Location

2014/11/20 10:25:31 32.950 -88.017 5.0 3.8 Alabama

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  2014/11/20 10:25:31:0  32.95  -88.02   5.0 3.8 Alabama
 
 Stations used:
   AG.CCAR AG.FCAR AG.LCAR CO.CASEE CO.HODGE ET.CPCT ET.FPAL 
   ET.SWET IM.TKL IU.WVT N4.143B N4.146B N4.152A N4.154A 
   N4.250A N4.255A N4.352A N4.451A N4.S44A N4.T42B N4.T45B 
   N4.T47A N4.T50A N4.V48A N4.V52A N4.V53A N4.W45B N4.W50A 
   N4.W52A N4.X48A N4.X51A N4.Y45B N4.Y49A N4.Y52A N4.Z47B 
   N4.Z51A NM.FVM NM.GNAR NM.HALT NM.HBAR NM.HENM NM.HICK 
   NM.LNXT NM.LPAR NM.MPH NM.PARM NM.PBMO NM.PEBM NM.PENM 
   NM.PLAL NM.PVMO NM.UALR NM.USIN NM.UTMT TA.TIGA TA.U40A 
   TA.W39A TA.X40A TA.X43A US.BRAL US.GOGA US.LRAL US.OXF 
   US.TZTN US.VBMS 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 5.31e+21 dyne-cm
  Mw = 3.75 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1      211    85   -170
   NP2      120    80    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.31e+21      4     345
    N   0.00e+00     79     237
    P  -5.31e+21     11      76

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.63e+21
       Mxy    -2.54e+21
       Mxz     8.26e+19
       Myy    -4.47e+21
       Myz    -1.01e+21
       Mzz    -1.58e+20
                                                     
                                                     
                                                     
                                                     
                      T ###########                  
                 ####   ##############-              
              #######################-----           
             #######################-------          
           #######################-----------        
          #######################-------------       
         ---####################---------------      
        ------#################--------------        
        --------#############---------------- P      
       ------------#########-----------------   -    
       ---------------#####----------------------    
       ------------------#-----------------------    
       ------------------###---------------------    
        ----------------#######-----------------     
        ---------------############-------------     
         -------------#################--------      
          -----------########################-       
           ---------#########################        
             ------########################          
              ----########################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.58e+20   8.26e+19   1.01e+21 
  8.26e+19   4.63e+21   2.54e+21 
  1.01e+21   2.54e+21  -4.47e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20141120102531/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 = 120
      DIP = 80
     RAKE = -5
       MW = 3.75
       HS = 5.0

The NDK file is 20141120102531.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2014/11/20 10:25:31:0  32.95  -88.02   5.0 3.8 Alabama
 
 Stations used:
   AG.CCAR AG.FCAR AG.LCAR CO.CASEE CO.HODGE ET.CPCT ET.FPAL 
   ET.SWET IM.TKL IU.WVT N4.143B N4.146B N4.152A N4.154A 
   N4.250A N4.255A N4.352A N4.451A N4.S44A N4.T42B N4.T45B 
   N4.T47A N4.T50A N4.V48A N4.V52A N4.V53A N4.W45B N4.W50A 
   N4.W52A N4.X48A N4.X51A N4.Y45B N4.Y49A N4.Y52A N4.Z47B 
   N4.Z51A NM.FVM NM.GNAR NM.HALT NM.HBAR NM.HENM NM.HICK 
   NM.LNXT NM.LPAR NM.MPH NM.PARM NM.PBMO NM.PEBM NM.PENM 
   NM.PLAL NM.PVMO NM.UALR NM.USIN NM.UTMT TA.TIGA TA.U40A 
   TA.W39A TA.X40A TA.X43A US.BRAL US.GOGA US.LRAL US.OXF 
   US.TZTN US.VBMS 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 5.31e+21 dyne-cm
  Mw = 3.75 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1      211    85   -170
   NP2      120    80    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.31e+21      4     345
    N   0.00e+00     79     237
    P  -5.31e+21     11      76

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.63e+21
       Mxy    -2.54e+21
       Mxz     8.26e+19
       Myy    -4.47e+21
       Myz    -1.01e+21
       Mzz    -1.58e+20
                                                     
                                                     
                                                     
                                                     
                      T ###########                  
                 ####   ##############-              
              #######################-----           
             #######################-------          
           #######################-----------        
          #######################-------------       
         ---####################---------------      
        ------#################--------------        
        --------#############---------------- P      
       ------------#########-----------------   -    
       ---------------#####----------------------    
       ------------------#-----------------------    
       ------------------###---------------------    
        ----------------#######-----------------     
        ---------------############-------------     
         -------------#################--------      
          -----------########################-       
           ---------#########################        
             ------########################          
              ----########################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.58e+20   8.26e+19   1.01e+21 
  8.26e+19   4.63e+21   2.54e+21 
  1.01e+21   2.54e+21  -4.47e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20141120102531/index.html
	
Moment 5.52e+14 N-m
Magnitude 3.8
Percent DC 90%
Depth 5.0 km
Updated 2014-11-20 11:24:30 UTC
Author us
Catalog us
Contributor Code us_b000sy7g_mwr
Principal Axes

Axis	Value	Plunge	Azimuth
T	5.652	5	344
N	-0.269	84	204
P	-5.383	4	74
Nodal Planes

Plane	Strike	Dip	Rake
NP1	29	89	174
NP2	119	84	1

        

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

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 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2
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   300    90     0   3.51 0.4326
WVFGRD96    2.0   120    80   -10   3.63 0.5586
WVFGRD96    3.0   120    85   -10   3.68 0.6086
WVFGRD96    4.0   120    80    -5   3.72 0.6304
WVFGRD96    5.0   120    80    -5   3.75 0.6352
WVFGRD96    6.0   120    70    -5   3.78 0.6322
WVFGRD96    7.0   120    70     0   3.80 0.6285
WVFGRD96    8.0   120    65    -5   3.83 0.6253
WVFGRD96    9.0   120    65    -5   3.84 0.6161
WVFGRD96   10.0   120    65    -5   3.85 0.6085
WVFGRD96   11.0   120    65     0   3.86 0.6015
WVFGRD96   12.0   120    70    -5   3.87 0.5946
WVFGRD96   13.0   120    70     0   3.88 0.5849
WVFGRD96   14.0   120    70     0   3.89 0.5723
WVFGRD96   15.0   120    70    -5   3.90 0.5574
WVFGRD96   16.0   120    70    -5   3.91 0.5417
WVFGRD96   17.0   120    70    -5   3.91 0.5248
WVFGRD96   18.0   120    70    -5   3.92 0.5067
WVFGRD96   19.0   120    70     0   3.93 0.4874
WVFGRD96   20.0   120    70     0   3.94 0.4677
WVFGRD96   21.0   120    70     0   3.94 0.4473
WVFGRD96   22.0   120    70     0   3.95 0.4262
WVFGRD96   23.0   120    70     0   3.95 0.4052
WVFGRD96   24.0   120    65     0   3.95 0.3842
WVFGRD96   25.0    35    70    20   3.95 0.3722
WVFGRD96   26.0    30    70    15   3.96 0.3647
WVFGRD96   27.0    30    75    15   3.97 0.3585
WVFGRD96   28.0    30    70    10   3.98 0.3548
WVFGRD96   29.0    30    75    10   3.99 0.3539

The best solution is

WVFGRD96    5.0   120    80    -5   3.75 0.6352

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 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 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. 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 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:

Last Changed Mon Dec 7 00:18:42 CST 2015