The ANSS event ID is ak0211kjmeea and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0211kjmeea/executive.
2021/02/03 19:06:36 62.106 -149.534 60.5 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/02/03 19:06:36:0 62.11 -149.53 60.5 3.6 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.EYAK AK.GHO AK.HIN AK.KNK AK.MCAR
AK.MCK AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF
AT.PMR
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
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 7.50e+21 dyne-cm
Mw = 3.85
Z = 60 km
Plane Strike Dip Rake
NP1 9 64 -114
NP2 235 35 -50
Principal Axes:
Axis Value Plunge Azimuth
T 7.50e+21 16 117
N 0.00e+00 22 20
P -7.50e+21 63 240
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.02e+21
Mxy -3.48e+21
Mxz 6.55e+20
Myy 4.37e+21
Myz 4.36e+21
Mzz -5.40e+21
###########---
################------
####################--------
############---------########-
###########-------------##########
#########----------------###########
########------------------############
#######--------------------#############
######---------------------#############
######----------------------##############
#####-----------------------##############
####---------- -----------##############
####---------- P ----------###############
##----------- ----------##############
##-----------------------######### ###
#-----------------------######### T ##
#---------------------########## #
--------------------##############
-----------------#############
---------------#############
-----------###########
-----#########
Global CMT Convention Moment Tensor:
R T P
-5.40e+21 6.55e+20 -4.36e+21
6.55e+20 1.02e+21 3.48e+21
-4.36e+21 3.48e+21 4.37e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210203190636/index.html
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STK = 235
DIP = 35
RAKE = -50
MW = 3.85
HS = 60.0
The NDK file is 20210203190636.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 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 2.0 210 45 90 3.19 0.4344
WVFGRD96 4.0 345 25 15 3.26 0.3397
WVFGRD96 6.0 345 20 15 3.30 0.4881
WVFGRD96 8.0 345 20 20 3.36 0.5284
WVFGRD96 10.0 350 25 30 3.35 0.5429
WVFGRD96 12.0 340 30 10 3.35 0.5490
WVFGRD96 14.0 340 30 10 3.36 0.5511
WVFGRD96 16.0 330 35 0 3.38 0.5506
WVFGRD96 18.0 330 30 0 3.39 0.5495
WVFGRD96 20.0 325 35 -5 3.42 0.5458
WVFGRD96 22.0 325 30 0 3.44 0.5388
WVFGRD96 24.0 330 25 20 3.46 0.5298
WVFGRD96 26.0 325 30 15 3.48 0.5191
WVFGRD96 28.0 65 90 -55 3.54 0.5076
WVFGRD96 30.0 245 90 55 3.56 0.4923
WVFGRD96 32.0 305 25 25 3.59 0.4805
WVFGRD96 34.0 300 25 20 3.60 0.4699
WVFGRD96 36.0 295 25 15 3.61 0.4549
WVFGRD96 38.0 250 85 45 3.63 0.4507
WVFGRD96 40.0 220 30 -65 3.74 0.4886
WVFGRD96 42.0 230 25 -50 3.78 0.5221
WVFGRD96 44.0 230 25 -50 3.80 0.5608
WVFGRD96 46.0 235 30 -45 3.82 0.5878
WVFGRD96 48.0 235 30 -45 3.83 0.6081
WVFGRD96 50.0 235 30 -45 3.84 0.6233
WVFGRD96 52.0 230 30 -55 3.83 0.6310
WVFGRD96 54.0 230 30 -55 3.84 0.6374
WVFGRD96 56.0 235 35 -45 3.86 0.6405
WVFGRD96 58.0 235 35 -45 3.86 0.6426
WVFGRD96 60.0 235 35 -50 3.85 0.6426
WVFGRD96 62.0 235 35 -50 3.85 0.6415
WVFGRD96 64.0 230 35 -55 3.85 0.6398
WVFGRD96 66.0 230 35 -55 3.85 0.6356
WVFGRD96 68.0 230 35 -55 3.85 0.6298
WVFGRD96 70.0 235 40 -50 3.86 0.6247
WVFGRD96 72.0 230 40 -55 3.86 0.6196
WVFGRD96 74.0 230 40 -55 3.86 0.6125
WVFGRD96 76.0 230 40 -55 3.86 0.6067
WVFGRD96 78.0 230 40 -55 3.86 0.6028
WVFGRD96 80.0 230 40 -55 3.86 0.5982
WVFGRD96 82.0 230 40 -55 3.87 0.5921
WVFGRD96 84.0 225 40 -65 3.85 0.5872
WVFGRD96 86.0 230 40 -60 3.86 0.5807
WVFGRD96 88.0 225 40 -65 3.86 0.5776
WVFGRD96 90.0 225 40 -65 3.86 0.5734
WVFGRD96 92.0 225 40 -65 3.86 0.5685
WVFGRD96 94.0 215 40 -80 3.86 0.5630
WVFGRD96 96.0 25 50 -95 3.86 0.5601
WVFGRD96 98.0 215 40 -80 3.86 0.5549
The best solution is
WVFGRD96 60.0 235 35 -50 3.85 0.6426
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 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