Location

Location ANSS

2020/01/21 02:07:59 18.002 -66.732 16.0 3.9 Puerto Rico

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2020/01/21 02:07:59:0  18.00  -66.73  16.0 3.9 Puerto Rico
 
 Stations used:
   GS.PR01 GS.PR03 GS.PR05 IU.SJG PR.AGPR PR.CRPR PR.EMPR 
   PR.PRSN 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 4.62e+21 dyne-cm
  Mw = 3.71 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1       25    70   -142
   NP2      280    55   -25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.62e+21      9     149
    N   0.00e+00     48      49
    P  -4.62e+21     41     248

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.95e+21
       Mxy    -2.91e+21
       Mxz     2.41e+20
       Myy    -1.12e+21
       Myz     2.48e+21
       Mzz    -1.84e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ####################--              
              #######################-----           
             ########################------          
           ##########################--------        
          ###########################---------       
         #######---------------#####-----------      
        ####------------------------#-----------     
        #--------------------------######-------     
       ----------------------------########------    
       ---------------------------###########----    
       --------------------------##############--    
       --------   --------------################-    
        ------- P -------------#################     
        -------   ------------##################     
         --------------------##################      
          ------------------##################       
           ----------------##################        
             ------------############   ###          
              ---------############## T ##           
                 ----################                
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.84e+21   2.41e+20  -2.48e+21 
  2.41e+20   2.95e+21   2.91e+21 
 -2.48e+21   2.91e+21  -1.12e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200121020759/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 = 280
      DIP = 55
     RAKE = -25
       MW = 3.71
       HS = 14.0

The NDK file is 20200121020759.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2020/01/21 02:07:59:0  18.00  -66.73  16.0 3.9 Puerto Rico
 
 Stations used:
   GS.PR01 GS.PR03 GS.PR05 IU.SJG PR.AGPR PR.CRPR PR.EMPR 
   PR.PRSN 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 4.62e+21 dyne-cm
  Mw = 3.71 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1       25    70   -142
   NP2      280    55   -25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.62e+21      9     149
    N   0.00e+00     48      49
    P  -4.62e+21     41     248

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.95e+21
       Mxy    -2.91e+21
       Mxz     2.41e+20
       Myy    -1.12e+21
       Myz     2.48e+21
       Mzz    -1.84e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ####################--              
              #######################-----           
             ########################------          
           ##########################--------        
          ###########################---------       
         #######---------------#####-----------      
        ####------------------------#-----------     
        #--------------------------######-------     
       ----------------------------########------    
       ---------------------------###########----    
       --------------------------##############--    
       --------   --------------################-    
        ------- P -------------#################     
        -------   ------------##################     
         --------------------##################      
          ------------------##################       
           ----------------##################        
             ------------############   ###          
              ---------############## T ##           
                 ----################                
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.84e+21   2.41e+20  -2.48e+21 
  2.41e+20   2.95e+21   2.91e+21 
 -2.48e+21   2.91e+21  -1.12e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200121020759/index.html
	

Magnitudes

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 +50
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   135    90    -5   3.18 0.2006
WVFGRD96    2.0   310    90     0   3.32 0.2926
WVFGRD96    3.0   130    90     0   3.39 0.3204
WVFGRD96    4.0   285    40    -5   3.42 0.3479
WVFGRD96    5.0   285    45    -5   3.45 0.4094
WVFGRD96    6.0   285    45   -10   3.49 0.4615
WVFGRD96    7.0   285    50    -5   3.51 0.5028
WVFGRD96    8.0   280    45   -25   3.60 0.5375
WVFGRD96    9.0   280    45   -30   3.63 0.5635
WVFGRD96   10.0   280    50   -25   3.64 0.5840
WVFGRD96   11.0   280    50   -30   3.67 0.5981
WVFGRD96   12.0   280    55   -30   3.69 0.6068
WVFGRD96   13.0   280    55   -25   3.70 0.6131
WVFGRD96   14.0   280    55   -25   3.71 0.6163
WVFGRD96   15.0   280    55   -25   3.73 0.6159
WVFGRD96   16.0   280    55   -25   3.74 0.6162
WVFGRD96   17.0   280    55   -25   3.75 0.6139
WVFGRD96   18.0   280    55   -25   3.77 0.6119
WVFGRD96   19.0   280    55   -25   3.78 0.6076
WVFGRD96   20.0   280    55   -25   3.79 0.6041
WVFGRD96   21.0   280    55   -20   3.79 0.5997
WVFGRD96   22.0   280    55   -20   3.80 0.5945
WVFGRD96   23.0   280    50   -25   3.82 0.5893
WVFGRD96   24.0   280    50   -20   3.81 0.5844
WVFGRD96   25.0   280    50   -20   3.82 0.5790
WVFGRD96   26.0   280    50   -20   3.83 0.5716
WVFGRD96   27.0   285    50   -15   3.83 0.5625
WVFGRD96   28.0   285    45   -15   3.83 0.5536
WVFGRD96   29.0   285    50    -5   3.82 0.5461

The best solution is

WVFGRD96   14.0   280    55   -25   3.71 0.6163

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 +50
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 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 Jan 20 21:29:02 CST 2020