2011/11/04 17:17:22 37.969 -77.884 0.8 2.50 Virginia
USGS/SLU Moment Tensor Solution
ENS 2011/11/04 17:17:22:5 37.97 -77.88 0.8 2.5 Virginia
Stations used:
ET.UOM1 ET.UOM3 GS.CVRD YC.IP01 YC.IP03
Filtering commands used:
taper w 0.05
transfer from none to none freqlimtis 0.3 0.4 0.8 1.5
Best Fitting Double Couple
Mo = 4.62e+18 dyne-cm
Mw = 1.71
Z = 1 km
Plane Strike Dip Rake
NP1 162 51 124
NP2 295 50 55
Principal Axes:
Axis Value Plunge Azimuth
T 4.62e+18 64 138
N 0.00e+00 26 319
P -4.62e+18 1 229
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.51e+18
Mxy -2.73e+18
Mxz -1.32e+18
Myy -2.22e+18
Myz 1.27e+18
Mzz 3.73e+18
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--------#################-------------
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-----------#########################------
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------------############ ###########----
-----------############ T ############--
------------########### ############--
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- --------######################
P ----------##################
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Global CMT Convention Moment Tensor:
R T P
3.73e+18 -1.32e+18 -1.27e+18
-1.32e+18 -1.51e+18 2.73e+18
-1.27e+18 2.73e+18 -2.22e+18
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20111104171722/index.html
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STK = 295
DIP = 50
RAKE = 55
MW = 1.71
HS = 1.0
The NDK file is 20111104171722.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:
taper w 0.05 transfer from none to none freqlimtis 0.3 0.4 0.8 1.5The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 295 50 55 1.71 0.4948
WVFGRD96 2.0 340 55 55 1.85 0.4490
WVFGRD96 3.0 355 55 60 1.96 0.4630
WVFGRD96 4.0 265 75 15 2.11 0.4304
WVFGRD96 5.0 265 75 20 2.15 0.4176
WVFGRD96 6.0 265 75 30 2.15 0.3789
WVFGRD96 7.0 350 90 75 2.04 0.3524
WVFGRD96 8.0 330 40 -45 2.22 0.3480
WVFGRD96 9.0 100 60 60 2.26 0.3519
WVFGRD96 10.0 325 40 -55 2.32 0.3553
WVFGRD96 11.0 325 40 -55 2.34 0.3548
WVFGRD96 12.0 325 40 -55 2.35 0.3432
WVFGRD96 13.0 335 45 -50 2.36 0.3180
WVFGRD96 14.0 145 75 -60 2.28 0.2980
WVFGRD96 15.0 150 80 -50 2.29 0.2891
WVFGRD96 16.0 150 80 -55 2.29 0.2899
WVFGRD96 17.0 145 75 -50 2.35 0.2909
WVFGRD96 18.0 270 35 50 2.39 0.2738
WVFGRD96 19.0 275 35 55 2.39 0.2341
The best solution is
WVFGRD96 1.0 295 50 55 1.71 0.4948
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
taper w 0.05 transfer from none to none freqlimtis 0.3 0.4 0.8 1.5
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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. |
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