Location

2006/05/24 04:20:27 32.35N 115.19W 5. 5.0 Baja California

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Mexico

Focal Mechanism

 SLU Moment Tensor Solution
 2006/05/24 04:20:27 32.35N 115.19W   5. 5.0 Baja California
 
 Best Fitting Double Couple
    Mo = 4.84e+23 dyne-cm
    Mw = 5.09 
    Z  = 12 km
     Plane   Strike  Dip  Rake
      NP1       35    45   -90
      NP2      215    45   -90
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   4.84e+23     -0     125
     N   0.00e+00     -0      35
     P  -4.84e+23     90     250



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     1.59e+23
       Mxy    -2.27e+23
       Mxz     3.33e+13
       Myy     3.25e+23
       Myz     7.22e+15
       Mzz    -4.84e+23
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ################-----------#           
             ##############--------------##          
           #############------------------###        
          ############--------------------####       
         ###########----------------------#####      
        ###########-----------------------######     
        ##########------------------------######     
       ##########------------------------########    
       #########--------   --------------########    
       ########--------- P -------------#########    
       ########---------   ------------##########    
        ######------------------------##########     
        ######-----------------------###########     
         #####----------------------###########      
          ####--------------------#########          
           ###------------------########### T        
             ##--------------##############          
              #-----------################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -4.84e+23   3.33e+13  -7.22e+15 
  3.33e+13   1.59e+23   2.27e+23 
 -7.22e+15   2.27e+23   3.25e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20060524042027/index.html
        
** SCSN Moment Tensor Solution Message **

REAL-TIME SOLUTION: OPERATOR REVIEWED
Reviewed On: 05/24/2006 17:46:8

Inversion Method: Complete Waveform
Number of Stations used: 6
Stations: CI.IRM CI.BAR CI.DVT AZ.YAQ CI.SWS CI.RXH 
                   
Real-Time Solution:
-------------------
Event ID    : 12245763
Magnitude   : 5.16
Depth (km)  : 6.0
Origin Time :  05/24/2006 04:20:26:010
Latitude    : 32.31
Longitude   : -115.23
Further Information at: http://pasadena.wr.usgs.gov/recenteqs/Quakes/ci12245763.htm
                            
SCSN Moment Tensor Solution:
----------------------------
Moment Magnitude     : 5.37
Depth (km)           : 5
Variance Reduction(%): 81.12
Quality Factor       : A
   (A : Mw, MT good enough for distribution)
   (B : Mw only good enough for distribution)
   (C : Solution needs review before distribution)
                                                  
                                             
Best Fitting Double Couple and CLVD Solution:
---------------------------------------------------
                                                                
Moment Tensor: Scale = 10**21 Dyne-cm
    Component   Value
       Mxx      436
       Mxy      -487
       Mxz      67.1
       Myy      886
       Myz      23.7
       Mzz      -1320

Best Fitting Double Couple Solution:              
--------------------------------------------------
 Moment Tensor: Scale = 10**24 Dyne-cm
    Component   Value
       Mxx      0.403
       Mxy     -0.646
       Mxz      0.058
       Myy      1.023
       Myz      0.040
       Mzz     -1.425
 
 Principle Axes:
    Axis    Value   Plunge   Azimuth
      T     1.429       0      122
      N     0.000       3       32
      P    -1.429      87      214
 
 Best Fitting Double-Couple:
    Mo = 1.43E+24 Dyne-cm
    Plane   Strike   Rake   Dip
     NP1       29     -94    45
     NP2      215     -86    45
 
 Moment Magnitude = 5.37
                                               
                                               
                    #######                    
              ###################              
           ####################---##           
         ################----------###         
       ###############-------------#####       
      ##############----------------#####      
     #############------------------######     
    ############--------------------#######    
    ###########---------------------#######    
   ###########----------------------########   
   ##########---------   ----------#########   
   #########---------- P ----------#########   
   ########-----------   ---------##########   
   ########----------------------###########   
    ######----------------------###########    
    ######---------------------############    
     #####--------------------#########        
      ####------------------########### T      
       ###----------------#############        
         ##------------###############         
           #--------################           
              ###################              
                    #######                    
                                               
     Lower Hemisphere Equiangle Projection
 
=============  Station Information ==============
                                                 
Name       Distance     Azimuth     VR      ZCore
-------------------------------------------------
CI.IRM      205.002      2.188   74.911     24.00
CI.BAR      141.595    287.227   82.649     20.00
CI.DVT       90.517    295.551   84.515     12.00
AZ.YAQ      141.825    312.342   80.599     19.00
CI.SWS       87.807    322.948   79.474     10.00
CI.RXH      103.562    339.298   82.430     11.00
        

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 215
      DIP = 45
     RAKE = -90
       MW = 5.09
       HS = 12

The surface-wave and waveform inversion solutions are equivalent. The waveform inversion is preferred.

