Location

2011/09/17 15:33:13 37.926 -77.987 4.6 2.60 Virginia

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2011/09/17 15:33:13:2 37.93 -77.99 4.6 2.6 Virginia Stations used: GS.CVRD GS.PTRD YC.IP01 YC.IP02 YC.IP03 YC.IP04 YC.IP05 YC.IP07 Filtering commands used: taper w 0.05 triangle w 0.5 transfer from none to none freqlimits 0.25 0.30 1 2 Best Fitting Double Couple Mo = 1.15e+20 dyne-cm Mw = 2.64 Z = 5 km Plane Strike Dip Rake NP1 19 64 146 NP2 125 60 30 Principal Axes: Axis Value Plunge Azimuth T 1.15e+20 41 340 N 0.00e+00 49 166 P -1.15e+20 3 73 Moment Tensor: (dyne-cm) Component Value Mxx 4.76e+19 Mxy -5.28e+19 Mxz 5.20e+19 Myy -9.73e+19 Myz -2.43e+19 Mzz 4.97e+19 ############## ###################--- ######################------ ######### ###########------- ########### T ###########--------- -########### ###########---------- ---########################---------- -----#######################---------- P -----#######################---------- --------#####################------------- ---------###################-------------- -----------#################-------------- ------------###############--------------- --------------###########--------------- ----------------########---------------- ------------------#####--------------- --------------------#--------------- ------------------########-------- ---------------############### ------------################ -------############### -############# Global CMT Convention Moment Tensor: R T P 4.97e+19 5.20e+19 2.43e+19 5.20e+19 4.76e+19 5.28e+19 2.43e+19 5.28e+19 -9.73e+19 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110917153313/index.html

Preferred Solution

      STK = 125
      DIP = 60
     RAKE = 30
       MW = 2.64
       HS = 5.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

SLUFM

USGS/SLU Moment Tensor Solution ENS 2011/09/17 15:33:13:2 37.93 -77.99 4.6 2.6 Virginia Stations used: GS.CVRD GS.PTRD YC.IP01 YC.IP02 YC.IP03 YC.IP04 YC.IP05 YC.IP07 Filtering commands used: taper w 0.05 triangle w 0.5 transfer from none to none freqlimits 0.25 0.30 1 2 Best Fitting Double Couple Mo = 1.15e+20 dyne-cm Mw = 2.64 Z = 5 km Plane Strike Dip Rake NP1 19 64 146 NP2 125 60 30 Principal Axes: Axis Value Plunge Azimuth T 1.15e+20 41 340 N 0.00e+00 49 166 P -1.15e+20 3 73 Moment Tensor: (dyne-cm) Component Value Mxx 4.76e+19 Mxy -5.28e+19 Mxz 5.20e+19 Myy -9.73e+19 Myz -2.43e+19 Mzz 4.97e+19 ############## ###################--- ######################------ ######### ###########------- ########### T ###########--------- -########### ###########---------- ---########################---------- -----#######################---------- P -----#######################---------- --------#####################------------- ---------###################-------------- -----------#################-------------- ------------###############--------------- --------------###########--------------- ----------------########---------------- ------------------#####--------------- --------------------#--------------- ------------------########-------- ---------------############### ------------################ -------############### -############# Global CMT Convention Moment Tensor: R T P 4.97e+19 5.20e+19 2.43e+19 5.20e+19 4.76e+19 5.28e+19 2.43e+19 5.28e+19 -9.73e+19 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110917153313/index.html

First motions and takeoff angles from an elocate run.

Waveform Inversion

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.

Location of broadband stations used for waveform inversion

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:

taper w 0.05
triangle w 0.5
transfer from none to none freqlimits 0.25 0.30 1 2

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   185    80   -35   2.18 0.3379
WVFGRD96    2.0   145    65    50   2.32 0.3003
WVFGRD96    3.0   315    60    65   2.50 0.4156
WVFGRD96    4.0   130    50    30   2.61 0.5853
WVFGRD96    5.0   125    60    30   2.64 0.6006
WVFGRD96    6.0   125    65    35   2.63 0.5077
WVFGRD96    7.0    25    40   -30   2.62 0.3501
WVFGRD96    8.0    30    35   -25   2.59 0.2468
WVFGRD96    9.0    30    50   -10   2.53 0.1876
WVFGRD96   10.0    40    50    10   2.53 0.1746
WVFGRD96   11.0    40    55    30   2.49 0.1431
WVFGRD96   12.0    45    55    35   2.46 0.1096
WVFGRD96   13.0   260    65   -40   2.43 0.1028
WVFGRD96   14.0    50    50     0   2.50 0.0897
WVFGRD96   15.0    50    50    -5   2.50 0.0793
WVFGRD96   16.0    60    65    -5   2.48 0.0594
WVFGRD96   17.0    70    45    40   2.44 0.0568
WVFGRD96   18.0    70    50    40   2.38 0.0371
WVFGRD96   19.0    60    70     0   2.41 0.0328

The best solution is

WVFGRD96    5.0   125    60    30   2.64 0.6006

The mechanism correspond to the best fit is

Figure 1. Waveform inversion focal mechanism

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

Figure 2. Depth sensitivity for waveform mechanism

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

taper w 0.05
triangle w 0.5
transfer from none to none freqlimits 0.25 0.30 1 2

Figure 3. Waveform comparison for selected depth

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:

• The origin time and epicentral distance are incorrect
• The velocity model used for the inversion is incorrect
• The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

 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.

Discussion

The Future

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.

Acknowledgements

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 Louis University, University of Memphis, Lamont Doherty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

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 

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

DATE=Sat Sep 17 19:32:30 CDT 2011