The ANSS event ID is ak018aaq66sw and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018aaq66sw/executive.
2018/08/12 16:44:40 69.471 -144.593 2.6 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/08/12 16:44:40:0 69.47 -144.59 2.6 4.1 Alaska
Stations used:
AK.FYU TA.D24K TA.D25K TA.D27M TA.E28M TA.E29M TA.F24K
TA.G23K TA.G24K TA.G27K TA.TOLK
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 0.07 n 3
Best Fitting Double Couple
Mo = 2.04e+22 dyne-cm
Mw = 4.14
Z = 13 km
Plane Strike Dip Rake
NP1 185 90 -5
NP2 275 85 -180
Principal Axes:
Axis Value Plunge Azimuth
T 2.04e+22 4 230
N 0.00e+00 85 5
P -2.04e+22 4 140
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.53e+21
Mxy 2.00e+22
Mxz 1.55e+20
Myy 3.53e+21
Myz -1.77e+21
Mzz 1.56e+14
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# ############------------------
T ############------------ -
############------------ P
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Global CMT Convention Moment Tensor:
R T P
1.56e+14 1.55e+20 1.77e+21
1.55e+20 -3.53e+21 -2.00e+22
1.77e+21 -2.00e+22 3.53e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180812164440/index.html
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STK = 185
DIP = 90
RAKE = -5
MW = 4.14
HS = 13.0
The NDK file is 20180812164440.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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 190 70 30 3.77 0.2727
WVFGRD96 2.0 195 65 40 3.93 0.3848
WVFGRD96 3.0 185 80 15 3.89 0.4266
WVFGRD96 4.0 5 90 -10 3.93 0.4653
WVFGRD96 5.0 5 90 -10 3.96 0.5003
WVFGRD96 6.0 5 90 -5 3.99 0.5300
WVFGRD96 7.0 5 90 -5 4.02 0.5575
WVFGRD96 8.0 5 90 -5 4.05 0.5841
WVFGRD96 9.0 5 90 -5 4.08 0.5966
WVFGRD96 10.0 5 90 -10 4.09 0.6061
WVFGRD96 11.0 185 85 -10 4.12 0.6134
WVFGRD96 12.0 185 85 -10 4.13 0.6193
WVFGRD96 13.0 185 90 -5 4.14 0.6223
WVFGRD96 14.0 185 90 -5 4.16 0.6221
WVFGRD96 15.0 185 90 -5 4.17 0.6198
WVFGRD96 16.0 185 90 -10 4.18 0.6148
WVFGRD96 17.0 185 90 -10 4.19 0.6076
WVFGRD96 18.0 185 90 -10 4.20 0.5993
WVFGRD96 19.0 5 75 5 4.22 0.5925
WVFGRD96 20.0 5 70 5 4.23 0.5827
WVFGRD96 21.0 10 25 25 4.38 0.5750
WVFGRD96 22.0 10 25 25 4.39 0.5717
WVFGRD96 23.0 10 25 25 4.40 0.5670
WVFGRD96 24.0 10 25 25 4.41 0.5607
WVFGRD96 25.0 10 25 25 4.41 0.5535
WVFGRD96 26.0 10 25 25 4.42 0.5454
WVFGRD96 27.0 5 25 15 4.43 0.5361
WVFGRD96 28.0 5 25 15 4.43 0.5262
WVFGRD96 29.0 365 25 20 4.44 0.5162
The best solution is
WVFGRD96 13.0 185 90 -5 4.14 0.6223
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 0.07 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 CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00