Location

Location ANSS

The ANSS event ID is usp000jaep and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000jaep/executive.

2011/11/06 09:39:57 35.469 -96.865 5.0 3.7 Oklahoma

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2011/11/06 09:39:57:0 35.47 -96.86 5.0 3.7 Oklahoma Stations used: TA.Q35A TA.Q36A TA.R34A TA.R37A TA.R38A TA.S34A TA.S35A TA.S36A TA.S37A TA.S39A TA.T34A TA.T35A TA.T36A TA.T39A TA.TUL1 TA.U35A TA.U36A TA.U39A TA.U40A TA.V35A TA.V36A TA.V37A TA.V38A TA.V39A TA.V40A TA.W35A TA.W36A TA.W37B TA.W39A TA.X35A TA.X36A TA.X38A TA.Y36A TA.Y38A TA.Y39A US.KSU1 US.MIAR Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 3.76e+21 dyne-cm Mw = 3.65 Z = 3 km Plane Strike Dip Rake NP1 320 90 -10 NP2 50 80 -180 Principal Axes: Axis Value Plunge Azimuth T 3.76e+21 7 5 N 0.00e+00 80 140 P -3.76e+21 7 275 Moment Tensor: (dyne-cm) Component Value Mxx 3.65e+21 Mxy 6.43e+20 Mxz 4.20e+20 Myy -3.65e+21 Myz 5.00e+20 Mzz 5.71e+13 ####### T #### ########### ######## -########################### ---########################### -------#########################-- ---------#######################---- ------------###################------- ---------------################--------- ----------------#############----------- ----------------#########-------------- P -----------------######---------------- -------------------##------------------ ----------------------#------------------- ------------------######---------------- ----------------#########--------------- ------------##############------------ --------###################--------- ----#######################------- ###########################--- ###########################- ###################### ############## Global CMT Convention Moment Tensor: R T P 5.71e+13 4.20e+20 -5.00e+20 4.20e+20 3.65e+21 -6.43e+20 -5.00e+20 -6.43e+20 -3.65e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20111106093957/index.html

Preferred Solution

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

      STK = 320
      DIP = 90
     RAKE = -10
       MW = 3.65
       HS = 3.0

The NDK file is 20111106093957.ndk The waveform inversion is preferred.

Magnitudes

Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and M_L by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for M_L is adjusted to remove a linear distance trend in residuals to give a regionally defined M_L. The defined M_L uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.

mLg Magnitude

Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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 0.06 n 3

The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   315    80   -30   3.52 0.3899
WVFGRD96    1.0   320    90   -10   3.52 0.4134
WVFGRD96    2.0   135    70   -15   3.62 0.4824
WVFGRD96    3.0   320    90   -10   3.65 0.4945
WVFGRD96    4.0   140    90    15   3.68 0.4851
WVFGRD96    5.0   140    85    20   3.70 0.4716
WVFGRD96    6.0   140    85    25   3.72 0.4636
WVFGRD96    7.0   140    85    25   3.74 0.4590
WVFGRD96    8.0   140    55     0   3.78 0.4568
WVFGRD96    9.0   140    55     0   3.78 0.4511
WVFGRD96   10.0   140    55     0   3.79 0.4445
WVFGRD96   11.0   140    60     5   3.79 0.4373
WVFGRD96   12.0   140    60     5   3.80 0.4305
WVFGRD96   13.0   140    60     0   3.81 0.4275
WVFGRD96   14.0   140    60    -5   3.81 0.4242
WVFGRD96   15.0   140    60    -5   3.82 0.4201
WVFGRD96   16.0   140    65    -5   3.83 0.4156
WVFGRD96   17.0   140    65   -10   3.83 0.4111
WVFGRD96   18.0   140    65   -10   3.84 0.4069
WVFGRD96   19.0   135    65   -15   3.85 0.4024
WVFGRD96   20.0   135    65   -15   3.86 0.3982
WVFGRD96   21.0   135    65   -15   3.87 0.3938
WVFGRD96   22.0   135    65   -15   3.88 0.3888
WVFGRD96   23.0   135    65   -15   3.88 0.3837
WVFGRD96   24.0   135    70   -15   3.89 0.3789
WVFGRD96   25.0   135    70   -15   3.90 0.3745
WVFGRD96   26.0   135    70   -15   3.90 0.3701
WVFGRD96   27.0   135    70   -15   3.91 0.3658
WVFGRD96   28.0   135    70   -15   3.92 0.3616
WVFGRD96   29.0   135    70   -15   3.93 0.3570

The best solution is

WVFGRD96    3.0   320    90   -10   3.65 0.4945

The mechanism corresponding 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 0.06 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. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

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

Last Changed Sat Apr 27 05:38:06 PM CDT 2024