The ANSS event ID is ak020lj73yv and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020lj73yv/executive.
2020/01/13 02:03:42 61.641 -147.909 25.8 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/01/13 02:03:42:0 61.64 -147.91 25.8 3.8 Alaska
Stations used:
AK.CUT AK.EYAK AK.GHO AK.GLI AK.HIN AK.KLU AK.KNK AK.L22K
AK.PAX AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN AT.PMR TA.M22K
TA.M24K TA.N25K
Filtering commands used:
cut o DIST/3.3 -30 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 = 4.47e+21 dyne-cm
Mw = 3.70
Z = 30 km
Plane Strike Dip Rake
NP1 343 69 -148
NP2 240 60 -25
Principal Axes:
Axis Value Plunge Azimuth
T 4.47e+21 5 110
N 0.00e+00 52 13
P -4.47e+21 38 204
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.81e+21
Mxy -2.46e+21
Mxz 1.83e+21
Myy 3.44e+21
Myz 1.28e+21
Mzz -1.63e+21
##------------
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###############---------------
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####################---#############
###################---################
################-------#################
#############-----------################
############--------------################
##########----------------################
########-------------------###############
######---------------------###############
####-----------------------##########
###------------------------########## T
##------------------------##########
----------- -----------###########
---------- P -----------##########
-------- -----------########
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Global CMT Convention Moment Tensor:
R T P
-1.63e+21 1.83e+21 -1.28e+21
1.83e+21 -1.81e+21 2.46e+21
-1.28e+21 2.46e+21 3.44e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200113020342/index.html
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STK = 240
DIP = 60
RAKE = -25
MW = 3.70
HS = 30.0
The NDK file is 20200113020342.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 -30 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 2.0 250 80 0 3.22 0.3970
WVFGRD96 4.0 245 70 -10 3.34 0.5032
WVFGRD96 6.0 245 60 10 3.40 0.5524
WVFGRD96 8.0 260 50 50 3.53 0.5968
WVFGRD96 10.0 255 50 45 3.53 0.6084
WVFGRD96 12.0 240 65 -25 3.53 0.6144
WVFGRD96 14.0 240 65 -20 3.55 0.6292
WVFGRD96 16.0 240 65 -20 3.57 0.6392
WVFGRD96 18.0 240 65 -20 3.59 0.6485
WVFGRD96 20.0 240 60 -20 3.60 0.6560
WVFGRD96 22.0 240 60 -20 3.63 0.6633
WVFGRD96 24.0 240 60 -15 3.64 0.6704
WVFGRD96 26.0 240 65 -25 3.67 0.6762
WVFGRD96 28.0 240 60 -25 3.68 0.6863
WVFGRD96 30.0 240 60 -25 3.70 0.6936
WVFGRD96 32.0 240 60 -25 3.72 0.6930
WVFGRD96 34.0 240 60 -20 3.73 0.6875
WVFGRD96 36.0 240 55 -20 3.74 0.6828
WVFGRD96 38.0 245 60 -20 3.77 0.6802
WVFGRD96 40.0 240 50 -20 3.85 0.6893
WVFGRD96 42.0 240 50 -20 3.87 0.6910
WVFGRD96 44.0 240 50 -20 3.89 0.6892
WVFGRD96 46.0 240 50 -20 3.90 0.6855
WVFGRD96 48.0 240 50 -20 3.92 0.6805
WVFGRD96 50.0 240 50 -20 3.93 0.6753
WVFGRD96 52.0 240 50 -20 3.94 0.6695
WVFGRD96 54.0 240 50 -20 3.96 0.6609
WVFGRD96 56.0 240 50 -20 3.97 0.6540
WVFGRD96 58.0 240 50 -20 3.98 0.6474
WVFGRD96 60.0 240 50 -20 3.99 0.6402
WVFGRD96 62.0 245 55 -10 3.99 0.6344
WVFGRD96 64.0 245 55 -5 3.99 0.6289
WVFGRD96 66.0 245 55 -5 4.00 0.6245
WVFGRD96 68.0 245 55 0 4.01 0.6194
WVFGRD96 70.0 245 55 0 4.02 0.6167
WVFGRD96 72.0 245 60 0 4.02 0.6136
WVFGRD96 74.0 245 60 0 4.02 0.6115
WVFGRD96 76.0 245 60 0 4.03 0.6076
WVFGRD96 78.0 245 60 0 4.04 0.6018
WVFGRD96 80.0 245 60 -5 4.04 0.5967
WVFGRD96 82.0 245 65 -5 4.04 0.5878
WVFGRD96 84.0 245 65 -5 4.05 0.5769
WVFGRD96 86.0 240 65 -5 4.04 0.5527
WVFGRD96 88.0 245 70 -5 4.04 0.5249
WVFGRD96 90.0 245 75 -5 4.04 0.5100
WVFGRD96 92.0 245 75 -5 4.04 0.4967
WVFGRD96 94.0 245 80 -5 4.04 0.4830
WVFGRD96 96.0 245 80 -5 4.04 0.4632
WVFGRD96 98.0 85 50 30 4.05 0.4526
The best solution is
WVFGRD96 30.0 240 60 -25 3.70 0.6936
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 -30 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