The ANSS event ID is ak0192mc2j6m and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0192mc2j6m/executive.
2019/02/26 04:09:41 66.272 -157.226 13.1 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/02/26 04:09:41:0 66.27 -157.23 13.1 4.0 Alaska
Stations used:
AK.ANM AK.BPAW AK.CAST AK.COLD AK.KTH AK.NEA2 AK.RDOG
AT.TTA TA.B20K TA.B21K TA.C16K TA.C18K TA.D19K TA.D22K
TA.D23K TA.E18K TA.E19K TA.E21K TA.E22K TA.E23K TA.E24K
TA.F15K TA.F17K TA.F19K TA.F20K TA.F21K TA.F24K TA.G16K
TA.G18K TA.G19K TA.G21K TA.G23K TA.G24K TA.H17K TA.H18K
TA.H19K TA.H21K TA.H24K TA.I17K TA.I20K TA.I21K TA.I23K
TA.J16K TA.J17K TA.J18K TA.J19K TA.J20K TA.K17K TA.K20K
TA.TOLK
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +60
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 = 1.14e+22 dyne-cm
Mw = 3.97
Z = 8 km
Plane Strike Dip Rake
NP1 350 85 15
NP2 259 75 175
Principal Axes:
Axis Value Plunge Azimuth
T 1.14e+22 14 215
N 0.00e+00 74 8
P -1.14e+22 7 123
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.72e+21
Mxy 1.02e+22
Mxz -1.44e+21
Myy -4.23e+21
Myz -2.68e+21
Mzz 5.10e+20
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######################------------ -
#####################------------ P
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## T #############------------
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Global CMT Convention Moment Tensor:
R T P
5.10e+20 -1.44e+21 2.68e+21
-1.44e+21 3.72e+21 -1.02e+22
2.68e+21 -1.02e+22 -4.23e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190226040941/index.html
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STK = 350
DIP = 85
RAKE = 15
MW = 3.97
HS = 8.0
The NDK file is 20190226040941.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 +60 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 1.0 175 85 0 3.56 0.3566
WVFGRD96 2.0 350 90 0 3.74 0.6077
WVFGRD96 3.0 170 90 0 3.81 0.7183
WVFGRD96 4.0 170 90 0 3.85 0.7817
WVFGRD96 5.0 350 85 -5 3.89 0.8155
WVFGRD96 6.0 170 90 -5 3.91 0.8299
WVFGRD96 7.0 350 85 10 3.94 0.8404
WVFGRD96 8.0 350 85 15 3.97 0.8462
WVFGRD96 9.0 170 90 -15 3.99 0.8437
WVFGRD96 10.0 170 90 -15 4.00 0.8393
WVFGRD96 11.0 170 90 -15 4.01 0.8358
WVFGRD96 12.0 170 90 -15 4.03 0.8294
WVFGRD96 13.0 350 90 15 4.04 0.8235
WVFGRD96 14.0 350 90 15 4.05 0.8152
WVFGRD96 15.0 170 90 -10 4.06 0.8075
WVFGRD96 16.0 350 90 10 4.07 0.7995
WVFGRD96 17.0 170 85 -5 4.08 0.7909
WVFGRD96 18.0 350 90 10 4.09 0.7795
WVFGRD96 19.0 170 85 5 4.09 0.7726
WVFGRD96 20.0 170 85 5 4.10 0.7622
WVFGRD96 21.0 170 85 5 4.11 0.7504
WVFGRD96 22.0 170 85 5 4.12 0.7382
WVFGRD96 23.0 170 80 10 4.12 0.7264
WVFGRD96 24.0 170 80 10 4.13 0.7145
WVFGRD96 25.0 170 80 10 4.13 0.7022
WVFGRD96 26.0 170 80 10 4.14 0.6897
WVFGRD96 27.0 170 80 10 4.15 0.6776
WVFGRD96 28.0 170 80 10 4.15 0.6666
WVFGRD96 29.0 170 80 10 4.16 0.6564
The best solution is
WVFGRD96 8.0 350 85 15 3.97 0.8462
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 +60 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