Location

2012/03/26 03:21:51 37.9090 -77.9813 7.4 3.10 Virginia

The location given was created using the CUS model and SLU phase readings given in elocate.txt. This location is the same at that given by CERI. The USGS times below are for reference.

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 2012/03/26 03:21:51:0 37.91 -77.98 7.4 3.1 Virginia Stations used: GS.CVRD GS.PTRD GS.SPRD YC.IP02 YC.IP03 YC.IP05 Filtering commands used: hp c 0.20 n 3 lp c 0.50 n 3 Best Fitting Double Couple Mo = 3.72e+20 dyne-cm Mw = 2.98 Z = 8 km Plane Strike Dip Rake NP1 175 60 55 NP2 49 45 135 Principal Axes: Axis Value Plunge Azimuth T 3.72e+20 59 33 N 0.00e+00 30 194 P -3.72e+20 9 289 Moment Tensor: (dyne-cm) Component Value Mxx 3.00e+19 Mxy 1.59e+20 Mxz 1.19e+20 Myy -2.94e+20 Myz 1.42e+20 Mzz 2.64e+20 ----########## -------############### ---------################### ---------##################### ----------#######################- -----------#######################-- ----------########## ##########--- P ---------########### T ##########---- ---------########### ##########---- -------------#######################------ -------------######################------- -------------######################------- --------------###################--------- -------------##################--------- -------------#################---------- -------------##############----------- -------------##########------------- ------------#######--------------- -----------##----------------- ###########----------------- ##########------------ #######------- Global CMT Convention Moment Tensor: R T P 2.64e+20 1.19e+20 -1.42e+20 1.19e+20 3.00e+19 -1.59e+20 -1.42e+20 -1.59e+20 -2.94e+20 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20120326032151/index.html

Preferred Solution

      STK = 175
      DIP = 60
     RAKE = 55
       MW = 2.98
       HS = 8.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

SLUFM

USGS/SLU Moment Tensor Solution ENS 2012/03/26 03:21:51:0 37.91 -77.98 7.4 3.1 Virginia Stations used: GS.CVRD GS.PTRD GS.SPRD YC.IP02 YC.IP03 YC.IP05 Filtering commands used: hp c 0.20 n 3 lp c 0.50 n 3 Best Fitting Double Couple Mo = 3.72e+20 dyne-cm Mw = 2.98 Z = 8 km Plane Strike Dip Rake NP1 175 60 55 NP2 49 45 135 Principal Axes: Axis Value Plunge Azimuth T 3.72e+20 59 33 N 0.00e+00 30 194 P -3.72e+20 9 289 Moment Tensor: (dyne-cm) Component Value Mxx 3.00e+19 Mxy 1.59e+20 Mxz 1.19e+20 Myy -2.94e+20 Myz 1.42e+20 Mzz 2.64e+20 ----########## -------############### ---------################### ---------##################### ----------#######################- -----------#######################-- ----------########## ##########--- P ---------########### T ##########---- ---------########### ##########---- -------------#######################------ -------------######################------- -------------######################------- --------------###################--------- -------------##################--------- -------------#################---------- -------------##############----------- -------------##########------------- ------------#######--------------- -----------##----------------- ###########----------------- ##########------------ #######------- Global CMT Convention Moment Tensor: R T P 2.64e+20 1.19e+20 -1.42e+20 1.19e+20 3.00e+19 -1.59e+20 -1.42e+20 -1.59e+20 -2.94e+20 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20120326032151/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:

hp c 0.20 n 3
lp c 0.50 n 3

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   350    50    60   2.58 0.2982
WVFGRD96    2.0   325    80   -40   2.60 0.3472
WVFGRD96    3.0   325    85   -35   2.67 0.3773
WVFGRD96    4.0   155    75    35   2.74 0.3968
WVFGRD96    5.0   170    45    65   2.85 0.4650
WVFGRD96    6.0   170    50    55   2.91 0.5375
WVFGRD96    7.0   175    55    60   2.95 0.5638
WVFGRD96    8.0   175    60    55   2.98 0.5649
WVFGRD96    9.0   170    65    50   2.99 0.5345
WVFGRD96   10.0   170    65    45   3.00 0.5010
WVFGRD96   11.0   165    70    40   3.00 0.4815
WVFGRD96   12.0   165    70    35   3.01 0.4658
WVFGRD96   13.0   165    75    35   3.02 0.4526
WVFGRD96   14.0   165    75    30   3.03 0.4453
WVFGRD96   15.0   165    75    30   3.04 0.4440
WVFGRD96   16.0   155    80    15   3.00 0.4450
WVFGRD96   17.0   155    85    15   3.00 0.4438
WVFGRD96   18.0   155    85    15   3.01 0.4430
WVFGRD96   19.0   155    85    10   3.02 0.4429

The best solution is

WVFGRD96    8.0   175    60    55   2.98 0.5649

The mechanism corresponding 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

hp c 0.20 n 3
lp c 0.50 n 3

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 L ouis University, Universityof Memphis, Lamont Doehrty 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=Mon Mar 26 09:11:09 CDT 2012