Location

2011/04/26 21:10:26 27.439 -111.598 15 5.60 Mexico, Gulf of Cortex

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  2011/04/26 21:10:26:1  27.44 -111.60  15.0 5.6 Mexico, Gulf of Cortex
 
 Stations used:
   CI.BAR CI.GLA CI.PASC II.PFO IU.ANMO IU.TUC MX.HPIG MX.HSIG 
   MX.SRIG TA.109C TA.121A TA.214A TA.W18A TA.Y12C TA.Y22D 
   US.MNTX US.WUAZ 
 
 Filtering commands used:
   hp c 0.01 n 3
   lp c 0.02 n 3
 
 Best Fitting Double Couple
  Mo = 7.16e+23 dyne-cm
  Mw = 5.17 
  Z  = 3 km
  Plane   Strike  Dip  Rake
   NP1      134    76   -164
   NP2       40    75   -15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.16e+23      0     267
    N   0.00e+00     69     176
    P  -7.16e+23     21     357

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -6.20e+23
       Mxy     7.04e+22
       Mxz    -2.40e+23
       Myy     7.12e+23
       Myz     7.88e+21
       Mzz    -9.27e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------   ----------              
              ------------ P ------------#           
             -------------   ------------##          
           ###--------------------------#####        
          #####------------------------#######       
         #######----------------------#########      
        #########--------------------###########     
        ###########-----------------############     
       #############---------------##############    
         #############------------###############    
       T ###############--------#################    
         #################-----##################    
        ####################-###################     
        ###################---##################     
         ################-------###############      
          #############------------###########       
           ##########-----------------#######        
             ######----------------------##          
              #---------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -9.27e+22  -2.40e+23  -7.88e+21 
 -2.40e+23  -6.20e+23  -7.04e+22 
 -7.88e+21  -7.04e+22   7.12e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110426211026/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 = 40
      DIP = 75
     RAKE = -15
       MW = 5.17
       HS = 3.0

The NDK file is 20110426211026.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
GCMT
 USGS/SLU Moment Tensor Solution
 ENS  2011/04/26 21:10:26:1  27.44 -111.60  15.0 5.6 Mexico, Gulf of Cortex
 
 Stations used:
   CI.BAR CI.GLA CI.PASC II.PFO IU.ANMO IU.TUC MX.HPIG MX.HSIG 
   MX.SRIG TA.109C TA.121A TA.214A TA.W18A TA.Y12C TA.Y22D 
   US.MNTX US.WUAZ 
 
 Filtering commands used:
   hp c 0.01 n 3
   lp c 0.02 n 3
 
 Best Fitting Double Couple
  Mo = 7.16e+23 dyne-cm
  Mw = 5.17 
  Z  = 3 km
  Plane   Strike  Dip  Rake
   NP1      134    76   -164
   NP2       40    75   -15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.16e+23      0     267
    N   0.00e+00     69     176
    P  -7.16e+23     21     357

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -6.20e+23
       Mxy     7.04e+22
       Mxz    -2.40e+23
       Myy     7.12e+23
       Myz     7.88e+21
       Mzz    -9.27e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------   ----------              
              ------------ P ------------#           
             -------------   ------------##          
           ###--------------------------#####        
          #####------------------------#######       
         #######----------------------#########      
        #########--------------------###########     
        ###########-----------------############     
       #############---------------##############    
         #############------------###############    
       T ###############--------#################    
         #################-----##################    
        ####################-###################     
        ###################---##################     
         ################-------###############      
          #############------------###########       
           ##########-----------------#######        
             ######----------------------##          
              #---------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -9.27e+22  -2.40e+23  -7.88e+21 
 -2.40e+23  -6.20e+23  -7.04e+22 
 -7.88e+21  -7.04e+22   7.12e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110426211026/index.html
	
USGS WPhase Moment Solution
GULF OF CALIFORNIA

11/04/26 21:10:26

Epicenter:  27.439 -111.598
MW 5.4

USGS/WPHASE CENTROID MOMENT TENSOR
11/04/26 21:10:26.00
Centroid:   27.439 -111.598
Depth  15         No. of sta:  9
Moment Tensor;   Scale 10**18 Nm
  Mrr=-0.01       Mtt=-0.13
  Mpp= 0.14       Mrt= 0.04
  Mrp=-0.02       Mtp= 0.00
 Principal axes:
  T  Val=  0.14  Plg= 7  Azm= 89
  N     = -0.00      72      334
  P     = -0.14      15      181

