Location

2008/10/13 04:10:09 39.2490 21.4440 38.0 4.00

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution 2008/10/13 04:10:09 39.2490 21.4440 38.0 4.00 Best Fitting Double Couple Mo = 8.81e+21 dyne-cm Mw = 3.93 Z = 19 km Plane Strike Dip Rake NP1 242 84 125 NP2 340 35 10 Principal Axes: Axis Value Plunge Azimuth T 8.81e+21 41 184 N 0.00e+00 34 58 P -8.81e+21 31 304 Moment Tensor: (dyne-cm) Component Value Mxx 3.03e+21 Mxy 3.35e+21 Mxz -6.50e+21 Myy -4.47e+21 Myz 2.92e+21 Mzz 1.44e+21 --############ ------------########## ------------------########## ----------------------######## -------------------------######### ---- ---------------------######## ----- P ----------------------###----- ------ ---------------------##-------- --------------------------#######------- -----------------------###########-------- --------------------###############------- ----------------###################------- -------------######################------- ---------#########################------ ------############################------ ---##############################----- ################ ############----- ############### T ############---- ############# ###########--- #########################--- ####################-- ############## Harvard Convention Moment Tensor: R T F 1.44e+21 -6.50e+21 -2.92e+21 -6.50e+21 3.03e+21 -3.35e+21 -2.92e+21 -3.35e+21 -4.47e+21 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20081013041009/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 = 340
      DIP = 35
     RAKE = 10
       MW = 3.93
       HS = 19.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution 2008/10/13 04:10:09 39.2490 21.4440 38.0 4.00 Best Fitting Double Couple Mo = 8.81e+21 dyne-cm Mw = 3.93 Z = 19 km Plane Strike Dip Rake NP1 242 84 125 NP2 340 35 10 Principal Axes: Axis Value Plunge Azimuth T 8.81e+21 41 184 N 0.00e+00 34 58 P -8.81e+21 31 304 Moment Tensor: (dyne-cm) Component Value Mxx 3.03e+21 Mxy 3.35e+21 Mxz -6.50e+21 Myy -4.47e+21 Myz 2.92e+21 Mzz 1.44e+21 --############ ------------########## ------------------########## ----------------------######## -------------------------######### ---- ---------------------######## ----- P ----------------------###----- ------ ---------------------##-------- --------------------------#######------- -----------------------###########-------- --------------------###############------- ----------------###################------- -------------######################------- ---------#########################------ ------############################------ ---##############################----- ################ ############----- ############### T ############---- ############# ###########--- #########################--- ####################-- ############## Harvard Convention Moment Tensor: R T F 1.44e+21 -6.50e+21 -2.92e+21 -6.50e+21 3.03e+21 -3.35e+21 -2.92e+21 -3.35e+21 -4.47e+21 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20081013041009/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 0.0125 n 3
lp c 0.033 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   120    40   -35   3.76 0.4458
WVFGRD96    1.0   120    45   -35   3.77 0.4553
WVFGRD96    2.0   120    40   -30   3.83 0.4773
WVFGRD96    3.0   120    45   -30   3.84 0.4736
WVFGRD96    4.0   120    45   -25   3.86 0.4623
WVFGRD96    5.0   120    45   -25   3.87 0.4523
WVFGRD96    6.0   315    30    -5   3.90 0.4551
WVFGRD96    7.0   320    30     0   3.89 0.4800
WVFGRD96    8.0   325    25     0   3.95 0.4997
WVFGRD96    9.0   330    25     0   3.94 0.5257
WVFGRD96   10.0   325    30    15   3.94 0.5499
WVFGRD96   11.0   330    30    20   3.94 0.5680
WVFGRD96   12.0   335    30    10   3.92 0.5834
WVFGRD96   13.0   340    30    15   3.92 0.5961
WVFGRD96   14.0   340    30    15   3.92 0.6058
WVFGRD96   15.0   340    35    20   3.93 0.6128
WVFGRD96   16.0   340    35    20   3.93 0.6187
WVFGRD96   17.0   340    35    15   3.92 0.6223
WVFGRD96   18.0   340    35    10   3.92 0.6245
WVFGRD96   19.0   340    35    10   3.93 0.6252
WVFGRD96   20.0   340    35    10   3.93 0.6241
WVFGRD96   21.0   345    35    15   3.94 0.6229
WVFGRD96   22.0   345    35    15   3.94 0.6200
WVFGRD96   23.0   340    40    10   3.94 0.6163
WVFGRD96   24.0   340    40    10   3.94 0.6127
WVFGRD96   25.0   345    40    15   3.95 0.6083
WVFGRD96   26.0   345    40    15   3.95 0.6030
WVFGRD96   27.0   345    40    15   3.95 0.5971
WVFGRD96   28.0   345    40    15   3.96 0.5907
WVFGRD96   29.0   345    45    15   3.96 0.5839
WVFGRD96   30.0   345    45    15   3.97 0.5778
WVFGRD96   31.0   345    45    15   3.97 0.5713
WVFGRD96   32.0   345    45    15   3.97 0.5644
WVFGRD96   33.0   345    45    15   3.97 0.5570
WVFGRD96   34.0   345    45    20   3.98 0.5492
WVFGRD96   35.0   345    50    20   3.99 0.5427
WVFGRD96   36.0   345    50    20   3.99 0.5361
WVFGRD96   37.0   345    50    20   4.00 0.5290
WVFGRD96   38.0   345    50    20   4.00 0.5213
WVFGRD96   39.0   345    50    20   4.00 0.5135

The best solution is

WVFGRD96   19.0   340    35    10   3.93 0.6252

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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 bandpass filter used in the processing and for the display was

hp c 0.0125 n 3
lp c 0.033 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.

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=Sun Oct 12 23:42:30 MDT 2008

Last Changed 2008/10/13