Location

Location ANSS

2020/08/09 12:07:37 36.479 -81.098 9.2 5.1 North Carolina

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2020/08/09 12:07:37:0  36.48  -81.10   9.2 5.1 North Carolina
 
 Stations used:
   CO.BIRD CO.CASEE CO.CSB CO.HAW CO.HODGE CO.JSC CO.PAULI 
   ET.CPCT IM.TKL N4.KMSC N4.O52A N4.O54A N4.P53A N4.P57A 
   N4.Q51A N4.Q52A N4.Q54A N4.Q56A N4.R49A N4.R50A N4.R55A 
   N4.R58B N4.S51A N4.S54A N4.S57A N4.T57A N4.T59A N4.U49A 
   N4.U54A N4.U56A N4.V53A N4.V55A N4.V58A N4.V61A N4.W50A 
   N4.W52A N4.W59A N4.X51A N4.Y52A N4.Y57A N4.Y58A N4.Y60A 
   OH.O53A OH.P51A OH.P52A US.BLA US.CBN US.GOGA US.MCWV 
   US.TZTN 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 4.42e+23 dyne-cm
  Mw = 5.03 
  Z  = 1 km
  Plane   Strike  Dip  Rake
   NP1      357    61   132
   NP2      115    50    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.42e+23     53     320
    N   0.00e+00     36     152
    P  -4.42e+23      6      58

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.11e+22
       Mxy    -2.74e+23
       Mxz     1.37e+23
       Myy    -2.48e+23
       Myz    -1.76e+23
       Mzz     2.80e+23
                                                     
                                                     
                                                     
                                                     
                     #######-------                  
                 #############---------              
              #################-----------           
             ###################-----------          
           ######################----------          
          ########################--------- P        
         ############   ##########---------   -      
        --########### T ###########-------------     
        --###########   ###########-------------     
       ----########################--------------    
       -----#######################--------------    
       ------#######################-------------    
       --------#####################-------------    
        ---------##################-------------     
        -----------################-------------     
         -------------#############------------      
          ----------------#########---------##       
           -----------------------###########        
             --------------------##########          
              -------------------#########           
                 ---------------#######              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.80e+23   1.37e+23   1.76e+23 
  1.37e+23  -3.11e+22   2.74e+23 
  1.76e+23   2.74e+23  -2.48e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200809120737/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 = 115
      DIP = 50
     RAKE = 40
       MW = 5.03
       HS = 1.0

The NDK file is 20200809120737.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMWR
 USGS/SLU Moment Tensor Solution
 ENS  2020/08/09 12:07:37:0  36.48  -81.10   9.2 5.1 North Carolina
 
 Stations used:
   CO.BIRD CO.CASEE CO.CSB CO.HAW CO.HODGE CO.JSC CO.PAULI 
   ET.CPCT IM.TKL N4.KMSC N4.O52A N4.O54A N4.P53A N4.P57A 
   N4.Q51A N4.Q52A N4.Q54A N4.Q56A N4.R49A N4.R50A N4.R55A 
   N4.R58B N4.S51A N4.S54A N4.S57A N4.T57A N4.T59A N4.U49A 
   N4.U54A N4.U56A N4.V53A N4.V55A N4.V58A N4.V61A N4.W50A 
   N4.W52A N4.W59A N4.X51A N4.Y52A N4.Y57A N4.Y58A N4.Y60A 
   OH.O53A OH.P51A OH.P52A US.BLA US.CBN US.GOGA US.MCWV 
   US.TZTN 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 4.42e+23 dyne-cm
  Mw = 5.03 
  Z  = 1 km
  Plane   Strike  Dip  Rake
   NP1      357    61   132
   NP2      115    50    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.42e+23     53     320
    N   0.00e+00     36     152
    P  -4.42e+23      6      58

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.11e+22
       Mxy    -2.74e+23
       Mxz     1.37e+23
       Myy    -2.48e+23
       Myz    -1.76e+23
       Mzz     2.80e+23
                                                     
                                                     
                                                     
                                                     
                     #######-------                  
                 #############---------              
              #################-----------           
             ###################-----------          
           ######################----------          
          ########################--------- P        
         ############   ##########---------   -      
        --########### T ###########-------------     
        --###########   ###########-------------     
       ----########################--------------    
       -----#######################--------------    
       ------#######################-------------    
       --------#####################-------------    
        ---------##################-------------     
        -----------################-------------     
         -------------#############------------      
          ----------------#########---------##       
           -----------------------###########        
             --------------------##########          
              -------------------#########           
                 ---------------#######              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.80e+23   1.37e+23   1.76e+23 
  1.37e+23  -3.11e+22   2.74e+23 
  1.76e+23   2.74e+23  -2.48e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200809120737/index.html
	
W-phase Moment Tensor (Mww)
Moment 6.549e+16 N-m
Magnitude 5.14 Mww
Depth 11.5 km
Percent DC 31%
Half Duration 0.97 s
Catalog US
Data Source US 2
Contributor US 2

Nodal Planes
Plane Strike Dip Rake
NP1 304 57 59
NP2 172 44 128

Principal Axes
Axis Value Plunge Azimuth
T 4.891e+16 N-m 63 160
N 2.549e+16 N-m 25 322
P -7.441e+16 N-m 7 55


        

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

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. 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).

Waveform Inversion using wvfgrd96

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:

cut o DIST/3.3 -40 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 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    1.0   115    50    40   5.03 0.7989
WVFGRD96    2.0   125    40    60   5.10 0.7513
WVFGRD96    3.0   325    60    85   5.11 0.6697
WVFGRD96    4.0   325    60    85   5.09 0.6061
WVFGRD96    5.0   100    70   -30   5.02 0.6131
WVFGRD96    6.0   100    70   -30   5.03 0.6235
WVFGRD96    7.0   100    70   -30   5.04 0.6301
WVFGRD96    8.0   275    60   -25   5.05 0.6379
WVFGRD96    9.0   275    60   -25   5.06 0.6418
WVFGRD96   10.0   270    55   -30   5.08 0.6366
WVFGRD96   11.0   270    55   -30   5.09 0.6338
WVFGRD96   12.0   270    60   -25   5.10 0.6297
WVFGRD96   13.0   270    60   -25   5.10 0.6235
WVFGRD96   14.0   270    60   -25   5.11 0.6157
WVFGRD96   15.0   270    60   -25   5.12 0.6062
WVFGRD96   16.0   270    60   -25   5.13 0.5963
WVFGRD96   17.0   270    60   -20   5.13 0.5852
WVFGRD96   18.0   270    60   -20   5.14 0.5739
WVFGRD96   19.0   270    60   -20   5.15 0.5626

The best solution is

WVFGRD96    1.0   115    50    40   5.03 0.7989

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

cut o DIST/3.3 -40 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 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:

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

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 Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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 Sun Aug 9 08:13:16 CDT 2020