2002/06/18 17:37:17 37.983 -87.795 17.0 4.9 Indiana
The program elocate was used to locate this event. The source depth was fixed at 17 km, which is the depth determined from the waveform inversion. Waveforms were downloaded using the IRIS FetchData mechanism on 11/27/2013. The following are noted: a) none of the PP stations in Indiana were used because of calibration and timing differences; b) I flipped the BLO waveform because of original problems with polarity and wiring.
The phase picks and location are given in the file elocate.txt. The computed take-off angles are used in the comparison of the P-wave first motions to the waveform focal mechanism.
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution ENS 2002/06/18 17:37:17:0 37.98 -87.79 17.0 4.9 Indiana Stations used: IU.CCM IU.WCI IU.WVT NM.BLO NM.MPH NM.PLAL NM.SIUC NM.SLM NM.UALR NM.UTMT SP.DWDAN US.ACSO US.BLA US.GOGA US.JFWS US.MIAR US.MYNC US.OXF Filtering commands used: cut a -30 a 180 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 6.84e+22 dyne-cm Mw = 4.49 Z = 17 km Plane Strike Dip Rake NP1 125 90 10 NP2 35 80 180 Principal Axes: Axis Value Plunge Azimuth T 6.84e+22 7 350 N 0.00e+00 80 125 P -6.84e+22 7 260 Moment Tensor: (dyne-cm) Component Value Mxx 6.33e+22 Mxy -2.30e+22 Mxz 9.73e+21 Myy -6.33e+22 Myz 6.81e+21 Mzz -1.04e+15 ## T ######### ###### ############# ##########################-- ##########################---- ###########################------- ---########################--------- -------####################----------- -----------################------------- -------------#############-------------- -----------------#########---------------- --------------------#####----------------- -------------------#------------------- P -------------------###----------------- ------------------#######------------- ------------------###########----------- ----------------###############------- -------------####################--- -----------####################### -------####################### ----######################## ###################### ############## Global CMT Convention Moment Tensor: R T P -1.04e+15 9.73e+21 -6.81e+21 9.73e+21 6.33e+22 2.30e+22 -6.81e+21 2.30e+22 -6.33e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20020618173717/index.html |
STK = 125 DIP = 90 RAKE = 10 MW = 4.49 HS = 17.0
The NDK file is 20020618173717.ndk The waveform inversion is preferred.
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution ENS 2002/06/18 17:37:17:0 37.98 -87.79 17.0 4.9 Indiana Stations used: IU.CCM IU.WCI IU.WVT NM.BLO NM.MPH NM.PLAL NM.SIUC NM.SLM NM.UALR NM.UTMT SP.DWDAN US.ACSO US.BLA US.GOGA US.JFWS US.MIAR US.MYNC US.OXF Filtering commands used: cut a -30 a 180 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 6.84e+22 dyne-cm Mw = 4.49 Z = 17 km Plane Strike Dip Rake NP1 125 90 10 NP2 35 80 180 Principal Axes: Axis Value Plunge Azimuth T 6.84e+22 7 350 N 0.00e+00 80 125 P -6.84e+22 7 260 Moment Tensor: (dyne-cm) Component Value Mxx 6.33e+22 Mxy -2.30e+22 Mxz 9.73e+21 Myy -6.33e+22 Myz 6.81e+21 Mzz -1.04e+15 ## T ######### ###### ############# ##########################-- ##########################---- ###########################------- ---########################--------- -------####################----------- -----------################------------- -------------#############-------------- -----------------#########---------------- --------------------#####----------------- -------------------#------------------- P -------------------###----------------- ------------------#######------------- ------------------###########----------- ----------------###############------- -------------####################--- -----------####################### -------####################### ----######################## ###################### ############## Global CMT Convention Moment Tensor: R T P -1.04e+15 9.73e+21 -6.81e+21 9.73e+21 6.33e+22 2.30e+22 -6.81e+21 2.30e+22 -6.33e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20020618173717/index.html |
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The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for 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 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 a -30 a 180 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 290 65 -25 4.18 0.3630 WVFGRD96 1.0 290 70 -25 4.19 0.3842 WVFGRD96 2.0 115 90 5 4.19 0.4167 WVFGRD96 3.0 300 85 5 4.23 0.4253 WVFGRD96 4.0 295 75 -15 4.24 0.4267 WVFGRD96 5.0 295 70 -15 4.26 0.4424 WVFGRD96 6.0 295 70 -15 4.27 0.4665 WVFGRD96 7.0 295 75 -15 4.29 0.4931 WVFGRD96 8.0 295 75 -15 4.31 0.5195 WVFGRD96 9.0 300 75 -10 4.35 0.5505 WVFGRD96 10.0 305 80 -10 4.39 0.5825 WVFGRD96 11.0 305 80 -10 4.41 0.6105 WVFGRD96 12.0 305 80 -10 4.42 0.6324 WVFGRD96 13.0 305 85 -10 4.44 0.6502 WVFGRD96 14.0 305 85 -10 4.46 0.6642 WVFGRD96 15.0 305 85 -10 4.47 0.6718 WVFGRD96 16.0 125 90 10 4.48 0.6737 WVFGRD96 17.0 305 85 -10 4.50 0.6754 WVFGRD96 18.0 305 85 -10 4.51 0.6716 WVFGRD96 19.0 125 90 10 4.52 0.6644 WVFGRD96 20.0 305 85 -10 4.54 0.6581 WVFGRD96 21.0 305 85 -10 4.55 0.6456 WVFGRD96 22.0 305 85 -10 4.55 0.6338 WVFGRD96 23.0 305 85 -10 4.56 0.6182 WVFGRD96 24.0 305 85 -10 4.57 0.5996 WVFGRD96 25.0 305 85 -10 4.57 0.5817 WVFGRD96 26.0 300 80 -5 4.57 0.5624 WVFGRD96 27.0 300 80 -5 4.57 0.5442 WVFGRD96 28.0 300 80 -5 4.58 0.5261 WVFGRD96 29.0 300 80 -5 4.58 0.5073
The best solution is
WVFGRD96 17.0 305 85 -10 4.50 0.6754
The mechanism correspond 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 and because the velocity model used in the predictions may not be perfect. 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 a -30 a 180 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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= 30.00 DIP= 84.99 RAKE= -170.00 OR STK= 299.12 DIP= 80.04 RAKE= -5.08 DEPTH = 19.0 km Mw = 4.58 Best Fit 0.8834 - P-T axis plot gives solutions with FIT greater than FIT90
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az Dist First motion
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. |
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. |
The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used
Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.
The fits to the waveforms with the given mechanism are show below:
This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:
hp c 0.02 n 3 lp c 0.10 n 3
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.
The CUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
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
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: