The ANSS event ID is ak0214icukav and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0214icukav/executive.
2021/04/08 17:10:18 63.201 -148.580 80.1 5.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/04/08 17:10:18:0 63.20 -148.58 80.1 5.5 Alaska
Stations used:
AK.BARN AK.CAST AK.CCB AK.CRQ AK.CUT AK.GLB AK.HIN AK.I23K
AK.J19K AK.K24K AK.M26K AK.M27K AK.MCK AK.PAX AK.POKR
AK.PPD AK.PPLA AK.RAG AK.SAW AK.SCM AK.SSN AK.TRF AK.WRH
AT.PMR AV.RED AV.SPCP IU.COLA TA.M22K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 2.48e+24 dyne-cm
Mw = 5.53
Z = 80 km
Plane Strike Dip Rake
NP1 280 65 -70
NP2 59 32 -126
Principal Axes:
Axis Value Plunge Azimuth
T 2.48e+24 18 355
N 0.00e+00 18 91
P -2.48e+24 64 224
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.00e+24
Mxy -4.18e+23
Mxz 1.41e+24
Myy -2.09e+23
Myz 6.14e+23
Mzz -1.79e+24
#### #######
######## T ###########
########### ##############
##############################
##################################
####################################
#####################################-
#######-------------##################--
#---------------------------##########--
---------------------------------######---
------------------------------------###---
--------------------------------------#---
--------------- -------------------####-
-------------- P ------------------#####
-------------- -----------------######
-------------------------------#######
----------------------------########
#------------------------#########
##-----------------###########
############################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-1.79e+24 1.41e+24 -6.14e+23
1.41e+24 2.00e+24 4.18e+23
-6.14e+23 4.18e+23 -2.09e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210408171018/index.html
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STK = 280
DIP = 65
RAKE = -70
MW = 5.53
HS = 80.0
The NDK file is 20210408171018.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 0.02 n 3 lp c 0.05 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 85 45 90 4.80 0.3070
WVFGRD96 4.0 85 50 85 4.89 0.2897
WVFGRD96 6.0 70 60 70 4.89 0.2382
WVFGRD96 8.0 75 75 80 4.94 0.2590
WVFGRD96 10.0 75 75 80 4.94 0.2908
WVFGRD96 12.0 70 85 70 4.94 0.3197
WVFGRD96 14.0 250 90 -70 4.95 0.3456
WVFGRD96 16.0 250 90 -70 4.97 0.3707
WVFGRD96 18.0 75 85 70 4.98 0.3948
WVFGRD96 20.0 250 90 -70 5.00 0.4154
WVFGRD96 22.0 250 90 -70 5.02 0.4352
WVFGRD96 24.0 75 85 70 5.04 0.4545
WVFGRD96 26.0 255 90 -70 5.06 0.4725
WVFGRD96 28.0 255 90 -75 5.07 0.4886
WVFGRD96 30.0 75 90 75 5.08 0.5020
WVFGRD96 32.0 75 90 75 5.10 0.5127
WVFGRD96 34.0 80 90 75 5.11 0.5204
WVFGRD96 36.0 80 90 75 5.11 0.5252
WVFGRD96 38.0 80 90 75 5.12 0.5273
WVFGRD96 40.0 260 90 -80 5.27 0.5259
WVFGRD96 42.0 260 85 -80 5.28 0.5267
WVFGRD96 44.0 80 90 80 5.29 0.5291
WVFGRD96 46.0 265 80 -80 5.31 0.5419
WVFGRD96 48.0 265 80 -80 5.32 0.5562
WVFGRD96 50.0 275 75 -75 5.35 0.5763
WVFGRD96 52.0 275 70 -75 5.37 0.6002
WVFGRD96 54.0 275 70 -75 5.39 0.6293
WVFGRD96 56.0 270 65 -80 5.41 0.6624
WVFGRD96 58.0 270 65 -80 5.42 0.6939
WVFGRD96 60.0 270 65 -80 5.43 0.7221
WVFGRD96 62.0 275 65 -75 5.45 0.7476
WVFGRD96 64.0 275 65 -75 5.46 0.7709
WVFGRD96 66.0 275 65 -75 5.47 0.7905
WVFGRD96 68.0 275 65 -75 5.48 0.8070
WVFGRD96 70.0 275 65 -75 5.49 0.8201
WVFGRD96 72.0 275 65 -75 5.50 0.8306
WVFGRD96 74.0 275 65 -75 5.50 0.8380
WVFGRD96 76.0 280 65 -70 5.52 0.8430
WVFGRD96 78.0 280 65 -70 5.52 0.8459
WVFGRD96 80.0 280 65 -70 5.53 0.8467
WVFGRD96 82.0 280 65 -70 5.53 0.8453
WVFGRD96 84.0 280 65 -70 5.54 0.8422
WVFGRD96 86.0 280 65 -70 5.54 0.8377
WVFGRD96 88.0 280 65 -70 5.54 0.8321
WVFGRD96 90.0 280 65 -70 5.55 0.8244
WVFGRD96 92.0 280 65 -70 5.55 0.8158
WVFGRD96 94.0 280 65 -70 5.55 0.8055
WVFGRD96 96.0 280 65 -70 5.55 0.7951
WVFGRD96 98.0 280 65 -70 5.56 0.7832
The best solution is
WVFGRD96 80.0 280 65 -70 5.53 0.8467
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 0.02 n 3 lp c 0.05 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