Location

Location ANSS

The ANSS event ID is nm610403 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nm610403/executive.

2014/06/04 21:19:24 35.580 -92.245 0.1 3.8 Arkansas

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/06/04 21:19:24:0 35.58 -92.25 0.1 3.8 Arkansas Stations used: AG.FCAR AG.HHAR AG.LCAR AG.WHAR AG.WLAR GS.KAN12 GS.OK027 GS.OK028 GS.OK029 IU.CCM IU.WVT N4.N38B N4.N41A N4.P38B N4.P40B N4.P43A N4.Q44B N4.R40B N4.S44A N4.T35B N4.T42B N4.T47A N4.V48A N4.Y49A N4.Z38B NM.CLTN NM.GLAT NM.GNAR NM.HALT NM.HENM NM.HICK NM.LNXT NM.MPH NM.OLIL NM.PEBM NM.PENM NM.PLAL NM.PVMO NM.SLM NM.UALR NM.USIN NM.UTMT OK.X37A TA.TUL1 TA.U40A TA.W39A TA.W41B TA.X40A TA.Z41A US.KSU1 US.LRAL US.MIAR Filtering commands used: cut o DIST/3.3 -50 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 6.53e+21 dyne-cm Mw = 3.81 Z = 2 km Plane Strike Dip Rake NP1 182 80 165 NP2 275 75 10 Principal Axes: Axis Value Plunge Azimuth T 6.53e+21 18 138 N 0.00e+00 72 331 P -6.53e+21 4 229 Moment Tensor: (dyne-cm) Component Value Mxx 5.16e+20 Mxy -6.17e+21 Mxz -1.12e+21 Myy -1.08e+21 Myz 1.57e+21 Mzz 5.67e+20 #######------- ###########----------- #############--------------- ##############---------------- ################------------------ ################-------------------- #################--------------------- ##################---------------------- ##################---------------------- ###----------------##########------------- -------------------#################------ ------------------######################-- ------------------######################## -----------------####################### -----------------####################### ----------------###################### ----------------############# #### - -----------############# T ### P -----------############# # -----------################ ---------############# ------######## Global CMT Convention Moment Tensor: R T P 5.67e+20 -1.12e+21 -1.57e+21 -1.12e+21 5.16e+20 6.17e+21 -1.57e+21 6.17e+21 -1.08e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140604211924/index.html

Preferred Solution

      STK = 275
      DIP = 75
     RAKE = 10
       MW = 3.81
       HS = 2.0

The NDK file is 20140604211924.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.

SLU

USGSMT

USGS/SLU Moment Tensor Solution ENS 2014/06/04 21:19:24:0 35.58 -92.25 0.1 3.8 Arkansas Stations used: AG.FCAR AG.HHAR AG.LCAR AG.WHAR AG.WLAR GS.KAN12 GS.OK027 GS.OK028 GS.OK029 IU.CCM IU.WVT N4.N38B N4.N41A N4.P38B N4.P40B N4.P43A N4.Q44B N4.R40B N4.S44A N4.T35B N4.T42B N4.T47A N4.V48A N4.Y49A N4.Z38B NM.CLTN NM.GLAT NM.GNAR NM.HALT NM.HENM NM.HICK NM.LNXT NM.MPH NM.OLIL NM.PEBM NM.PENM NM.PLAL NM.PVMO NM.SLM NM.UALR NM.USIN NM.UTMT OK.X37A TA.TUL1 TA.U40A TA.W39A TA.W41B TA.X40A TA.Z41A US.KSU1 US.LRAL US.MIAR Filtering commands used: cut o DIST/3.3 -50 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 6.53e+21 dyne-cm Mw = 3.81 Z = 2 km Plane Strike Dip Rake NP1 182 80 165 NP2 275 75 10 Principal Axes: Axis Value Plunge Azimuth T 6.53e+21 18 138 N 0.00e+00 72 331 P -6.53e+21 4 229 Moment Tensor: (dyne-cm) Component Value Mxx 5.16e+20 Mxy -6.17e+21 Mxz -1.12e+21 Myy -1.08e+21 Myz 1.57e+21 Mzz 5.67e+20 #######------- ###########----------- #############--------------- ##############---------------- ################------------------ ################-------------------- #################--------------------- ##################---------------------- ##################---------------------- ###----------------##########------------- -------------------#################------ ------------------######################-- ------------------######################## -----------------####################### -----------------####################### ----------------###################### ----------------############# #### - -----------############# T ### P -----------############# # -----------################ ---------############# ------######## Global CMT Convention Moment Tensor: R T P 5.67e+20 -1.12e+21 -1.57e+21 -1.12e+21 5.16e+20 6.17e+21 -1.57e+21 6.17e+21 -1.08e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140604211924/index.html

Regional Moment Tensor (Mwr) Moment magnitude derived from a moment tensor inversion of complete waveforms at regional distances (less than ~8 degrees), generally used for the analysis of small to moderate size earthquakes (typically Mw 3.5-6.0) crust or upper mantle earthquakes. Moment 5.83e+14 N-m Magnitude 3.8 Percent DC 75% Depth 8.0 km Updated 2014-06-04 21:51:27 UTC Author us Catalog us Contributor us Code us_c000rb2u_mwr Principal Axes Axis Value Plunge Azimuth T 5.470 15 313 N 0.662 65 187 P -6.133 19 48 Nodal Planes Plane Strike Dip Rake NP1 181 87 -156 NP2 90 66 -3

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.

ML Magnitude

Left: ML computed using the IASPEI formula for Horizontal components. Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Left: ML computed using the IASPEI formula for Vertical components (research). Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Context

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:

cut o DIST/3.3 -50 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   270    80   -20   3.79 0.6038
WVFGRD96    2.0   275    75    10   3.81 0.6118
WVFGRD96    3.0   275    65    10   3.84 0.6001
WVFGRD96    4.0   275    65     5   3.85 0.5823
WVFGRD96    5.0    90    65    -5   3.85 0.5767
WVFGRD96    6.0    90    65    -5   3.86 0.5758
WVFGRD96    7.0    90    65    -5   3.87 0.5744
WVFGRD96    8.0    90    70    -5   3.88 0.5738
WVFGRD96    9.0    90    70    -5   3.89 0.5743
WVFGRD96   10.0    90    65    -5   3.90 0.5735
WVFGRD96   11.0    90    65    -5   3.91 0.5700
WVFGRD96   12.0    90    70    -5   3.92 0.5657
WVFGRD96   13.0    90    70    -5   3.93 0.5605
WVFGRD96   14.0    90    70    -5   3.93 0.5560
WVFGRD96   15.0    95    70     5   3.94 0.5519
WVFGRD96   16.0    95    70     5   3.95 0.5460
WVFGRD96   17.0    95    70     5   3.96 0.5400
WVFGRD96   18.0    95    70     5   3.96 0.5334
WVFGRD96   19.0    95    70    -5   3.97 0.5264
WVFGRD96   20.0    95    70    -5   3.98 0.5184
WVFGRD96   21.0    95    70    -5   3.99 0.5113
WVFGRD96   22.0    95    70    -5   3.99 0.5052
WVFGRD96   23.0    95    70    -5   4.00 0.4980
WVFGRD96   24.0   275    75     5   3.99 0.4864
WVFGRD96   25.0   275    75     5   4.00 0.4814
WVFGRD96   26.0   275    75    10   4.00 0.4756
WVFGRD96   27.0   275    75    10   4.01 0.4720
WVFGRD96   28.0    95    75     0   4.02 0.4710
WVFGRD96   29.0    95    75     0   4.02 0.4693

The best solution is

WVFGRD96    2.0   275    75    10   3.81 0.6118

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

cut o DIST/3.3 -50 o DIST/3.3 +50
rtr
taper w 0.1
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. 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)

 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 CUS.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
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