The ANSS event ID is ak020idodhm and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020idodhm/executive.
2020/01/11 11:36:33 63.204 -151.415 14.6 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/01/11 11:36:33:0 63.20 -151.41 14.6 3.9 Alaska
Stations used:
AK.BPAW AK.CCB AK.CUT AK.DHY AK.DIV AK.DOT AK.EYAK AK.GHO
AK.GLB AK.GLI AK.HDA AK.HIN AK.I23K AK.J20K AK.K24K AK.KNK
AK.KTH AK.L18K AK.L19K AK.M20K AK.MCK AK.N18K AK.N19K
AK.O18K AK.PAX AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM
AK.SCRK AK.SKN AK.SLK AK.SSN AK.SWD AK.TRF AK.WRH AT.PMR
AV.ILSW IM.IL31 IU.COLA TA.G23K TA.H18K TA.M22K TA.M24K
TA.P19K TA.POKR
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 = 6.76e+21 dyne-cm
Mw = 3.82
Z = 14 km
Plane Strike Dip Rake
NP1 60 55 120
NP2 195 45 55
Principal Axes:
Axis Value Plunge Azimuth
T 6.76e+21 65 28
N 0.00e+00 24 221
P -6.76e+21 5 129
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.74e+21
Mxy 3.76e+21
Mxz 2.65e+21
Myy -3.80e+21
Myz 7.10e+20
Mzz 5.54e+21
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----------########### T ############----
----------############ ###########------
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-------##########--------------- -
######------------------------- P
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Global CMT Convention Moment Tensor:
R T P
5.54e+21 2.65e+21 -7.10e+20
2.65e+21 -1.74e+21 -3.76e+21
-7.10e+20 -3.76e+21 -3.80e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200111113633/index.html
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STK = 195
DIP = 45
RAKE = 55
MW = 3.82
HS = 14.0
The NDK file is 20200111113633.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 1.0 120 45 -90 3.37 0.3181
WVFGRD96 2.0 295 45 -90 3.55 0.4856
WVFGRD96 3.0 170 60 -25 3.56 0.4195
WVFGRD96 4.0 180 35 0 3.65 0.4493
WVFGRD96 5.0 185 30 10 3.69 0.5370
WVFGRD96 6.0 190 30 10 3.70 0.6025
WVFGRD96 7.0 195 35 25 3.71 0.6408
WVFGRD96 8.0 190 25 5 3.75 0.6532
WVFGRD96 9.0 200 35 45 3.79 0.6724
WVFGRD96 10.0 200 40 55 3.81 0.7018
WVFGRD96 11.0 200 40 60 3.81 0.7240
WVFGRD96 12.0 195 45 55 3.81 0.7381
WVFGRD96 13.0 195 45 55 3.82 0.7461
WVFGRD96 14.0 195 45 55 3.82 0.7491
WVFGRD96 15.0 195 45 55 3.83 0.7488
WVFGRD96 16.0 190 50 50 3.83 0.7467
WVFGRD96 17.0 190 50 50 3.84 0.7429
WVFGRD96 18.0 190 50 50 3.84 0.7372
WVFGRD96 19.0 190 50 50 3.85 0.7296
WVFGRD96 20.0 190 50 50 3.86 0.7204
WVFGRD96 21.0 190 50 50 3.87 0.7084
WVFGRD96 22.0 190 50 50 3.88 0.6961
WVFGRD96 23.0 190 50 50 3.89 0.6817
WVFGRD96 24.0 190 50 50 3.90 0.6654
WVFGRD96 25.0 190 50 50 3.90 0.6477
WVFGRD96 26.0 190 50 50 3.91 0.6290
WVFGRD96 27.0 190 50 50 3.91 0.6096
WVFGRD96 28.0 190 50 45 3.92 0.5899
WVFGRD96 29.0 190 50 45 3.92 0.5696
The best solution is
WVFGRD96 14.0 195 45 55 3.82 0.7491
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