The ANSS event ID is us1000bsvk and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us1000bsvk/executive.
2017/12/19 00:10:15 59.841 -136.733 10.0 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/12/19 00:10:15:0 59.84 -136.73 10.0 4.0 Alaska
Stations used:
AK.BARN AK.BCP AK.CTG AK.GLB AK.HMT AK.ISLE AK.JIS AK.LOGN
AK.MCAR AK.MESA AK.PIN AK.SSP AK.TABL AK.VRDI AK.WAX AT.SIT
AT.SKAG AT.YKU2 CN.DLBC CN.HYT CN.WHY NY.FARO NY.MAYO
NY.WTLY TA.M30M TA.M31M TA.N31M TA.O30N TA.P32M TA.P33M
TA.R32K TA.R33M TA.S31K TA.S34M
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 = 1.26e+22 dyne-cm
Mw = 4.00
Z = 10 km
Plane Strike Dip Rake
NP1 155 60 90
NP2 335 30 90
Principal Axes:
Axis Value Plunge Azimuth
T 1.26e+22 75 65
N 0.00e+00 -0 335
P -1.26e+22 15 245
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.95e+21
Mxy -4.18e+21
Mxz 2.66e+21
Myy -8.96e+21
Myz 5.70e+21
Mzz 1.09e+22
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--###########---------
----###############---------
-----##################-------
-------###################--------
--------#####################-------
---------######################-------
----------#######################-------
----------########################------
------------########## ###########------
------------########## T ###########------
-------------######### ###########------
--------------######################------
-- ---------#####################-----
-- P ----------####################-----
- -----------###################----
---------------#################----
----------------###############---
----------------############--
-----------------#########--
-----------------####-
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Global CMT Convention Moment Tensor:
R T P
1.09e+22 2.66e+21 -5.70e+21
2.66e+21 -1.95e+21 4.18e+21
-5.70e+21 4.18e+21 -8.96e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171219001015/index.html
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STK = 335
DIP = 30
RAKE = 90
MW = 4.00
HS = 10.0
The NDK file is 20171219001015.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 1.0 335 45 85 3.64 0.3224
WVFGRD96 2.0 335 45 85 3.80 0.4812
WVFGRD96 3.0 130 55 40 3.79 0.4343
WVFGRD96 4.0 295 40 5 3.83 0.4596
WVFGRD96 5.0 290 25 30 3.91 0.5122
WVFGRD96 6.0 315 25 65 3.93 0.5575
WVFGRD96 7.0 325 25 80 3.94 0.5917
WVFGRD96 8.0 330 25 85 4.00 0.6168
WVFGRD96 9.0 330 25 85 4.00 0.6279
WVFGRD96 10.0 335 30 90 4.00 0.6334
WVFGRD96 11.0 335 30 90 4.00 0.6312
WVFGRD96 12.0 335 30 90 3.99 0.6250
WVFGRD96 13.0 310 35 55 3.99 0.6187
WVFGRD96 14.0 305 40 45 3.99 0.6156
WVFGRD96 15.0 300 45 35 3.99 0.6114
WVFGRD96 16.0 280 55 -30 3.99 0.6052
WVFGRD96 17.0 300 45 35 4.01 0.6001
WVFGRD96 18.0 300 45 35 4.01 0.5925
WVFGRD96 19.0 295 50 30 4.03 0.5841
WVFGRD96 20.0 295 50 30 4.03 0.5741
WVFGRD96 21.0 300 50 35 4.05 0.5641
WVFGRD96 22.0 300 50 35 4.06 0.5516
WVFGRD96 23.0 300 50 35 4.06 0.5378
WVFGRD96 24.0 125 55 40 4.06 0.5264
WVFGRD96 25.0 125 50 40 4.07 0.5136
WVFGRD96 26.0 125 50 40 4.07 0.4999
WVFGRD96 27.0 125 50 40 4.08 0.4856
WVFGRD96 28.0 125 50 40 4.08 0.4707
WVFGRD96 29.0 125 50 40 4.09 0.4561
The best solution is
WVFGRD96 10.0 335 30 90 4.00 0.6334
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