Location

2007/03/02 04:40:00 37.93N 122.14W 5. 4.2 California

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for California

Focal Mechanism

SLUUC Berkeley
 SLU Moment Tensor Solution
 2007/03/02 04:40:00 37.93N 122.14W   5. 4.2 California
 
 Best Fitting Double Couple
    Mo = 3.55e+22 dyne-cm
    Mw = 4.30 
    Z  = 12 km
     Plane   Strike  Dip  Rake
      NP1      348    80   170
      NP2       80    80    10
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.55e+22     14     304
     N   0.00e+00     76     125
     P  -3.55e+22      0      34



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.38e+22
       Mxy    -3.20e+22
       Mxz     4.65e+21
       Myy     1.17e+22
       Myz    -6.98e+21
       Mzz     2.11e+21
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 ########------------ P              
              ############-----------   --           
             ##############----------------          
           #   ############------------------        
          ## T #############------------------       
         ###   ##############------------------      
        #####################-------------------     
        ######################------------------     
       #######################-----------------##    
       #######################------------#######    
       ########################-----#############    
       ####################----##################    
        -----------------------#################     
        ------------------------################     
         -----------------------###############      
          ----------------------##############       
           ---------------------#############        
             -------------------###########          
              ------------------##########           
                 ---------------#######              
                     -----------###                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  2.11e+21   4.65e+21   6.98e+21 
  4.65e+21  -1.38e+22   3.20e+22 
  6.98e+21   3.20e+22   1.17e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070302044000/index.html
        
This is a preliminary UCB moment tensor solution for the event located
2km NE from Lafayette, CA; 7.9010N 122.0983W; Z=16.6km;  ML=4.4;
(USGS/UCB Joint Notification System) on March 2, 2007 at 04:40:00 UTC.
Other information about this event can be viewed at:
http://earthquake.usgs.gov/recenteqsus/Quakes/nc40194055.php

Reviewed by:
kim
UCB Seismological Laboratory

Inversion method:   complete waveform
Stations used:      CMB, CVS, MCCM, WENL, Q03C
 
 Berkeley Moment Tensor Solution
 
 Best Fitting Double-Couple:
    Mo = 2.60E+22 Dyne-cm
    Mw = 4.21
    Z  = 11.0
    Plane   Strike   Rake   Dip
     NP1      353    -177    88
     NP2      263      -2    87
 
 Principal Axes:
    Axis    Value   Plunge   Azimuth
      T     2.600       1      128
      N     0.000      86       27
      P    -2.600       4      218
 
 Event Date/Time: March 2, 2007 at 04:40:00 UTC
 Event ID:        nc40194055
 Moment Tensor: Scale = 10**22 Dyne-cm
    Component   Value
       Mxx     -0.628
       Mxy     -2.517
       Mxz      0.106
       Myy      0.637
       Myz      0.124
       Mzz     -0.009
 
                                               
                                               
                    #------                    
              #######------------              
           ###########--------------           
         #############----------------         
       ###############------------------       
      ################-------------------      
     ##################-------------------     
    ###################--------------------    
    ###################--------------------    
   ####################---------------------   
   #####################----------##########   
   ###############------####################   
   ####-----------------####################   
   ---------------------####################   
    ---------------------##################    
    ---------------------##################    
     --------------------#################     
      -------------------#############         
       ---   ------------############# T       
         - P -------------############         
             -------------##########           
              ------------#######              
                    ------#                    
                                               
     Lower Hemisphere Equiangle Projection
 

	

Preferred Solution

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

      STK = 80
      DIP = 80
     RAKE = 10
       MW = 4.30
       HS = 12

The surface-wave agrees with the waveform inversion solution. The waveform inversion solution is used to define the event. It also agrees with the UCB moment tensor.

