Location

2008/08/17 05:56:59 41.1830 -124.1970 17.0 4.60 California

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for

Focal Mechanism

 SLU Moment Tensor Solution
 2008/08/17 05:56:59  41.1830  -124.1970  17.0  4.60 California
 
 Best Fitting Double Couple
    Mo = 7.59e+22 dyne-cm
    Mw = 4.52 
    Z  = 20 km
     Plane   Strike  Dip  Rake
      NP1      350    90   -75
      NP2       80    15   -180
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   7.59e+22     43      65
     N   0.00e+00     15     170
     P  -7.59e+22     43     274



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     6.72e+21
       Mxy     1.84e+22
       Mxz     1.27e+22
       Myy    -6.72e+21
       Myz     7.22e+22
       Mzz    -1.78e+15
                                                     
                                                     
                                                     
                                                     
                     ----##########                  
                 --------##############              
              -----------#################           
             -------------#################          
           ---------------###################        
          ----------------####################       
         ------------------####################      
        -------------------##########   ########     
        -------------------########## T ########     
       --------   ----------#########   ########-    
       -------- P ----------####################-    
       --------   ----------####################-    
       ---------------------####################-    
        ---------------------##################-     
        #--------------------#################--     
         #-------------------################--      
          #-------------------##############--       
           ##-----------------############---        
             ##---------------##########---          
              ####-------------######-----           
                 #######---------------              
                     ##########----                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.78e+15   1.27e+22  -7.22e+22 
  1.27e+22   6.72e+21  -1.84e+22 
 -7.22e+22  -1.84e+22  -6.72e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080817055659/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 = 350
      DIP = 90
     RAKE = -75
       MW = 4.52
       HS = 20.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
UCB
 SLU Moment Tensor Solution
 2008/08/17 05:56:59  41.1830  -124.1970  17.0  4.60 California
 
 Best Fitting Double Couple
    Mo = 7.59e+22 dyne-cm
    Mw = 4.52 
    Z  = 20 km
     Plane   Strike  Dip  Rake
      NP1      350    90   -75
      NP2       80    15   -180
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   7.59e+22     43      65
     N   0.00e+00     15     170
     P  -7.59e+22     43     274



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     6.72e+21
       Mxy     1.84e+22
       Mxz     1.27e+22
       Myy    -6.72e+21
       Myz     7.22e+22
       Mzz    -1.78e+15
                                                     
                                                     
                                                     
                                                     
                     ----##########                  
                 --------##############              
              -----------#################           
             -------------#################          
           ---------------###################        
          ----------------####################       
         ------------------####################      
        -------------------##########   ########     
        -------------------########## T ########     
       --------   ----------#########   ########-    
       -------- P ----------####################-    
       --------   ----------####################-    
       ---------------------####################-    
        ---------------------##################-     
        #--------------------#################--     
         #-------------------################--      
          #-------------------##############--       
           ##-----------------############---        
             ##---------------##########---          
              ####-------------######-----           
                 #######---------------              
                     ##########----                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.78e+15   1.27e+22  -7.22e+22 
  1.27e+22   6.72e+21  -1.84e+22 
 -7.22e+22  -1.84e+22  -6.72e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080817055659/index.html
	
This is a preliminary NCSS moment tensor solution for the event located
14 km NNW of Trinidad, CA; 41.1830N 124.1968W; Z=17.0km;  ML=4.50;
(USGS/UCB Joint Notification System) on  08/17/2008 05:56:59:580 UTC.
Other information about this event can be viewed at:
http://earthquake.usgs.gov/recenteqsus/Quakes/nc51207076.php

Reviewed by:
Dreger
UCB Seismological Laboratory

Inversion method:   complete waveform
Stations used:      BK.WDC BK.YBH BK.GASB BK.HUMO BK.JCC NC.KRMB 
 
 Berkeley Moment Tensor Solution
 
 Best Fitting Double-Couple:
    Mo = 1.10E+23 Dyne-cm
    Mw = 4.63
    Z  = 11 km
    Plane   Strike   Rake   Dip
     NP1      159      79    84
     NP2       41     151    13
 
 Event Date/Time:  08/17/2008 05:56:59:580
 Event ID:        51207076
 
                                                                   
                                                                   
