Location

Location ANSS

The ANSS event ID is usp000gzsh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000gzsh/executive.

2009/07/29 10:00:36 36.799 -104.831 5.0 4.1 New Mexico

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2009/07/29 10:00:36:0  36.80 -104.83   5.0 4.1 New Mexico
 
 Stations used:
   IU.ANMO IW.SMCO TA.MSTX TA.O22A TA.O23A TA.O24A TA.P22A 
   TA.P23A TA.P24A TA.P25A TA.Q20A TA.Q22A TA.Q23A TA.Q24A 
   TA.Q25A TA.R20A TA.R21A TA.R22A TA.R25A TA.S20A TA.S21A 
   TA.S22A TA.S23A TA.S24A TA.S25A TA.T21A TA.T22A TA.T23A 
   TA.T25A TA.U21A TA.U22A TA.U23A TA.U24A TA.U25A TA.U26A 
   TA.V21A TA.V22A TA.V24A TA.V25A TA.V26A TA.W20A TA.W21A 
   TA.W23A TA.W24A TA.W25A TA.W26A TA.W27A TA.X21A TA.X26A 
   TA.X27A TA.Y22A TA.Y23A TA.Y24A TA.Y25A TA.Y26A TA.Y27A 
   TA.Z22A TA.Z25A TA.Z26A US.AMTX US.ISCO US.MVCO US.SDCO 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 9.89e+21 dyne-cm
  Mw = 3.93 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1       40    75    35
   NP2      300    56   162
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.89e+21     35     265
    N   0.00e+00     52      60
    P  -9.89e+21     12     166

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -8.87e+21
       Mxy     2.75e+21
       Mxz     1.55e+21
       Myy     6.04e+21
       Myz    -5.11e+21
       Mzz     2.84e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ---------------------------#           
             ----------------------------##          
           -----------------------------#####        
          ################-------------#######       
         #####################--------#########      
        ##########################---###########     
        ########################################     
       ############################----##########    
       ######   ##################------#########    
       ###### T ################----------#######    
       ######   ###############------------######    
        #####################---------------####     
        ###################------------------###     
         ################---------------------#      
          #############-----------------------       
           ##########------------------------        
             ######------------------------          
              #----------------   --------           
                 -------------- P -----              
                     ----------   -                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.84e+21   1.55e+21   5.11e+21 
  1.55e+21  -8.87e+21  -2.75e+21 
  5.11e+21  -2.75e+21   6.04e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090729100036/index.html
        

Preferred Solution

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

      STK = 40
      DIP = 75
     RAKE = 35
       MW = 3.93
       HS = 15.0

The NDK file is 20090729100036.ndk The waveform inversion is preferred.

Magnitudes

Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.

ML Magnitude


Left: ML computed using the IASPEI formula for Horizontal components. Center: 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. Right: Residuals from new relation as a function of distance and azimuth.


Left: ML computed using the IASPEI formula for Vertical components (research). Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. 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). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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:

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5     0    55   -90   3.63 0.3781
WVFGRD96    1.0     0    50   -90   3.66 0.3790
WVFGRD96    2.0    -5    50   -95   3.77 0.4739
WVFGRD96    3.0     5    55   -80   3.83 0.4969
WVFGRD96    4.0    15    65   -65   3.85 0.5235
WVFGRD96    5.0    15    65   -65   3.86 0.5377
WVFGRD96    6.0    15    65   -60   3.85 0.5429
WVFGRD96    7.0    25    70   -50   3.84 0.5436
WVFGRD96    8.0    15    65   -65   3.92 0.5804
WVFGRD96    9.0    25    75   -50   3.89 0.5720
WVFGRD96   10.0    30    80   -45   3.89 0.5700
WVFGRD96   11.0    40    75    40   3.90 0.5762
WVFGRD96   12.0    40    75    40   3.91 0.5835
WVFGRD96   13.0    40    75    40   3.92 0.5883
WVFGRD96   14.0    40    75    35   3.92 0.5913
WVFGRD96   15.0    40    75    35   3.93 0.5918
WVFGRD96   16.0    40    75    35   3.94 0.5905
WVFGRD96   17.0    40    75    35   3.94 0.5878
WVFGRD96   18.0    40    75    35   3.95 0.5833
WVFGRD96   19.0    40    75    30   3.96 0.5780
WVFGRD96   20.0    40    75    30   3.96 0.5724
WVFGRD96   21.0    40    75    30   3.97 0.5647
WVFGRD96   22.0    40    75    30   3.98 0.5572
WVFGRD96   23.0    40    75    30   3.98 0.5487
WVFGRD96   24.0    40    75    30   3.99 0.5399
WVFGRD96   25.0    35    80   -30   3.99 0.5339
WVFGRD96   26.0    30    75   -30   4.00 0.5278
WVFGRD96   27.0    30    75   -30   4.01 0.5214
WVFGRD96   28.0    30    75   -30   4.01 0.5145
WVFGRD96   29.0    35    80   -25   4.02 0.5071

The best solution is

WVFGRD96   15.0    40    75    35   3.93 0.5918

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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3 
Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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=      21.13
  DIP=      71.25
 RAKE=     -54.00
  
             OR
  
  STK=     135.00
  DIP=      40.00
 RAKE=    -149.99
 
 
DEPTH = 2.0 km
 
Mw = 4.08
Best Fit 0.8218 - P-T axis plot gives solutions with FIT greater than FIT90

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