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
br c 0.12 0.2 n 4 p 2
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   175    60    45   3.73 0.1892
WVFGRD96    1.0   105    90    30   3.76 0.2057
WVFGRD96    2.0   250    85    -5   3.88 0.3064
WVFGRD96    3.0    70    90     0   3.98 0.3836
WVFGRD96    4.0    75    85    10   4.05 0.4651
WVFGRD96    5.0    80    75    15   4.12 0.5372
WVFGRD96    6.0    80    80    20   4.17 0.5977
WVFGRD96    7.0    80    80    15   4.20 0.6436
WVFGRD96    8.0    80    80    15   4.24 0.6761
WVFGRD96    9.0    80    80    10   4.26 0.6958
WVFGRD96   10.0    80    80     0   4.27 0.7065
WVFGRD96   11.0    80    80    10   4.28 0.7137
WVFGRD96   12.0    80    80    10   4.30 0.7189
WVFGRD96   13.0    80    80    10   4.30 0.7181
WVFGRD96   14.0    80    85    10   4.32 0.7179
WVFGRD96   15.0    80    85    10   4.33 0.7178
WVFGRD96   16.0    80    85    10   4.34 0.7107
WVFGRD96   17.0    80    85    15   4.34 0.7108
WVFGRD96   18.0    80    85    15   4.35 0.7031
WVFGRD96   19.0    80    85    15   4.36 0.7032

The best solution is

WVFGRD96   12.0    80    80    10   4.30 0.7189

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
br c 0.12 0.2 n 4 p 2
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

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=     345.73
  DIP=      80.95
 RAKE=     154.66
  
             OR
  
  STK=      79.99
  DIP=      65.00
 RAKE=      10.00
 
 
DEPTH = 12.0 km
 
Mw = 4.22
Best Fit 0.8135 - 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
BDM        83   24 eP_+
WENL      135   48 iP_C
CVS       329   54 iP_C
JRSC      189   59 iP_D
MCCM      291   70 iP_C
FARB      252   80 iP_D
MNRC      346  109 eP_+
PACP      143  127 iP_C
HOPS      326  144 iP_C
CMB        85  154 iP_D
ORV        17  189 eP_-
GASB      345  198 eP_X
KCC       104  258 iP_C
PKD       147  262 iP_C
WDC       353  297 eP_-
JCC       334  360 eP_X
YBH       354  425 eP_X
TPH        86  432 ePn
DAC       113  443 ePn
MOD        19  468 eP_X

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)   
TPH	   86	  432
DAC	  113	  443
BMN	   55	  508
TPNV	  100	  532
WVOR	   30	  582
ELK	   60	  672
MVU	   83	  871
HLID	   44	  903
HAWA	   12	  964
LON	    2	  980
HWUT	   62	  993
WUAZ	  102	  997
AHID	   57	 1079
REDW	   54	 1128
MOOW	   52	 1156
LOHW	   53	 1159
BW06	   59	 1194
MSO	   32	 1196
MVCO	   90	 1206
BOZ	   42	 1222
LKWY	   49	 1227
RLMT	   49	 1336
RWWY	   67	 1343
ISCO	   77	 1447
SDCO	   86	 1463
PHWY	   70	 1478
EGMT	   38	 1506
LAO	   48	 1628
MNTX	  109	 1678
CBKS	   80	 1955
JCT	  105	 2211
KVTX	  110	 2537
MIAR	   90	 2584
NATX	   97	 2594
FVM	   80	 2773
SLM	   78	 2778
PVMO	   83	 2871
SIUC	   80	 2883
MPH	   86	 2887
OXF	   87	 2952
USIN	   79	 3012
PLAL	   86	 3054
BLO	   76	 3088
LRAL	   89	 3215
EGAK	  343	 3238
BRAL	   93	 3292
ACSO	   73	 3370
TZTN	   81	 3403
BLA	   78	 3655
MVL	   72	 3931
LBNH	   64	 4210

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:

hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.2 n 4 p 2

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.

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 Fri Mar 2 20:16:28 CST 2007