Location

2011/03/09 20:48:31 35.243 -92.397 6.0 3.50 Arkansas

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 2011/03/09 20:48:31:0 35.24 -92.40 6.0 3.5 Arkansas Stations used: AG.WHAR NM.X102 NM.X201 NM.X301 Filtering commands used: hp c 0.02 n 3 lp c 0.25 n 3 Best Fitting Double Couple Mo = 1.29e+21 dyne-cm Mw = 3.34 Z = 4 km Plane Strike Dip Rake NP1 224 85 165 NP2 315 75 5 Principal Axes: Axis Value Plunge Azimuth T 1.29e+21 14 178 N 0.00e+00 74 26 P -1.29e+21 7 270 Moment Tensor: (dyne-cm) Component Value Mxx 1.21e+21 Mxy -2.81e+19 Mxz -3.04e+20 Myy -1.27e+21 Myz 1.66e+20 Mzz 5.61e+19 ############## ###################### ############################ ---#########################-- --------####################------ ------------###############--------- ---------------###########------------ -------------------######--------------- ---------------------##----------------- -------------------##------------------ P -----------------#####----------------- ---------------#########--------------- -----------------############------------- --------------###############----------- ------------##################---------- ----------#####################------- -------########################----- -----##########################--- -############################- ############# ############ ########## T ######### ###### ##### Global CMT Convention Moment Tensor: R T P 5.61e+19 -3.04e+20 -1.66e+20 -3.04e+20 1.21e+21 2.81e+19 -1.66e+20 2.81e+19 -1.27e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110309204831/index.html

Preferred Solution

      STK = 315
      DIP = 75
     RAKE = 5
       MW = 3.34
       HS = 4.0

The NDK file is 20110309204831.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

SLUFM

USGS/SLU Moment Tensor Solution ENS 2011/03/09 20:48:31:0 35.24 -92.40 6.0 3.5 Arkansas Stations used: AG.WHAR NM.X102 NM.X201 NM.X301 Filtering commands used: hp c 0.02 n 3 lp c 0.25 n 3 Best Fitting Double Couple Mo = 1.29e+21 dyne-cm Mw = 3.34 Z = 4 km Plane Strike Dip Rake NP1 224 85 165 NP2 315 75 5 Principal Axes: Axis Value Plunge Azimuth T 1.29e+21 14 178 N 0.00e+00 74 26 P -1.29e+21 7 270 Moment Tensor: (dyne-cm) Component Value Mxx 1.21e+21 Mxy -2.81e+19 Mxz -3.04e+20 Myy -1.27e+21 Myz 1.66e+20 Mzz 5.61e+19 ############## ###################### ############################ ---#########################-- --------####################------ ------------###############--------- ---------------###########------------ -------------------######--------------- ---------------------##----------------- -------------------##------------------ P -----------------#####----------------- ---------------#########--------------- -----------------############------------- --------------###############----------- ------------##################---------- ----------#####################------- -------########################----- -----##########################--- -############################- ############# ############ ########## T ######### ###### ##### Global CMT Convention Moment Tensor: R T P 5.61e+19 -3.04e+20 -1.66e+20 -3.04e+20 1.21e+21 2.81e+19 -1.66e+20 2.81e+19 -1.27e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110309204831/index.html

First motions and takeoff angles from an elocate run.

Magnitudes

mLg Magnitude

(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

ML Magnitude

(a) ML computed using the IASPEI formula for Horizontal components; (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.

(a) ML computed using the IASPEI formula for Vertical components (research); (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.

Context

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 0.25 n 3

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   320    60    15   3.27 0.7524
WVFGRD96    1.0   315    75    10   3.26 0.7614
WVFGRD96    2.0   315    80    -5   3.30 0.7681
WVFGRD96    3.0   315    80     0   3.33 0.8023
WVFGRD96    4.0   315    75     5   3.34 0.8259
WVFGRD96    5.0   315    75     5   3.37 0.8155
WVFGRD96    6.0   315    70     5   3.40 0.8243
WVFGRD96    7.0   315    65     5   3.42 0.8075
WVFGRD96    8.0   315    60     5   3.44 0.8039
WVFGRD96    9.0   320    45    10   3.44 0.7757
WVFGRD96   10.0   320    45     5   3.48 0.7756
WVFGRD96   11.0   325    40    15   3.49 0.7665
WVFGRD96   12.0   325    40    15   3.51 0.7425
WVFGRD96   13.0   325    40    10   3.53 0.7508
WVFGRD96   14.0   325    40    10   3.54 0.7416
WVFGRD96   15.0   330    30    10   3.55 0.7304
WVFGRD96   16.0   325    40     0   3.56 0.7206
WVFGRD96   17.0   325    20    -5   3.58 0.7182
WVFGRD96   18.0   315     5   -20   3.61 0.7322
WVFGRD96   19.0   315     5   -20   3.61 0.7405
WVFGRD96   20.0    65    90   -95   3.65 0.7384
WVFGRD96   21.0    70    90   -85   3.68 0.7344
WVFGRD96   22.0    70    90   -80   3.70 0.7357
WVFGRD96   23.0    70    90   -80   3.70 0.7297
WVFGRD96   24.0   255    90    75   3.72 0.7197
WVFGRD96   25.0   255    90    75   3.73 0.7160
WVFGRD96   26.0   330    -5   -10   3.72 0.7221
WVFGRD96   27.0   360    -5    20   3.71 0.7291
WVFGRD96   28.0   250     0   -90   3.72 0.7337
WVFGRD96   29.0   250     0   -90   3.72 0.7337

The best solution is

WVFGRD96    4.0   315    75     5   3.34 0.8259

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 0.25 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

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, 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, the Iris stations and the Transportable Array of EarthScope.

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: