The ANSS event ID is ak0228qeea2m and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0228qeea2m/executive.
2022/07/09 10:56:01 60.857 -146.845 24.1 4.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2022/07/09 10:56:01:0 60.86 -146.85 24.1 4.4 Alaska
Stations used:
AK.BARN AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CCB AK.CNP AK.CUT
AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.HIN
AK.HOM AK.J26L AK.KLU AK.KNK AK.L26K AK.LOGN AK.M26K
AK.MCAR AK.MCK AK.NEA2 AK.P23K AK.PAX AK.RC01 AK.RND AK.SAW
AK.SCM AK.SWD AK.VRDI AT.PMR AV.SPCP
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
Best Fitting Double Couple
Mo = 3.43e+22 dyne-cm
Mw = 4.29
Z = 33 km
Plane Strike Dip Rake
NP1 331 67 -153
NP2 230 65 -25
Principal Axes:
Axis Value Plunge Azimuth
T 3.43e+22 2 100
N 0.00e+00 55 8
P -3.43e+22 35 191
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.12e+22
Mxy -1.04e+22
Mxz 1.56e+22
Myy 3.23e+22
Myz 4.07e+21
Mzz -1.11e+22
--------------
##--------------------
########--------------------
############------------------
###############----------#########
##################----##############
######################################
##################-----#################
################--------################
###############-----------################
#############--------------###############
###########-----------------############
##########-------------------########### T
#######---------------------###########
######-----------------------###########
#####-----------------------##########
###-------------------------########
#------------ -----------#######
----------- P -----------#####
---------- -----------####
---------------------#
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Global CMT Convention Moment Tensor:
R T P
-1.11e+22 1.56e+22 -4.07e+21
1.56e+22 -2.12e+22 1.04e+22
-4.07e+21 1.04e+22 3.23e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220709105601/index.html
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STK = 230
DIP = 65
RAKE = -25
MW = 4.29
HS = 33.0
The NDK file is 20220709105601.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 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 90 90 0 3.58 0.2238
WVFGRD96 2.0 280 45 85 3.81 0.3030
WVFGRD96 3.0 60 55 0 3.74 0.3239
WVFGRD96 4.0 60 55 5 3.79 0.3529
WVFGRD96 5.0 60 55 5 3.82 0.3799
WVFGRD96 6.0 60 60 10 3.85 0.4048
WVFGRD96 7.0 60 60 10 3.88 0.4260
WVFGRD96 8.0 60 55 15 3.94 0.4487
WVFGRD96 9.0 60 55 15 3.96 0.4638
WVFGRD96 10.0 60 55 15 3.98 0.4768
WVFGRD96 11.0 60 55 15 3.99 0.4888
WVFGRD96 12.0 60 60 20 4.01 0.4989
WVFGRD96 13.0 60 60 15 4.02 0.5072
WVFGRD96 14.0 50 65 -25 4.04 0.5256
WVFGRD96 15.0 50 65 -25 4.06 0.5406
WVFGRD96 16.0 50 65 -25 4.07 0.5546
WVFGRD96 17.0 235 70 -15 4.09 0.5697
WVFGRD96 18.0 235 70 -15 4.11 0.5880
WVFGRD96 19.0 235 70 -15 4.13 0.6056
WVFGRD96 20.0 235 70 -20 4.14 0.6234
WVFGRD96 21.0 230 65 -20 4.16 0.6401
WVFGRD96 22.0 230 65 -20 4.18 0.6562
WVFGRD96 23.0 230 65 -20 4.19 0.6713
WVFGRD96 24.0 230 65 -20 4.20 0.6857
WVFGRD96 25.0 230 65 -20 4.21 0.7006
WVFGRD96 26.0 230 65 -25 4.22 0.7147
WVFGRD96 27.0 230 65 -25 4.23 0.7271
WVFGRD96 28.0 230 65 -25 4.25 0.7382
WVFGRD96 29.0 230 65 -25 4.25 0.7470
WVFGRD96 30.0 230 65 -25 4.26 0.7538
WVFGRD96 31.0 230 65 -25 4.27 0.7576
WVFGRD96 32.0 230 65 -25 4.28 0.7593
WVFGRD96 33.0 230 65 -25 4.29 0.7593
WVFGRD96 34.0 230 65 -25 4.30 0.7571
WVFGRD96 35.0 230 65 -25 4.30 0.7532
WVFGRD96 36.0 230 65 -25 4.31 0.7479
WVFGRD96 37.0 230 65 -25 4.32 0.7412
WVFGRD96 38.0 235 70 -20 4.33 0.7369
WVFGRD96 39.0 235 70 -20 4.35 0.7338
WVFGRD96 40.0 230 65 -25 4.39 0.7380
WVFGRD96 41.0 230 65 -25 4.41 0.7418
WVFGRD96 42.0 230 65 -25 4.42 0.7429
WVFGRD96 43.0 230 65 -25 4.42 0.7402
WVFGRD96 44.0 230 65 -25 4.43 0.7373
WVFGRD96 45.0 230 65 -25 4.44 0.7325
WVFGRD96 46.0 230 65 -25 4.44 0.7269
WVFGRD96 47.0 235 70 -20 4.45 0.7227
WVFGRD96 48.0 235 70 -20 4.45 0.7171
WVFGRD96 49.0 235 70 -20 4.46 0.7117
WVFGRD96 50.0 235 70 -20 4.46 0.7058
WVFGRD96 51.0 235 70 -20 4.46 0.7003
WVFGRD96 52.0 235 70 -20 4.47 0.6928
WVFGRD96 53.0 235 70 -20 4.47 0.6872
WVFGRD96 54.0 235 70 -20 4.47 0.6805
WVFGRD96 55.0 235 75 -20 4.47 0.6745
WVFGRD96 56.0 235 75 -20 4.47 0.6688
WVFGRD96 57.0 235 75 -20 4.48 0.6638
WVFGRD96 58.0 235 75 -20 4.48 0.6580
WVFGRD96 59.0 235 75 -20 4.48 0.6523
The best solution is
WVFGRD96 33.0 230 65 -25 4.29 0.7593
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
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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