2011/08/23 17:51:05 37.936 77.933 6 5.80 Virginia
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution ENS 2011/08/23 17:51:05:0 37.94 77.93 6.0 5.8 Virginia Stations used: IU.HRV IU.SSPA IU.WCI IU.WVT LD.ACCN LD.FRNY LD.WVNY NM.BLO NM.OLIL NM.USIN NM.UTMT PE.PSUB TA.Q44A TA.R44A TA.S44A TA.S45A TA.SFIN TA.TIGA TA.Y47A US.AAM US.ACSO US.BINY US.BLA US.CNNC US.ERPA US.GLMI US.GOGA US.LBNH US.LONY US.LRAL US.MCWV US.TZTN Filtering commands used: hp c 0.01 n 3 lp c 0.03 n 3 Best Fitting Double Couple Mo = 3.76e+24 dynecm Mw = 5.65 Z = 6 km Plane Strike Dip Rake NP1 28 50 113 NP2 175 45 65 Principal Axes: Axis Value Plunge Azimuth T 3.76e+24 72 4 N 0.00e+00 17 193 P 3.76e+24 3 102 Moment Tensor: (dynecm) Component Value Mxx 1.69e+23 Mxy 8.10e+23 Mxz 1.12e+24 Myy 3.58e+24 Myz 9.79e+22 Mzz 3.41e+24 ########### ################ ################## #################### ##################### ###################### ###################### ########## ########## ########## T ########## ########## ########## ###################### ###################### ##################### #################### P ################## ################# ############## ########### ######## ### ### ###### Global CMT Convention Moment Tensor: R T P 3.41e+24 1.12e+24 9.79e+22 1.12e+24 1.69e+23 8.10e+23 9.79e+22 8.10e+23 3.58e+24 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110823175105/index.html 
STK = 175 DIP = 45 RAKE = 65 MW = 5.65 HS = 6.0
The NDK file is 20110823175105.ndk The waveform inversion is preferred.
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution ENS 2011/08/23 17:51:05:0 37.94 77.93 6.0 5.8 Virginia Stations used: IU.HRV IU.SSPA IU.WCI IU.WVT LD.ACCN LD.FRNY LD.WVNY NM.BLO NM.OLIL NM.USIN NM.UTMT PE.PSUB TA.Q44A TA.R44A TA.S44A TA.S45A TA.SFIN TA.TIGA TA.Y47A US.AAM US.ACSO US.BINY US.BLA US.CNNC US.ERPA US.GLMI US.GOGA US.LBNH US.LONY US.LRAL US.MCWV US.TZTN Filtering commands used: hp c 0.01 n 3 lp c 0.03 n 3 Best Fitting Double Couple Mo = 3.76e+24 dynecm Mw = 5.65 Z = 6 km Plane Strike Dip Rake NP1 28 50 113 NP2 175 45 65 Principal Axes: Axis Value Plunge Azimuth T 3.76e+24 72 4 N 0.00e+00 17 193 P 3.76e+24 3 102 Moment Tensor: (dynecm) Component Value Mxx 1.69e+23 Mxy 8.10e+23 Mxz 1.12e+24 Myy 3.58e+24 Myz 9.79e+22 Mzz 3.41e+24 ########### ################ ################## #################### ##################### ###################### ###################### ########## ########## ########## T ########## ########## ########## ###################### ###################### ##################### #################### P ################## ################# ############## ########### ######## ### ### ###### Global CMT Convention Moment Tensor: R T P 3.41e+24 1.12e+24 9.79e+22 1.12e+24 1.69e+23 8.10e+23 9.79e+22 8.10e+23 3.58e+24 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110823175105/index.html 

