Location

2013/05/22 17:19:39 35.299 -92.715 2.0 3.00

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2013/05/22 17:19:39:0 35.30 -92.71 2.0 3.0 Stations used: Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 7.67e+20 dyne-cm Mw = 3.19 Z = 8 km Plane Strike Dip Rake NP1 130 60 45 NP2 13 52 141 Principal Axes: Axis Value Plunge Azimuth T 7.67e+20 52 346 N 0.00e+00 38 157 P -7.67e+20 5 250 Moment Tensor: (dyne-cm) Component Value Mxx 1.87e+20 Mxy -3.13e+20 Mxz 3.82e+20 Myy -6.57e+20 Myz -3.34e+19 Mzz 4.70e+20 #############- ##################---- ######################------ ########################------ -############ ##########-------- --############ T ###########-------- ----########### ###########--------- ------########################---------- -------#######################---------- ---------######################----------- ----------#####################----------- -----------####################----------- -------------#################------------ -----------###############----------- P -------------############------------ ---------------#########------------ -------------------#####------------ ----------------------#----------- -------------------########### ----------------############ -----------########### ----########## Global CMT Convention Moment Tensor: R T P 4.70e+20 3.82e+20 3.34e+19 3.82e+20 1.87e+20 3.13e+20 3.34e+19 3.13e+20 -6.57e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130522171939/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 = 130
      DIP = 60
     RAKE = 45
       MW = 3.19
       HS = 8.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2013/05/22 17:19:39:0 35.30 -92.71 2.0 3.0 Stations used: Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 7.67e+20 dyne-cm Mw = 3.19 Z = 8 km Plane Strike Dip Rake NP1 130 60 45 NP2 13 52 141 Principal Axes: Axis Value Plunge Azimuth T 7.67e+20 52 346 N 0.00e+00 38 157 P -7.67e+20 5 250 Moment Tensor: (dyne-cm) Component Value Mxx 1.87e+20 Mxy -3.13e+20 Mxz 3.82e+20 Myy -6.57e+20 Myz -3.34e+19 Mzz 4.70e+20 #############- ##################---- ######################------ ########################------ -############ ##########-------- --############ T ###########-------- ----########### ###########--------- ------########################---------- -------#######################---------- ---------######################----------- ----------#####################----------- -----------####################----------- -------------#################------------ -----------###############----------- P -------------############------------ ---------------#########------------ -------------------#####------------ ----------------------#----------- -------------------########### ----------------############ -----------########### ----########## Global CMT Convention Moment Tensor: R T P 4.70e+20 3.82e+20 3.34e+19 3.82e+20 1.87e+20 3.13e+20 3.34e+19 3.13e+20 -6.57e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130522171939/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 0.02 n 3
lp c 0.10 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   275    30   -45   3.12 0.0302
WVFGRD96    1.0   275    30   -45   3.17 0.0318
WVFGRD96    2.0   300    45   -45   3.20 0.0344
WVFGRD96    3.0   280    55   -50   3.20 0.0334
WVFGRD96    4.0    65    60   -95   3.22 0.0335
WVFGRD96    5.0   130    60    50   3.18 0.0352
WVFGRD96    6.0   130    55    50   3.19 0.0369
WVFGRD96    7.0   130    60    45   3.19 0.0378
WVFGRD96    8.0   130    60    45   3.19 0.0379
WVFGRD96    9.0   130    60    45   3.19 0.0377
WVFGRD96   10.0   125    65    45   3.21 0.0370
WVFGRD96   11.0   125    65    45   3.21 0.0365
WVFGRD96   12.0   175    45    25   3.23 0.0360
WVFGRD96   13.0   175    45    20   3.23 0.0356
WVFGRD96   14.0   165    45     0   3.23 0.0352
WVFGRD96   15.0   160    50   -10   3.25 0.0348
WVFGRD96   16.0   165    45   -30   3.25 0.0345
WVFGRD96   17.0   165    45   -30   3.26 0.0342
WVFGRD96   18.0   165    45   -35   3.27 0.0338
WVFGRD96   19.0   155    45   -40   3.28 0.0336
WVFGRD96   20.0   150    45   -40   3.31 0.0333
WVFGRD96   21.0   150    45   -40   3.32 0.0331
WVFGRD96   22.0   150    45   -40   3.32 0.0326
WVFGRD96   23.0   150    45   -40   3.33 0.0319
WVFGRD96   24.0   150    45   -40   3.34 0.0312
WVFGRD96   25.0   135    40   -40   3.35 0.0307
WVFGRD96   26.0   275    35    55   3.37 0.0309
WVFGRD96   27.0   275    35    50   3.38 0.0309
WVFGRD96   28.0   275    35    50   3.39 0.0309
WVFGRD96   29.0   275    35    50   3.39 0.0306
WVFGRD96   30.0   275    35    50   3.40 0.0301
WVFGRD96   31.0   100    65    70   3.36 0.0297
WVFGRD96   32.0   100    65    70   3.36 0.0296
WVFGRD96   33.0   110    65    80   3.38 0.0293
WVFGRD96   34.0   110    65    85   3.39 0.0290
WVFGRD96   35.0   110    65    85   3.39 0.0285
WVFGRD96   36.0   210    50    50   3.40 0.0285
WVFGRD96   37.0   210    50    50   3.41 0.0287
WVFGRD96   38.0   210    50    50   3.42 0.0289
WVFGRD96   39.0   210    50    50   3.43 0.0290
WVFGRD96   40.0   295    30    90   3.52 0.0286
WVFGRD96   41.0   290    30    85   3.53 0.0283
WVFGRD96   42.0   285    30    75   3.54 0.0279
WVFGRD96   43.0   285    30    75   3.55 0.0276
WVFGRD96   44.0   285    30    75   3.55 0.0273
WVFGRD96   45.0   285    30    75   3.56 0.0270
WVFGRD96   46.0   325    45   -40   3.51 0.0270
WVFGRD96   47.0   325    45   -40   3.52 0.0273
WVFGRD96   48.0   325    45   -40   3.52 0.0275
WVFGRD96   49.0   325    45   -40   3.53 0.0277

The best solution is

WVFGRD96    8.0   130    60    45   3.19 0.0379

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 0.02 n 3
lp c 0.10 n 3

Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.

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

Acknowledgements

Velocity Model

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
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:

Last Changed Fri Mar 14 19:29:29 CDT 2014