Location

2012/08/26 20:57:58 33.024 -115.549 9.0 5.50 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 20:57:58:0  33.02 -115.55   9.0 5.5 California
 
 Best Fitting Double Couple
  Mo = 3.27e+24 dyne-cm
  Mw = 5.61 
  Z  = 4 km
  Plane   Strike  Dip  Rake
   NP1      235    90    50
   NP2      145    40   180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.27e+24     33     112
    N   0.00e+00     40     235
    P  -3.27e+24     33     358

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.98e+24
       Mxy    -7.20e+23
       Mxz    -2.05e+24
       Myy     1.98e+24
       Myz     1.44e+24
       Mzz    -2.19e+17
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              #-----------   -------------           
             ##----------- P --------------          
           ###------------   ----------------        
          ####-----------------------------###       
         ####---------------------------#######      
        #####------------------------###########     
        #####----------------------#############     
       #######------------------#################    
       #######---------------####################    
       ########------------######################    
       ########---------################   ######    
        ########-----################### T #####     
        #########--#####################   #####     
         #######---############################      
          ###-------##########################       
           -----------#######################        
             ------------##################          
              --------------##############           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.19e+17  -2.05e+24  -1.44e+24 
 -2.05e+24  -1.98e+24   7.20e+23 
 -1.44e+24   7.20e+23   1.98e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120826205758/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 = 50
       MW = 5.61
       HS = 4.0

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 20:57:58:0  33.02 -115.55   9.0 4.3 California
 
 Best Fitting Double Couple
  Mo = 3.27e+24 dyne-cm
  Mw = 5.61 
  Z  = 4 km
  Plane   Strike  Dip  Rake
   NP1      235    90    50
   NP2      145    40   180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.27e+24     33     112
    N   0.00e+00     40     235
    P  -3.27e+24     33     358

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.98e+24
       Mxy    -7.20e+23
       Mxz    -2.05e+24
       Myy     1.98e+24
       Myz     1.44e+24
       Mzz    -2.19e+17
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              #-----------   -------------           
             ##----------- P --------------          
           ###------------   ----------------        
          ####-----------------------------###       
         ####---------------------------#######      
        #####------------------------###########     
        #####----------------------#############     
       #######------------------#################    
       #######---------------####################    
       ########------------######################    
       ########---------################   ######    
        ########-----################### T #####     
        #########--#####################   #####     
         #######---############################      
          ###-------##########################       
           -----------#######################        
             ------------##################          
              --------------##############           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.19e+17  -2.05e+24  -1.44e+24 
 -2.05e+24  -1.98e+24   7.20e+23 
 -1.44e+24   7.20e+23   1.98e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120826205758/index.html
	
Global CMT Project Moment Tensor Solution

August 26, 2012, SOUTHERN CALIFORNIA, MW=5.5

Meredith Nettles
Goran Ekstrom

CENTROID-MOMENT-TENSOR  SOLUTION
GCMT EVENT:     C201208262057A  
DATA: II IU DK CU MN G  IC LD GE
L.P.BODY WAVES: 92S, 144C, T= 40
SURFACE WAVES: 142S, 282C, T= 50
TIMESTAMP:      Q-20120826204635
CENTROID LOCATION:
ORIGIN TIME:      20:58:00.8 0.1
LAT:33.02N 0.01;LON:115.59W 0.01
DEP: 12.2  0.8;TRIANG HDUR:  1.4
MOMENT TENSOR: SCALE 10**24 D-CM
RR=-0.513 0.033; TT=-1.740 0.031
PP= 2.250 0.036; RT= 0.512 0.094
RP=-0.310 0.084; TP= 1.050 0.028
PRINCIPAL AXES:
1.(T) VAL=  2.520;PLG= 3;AZM=103
2.(N)      -0.329;    71;      3
3.(P)      -2.194;    19;    195
BEST DBLE.COUPLE:M0= 2.36*10**24
NP1: STRIKE=237;DIP=74;SLIP= -11
NP2: STRIKE=331;DIP=79;SLIP=-164

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


Global CMT Project Catalog Search 
        

Moment Tensor Diagram

Variance Reduction vs Depth plot
Hypocentral Location:
Event ID15200401
Origin Time2012/08/26 20:57:58
Latitude33.0262
Longitude-115.5395
Depth (TT)8.8 km
Depth (MT; not authoritative)5 km

Magnitudes:
Me5.57 (not authoritative)
Ml5.38 (not authoritative)
Mw5.44 (authoritative)

Deviatoric Solution:
Scale1.0e+24 Dyne-cm
AxisValuePlungeAzimuth
T2.0493282
N-0.51458187
P-1.5333214

Source Composition:
TypePercent
DC50
CLVD50
Iso(null)
Moment Tensor:
Moment1.79e+24 Dyne-cm
Scale1.0e+24 Dyne-cm
Mxx-1.098
Mxy-0.687
Mxz-0.414
Myy1.887
Myz-0.246
Mzz-0.789
Variance Reduction89%

Best-fit Double Couple Solution
PlaneStrikeRakeDip
NP1152-15470
NP253-2266

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


        
USGS WPhase Moment Solution

12/08/26 20:57:58

Epicenter:  33.024 -115.549
MW 5.5

USGS/WPHASE CENTROID MOMENT TENSOR
12/08/26 20:57:58.00
Centroid:   32.323 -115.787
Depth  11         No. of sta: 24
Moment Tensor;   Scale 10**17 Nm
  Mrr=-0.96       Mtt=-1.19
  Mpp= 2.15       Mrt=-1.42
  Mrp=-0.69       Mtp= 0.69
 Principal axes:
  T  Val=  2.58  Plg=16  Azm=106
  N     = -0.08      42      212
  P     = -2.50      42        0

Best Double Couple:Mo=2.5*10**17
 NP1:Strike=154 Dip=47 Slip=-158
 NP2:        48     74       -45

        

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=     234.99
  DIP=      90.00
 RAKE=      50.00
  
             OR
  
  STK=     144.98
  DIP=      40.00
 RAKE=     179.99
 
 
DEPTH = 4.0 km
 
Mw = 5.61
Best Fit 0.8648 - 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

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.

Appendix A


Spectra fit plots to each station

Velocity Model

The CUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 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 Tue Aug 28 12:43:14 CDT 2012