Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak0193ycm3x7 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0193ycm3x7/executive.
2019/03/27 07:38:40 66.313 -157.214 8.9 4.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/03/27 07:38:40:0 66.31 -157.21 8.9 4.4 Alaska
Stations used:
AK.CAST AK.CHUM AK.COLD AK.CUT AK.GCSA AK.HDA AK.NEA2
AK.PPD AK.SKN AK.TNA AV.STLK IU.COLA TA.B20K TA.C17K
TA.C18K TA.C21K TA.D17K TA.D19K TA.D20K TA.D22K TA.D23K
TA.D24K TA.D25K TA.E18K TA.E19K TA.E20K TA.E22K TA.E23K
TA.E24K TA.E25K TA.F14K TA.F15K TA.F17K TA.F18K TA.F19K
TA.F20K TA.F21K TA.F24K TA.F25K TA.F26K TA.G15K TA.G16K
TA.G17K TA.G18K TA.G21K TA.G23K TA.G24K TA.G26K TA.H16K
TA.H17K TA.H18K TA.H19K TA.H20K TA.H21K TA.H22K TA.H24K
TA.I17K TA.I20K TA.I23K TA.J16K TA.J17K TA.J18K TA.J19K
TA.J20K TA.J25K TA.K17K TA.K20K TA.K24K TA.L16K TA.L17K
TA.L18K TA.L19K TA.L20K TA.M19K TA.M20K TA.N19K TA.POKR
TA.TOLK XV.F2TN XV.F6TP XV.F7TV XV.F8KN XV.FAPT XV.FNN1
XV.FNN2 XV.FPAP XV.FTGH
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 0.08 n 3
Best Fitting Double Couple
Mo = 1.91e+22 dyne-cm
Mw = 4.12
Z = 11 km
Plane Strike Dip Rake
NP1 170 85 -30
NP2 263 60 -174
Principal Axes:
Axis Value Plunge Azimuth
T 1.91e+22 17 220
N 0.00e+00 60 341
P -1.91e+22 24 122
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.67e+21
Mxy 1.57e+22
Mxz -2.13e+20
Myy -4.02e+21
Myz -9.49e+21
Mzz -1.65e+21
---###########
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-----------#################
------------##################
--------------####################
---------------#####################
----------------######################
--------------####---------------#######
---------#########--------------------##
------#############----------------------#
----################----------------------
--##################----------------------
####################----------------------
####################--------------------
####################------------ -----
###################------------ P ----
###################----------- ---
#### ###########----------------
## T ############-------------
# ############------------
##############--------
##########----
Global CMT Convention Moment Tensor:
R T P
-1.65e+21 -2.13e+20 9.49e+21
-2.13e+20 5.67e+21 -1.57e+22
9.49e+21 -1.57e+22 -4.02e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190327073840/index.html
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STK = 170
DIP = 85
RAKE = -30
MW = 4.12
HS = 11.0
The NDK file is 20190327073840.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 350 90 0 3.70 0.3614
WVFGRD96 2.0 350 85 -5 3.82 0.4596
WVFGRD96 3.0 350 90 15 3.88 0.4896
WVFGRD96 4.0 165 80 -40 3.96 0.5199
WVFGRD96 5.0 165 80 -40 3.99 0.5626
WVFGRD96 6.0 165 80 -35 4.01 0.5963
WVFGRD96 7.0 165 80 -35 4.03 0.6227
WVFGRD96 8.0 165 75 -40 4.09 0.6477
WVFGRD96 9.0 165 80 -35 4.09 0.6591
WVFGRD96 10.0 165 80 -35 4.11 0.6646
WVFGRD96 11.0 170 85 -30 4.12 0.6660
WVFGRD96 12.0 170 85 -30 4.13 0.6626
WVFGRD96 13.0 350 90 30 4.14 0.6535
WVFGRD96 14.0 170 85 -25 4.15 0.6453
WVFGRD96 15.0 170 90 -25 4.16 0.6341
WVFGRD96 16.0 170 90 -25 4.17 0.6210
WVFGRD96 17.0 350 90 25 4.18 0.6065
WVFGRD96 18.0 170 90 -25 4.19 0.5912
WVFGRD96 19.0 350 90 25 4.19 0.5754
WVFGRD96 20.0 350 90 25 4.20 0.5587
WVFGRD96 21.0 170 90 -25 4.21 0.5419
WVFGRD96 22.0 170 90 -25 4.21 0.5252
WVFGRD96 23.0 350 90 25 4.22 0.5084
WVFGRD96 24.0 350 90 25 4.22 0.4917
WVFGRD96 25.0 170 90 -25 4.22 0.4748
WVFGRD96 26.0 350 90 25 4.23 0.4578
WVFGRD96 27.0 350 85 25 4.23 0.4415
WVFGRD96 28.0 350 85 25 4.23 0.4259
WVFGRD96 29.0 350 85 25 4.23 0.4105
The best solution is
WVFGRD96 11.0 170 85 -30 4.12 0.6660
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 0.08 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