Location

2011/03/31 05:41:31 35.534 -96.575 5.0 3.50 Oklahoma

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 2011/03/31 05:41:31:0 35.53 -96.57 5.0 3.5 Oklahoma Stations used: TA.T34A TA.T35A TA.TUL1 TA.U34A TA.U37A TA.V35A TA.V36A TA.W35A TA.W36A TA.W38A TA.X36A TA.X37A TA.Y36A TA.Y37A Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 7.41e+20 dyne-cm Mw = 3.18 Z = 4 km Plane Strike Dip Rake NP1 130 90 20 NP2 40 70 180 Principal Axes: Axis Value Plunge Azimuth T 7.41e+20 14 357 N 0.00e+00 70 130 P -7.41e+20 14 263 Moment Tensor: (dyne-cm) Component Value Mxx 6.86e+20 Mxy -1.21e+20 Mxz 1.94e+20 Myy -6.86e+20 Myz 1.63e+20 Mzz -2.22e+13 #### ####### ######## T ########### ########### #############- ############################-- ---###########################---- ------########################------ ---------#####################-------- ------------##################---------- --------------###############----------- ------------------###########------------- --------------------########-------------- -- -----------------#####--------------- -- P -------------------#----------------- - -------------------###-------------- ---------------------#######------------ ------------------###########--------- ---------------###############------ ------------###################--- -------####################### -########################### ###################### ############## Global CMT Convention Moment Tensor: R T P -2.22e+13 1.94e+20 -1.63e+20 1.94e+20 6.86e+20 1.21e+20 -1.63e+20 1.21e+20 -6.86e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110331054131/index.html

Preferred Solution

      STK = 130
      DIP = 90
     RAKE = 20
       MW = 3.18
       HS = 4.0

The NDK file is 20110331054131.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 2011/03/31 05:41:30:0 35.43 -96.53 13.0 3.5 Oklahoma Stations used: TA.T34A TA.TUL1 TA.U34A TA.V35A TA.V36A TA.W35A TA.W36A TA.W38A TA.X36A TA.X37A Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 8.22e+20 dyne-cm Mw = 3.21 Z = 4 km Plane Strike Dip Rake NP1 300 85 -10 NP2 31 80 -175 Principal Axes: Axis Value Plunge Azimuth T 8.22e+20 3 346 N 0.00e+00 79 94 P -8.22e+20 11 255 Moment Tensor: (dyne-cm) Component Value Mxx 7.17e+20 Mxy -3.93e+20 Mxz 8.65e+19 Myy -6.92e+20 Myz 1.31e+20 Mzz -2.48e+19 T ########### #### ############### ########################---- #########################----- ##########################-------- -#########################---------- ------####################------------ -----------###############-------------- --------------###########--------------- -------------------#######---------------- ----------------------##------------------ -----------------------##----------------- - ------------------######-------------- P -----------------##########---------- ----------------##############------- -----------------#################---- --------------###################### ------------###################### --------###################### ------###################### -##################### ############## Global CMT Convention Moment Tensor: R T P -2.48e+19 8.65e+19 -1.31e+20 8.65e+19 7.17e+20 3.93e+20 -1.31e+20 3.93e+20 -6.92e+20 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20110331054130/index.html USGS/SLU Moment Tensor Solution ENS 2011/03/31 05:41:31:0 35.53 -96.57 5.0 3.5 Oklahoma Stations used: TA.T34A TA.T35A TA.TUL1 TA.U34A TA.U37A TA.V35A TA.V36A TA.W35A TA.W36A TA.W38A TA.X36A TA.X37A TA.Y36A TA.Y37A Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 7.41e+20 dyne-cm Mw = 3.18 Z = 4 km Plane Strike Dip Rake NP1 130 90 20 NP2 40 70 180 Principal Axes: Axis Value Plunge Azimuth T 7.41e+20 14 357 N 0.00e+00 70 130 P -7.41e+20 14 263 Moment Tensor: (dyne-cm) Component Value Mxx 6.86e+20 Mxy -1.21e+20 Mxz 1.94e+20 Myy -6.86e+20 Myz 1.63e+20 Mzz -2.22e+13 #### ####### ######## T ########### ########### #############- ############################-- ---###########################---- ------########################------ ---------#####################-------- ------------##################---------- --------------###############----------- ------------------###########------------- --------------------########-------------- -- -----------------#####--------------- -- P -------------------#----------------- - -------------------###-------------- ---------------------#######------------ ------------------###########--------- ---------------###############------ ------------###################--- -------####################### -########################### ###################### ############## Global CMT Convention Moment Tensor: R T P -2.22e+13 1.94e+20 -1.63e+20 1.94e+20 6.86e+20 1.21e+20 -1.63e+20 1.21e+20 -6.86e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110331054131/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 0.10 n 3
br c 0.12 0.25 n 4 p 2

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   295    70   -25   2.92 0.3123
WVFGRD96    1.0   300    80   -10   2.94 0.3479
WVFGRD96    2.0   310    85     0   3.08 0.5103
WVFGRD96    3.0   310    85   -10   3.14 0.5497
WVFGRD96    4.0   130    90    20   3.18 0.5558
WVFGRD96    5.0   135    70    25   3.22 0.5523
WVFGRD96    6.0   140    65    30   3.25 0.5504
WVFGRD96    7.0   140    65    25   3.26 0.5458
WVFGRD96    8.0   140    60    30   3.31 0.5408
WVFGRD96    9.0   140    60    25   3.31 0.5361
WVFGRD96   10.0   140    65    25   3.31 0.5313
WVFGRD96   11.0   135    70    20   3.32 0.5271
WVFGRD96   12.0   315    75    30   3.35 0.5223
WVFGRD96   13.0   315    75    30   3.36 0.5190
WVFGRD96   14.0   315    75    30   3.38 0.5143
WVFGRD96   15.0   315    75    25   3.38 0.5090
WVFGRD96   16.0   125    70   -25   3.39 0.5039
WVFGRD96   17.0   125    70   -25   3.40 0.4978
WVFGRD96   18.0   130    70   -20   3.39 0.4914
WVFGRD96   19.0   130    70   -20   3.40 0.4840
WVFGRD96   20.0   130    70   -20   3.40 0.4762
WVFGRD96   21.0   130    70   -20   3.41 0.4693
WVFGRD96   22.0   130    70   -20   3.42 0.4621
WVFGRD96   23.0   130    70   -20   3.42 0.4550
WVFGRD96   24.0   130    70   -20   3.43 0.4494
WVFGRD96   25.0   140    60    20   3.44 0.4441
WVFGRD96   26.0   140    60    25   3.46 0.4388
WVFGRD96   27.0   140    60    20   3.46 0.4342
WVFGRD96   28.0   140    60    20   3.46 0.4295
WVFGRD96   29.0   140    60    20   3.47 0.4244

The best solution is

WVFGRD96    4.0   130    90    20   3.18 0.5558

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
br c 0.12 0.25 n 4 p 2

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 WUS model 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: