2010/10/14 10:07:34 35.299 -92.350 4.6 3.50 Arkansas
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution ENS 2010/10/14 10:07:34:8 35.30 -92.35 4.6 3.5 Arkansas Stations used: AG.FCAR AG.LCAR AG.WLAR NM.MGMO NM.UALR TA.S35A TA.S36A TA.T35A TA.T37A TA.V34A TA.W36A TA.W37A TA.W38A TA.X37A TA.X38A TA.Y36A TA.Y38A TA.Y39A TA.Z38A Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.53e+21 dyne-cm Mw = 3.39 Z = 4 km Plane Strike Dip Rake NP1 115 90 -5 NP2 205 85 -180 Principal Axes: Axis Value Plunge Azimuth T 1.53e+21 4 160 N 0.00e+00 85 295 P -1.53e+21 4 70 Moment Tensor: (dyne-cm) Component Value Mxx 1.17e+21 Mxy -9.80e+20 Mxz -1.21e+20 Myy -1.17e+21 Myz -5.64e+19 Mzz 1.17e+13 ############## ###################--- #####################------- #####################--------- ######################------------ ######################-------------- -#####################-------------- ------################--------------- P ----------###########---------------- ---------------######--------------------- -------------------#---------------------- -------------------####------------------- ------------------#########--------------- -----------------#############---------- ----------------##################------ --------------#######################- ------------######################## ----------######################## --------###################### ------###################### ---############# ### ############ T Global CMT Convention Moment Tensor: R T P 1.17e+13 -1.21e+20 5.64e+19 -1.21e+20 1.17e+21 9.80e+20 5.64e+19 9.80e+20 -1.17e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101014100734/index.html |
STK = 115 DIP = 90 RAKE = -5 MW = 3.39 HS = 4.0
This is a marginal event because of the low S/N for the Z and R componets. The depth, mechanism and moment agree with others in the sequence. So this is acceptable.
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution ENS 2010/10/14 10:07:34:8 35.30 -92.35 4.6 3.5 Arkansas Stations used: AG.FCAR AG.LCAR AG.WLAR NM.MGMO NM.UALR TA.S35A TA.S36A TA.T35A TA.T37A TA.V34A TA.W36A TA.W37A TA.W38A TA.X37A TA.X38A TA.Y36A TA.Y38A TA.Y39A TA.Z38A Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.53e+21 dyne-cm Mw = 3.39 Z = 4 km Plane Strike Dip Rake NP1 115 90 -5 NP2 205 85 -180 Principal Axes: Axis Value Plunge Azimuth T 1.53e+21 4 160 N 0.00e+00 85 295 P -1.53e+21 4 70 Moment Tensor: (dyne-cm) Component Value Mxx 1.17e+21 Mxy -9.80e+20 Mxz -1.21e+20 Myy -1.17e+21 Myz -5.64e+19 Mzz 1.17e+13 ############## ###################--- #####################------- #####################--------- ######################------------ ######################-------------- -#####################-------------- ------################--------------- P ----------###########---------------- ---------------######--------------------- -------------------#---------------------- -------------------####------------------- ------------------#########--------------- -----------------#############---------- ----------------##################------ --------------#######################- ------------######################## ----------######################## --------###################### ------###################### ---############# ### ############ T Global CMT Convention Moment Tensor: R T P 1.17e+13 -1.21e+20 5.64e+19 -1.21e+20 1.17e+21 9.80e+20 5.64e+19 9.80e+20 -1.17e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101014100734/index.html |
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:
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 0.5 110 70 -10 3.30 0.2797 WVFGRD96 1.0 110 65 -10 3.34 0.2964 WVFGRD96 2.0 110 70 -15 3.37 0.3177 WVFGRD96 3.0 115 85 -5 3.38 0.3263 WVFGRD96 4.0 115 90 -5 3.39 0.3266 WVFGRD96 5.0 115 90 -5 3.40 0.3239 WVFGRD96 6.0 115 90 -5 3.41 0.3206 WVFGRD96 7.0 115 90 -5 3.42 0.3169 WVFGRD96 8.0 110 80 -10 3.42 0.3140 WVFGRD96 9.0 110 80 -10 3.43 0.3119 WVFGRD96 10.0 115 65 10 3.46 0.3113 WVFGRD96 11.0 115 65 10 3.47 0.3101 WVFGRD96 12.0 115 65 10 3.48 0.3079 WVFGRD96 13.0 115 65 10 3.48 0.3048 WVFGRD96 14.0 115 65 10 3.49 0.3020 WVFGRD96 15.0 115 70 10 3.50 0.2990 WVFGRD96 16.0 115 70 10 3.50 0.2959 WVFGRD96 17.0 115 70 10 3.51 0.2921 WVFGRD96 18.0 115 70 10 3.52 0.2885 WVFGRD96 19.0 115 65 10 3.53 0.2854 WVFGRD96 20.0 115 65 10 3.54 0.2828 WVFGRD96 21.0 115 65 10 3.55 0.2805 WVFGRD96 22.0 110 50 -15 3.59 0.2784 WVFGRD96 23.0 110 50 -15 3.60 0.2774 WVFGRD96 24.0 115 50 -5 3.61 0.2768 WVFGRD96 25.0 115 50 -5 3.62 0.2767 WVFGRD96 26.0 115 50 -5 3.62 0.2767 WVFGRD96 27.0 115 50 -5 3.63 0.2763 WVFGRD96 28.0 115 50 -5 3.64 0.2761 WVFGRD96 29.0 115 50 -5 3.64 0.2767
The best solution is
WVFGRD96 4.0 115 90 -5 3.39 0.3266
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
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.
Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.
Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, 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:
DATE=Thu Oct 14 10:41:20 CDT 2010