The ANSS event ID is nm605194 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nm605194/executive.
2003/06/06 12:29:33 36.879 -88.996 2.5 4 Kentucky
USGS/SLU Moment Tensor Solution
ENS 2003/06/06 12:29:33:0 36.88 -89.00 2.5 4.0 Kentucky
Stations used:
_.BLO _.CCM _.LRAL _.MPH _.OXF
_.PVMO _.SIUC _.SLM _.UALR _.USIN
_.UTMT _.WCI _.WVT
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 9.55e+21 dyne-cm
Mw = 3.92
Z = 2 km
Plane Strike Dip Rake
NP1 345 90 15
NP2 255 75 180
Principal Axes:
Axis Value Plunge Azimuth
T 9.55e+21 11 211
N 0.00e+00 75 345
P -9.55e+21 11 119
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.61e+21
Mxy 7.99e+21
Mxz -6.40e+20
Myy -4.61e+21
Myz -2.39e+21
Mzz -2.16e+14
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#####################------------- P -
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### T ############----------
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Global CMT Convention Moment Tensor:
R T P
-2.16e+14 -6.40e+20 2.39e+21
-6.40e+20 4.61e+21 -7.99e+21
2.39e+21 -7.99e+21 -4.61e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20030606122933/index.html
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STK = 345
DIP = 90
RAKE = 15
MW = 3.92
HS = 2.0
The NDK file is 20030606122933.ndk The waveform inversion is preferred.
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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 +30 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 1.0 345 80 15 3.89 0.5284
WVFGRD96 2.0 345 90 15 3.92 0.5315
WVFGRD96 3.0 345 90 25 3.95 0.5204
WVFGRD96 4.0 165 90 -25 3.96 0.5065
WVFGRD96 5.0 165 85 -30 3.98 0.4939
WVFGRD96 6.0 165 85 -30 3.98 0.4825
WVFGRD96 7.0 165 90 -25 3.97 0.4722
WVFGRD96 8.0 340 85 -30 3.96 0.4633
WVFGRD96 9.0 340 85 -30 3.96 0.4570
WVFGRD96 10.0 340 85 -30 3.98 0.4505
WVFGRD96 11.0 340 85 -30 3.98 0.4442
WVFGRD96 12.0 335 80 -30 3.97 0.4383
WVFGRD96 13.0 340 85 -25 3.99 0.4326
WVFGRD96 14.0 340 85 -25 3.99 0.4272
WVFGRD96 15.0 340 85 -25 3.99 0.4217
WVFGRD96 16.0 340 85 -25 4.00 0.4166
WVFGRD96 17.0 160 90 25 4.00 0.4103
WVFGRD96 18.0 160 90 25 4.00 0.4059
WVFGRD96 19.0 160 90 25 4.01 0.4021
WVFGRD96 20.0 160 90 25 4.02 0.3986
WVFGRD96 21.0 160 90 25 4.03 0.3957
WVFGRD96 22.0 160 90 25 4.03 0.3927
WVFGRD96 23.0 340 90 -25 4.04 0.3901
WVFGRD96 24.0 160 90 25 4.05 0.3875
WVFGRD96 25.0 160 90 25 4.05 0.3847
WVFGRD96 26.0 160 90 25 4.06 0.3815
WVFGRD96 27.0 340 90 -25 4.06 0.3778
WVFGRD96 28.0 160 90 25 4.07 0.3736
WVFGRD96 29.0 160 90 25 4.07 0.3687
The best solution is
WVFGRD96 2.0 345 90 15 3.92 0.5315
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 +30 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 following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 164.99
DIP= 85.00
RAKE= 14.99
OR
STK= 73.65
DIP= 75.06
RAKE= 174.82
DEPTH = 2.0 km
Mw = 4.02
Best Fit - P-T axis plot gives solutions with FIT greater than FIT90
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Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.
Digital data were collected, instrument response removed and traces converted
to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.
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| Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. 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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled. |
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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