                            ###########                            
                      -######################                      
                  -----##########################                  
               ---------############################               
             -----------##############################             
           --------------##############################-           
          ---------------###############################-          
         -----------------###############################-         
       -------------------################################--       
      ---------------------################################--      
      ---------------------################################--      
     -----------------------###############   ##############--     
    -------------------------############## T ##############---    
    -------------------------##############   ##############---    
    --------------------------##############################---    
    ---------------------------#############################---    
   ----------------------------#############################----   
    -----------   --------------############################---    
    ----------- P ---------------###########################---    
    -----------   ----------------#########################----    
    ------------------------------#########################----    
     ------------------------------#######################----     
      ------------------------------#####################----      
      -------------------------------###################-----      
       -------------------------------#################-----       
         -----------------------------###############-----         
          -----------------------------#############-----          
           -----------------------------##########------           
             ----------------------------######-------             
               #--------------------------##--------               
                  ##--------------------###------                  
                      ######################-                      
                            ###########                            
                                                                   
     Lower Hemisphere Equiangle Projection
 

Deviatoric Solution:
 Principal Axes:
    Axis    Value   Plunge   Azimuth
      T    10.656      50       56
      N     0.536      11      160
      P   -11.192      38      259
 
 Source Composition:
    Type   Percent
     DC      90.4
     CLVD     9.6
     Iso      0.0
 
 Moment Tensor: Scale = 10**22 Dyne-cm
    Component   Value
       Mxx      1.544
       Mxy      0.542
       Mxz      3.890
       Myy     -3.479
       Myz      9.747
       Mzz      1.935
 
                                                                   
                                                                   
                            ###########                            
                      #######################                      
                  ---############################                  
               -------##############################               
             ----------##############################-             
           -------------###############################-           
          ---------------###############################-          
         ----------------################################-         
       -------------------################################--       
      ---------------------################################--      
      ---------------------################################--      
     -----------------------###############   ##############--     
    -------------------------############## T ##############---    
    --------------------------#############   ##############---    
    --------------------------##############################---    
    ---------------------------#############################---    
   -----------------------------############################----   
    -----------   --------------############################---    
    ----------- P ---------------###########################---    
    -----------   ----------------#########################----    
    ------------------------------#########################----    
     ------------------------------#######################----     
      ------------------------------#####################----      
      ------------------------------####################-----      
       ------------------------------##################-----       
         -----------------------------################----         
          ----------------------------###############----          
           ---------------------------#############-----           
             -------------------------############----             
               #---------------------###########----               
                  ###-------------##############-                  
                      #######################                      
                            ###########                            
                                                                   
     Lower Hemisphere Equiangle Projection
 

        

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:

hp c 0.02 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    0.5   170    45    90   3.97 0.2682
WVFGRD96    1.0   170    45    90   4.02 0.2526
WVFGRD96    2.0   350    45    90   4.15 0.3113
WVFGRD96    3.0   145    50   -85   4.20 0.2106
WVFGRD96    4.0   170    80    80   4.15 0.1940
WVFGRD96    5.0   310     5    45   4.16 0.2513
WVFGRD96    6.0   175    85    80   4.18 0.3105
WVFGRD96    7.0   170    90    80   4.19 0.3610
WVFGRD96    8.0   350    90   -80   4.29 0.3988
WVFGRD96    9.0   175    90    80   4.31 0.4525
WVFGRD96   10.0   350    90   -75   4.33 0.5008
WVFGRD96   11.0   175    90    75   4.36 0.5436
WVFGRD96   12.0   175    90    75   4.38 0.5811
WVFGRD96   13.0   175    90    75   4.40 0.6136
WVFGRD96   14.0   355    90   -70   4.42 0.6412
WVFGRD96   15.0   175    90    70   4.44 0.6639
WVFGRD96   16.0   175    90    70   4.46 0.6816
WVFGRD96   17.0   170    90    75   4.47 0.6950
WVFGRD96   18.0   350    90   -75   4.49 0.7043
WVFGRD96   19.0   350    90   -75   4.50 0.7095
WVFGRD96   20.0   350    90   -75   4.52 0.7110
WVFGRD96   21.0   350    90   -75   4.54 0.7092
WVFGRD96   22.0   170    90    75   4.56 0.7038
WVFGRD96   23.0   170    90    80   4.56 0.6955
WVFGRD96   24.0   170    90    80   4.58 0.6848
WVFGRD96   25.0   350    90   -85   4.59 0.6717
WVFGRD96   26.0   170    90    85   4.60 0.6562
WVFGRD96   27.0   170    90    85   4.61 0.6382
WVFGRD96   28.0   170     0   -90   4.62 0.6164
WVFGRD96   29.0   180     0   -80   4.62 0.5940

The best solution is

WVFGRD96   20.0   350    90   -75   4.52 0.7110

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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 bandpass filter used in the processing and for the display was

hp c 0.02 n 3
lp c 0.10 n 3
Figure 3. Waveform comparison for selected depth
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.

Discussion

The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS 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:

DATE=Mon Aug 18 10:32:30 CDT 2008

Last Changed 2008/08/17