Location

2012/10/21 06:55:09 36.311 -120.856 9.4 5.30 California

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  2012/10/21 06:55:09:0  36.31 -120.86   9.4 5.3 California
 
 Stations used:
   BK.MCCM BK.SAO CI.MWC CI.PASC CI.SNCC LB.BMN NC.AFD NC.PAGB 
   NN.BEK NN.KVN NN.OMMB NN.RYN NN.SHP NN.WAK NN.YER TA.109C 
   TA.R11A US.TPNV UU.CCUT UU.LCMT UU.PSUT UU.SZCU UW.TREE 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.03 n 3
 
 Best Fitting Double Couple
  Mo = 1.08e+24 dyne-cm
  Mw = 5.29 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1       65    85    15
   NP2      334    75   175
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.08e+24     14     290
    N   0.00e+00     74      83
    P  -1.08e+24      7     198

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -8.39e+23
       Mxy    -6.52e+23
       Mxz     2.12e+23
       Myy     7.90e+23
       Myz    -1.99e+23
       Mzz     4.87e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ###-------------------              
              ########--------------------           
             ###########-------------------          
           ##############--------------------        
          ################--------------------       
         #   ###############------------------#      
        ## T ################--------------#####     
        ##   #################----------########     
       ########################------############    
       #########################-################    
       ######################----################    
       ##################--------################    
        #############-------------##############     
        ########-------------------#############     
         ##------------------------############      
          --------------------------##########       
           -------------------------#########        
             ------------------------######          
              -----------------------#####           
                 ---   --------------##              
                     P ------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.87e+22   2.12e+23   1.99e+23 
  2.12e+23  -8.39e+23   6.52e+23 
  1.99e+23   6.52e+23   7.90e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20121021065509/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 = 65
      DIP = 85
     RAKE = 15
       MW = 5.29
       HS = 11.0

The waveform inversion is preferred. This earthquake was studied obtain surface-wave dispersion for tomography.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
UCB
 USGS/SLU Moment Tensor Solution
 ENS  2012/10/21 06:55:09:0  36.31 -120.86   9.4 5.3 California
 
 Stations used:
   BK.MCCM BK.SAO CI.MWC CI.PASC CI.SNCC LB.BMN NC.AFD NC.PAGB 
   NN.BEK NN.KVN NN.OMMB NN.RYN NN.SHP NN.WAK NN.YER TA.109C 
   TA.R11A US.TPNV UU.CCUT UU.LCMT UU.PSUT UU.SZCU UW.TREE 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.03 n 3
 
 Best Fitting Double Couple
  Mo = 1.08e+24 dyne-cm
  Mw = 5.29 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1       65    85    15
   NP2      334    75   175
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.08e+24     14     290
    N   0.00e+00     74      83
    P  -1.08e+24      7     198

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -8.39e+23
       Mxy    -6.52e+23
       Mxz     2.12e+23
       Myy     7.90e+23
       Myz    -1.99e+23
       Mzz     4.87e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ###-------------------              
              ########--------------------           
             ###########-------------------          
           ##############--------------------        
          ################--------------------       
         #   ###############------------------#      
        ## T ################--------------#####     
        ##   #################----------########     
       ########################------############    
       #########################-################    
       ######################----################    
       ##################--------################    
        #############-------------##############     
        ########-------------------#############     
         ##------------------------############      
          --------------------------##########       
           -------------------------#########        
             ------------------------######          
              -----------------------#####           
                 ---   --------------##              
                     P ------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.87e+22   2.12e+23   1.99e+23 
  2.12e+23  -8.39e+23   6.52e+23 
  1.99e+23   6.52e+23   7.90e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20121021065509/index.html
	
USGS WPhase Moment Solution

12/10/21  6:55:09

Epicenter:  36.311 -120.856
MW 5.4

USGS/WPHASE CENTROID MOMENT TENSOR
12/10/21 06:55: 9.00
Centroid:   36.211 -120.980
Depth  11         No. of sta: 74
Moment Tensor;   Scale 10**17 Nm
  Mrr= 0.15       Mtt=-1.18
  Mpp= 1.03       Mrt= 0.42
  Mrp= 0.48       Mtp= 0.78
 Principal axes:
  T  Val=  1.53  Plg=23  Azm=289
  N     = -0.06      64       84
  P     = -1.47       9      195

Best Double Couple:Mo=1.5*10**17
 NP1:Strike=330 Dip=66 Slip= 170
 NP2:        65     81        24


        

NCSS Moment Tensor Solution

Solution reviewed by Cheng Cheng

Moment Tensor Diagram
Hypocentral Location:
Event ID71863625
Origin Time2012/10/21 06:55:10
Latitude36.3112
Longitude-120.8563
Depth (TT)9.4 km
Depth (MT; not authoritative)8 km fixed

Magnitudes:
Md5.53 (not authoritative)
Ml5.34 (not authoritative)
Mw5.29 (authoritative)

Principal Axes:
Scale1.0E+23 Dyne-cm
AxisValuePlungeAzimuth
T10.63314.605285.527
N0.26969.214152.178
P-10.90214.46119.380

Source Composition:
TypePercent
DC95
CLVD5
Iso(null)
Moment Tensor:
Mo1.08e+24 Dyne-cm
Mxx-8.357e+23 Dyne-cm
Mxy-5.782e+23 Dyne-cm
Mxz-1.872e+23 Dyne-cm
Myy8.125e+23 Dyne-cm
Myz-3.333e+23 Dyne-cm
Mzz2.316e+22 Dyne-cm
Variance Reduction89%
Version number4
Solution time2012/10/21 08:41:24 UTC

Best-fit Double Couple Solution
PlaneStrikeRakeDip
NP133215990
NP262069

Waveform data (solid line) and synthetic data (dashed line) from the moment tensor inversion:


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 0.03 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   240    80   -20   5.05 0.4247
WVFGRD96    1.0   240    80   -20   5.08 0.4527
WVFGRD96    2.0   240    80   -20   5.14 0.5418
WVFGRD96    3.0   240    80   -20   5.17 0.5801
WVFGRD96    4.0   240    80   -20   5.19 0.6032
WVFGRD96    5.0   240    80   -20   5.21 0.6244
WVFGRD96    6.0   245    90   -15   5.23 0.6375
WVFGRD96    7.0   245    90   -15   5.24 0.6526
WVFGRD96    8.0   245    90   -15   5.26 0.6654
WVFGRD96    9.0   245    90   -15   5.27 0.6674
WVFGRD96   10.0   245    90   -15   5.28 0.6667
WVFGRD96   11.0    65    85    15   5.29 0.6710
WVFGRD96   12.0   240    85   -15   5.28 0.6642
WVFGRD96   13.0   240    90   -15   5.29 0.6603
WVFGRD96   14.0    65    80    15   5.30 0.6608
WVFGRD96   15.0   240    90   -15   5.30 0.6557
WVFGRD96   16.0    60    90    15   5.30 0.6543
WVFGRD96   17.0    60    90    15   5.31 0.6489
WVFGRD96   18.0   240    90   -15   5.31 0.6437
WVFGRD96   19.0    60    90    15   5.32 0.6421
WVFGRD96   20.0    60    80    10   5.33 0.6375
WVFGRD96   21.0    60    80    10   5.33 0.6369
WVFGRD96   22.0    60    80    10   5.34 0.6328
WVFGRD96   23.0    60    80    10   5.34 0.6284
WVFGRD96   24.0    60    80    10   5.35 0.6265
WVFGRD96   25.0    60    85    10   5.35 0.6217
WVFGRD96   26.0    60    80    10   5.36 0.6169
WVFGRD96   27.0    60    75     5   5.37 0.6148
WVFGRD96   28.0   240    80   -10   5.37 0.6090
WVFGRD96   29.0   240    80   -10   5.37 0.6044

The best solution is

WVFGRD96   11.0    65    85    15   5.29 0.6710

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 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

hp c 0.02 n 3
lp c 0.03 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.

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.

Surface-Wave Focal Mechanism

The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=     150.00
  DIP=      90.00
 RAKE=    -155.00
  
             OR
  
  STK=      60.00
  DIP=      65.00
 RAKE=       0.00
 
 
DEPTH = 9.0 km
 
Mw = 5.41
Best Fit 0.9302 - P-T axis plot gives solutions with FIT greater than FIT90

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az    Dist   First motion

Surface-wave analysis

Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.

Data preparation

Digital data were collected, instrument response removed and traces converted to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively. These were input to the search program which examined all depths between 1 and 25 km and all possible mechanisms.
Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here.


Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. 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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

Broadband station distribution

The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used

Waveform comparison for this mechanism

Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.

The fits to the waveforms with the given mechanism are show below:

This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:

hp c 0.02 n 3
lp c 0.03 n 3

The mean observed group velocities and anelastic attenuation coefficients are compared to the predictions of the CUS model in the next figures. The CUS model was used since most of the travel paths are in the east.

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 Louis University, University of Memphis, Lamont Doherty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

Appendix A


Spectra fit plots to each station

Velocity Model

The WUS used for the waveform synthetic seismograms 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=Tue Oct 23 10:19:49 CDT 2012

Last Changed 2012/10/21