The ANSS event ID is ak0215iqq51a and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0215iqq51a/executive.
2021/04/30 17:19:24 64.830 -152.396 19.4 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/04/30 17:19:24:0 64.83 -152.40 19.4 4.3 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.DOT AK.E24K AK.F20K
AK.F21K AK.FYU AK.G19K AK.G23K AK.G24K AK.H17K AK.H21K
AK.H22K AK.H24K AK.HDA AK.I23K AK.J17K AK.J19K AK.J20K
AK.J25K AK.K20K AK.K24K AK.KNK AK.L18K AK.L19K AK.L20K
AK.M20K AK.MCK AK.MLY AK.NEA2 AK.PAX AK.POKR AK.PPD AK.PPLA
AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN
AK.TRF AK.WRH AT.PMR AV.SPCP IM.IL31 IU.COLA TA.E23K
TA.F19K TA.F24K TA.F25K TA.G18K TA.H19K TA.I17K TA.I20K
TA.J16K TA.J18K TA.K17K TA.M22K
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 = 1.60e+22 dyne-cm
Mw = 4.07
Z = 17 km
Plane Strike Dip Rake
NP1 291 64 -146
NP2 185 60 -30
Principal Axes:
Axis Value Plunge Azimuth
T 1.60e+22 3 57
N 0.00e+00 49 324
P -1.60e+22 41 150
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.04e+21
Mxy 1.12e+22
Mxz 7.27e+21
Myy 8.98e+21
Myz -3.39e+21
Mzz -6.94e+21
-------#######
----------############
-----------#################
-----------###################
------------#####################
-------------##################### T
--#################################
#############--------###################
#############-------------##############
##############----------------############
#############--------------------#########
#############----------------------#######
#############------------------------#####
############--------------------------##
############---------------------------#
############------------ -----------
###########------------ P ----------
##########------------ ---------
#########---------------------
#########-------------------
#######---------------
####----------
Global CMT Convention Moment Tensor:
R T P
-6.94e+21 7.27e+21 3.39e+21
7.27e+21 -2.04e+21 -1.12e+22
3.39e+21 -1.12e+22 8.98e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210430171924/index.html
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STK = 185
DIP = 60
RAKE = -30
MW = 4.07
HS = 17.0
The NDK file is 20210430171924.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 1.0 15 85 -5 3.48 0.2450
WVFGRD96 2.0 240 45 90 3.72 0.3093
WVFGRD96 3.0 20 65 10 3.67 0.3058
WVFGRD96 4.0 20 55 10 3.72 0.3198
WVFGRD96 5.0 200 80 50 3.76 0.3476
WVFGRD96 6.0 200 80 45 3.78 0.3852
WVFGRD96 7.0 200 45 15 3.81 0.4257
WVFGRD96 8.0 200 45 15 3.88 0.4605
WVFGRD96 9.0 205 45 30 3.91 0.5000
WVFGRD96 10.0 180 50 -35 3.94 0.5462
WVFGRD96 11.0 185 55 -30 3.96 0.5858
WVFGRD96 12.0 185 60 -30 3.98 0.6169
WVFGRD96 13.0 185 60 -30 4.00 0.6417
WVFGRD96 14.0 185 60 -30 4.02 0.6597
WVFGRD96 15.0 185 60 -30 4.04 0.6714
WVFGRD96 16.0 185 60 -30 4.05 0.6777
WVFGRD96 17.0 185 60 -30 4.07 0.6787
WVFGRD96 18.0 185 60 -30 4.08 0.6752
WVFGRD96 19.0 185 65 -30 4.09 0.6681
WVFGRD96 20.0 185 65 -30 4.10 0.6578
WVFGRD96 21.0 185 60 -30 4.11 0.6443
WVFGRD96 22.0 185 60 -25 4.12 0.6291
WVFGRD96 23.0 185 60 -25 4.13 0.6119
WVFGRD96 24.0 185 60 -25 4.14 0.5929
WVFGRD96 25.0 185 60 -25 4.14 0.5730
WVFGRD96 26.0 185 60 -25 4.15 0.5526
WVFGRD96 27.0 185 60 -25 4.15 0.5314
WVFGRD96 28.0 185 60 -30 4.15 0.5094
WVFGRD96 29.0 185 60 -30 4.16 0.4871
The best solution is
WVFGRD96 17.0 185 60 -30 4.07 0.6787
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