 5 4.78 0.4994
WVFGRD96 72.0 35 55 5 4.79 0.5086
WVFGRD96 74.0 35 55 5 4.80 0.5166
WVFGRD96 76.0 35 55 5 4.80 0.5240
WVFGRD96 78.0 35 50 5 4.81 0.5314
WVFGRD96 80.0 35 50 5 4.82 0.5383
WVFGRD96 82.0 35 50 5 4.82 0.5438
WVFGRD96 84.0 35 50 5 4.83 0.5496
WVFGRD96 86.0 35 50 5 4.83 0.5538
WVFGRD96 88.0 35 50 5 4.84 0.5584
WVFGRD96 90.0 35 50 10 4.84 0.5619
WVFGRD96 92.0 35 50 10 4.85 0.5651
WVFGRD96 94.0 35 50 10 4.85 0.5678
WVFGRD96 96.0 35 50 10 4.85 0.5695
WVFGRD96 98.0 35 45 10 4.86 0.5718
WVFGRD96 100.0 35 45 10 4.86 0.5729
WVFGRD96 102.0 35 45 10 4.87 0.5741
WVFGRD96 104.0 35 45 10 4.87 0.5742
WVFGRD96 106.0 40 45 10 4.88 0.5746
WVFGRD96 108.0 40 45 10 4.88 0.5743
WVFGRD96 110.0 40 45 10 4.88 0.5735
WVFGRD96 112.0 40 45 10 4.88 0.5720
WVFGRD96 114.0 40 45 10 4.89 0.5702
WVFGRD96 116.0 40 45 10 4.89 0.5679
WVFGRD96 118.0 40 45 10 4.89 0.5661
WVFGRD96 120.0 40 45 10 4.89 0.5645
WVFGRD96 122.0 40 45 10 4.89 0.5613
WVFGRD96 124.0 40 45 10 4.90 0.5587
WVFGRD96 126.0 40 45 10 4.90 0.5558
WVFGRD96 128.0 40 45 10 4.90 0.5531
WVFGRD96 130.0 40 45 10 4.90 0.5501
WVFGRD96 132.0 40 45 10 4.90 0.5458
WVFGRD96 134.0 35 45 10 4.90 0.5433
WVFGRD96 136.0 35 50 10 4.90 0.5398
WVFGRD96 138.0 35 50 10 4.90 0.5362
The best solution is
WVFGRD96 106.0 40 45 10 4.88 0.5746
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 0.07 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