August 23, 2011, VIRGINIA, MW=5.8 Meredith Nettles Howard Koss CENTROIDMOMENTTENSOR SOLUTION GCMT EVENT: C201108231751A DATA: II DK IU CU MN G GE LD L.P.BODY WAVES: 65S, 107C, T= 40 MANTLE WAVES: 18S, 19C, T=125 SURFACE WAVES: 82S, 142C, T= 50 TIMESTAMP: Q20110823145453 CENTROID LOCATION: ORIGIN TIME: 17:51:07.3 0.2 LAT:37.84N 0.01;LON: 77.96W 0.01 DEP: 12.0 FIX;TRIANG HDUR: 1.9 MOMENT TENSOR: SCALE 10**24 DCM RR= 5.200 0.084; TT= 0.326 0.084 PP=5.520 0.089; RT= 2.080 0.222 RP=0.542 0.274; TP=0.880 0.079 PRINCIPAL AXES: 1.(T) VAL= 6.025;PLG=69;AZM= 11 2.(N) 0.364; 21; 187 3.(P) 5.655; 1; 278 BEST DBLE.COUPLE:M0= 5.84*10**24 NP1: STRIKE= 28;DIP=47;SLIP= 119 NP2: STRIKE=169;DIP=50;SLIP= 62 ######### ############## ################ ################# ################# ######## ######## ######## T ######## P ######## ####### ################## ################ ################ ############### ############ ########## ####### ### ### ####### 
(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.
(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.
(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.
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.

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 0.01 n 3 lp c 0.03 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 155 40 20 5.56 0.6533 WVFGRD96 1.0 160 30 25 5.62 0.6720 WVFGRD96 2.0 160 45 35 5.56 0.7194 WVFGRD96 3.0 165 45 45 5.59 0.7637 WVFGRD96 4.0 170 45 55 5.61 0.7957 WVFGRD96 5.0 170 45 55 5.63 0.8177 WVFGRD96 6.0 175 45 65 5.65 0.8220 WVFGRD96 7.0 175 45 65 5.65 0.8081 WVFGRD96 8.0 175 45 65 5.65 0.7749 WVFGRD96 9.0 175 45 65 5.65 0.7317 WVFGRD96 10.0 170 45 55 5.64 0.7134 WVFGRD96 11.0 165 50 50 5.62 0.6680 WVFGRD96 12.0 155 60 30 5.57 0.6338 WVFGRD96 13.0 155 60 25 5.56 0.6107 WVFGRD96 14.0 150 65 15 5.55 0.5963 WVFGRD96 15.0 145 65 15 5.55 0.5953 WVFGRD96 16.0 145 65 15 5.56 0.5979 WVFGRD96 17.0 145 70 20 5.56 0.6008 WVFGRD96 18.0 145 70 20 5.57 0.6035 WVFGRD96 19.0 325 70 15 5.57 0.5988 WVFGRD96 20.0 145 70 20 5.58 0.6002 WVFGRD96 21.0 145 70 20 5.58 0.6017 WVFGRD96 22.0 145 70 20 5.59 0.6031 WVFGRD96 23.0 325 70 20 5.60 0.5977 WVFGRD96 24.0 145 70 20 5.60 0.6043 WVFGRD96 25.0 145 70 20 5.60 0.6042 WVFGRD96 26.0 145 70 20 5.61 0.6035 WVFGRD96 27.0 145 70 20 5.61 0.6027 WVFGRD96 28.0 145 70 20 5.62 0.6016 WVFGRD96 29.0 145 70 20 5.62 0.6001
The best solution is
WVFGRD96 6.0 175 45 65 5.65 0.8220
The mechanism correspond to the best fit is

The best fit as a function of depth is given in the following figure:

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 observedpredicted 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 0.01 n 3 lp c 0.03 n 3

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 surfacewave spectral amplitudes of the Love and Rayleigh waves.

The surfacewave determined focal mechanism is shown here.
NODAL PLANES STK= 49.64 DIP= 67.48 RAKE= 135.90 OR STK= 159.99 DIP= 50.00 RAKE= 30.00 DEPTH = 7.0 km Mw = 5.75 Best Fit 0.8343  PT axis plot gives solutions with FIT greater than FIT90
The Pwave 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 surfacewave 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 Lovewave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.

Pressuretension axis trends. Since the surfacewave 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 Taxes 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 0180 degrees are sampled. 
The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used
Since the analysis of the surfacewave radiation patterns uses only spectral amplitudes and because the surfavewave radiation patterns have a 180 degree symmetry, each surfacewave solution consists of four possible focal mechanisms corresponding to the interchange of the P and Taxes and a roation of the mechanism by 180 degrees. To select one mechanism, Pwave first motion can be used. This was not possible in this case because all the Pwave first motions were emergent ( a feature of the Pwave 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, Rradial 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 0.01 n 3 lp c 0.03 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 model 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 1D 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 0.172E02 0.387E02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E02 0.363E02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E02 0.336E02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E04 0.000E04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E02 0.431E02 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: