Location

2012/08/26 19:31:23 33.016 -115.564 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  2012/08/26 19:31:23:0  33.02 -115.56   9.4 5.3 California
 
 Stations used:
   CI.BAR CI.GLA CI.GSC CI.ISA CI.MWC CI.OSI CI.PASC CI.SNCC 
   II.PFO IU.TUC LB.DAC NN.SHP TA.109C TA.214A TA.Y12C US.TPNV 
 
 Filtering commands used:
   hp c 0.01 n 3
   lp c 0.03 n 3
 
 Best Fitting Double Couple
  Mo = 9.77e+23 dyne-cm
  Mw = 5.26 
  Z  = 7 km
  Plane   Strike  Dip  Rake
   NP1      235    90    -5
   NP2      325    85   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.77e+23      4     280
    N   0.00e+00     85      55
    P  -9.77e+23      4     190

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.15e+23
       Mxy    -3.33e+23
       Mxz     6.98e+22
       Myy     9.15e+23
       Myz    -4.89e+22
       Mzz     7.45e+15
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ###-------------------------           
             ######------------------------          
           ##########------------------------        
          ############---------------------###       
         ##############-----------------#######      
          ###############------------###########     
        T ################---------#############     
          ##################----#################    
       ##########################################    
       ####################---###################    
       #################-------##################    
        #############------------###############     
        ###########---------------##############     
         #######-------------------############      
          ###-----------------------##########       
           --------------------------########        
             -------------------------#####          
              -------------------------###           
                 ------   -------------              
                     -- P ---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  7.45e+15   6.98e+22   4.89e+22 
  6.98e+22  -9.15e+23   3.33e+23 
  4.89e+22   3.33e+23   9.15e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120826193123/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 = 235
      DIP = 90
     RAKE = -5
       MW = 5.26
       HS = 7.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
GCMT
SCAL
USGSW
 USGS/SLU Moment Tensor Solution
 ENS  2012/08/26 19:31:23:0  33.02 -115.56   9.4 5.3 California
 
 Stations used:
   CI.BAR CI.GLA CI.GSC CI.ISA CI.MWC CI.OSI CI.PASC CI.SNCC 
   II.PFO IU.TUC LB.DAC NN.SHP TA.109C TA.214A TA.Y12C US.TPNV 
 
 Filtering commands used:
   hp c 0.01 n 3
   lp c 0.03 n 3
 
 Best Fitting Double Couple
  Mo = 9.77e+23 dyne-cm
  Mw = 5.26 
  Z  = 7 km
  Plane   Strike  Dip  Rake
   NP1      235    90    -5
   NP2      325    85   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.77e+23      4     280
    N   0.00e+00     85      55
    P  -9.77e+23      4     190

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.15e+23
       Mxy    -3.33e+23
       Mxz     6.98e+22
       Myy     9.15e+23
       Myz    -4.89e+22
       Mzz     7.45e+15
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ###-------------------------           
             ######------------------------          
           ##########------------------------        
          ############---------------------###       
         ##############-----------------#######      
          ###############------------###########     
        T ################---------#############     
          ##################----#################    
       ##########################################    
       ####################---###################    
       #################-------##################    
        #############------------###############     
        ###########---------------##############     
         #######-------------------############      
          ###-----------------------##########       
           --------------------------########        
             -------------------------#####          
              -------------------------###           
                 ------   -------------              
                     -- P ---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  7.45e+15   6.98e+22   4.89e+22 
  6.98e+22  -9.15e+23   3.33e+23 
  4.89e+22   3.33e+23   9.15e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120826193123/index.html
	
Global CMT Project Moment Tensor Solution

August 26, 2012, SOUTHERN CALIFORNIA, MW=5.4

Meredith Nettles
Goran Ekstrom

CENTROID-MOMENT-TENSOR  SOLUTION
GCMT EVENT:     C201208261930A  
DATA: II IU CU MN G  IC LD DK GE
L.P.BODY WAVES: 73S,  95C, T= 40
SURFACE WAVES: 136S, 261C, T= 50
TIMESTAMP:      Q-20120826215307
CENTROID LOCATION:
ORIGIN TIME:      19:31:25.5 0.2
LAT:33.00N 0.01;LON:115.58W 0.01
DEP: 12.0  FIX;TRIANG HDUR:  1.2
MOMENT TENSOR: SCALE 10**24 D-CM
RR=-0.420 0.029; TT=-1.230 0.028
PP= 1.650 0.022; RT= 0.437 0.080
RP=-0.373 0.071; TP= 0.450 0.024
PRINCIPAL AXES:
1.(T) VAL=  1.762;PLG= 8;AZM= 97
2.(N)      -0.241;    64;    350
3.(P)      -1.521;    24;    191
BEST DBLE.COUPLE:M0= 1.64*10**24
NP1: STRIKE=232;DIP=67;SLIP= -12
NP2: STRIKE=326;DIP=79;SLIP=-156

            -----------           
        -------------------       
      #####------------------     
    ########----------------###   
   ###########--------##########  
  ##############---############## 
  ##############-################ 
 ############-----################
 ##########---------##############
 #########-----------##########   
 #######--------------######### T 
  #####----------------########   
  ####------------------######### 
   ##--------------------#######  
    #---------   --------######   
      -------- P ---------###     
        ------   ----------       
            -----------           



        

SCSN Moment Tensor Solution

Computer-generated solution; not reviewed

Moment Tensor Diagram

Variance Reduction vs Depth plot
Hypocentral Location:
Event ID15199681
Origin Time2012/08/26 19:31:23
Latitude33.0193
Longitude-115.5632
Depth (TT)8.9 km
Depth (MT; not authoritative)5 km

Magnitudes:
Me5.39 (not authoritative)
Ml5.37 (not authoritative)
Mw5.32 (authoritative)

Deviatoric Solution:
Scale1.0e+24 Dyne-cm
AxisValuePlungeAzimuth
T1.3797278
N-0.42977157
P-0.954119

Source Composition:
TypePercent
DC38
CLVD62
Iso(null)
Moment Tensor:
Moment1.17e+24 Dyne-cm
Scale1.0e+24 Dyne-cm
Mxx-0.890
Mxy-0.312
Mxz-0.073
Myy1.315
Myz-0.227
Mzz-0.424
Variance Reduction89%

Best-fit Double Couple Solution
PlaneStrikeRakeDip
NP1144-16787
NP253-377

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


        
12/08/26 19:31:23

Epicenter:  33.019 -115.563
MW 5.4

USGS/WPHASE CENTROID MOMENT TENSOR
12/08/26 19:31:23.00
Centroid:   33.119 -115.682
Depth  11         No. of sta: 15
Moment Tensor;   Scale 10**17 Nm
  Mrr=-0.51       Mtt=-0.85
  Mpp= 1.36       Mrt= 0.98
  Mrp=-0.28       Mtp= 0.51
 Principal axes:
  T  Val=  1.47  Plg= 2  Azm=101
  N     =  0.31      51        8
  P     = -1.78      38      193

Best Double Couple:Mo=1.6*10**17
 NP1:Strike=230 Dip=62 Slip= -27
 NP2:       334     66      -148

Moment Tensor Solution

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 0.01 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    50    70   -15   5.09 0.4946
WVFGRD96    1.0    50    75   -15   5.12 0.5250
WVFGRD96    2.0    50    70   -15   5.18 0.6188
WVFGRD96    3.0    50    70   -15   5.21 0.6526
WVFGRD96    4.0    50    70   -20   5.24 0.6687
WVFGRD96    5.0    50    70   -15   5.25 0.6736
WVFGRD96    6.0   235    90    -5   5.25 0.6728
WVFGRD96    7.0   235    90    -5   5.26 0.6758
WVFGRD96    8.0   230    75   -20   5.29 0.6756
WVFGRD96    9.0   235    90   -10   5.28 0.6647
WVFGRD96   10.0   235    80    20   5.30 0.6542
WVFGRD96   11.0    50    90   -25   5.31 0.6492
WVFGRD96   12.0    55    75    20   5.31 0.6477
WVFGRD96   13.0    55    75    20   5.31 0.6427
WVFGRD96   14.0    55    75    20   5.31 0.6385
WVFGRD96   15.0    60    70    25   5.33 0.6338
WVFGRD96   16.0    60    70    25   5.33 0.6288
WVFGRD96   17.0    60    70    25   5.33 0.6244
WVFGRD96   18.0    60    70    25   5.34 0.6185
WVFGRD96   19.0    60    70    25   5.34 0.6121
WVFGRD96   20.0    60    70    20   5.34 0.6061
WVFGRD96   21.0    60    70    25   5.35 0.6012
WVFGRD96   22.0    55    80    20   5.34 0.5959
WVFGRD96   23.0    55    80    20   5.34 0.5913
WVFGRD96   24.0    55    80    20   5.35 0.5868
WVFGRD96   25.0    55    80    20   5.35 0.5825
WVFGRD96   26.0    55    85    15   5.36 0.5786
WVFGRD96   27.0    55    85    15   5.36 0.5750
WVFGRD96   28.0    55    85    15   5.37 0.5714
WVFGRD96   29.0    55    85    15   5.37 0.5675

The best solution is

WVFGRD96    7.0   235    90    -5   5.26 0.6758

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 0.01 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=     228.82
  DIP=      67.47
 RAKE=     -27.23
  
             OR
  
  STK=     329.97
  DIP=      65.00
 RAKE=    -154.99
 
 
DEPTH = 4.0 km
 
Mw = 5.40
Best Fit 0.8746 - 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:

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 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=Wed Aug 29 07:10:51 CDT 2012

Last Changed 2012/08/26