Location

2007/09/24 06:20:54 45.10N 123.03W 22.0 3.6 Oregon

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for

Focal Mechanism

 SLU Moment Tensor Solution
 2007/09/24 06:20:54 45.10N 123.03W 22.0 3.6 Oregon
 
 Best Fitting Double Couple
    Mo = 3.76e+21 dyne-cm
    Mw = 3.65 
    Z  = 20 km
     Plane   Strike  Dip  Rake
      NP1      125    90   -45
      NP2      215    45   -180
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.76e+21     30     180
     N   0.00e+00     45     305
     P  -3.76e+21     30      70



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     2.50e+21
       Mxy    -9.09e+20
       Mxz    -2.18e+21
       Myy    -2.50e+21
       Myz    -1.52e+21
       Mzz    -6.47e+13
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ################------------           
             ##############----------------          
           #############---------------------        
          ---#########------------------------       
         -------####---------------------------      
        --------------------------------   -----     
        ----------###------------------- P -----     
       ----------#######----------------   ------    
       ---------###########----------------------    
       --------##############--------------------    
       --------#################-----------------    
        ------#####################-------------     
        ------#######################-----------     
         -----##########################-------      
          ----#############################---       
           ---#############   ###############        
             --############ T #############          
              -############   ############           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -6.47e+13  -2.18e+21   1.52e+21 
 -2.18e+21   2.50e+21   9.09e+20 
  1.52e+21   9.09e+20  -2.50e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070924062054/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 = 125
      DIP = 90
     RAKE = -45
       MW = 3.65
       HS = 20

This is a small event with little Love wave. The two techniques give the same solution

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 0.11 0.20 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   120    45    90   3.18 0.2709
WVFGRD96    1.0   295    45    90   3.23 0.2750
WVFGRD96    2.0   120    45    90   3.35 0.3407
WVFGRD96    3.0   290    90     0   3.42 0.2960
WVFGRD96    4.0   115    70    20   3.47 0.2859
WVFGRD96    5.0    20    35     0   3.44 0.2974
WVFGRD96    6.0    35    15   -10   3.47 0.3583
WVFGRD96    7.0    50    10    10   3.46 0.4032
WVFGRD96    8.0   310    85    90   3.54 0.4284
WVFGRD96    9.0   320    75    75   3.58 0.4530
WVFGRD96   10.0   310    75    80   3.55 0.4690
WVFGRD96   11.0   310    70    80   3.56 0.4808
WVFGRD96   12.0   310    70    85   3.56 0.4884
WVFGRD96   13.0   135    20    95   3.56 0.4937
WVFGRD96   14.0   130    70   -90   3.55 0.4977
WVFGRD96   15.0   130    70   -90   3.56 0.5023
WVFGRD96   16.0   125    70   -90   3.56 0.5058
WVFGRD96   17.0   120    85   -45   3.62 0.5096
WVFGRD96   18.0   125    90   -45   3.63 0.5125
WVFGRD96   19.0   125    90   -45   3.64 0.5138
WVFGRD96   20.0   125    90   -45   3.65 0.5138
WVFGRD96   21.0   125    90   -45   3.67 0.5127
WVFGRD96   22.0   125    90   -40   3.70 0.5101
WVFGRD96   23.0   125    90   -45   3.69 0.5066
WVFGRD96   24.0   305    90    45   3.70 0.5022
WVFGRD96   25.0   305    90    45   3.71 0.4964
WVFGRD96   26.0   305    90    45   3.72 0.4892
WVFGRD96   27.0   305    85    45   3.73 0.4815
WVFGRD96   28.0   305    85    45   3.74 0.4729
WVFGRD96   29.0   305    85    45   3.75 0.4631

The best solution is

WVFGRD96   20.0   125    90   -45   3.65 0.5138

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 0.11 0.20 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=     141.72
  DIP=      81.69
 RAKE=     -66.33
  
             OR
  
  STK=     249.98
  DIP=      25.01
 RAKE=    -159.99
 
 
DEPTH = 16.0 km
 
Mw = 3.65
Best Fit 0.8962 - 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
HEBO      283   58 -12345
J03A      178  154 -12345
LON        27  206 -12345
NLWA      346  263 -12345
GNW         3  275 -12345
HUMO      179  277 -12345
HAWA       61  308 -12345
M04C      165  381 -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.

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

Sta Az(deg)    Dist(km)   
J03A	  178	  192
LON	   27	  206
GNW	    3	  275
HUMO	  179	  277
HAWA	   61	  308
YBH	  176	  375
M04C	  165	  381
F09A	   79	  407
MOD	  147	  418
BMO	   92	  452
O01C	  187	  555
NEW	   50	  572
D12A	   67	  632
HLID	  101	  708
MSO	   72	  730
C14A	   64	  772
A13A	   54	  781
CMB	  163	  815
A15A	   58	  892
MOOW	   94	  989
EGMT	   68	 1067
BW06	   99	 1111

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 0.02 n 3
lp c 0.10 n 3
br c 0.11 0.20 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. 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


Spectra fit plots to each station

Velocity Model

The WUS 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:

DATE=Sat Sep 29 15:46:55 CDT 2007

Last Changed 2007/09/24