Best Double Couple:Mo=0.1*10**18
 NP1:Strike=224 Dip=73 Slip=  -5
 NP2:       316     84      -163

        
April 26, 2011, GULF OF CALIFORNIA, MW=5.4

Meredith Nettles
Goran Ekstrom

CENTROID-MOMENT-TENSOR  SOLUTION
GCMT EVENT:     C201104262110A  
DATA: II CU IU IC G  GE 
L.P.BODY WAVES: 65S, 103C, T= 40
MANTLE WAVES:   22S,  23C, T=125
SURFACE WAVES: 102S, 189C, T= 50
TIMESTAMP:      Q-20110426211532
CENTROID LOCATION:
ORIGIN TIME:      21:10:27.7 0.2
LAT:27.72N 0.01;LON:111.53W 0.01
DEP: 18.7  0.8;TRIANG HDUR:  1.3
MOMENT TENSOR: SCALE 10**24 D-CM
RR=-0.089 0.027; TT=-1.630 0.028
PP= 1.720 0.030; RT= 0.090 0.057
RP= 0.353 0.058; TP=-0.542 0.024
PRINCIPAL AXES:
1.(T) VAL=  1.863;PLG=10;AZM=261
2.(N)      -0.134;    79;     53
3.(P)      -1.728;     5;    171
BEST DBLE.COUPLE:M0= 1.80*10**24
NP1: STRIKE=306;DIP=79;SLIP= 177
NP2: STRIKE= 36;DIP=87;SLIP=  11

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

        

Magnitudes

ML Magnitude


(a) ML computed using the IASPEI formula for Horizontal components; (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.


(a) ML computed using the IASPEI formula for Vertical components (research); (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

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 0.02 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   220    60   -10   5.14 0.6443
WVFGRD96    1.0    40    80   -15   5.13 0.6637
WVFGRD96    2.0    40    75   -15   5.16 0.6913
WVFGRD96    3.0    40    75   -15   5.17 0.6942
WVFGRD96    4.0    40    80   -15   5.17 0.6886
WVFGRD96    5.0    25    90     0   5.17 0.6813
WVFGRD96    6.0    25    90     0   5.17 0.6724
WVFGRD96    7.0   205    85    -5   5.17 0.6657
WVFGRD96    8.0    25    90    10   5.19 0.6639
WVFGRD96    9.0    25    80    10   5.21 0.6581
WVFGRD96   10.0    45    80    20   5.25 0.6514
WVFGRD96   11.0    45    80    20   5.26 0.6483
WVFGRD96   12.0    45    80    20   5.26 0.6448
WVFGRD96   13.0    45    80    20   5.27 0.6454
WVFGRD96   14.0    45    75    15   5.28 0.6442
WVFGRD96   15.0    45    75    15   5.29 0.6419
WVFGRD96   16.0    45    75    15   5.29 0.6379
WVFGRD96   17.0    45    75    15   5.30 0.6374
WVFGRD96   18.0    45    75    15   5.31 0.6329
WVFGRD96   19.0    45    75    10   5.30 0.6307
WVFGRD96   20.0    45    75    10   5.31 0.6278
WVFGRD96   21.0    45    75    10   5.31 0.6218
WVFGRD96   22.0    45    75    10   5.32 0.6188
WVFGRD96   23.0    40    65   -10   5.31 0.6143
WVFGRD96   24.0    40    70   -10   5.30 0.6121
WVFGRD96   25.0    40    70   -10   5.30 0.6115
WVFGRD96   26.0    40    70   -10   5.31 0.6078
WVFGRD96   27.0    40    70   -10   5.32 0.6066
WVFGRD96   28.0    40    70   -10   5.33 0.6053
WVFGRD96   29.0    40    70   -15   5.33 0.6008

The best solution is

WVFGRD96    3.0    40    75   -15   5.17 0.6942

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 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 0.02 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=     305.66
  DIP=      87.50
 RAKE=     119.90
  
             OR
  
  STK=      40.00
  DIP=      30.00
 RAKE=       5.00
 
 
DEPTH = 4.0 km
 
Mw = 5.50
Best Fit 0.8509 - 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 0.01 n 3
lp c 0.02 n 3

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 WUS model 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:

Last Changed Sun Dec 6 20:08:23 CST 2015