The ANSS event ID is ak0216xxdp9j and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0216xxdp9j/executive.
2021/05/31 12:14:44 62.467 -148.283 44.1 4.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/05/31 12:14:44:0 62.47 -148.28 44.1 4.6 Alaska
Stations used:
AK.BARN AK.CCB AK.CRQ AK.CUT AK.DHY AK.DIV AK.DOT AK.EYAK
AK.FID AK.FIRE AK.GLI AK.HDA AK.K24K AK.KLU AK.L19K AK.L20K
AK.M20K AK.MCK AK.MLY AK.N19K AK.NEA2 AK.P23K AK.PAX
AK.POKR AK.PPLA AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM
AK.SCRK AK.SKN AK.SLK AK.TRF AT.MENT AT.PMR AV.STLK IM.IL31
IU.COLA
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 0.10 n 3
Best Fitting Double Couple
Mo = 5.56e+22 dyne-cm
Mw = 4.43
Z = 60 km
Plane Strike Dip Rake
NP1 250 65 -30
NP2 354 63 -152
Principal Axes:
Axis Value Plunge Azimuth
T 5.56e+22 1 302
N 0.00e+00 52 34
P -5.56e+22 38 211
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.24e+21
Mxy -4.03e+22
Mxz 2.37e+22
Myy 3.05e+22
Myz 1.30e+22
Mzz -2.13e+22
######--------
############----------
################------------
##################------------
####################-------------
T #####################-------------
#####################---#######----
###################-------##############
###############-----------##############
############----------------##############
#########-------------------##############
#######---------------------##############
#####-----------------------##############
##-------------------------#############
#--------------------------#############
----------- ------------############
---------- P ------------###########
--------- -----------###########
---------------------#########
-------------------#########
---------------#######
----------####
Global CMT Convention Moment Tensor:
R T P
-2.13e+22 2.37e+22 -1.30e+22
2.37e+22 -9.24e+21 4.03e+22
-1.30e+22 4.03e+22 3.05e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210531121444/index.html
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STK = 250
DIP = 65
RAKE = -30
MW = 4.43
HS = 60.0
The NDK file is 20210531121444.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 180 60 35 3.52 0.1581
WVFGRD96 4.0 165 65 -20 3.58 0.1877
WVFGRD96 6.0 350 65 5 3.64 0.2139
WVFGRD96 8.0 355 65 25 3.74 0.2387
WVFGRD96 10.0 355 70 20 3.79 0.2539
WVFGRD96 12.0 355 70 20 3.83 0.2596
WVFGRD96 14.0 255 80 -20 3.86 0.2709
WVFGRD96 16.0 255 80 -15 3.90 0.2893
WVFGRD96 18.0 255 80 -15 3.94 0.3088
WVFGRD96 20.0 255 80 -15 3.97 0.3298
WVFGRD96 22.0 255 75 -15 4.01 0.3547
WVFGRD96 24.0 255 75 -15 4.04 0.3778
WVFGRD96 26.0 255 75 -15 4.06 0.3974
WVFGRD96 28.0 255 70 -15 4.09 0.4176
WVFGRD96 30.0 255 70 -15 4.11 0.4372
WVFGRD96 32.0 255 70 -15 4.13 0.4536
WVFGRD96 34.0 250 65 -20 4.15 0.4685
WVFGRD96 36.0 250 65 -25 4.17 0.4823
WVFGRD96 38.0 255 65 -20 4.20 0.4913
WVFGRD96 40.0 250 55 -25 4.29 0.5139
WVFGRD96 42.0 250 55 -25 4.31 0.5204
WVFGRD96 44.0 250 60 -30 4.33 0.5301
WVFGRD96 46.0 250 60 -30 4.35 0.5422
WVFGRD96 48.0 250 60 -30 4.37 0.5514
WVFGRD96 50.0 250 60 -30 4.38 0.5603
WVFGRD96 52.0 250 60 -30 4.40 0.5661
WVFGRD96 54.0 250 65 -30 4.40 0.5703
WVFGRD96 56.0 250 65 -30 4.41 0.5749
WVFGRD96 58.0 250 65 -30 4.42 0.5782
WVFGRD96 60.0 250 65 -30 4.43 0.5799
WVFGRD96 62.0 250 65 -30 4.44 0.5791
WVFGRD96 64.0 250 65 -30 4.44 0.5788
WVFGRD96 66.0 250 65 -30 4.45 0.5761
WVFGRD96 68.0 250 65 -30 4.45 0.5752
WVFGRD96 70.0 250 65 -30 4.45 0.5732
WVFGRD96 72.0 250 70 -30 4.45 0.5704
WVFGRD96 74.0 250 70 -30 4.46 0.5679
WVFGRD96 76.0 250 70 -30 4.46 0.5656
WVFGRD96 78.0 250 70 -30 4.46 0.5600
WVFGRD96 80.0 250 70 -30 4.47 0.5561
WVFGRD96 82.0 250 70 -30 4.47 0.5510
WVFGRD96 84.0 250 70 -30 4.47 0.5462
WVFGRD96 86.0 250 70 -30 4.47 0.5407
WVFGRD96 88.0 255 75 -25 4.46 0.5367
WVFGRD96 90.0 255 75 -25 4.46 0.5328
WVFGRD96 92.0 255 80 -25 4.46 0.5295
WVFGRD96 94.0 255 80 -25 4.47 0.5269
WVFGRD96 96.0 255 80 -25 4.47 0.5241
WVFGRD96 98.0 255 80 -20 4.46 0.5210
The best solution is
WVFGRD96 60.0 250 65 -30 4.43 0.5799
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 0.10 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