Location

2011/08/18 11:57:43 42.618 42.961 10.0 5.10 Georgia

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2011/08/18 11:57:43:0 42.62 42.96 10.0 5.1 Georgia Stations used: GE.MALT II.KIV IM.BR131 IM.BR231 IU.ANTO IU.GNI TU.ARTV TU.KOPT TU.TOKA Filtering commands used: hp c 0.01 n 3 lp c 0.03 n 3 Best Fitting Double Couple Mo = 2.07e+23 dyne-cm Mw = 4.81 Z = 10 km Plane Strike Dip Rake NP1 245 55 45 NP2 125 55 135 Principal Axes: Axis Value Plunge Azimuth T 2.07e+23 55 95 N 0.00e+00 35 275 P -2.07e+23 0 5 Moment Tensor: (dyne-cm) Component Value Mxx -2.04e+23 Mxy -2.43e+22 Mxz -9.87e+21 Myy 6.71e+22 Myz 9.70e+22 Mzz 1.37e+23 ------- P ---- ----------- -------- ---------------------------- ------------------------------ ---------------------------------- #-------------------##############-- ##--------------###################### ####---------########################### #####------############################# #######--################################# #######-#################### ########### ######---################### T ########### #####------################# ########### ###---------############################ ##------------########################## ---------------####################### ------------------################## ----------------------############ ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 1.37e+23 -9.87e+21 -9.70e+22 -9.87e+21 -2.04e+23 2.43e+22 -9.70e+22 2.43e+22 6.71e+22 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20110818115743/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 245
      DIP = 55
     RAKE = 45
       MW = 4.81
       HS = 10.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2011/08/18 11:57:43:0 42.62 42.96 10.0 5.1 Georgia Stations used: GE.MALT II.KIV IM.BR131 IM.BR231 IU.ANTO IU.GNI TU.ARTV TU.KOPT TU.TOKA Filtering commands used: hp c 0.01 n 3 lp c 0.03 n 3 Best Fitting Double Couple Mo = 2.07e+23 dyne-cm Mw = 4.81 Z = 10 km Plane Strike Dip Rake NP1 245 55 45 NP2 125 55 135 Principal Axes: Axis Value Plunge Azimuth T 2.07e+23 55 95 N 0.00e+00 35 275 P -2.07e+23 0 5 Moment Tensor: (dyne-cm) Component Value Mxx -2.04e+23 Mxy -2.43e+22 Mxz -9.87e+21 Myy 6.71e+22 Myz 9.70e+22 Mzz 1.37e+23 ------- P ---- ----------- -------- ---------------------------- ------------------------------ ---------------------------------- #-------------------##############-- ##--------------###################### ####---------########################### #####------############################# #######--################################# #######-#################### ########### ######---################### T ########### #####------################# ########### ###---------############################ ##------------########################## ---------------####################### ------------------################## ----------------------############ ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 1.37e+23 -9.87e+21 -9.70e+22 -9.87e+21 -2.04e+23 2.43e+22 -9.70e+22 2.43e+22 6.71e+22 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20110818115743/index.html

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 0.01 n 3
lp c 0.03 n 3

The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   230    80   -20   4.53 0.3055
WVFGRD96    1.0   230    85   -15   4.54 0.3226
WVFGRD96    2.0   235    85    40   4.63 0.3691
WVFGRD96    3.0   235    80    30   4.63 0.4040
WVFGRD96    4.0   240    70    35   4.66 0.4341
WVFGRD96    5.0   240    65    35   4.69 0.4596
WVFGRD96    6.0   240    65    35   4.71 0.4818
WVFGRD96    7.0   245    60    40   4.74 0.5002
WVFGRD96    8.0   250    55    50   4.78 0.5242
WVFGRD96    9.0   245    55    45   4.80 0.5357
WVFGRD96   10.0   245    55    45   4.81 0.5397
WVFGRD96   11.0   240    60    35   4.81 0.5331
WVFGRD96   12.0   240    60    35   4.81 0.5217
WVFGRD96   13.0   235    70    25   4.80 0.5082
WVFGRD96   14.0   235    70    25   4.80 0.4953
WVFGRD96   15.0   235    70    25   4.80 0.4799
WVFGRD96   16.0   230    80    15   4.80 0.4657
WVFGRD96   17.0   230    80    15   4.80 0.4523
WVFGRD96   18.0    50    80   -15   4.79 0.4445
WVFGRD96   19.0    50    80   -15   4.79 0.4391
WVFGRD96   20.0    50    80   -15   4.80 0.4374
WVFGRD96   21.0    50    75   -15   4.80 0.4360
WVFGRD96   22.0    50    75   -15   4.81 0.4349
WVFGRD96   23.0    50    75   -15   4.81 0.4339
WVFGRD96   24.0    50    75   -15   4.82 0.4335
WVFGRD96   25.0    50    75   -15   4.82 0.4330
WVFGRD96   26.0    50    75   -15   4.83 0.4329
WVFGRD96   27.0    50    75   -15   4.84 0.4324
WVFGRD96   28.0    50    75   -15   4.84 0.4322
WVFGRD96   29.0    50    75   -15   4.85 0.4322

The best solution is

WVFGRD96   10.0   245    55    45   4.81 0.5397

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

hp c 0.01 n 3
lp c 0.03 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)

Assuming only a mislocation, the time shifts are fit to a functional form:

 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 WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
Model after     8 iterations
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
     1.9000     3.4065     2.0089     2.2150  0.302E-02  0.679E-02   0.00       0.00       1.00       1.00    
     6.1000     5.5445     3.2953     2.6089  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
    13.0000     6.2708     3.7396     2.7812  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
    19.0000     6.4075     3.7680     2.8223  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
     0.0000     7.9000     4.6200     3.2760  0.164E-10  0.370E-10   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=Thu Aug 18 10:00:05 MDT 2011

Last Changed 2011/08/18