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 3
lp c 0.04 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   215    40   -90   5.00 0.2830
WVFGRD96    1.0    35    45   -90   5.03 0.2820
WVFGRD96    2.0   215    45   -90   5.09 0.3056
WVFGRD96    3.0    40    50   -80   5.14 0.2883
WVFGRD96    4.0    45    50   -70   5.17 0.2661
WVFGRD96    5.0    45    60   -75   5.19 0.2414
WVFGRD96    6.0    40    65   -80   5.20 0.2283
WVFGRD96    7.0   230    25   -55   5.19 0.2274
WVFGRD96    8.0   230    25   -55   5.22 0.2340
WVFGRD96    9.0   235    20   -50   5.22 0.2384
WVFGRD96   10.0   235    20   -50   5.22 0.2421
WVFGRD96   11.0   240    20   -45   5.21 0.2431
WVFGRD96   12.0   240    20   -45   5.21 0.2439
WVFGRD96   13.0   360    80   -80   5.18 0.2447
WVFGRD96   14.0   360    80   -80   5.18 0.2447
WVFGRD96   15.0   360    80   -80   5.17 0.2467
WVFGRD96   16.0   360    85   -75   5.17 0.2468
WVFGRD96   17.0   360    85   -75   5.17 0.2479
WVFGRD96   18.0   360    85   -75   5.17 0.2476
WVFGRD96   19.0   125    20   -10   5.16 0.2485

The best solution is

WVFGRD96    2.0   215    45   -90   5.09 0.3056

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 0.01 3
lp c 0.04 3
Figure 3. Waveform comparison for depth of 8 km
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.

Surface-Wave Focal Mechanism


  NODAL PLANES 

  
  STK=      10.00
  DIP=      55.00
 RAKE=    -120.00
  
             OR
  
  STK=     235.18
  DIP=      44.81
 RAKE=     -54.48
 
 
DEPTH = 1.0 km
 
Mw = 5.29
Best Fit 0.8217 - 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(deg)    Dist(km)   First motion
GLA        23   85 P
BAR       285  144 -12345
MWC       309  338 -12345
GSC       336  360 -12345
TUC        89  415 iP
ISA       321  477 -12345
DAC       334  489 -12345
TPH       344  662 -12345
MNV       339  727 -12345
MVU        21  735 -12345

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.

The velocity model used for the search is a modified Utah model .

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 distributiuon

Sta Az(deg)    Dist(km)   
BAR	  285	  144
MWC	  309	  338
GSC	  336	  360
TUC	   89	  415
SNCC	  285	  418
ISA	  321	  477
DAC	  334	  489
TPH	  344	  662
MNV	  339	  727
MVU	   21	  735
SAO	  312	  755
MNTX	   92	  929
SDCO	   53	 1067
HWUT	   16	 1077
WDC	  326	 1125
LTX	  104	 1151
WVOR	  346	 1160
ISCO	   44	 1194
AHID	   16	 1211
HLID	    3	 1247
RWWY	   32	 1255
BW06	   22	 1258
REDW	   16	 1280
AMTX	   76	 1303
LOHW	   16	 1313
PHWY	   38	 1318
MOOW	   16	 1324
HUMO	  331	 1329
IMW	   15	 1335
LKWY	   16	 1419
JCT	   94	 1476
WMOK	   76	 1545
CBKS	   58	 1571
HAWA	  348	 1603
MSO	    3	 1612
LON	  342	 1696
LAO	   23	 1767
KSU1	   61	 1836
NATX	   86	 1936
UALR	   76	 2134
CCM	   67	 2263
LTL	   88	 2325
FVM	   68	 2333
MPH	   76	 2354
SLM	   66	 2361
PVMO	   72	 2380
OXF	   77	 2403
SIUC	   69	 2435
UTMT	   72	 2454
WVT	   73	 2546
USIN	   68	 2574
EYMN	   42	 2641
WCI	   68	 2696
ACSO	   64	 3004
ALLY	   62	 3260
MCWV	   66	 3266
NHSC	   79	 3267
DWPF	   89	 3269
ERPA	   61	 3280
SSPA	   64	 3439
CBN	   68	 3477
SDMD	   66	 3523
NCB	   58	 3771
HRV	   61	 3976

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 velocity model used for the waveform fit is a modified Utah model .

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

Appendix A

The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the theoretical predictions as a function of period for the mechanism given above. The modified Utah model earth model was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.

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 Thu May 25 00:35:13 CDT 2006