Location

2005/08/12 20:53:48 42.20N 120.04W 5. 3.6 Oregon

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Pacific Northwest US

Focal Mechanism

 SLU Moment Tensor Solution
 2005/08/12 20:53:48 42.20N 120.04W   5. 3.6 Oregon
 
 Best Fitting Double Couple
    Mo = 9.44e+21 dyne-cm
    Mw = 3.95 
    Z  = 13 km
     Plane   Strike  Dip  Rake
      NP1      153    84   -125
      NP2       55    35   -10
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   9.44e+21     31     271
     N   0.00e+00     34     157
     P  -9.44e+21     41      31



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -3.98e+21
       Mxy    -2.55e+21
       Mxz    -3.91e+21
       Myy     5.52e+21
       Myz    -6.56e+21
       Mzz    -1.54e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ##--------------------              
              ######----------------------           
             #######-----------------------          
           ##########------------   ---------        
          ############----------- P ---------#       
         #############-----------   ----------#      
        ###############-----------------------##     
        ################----------------------##     
       #####   ##########--------------------####    
       ##### T ###########-------------------####    
       #####   ############-----------------#####    
       #####################---------------######    
        #####################-------------######     
        ######################-----------#######     
         ######################--------########      
          ######################-----#########       
           ######################-###########        
             -#################---#########          
              ---------------------#######           
                 --------------------##              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.54e+21  -3.91e+21   6.56e+21 
 -3.91e+21  -3.98e+21   2.55e+21 
  6.56e+21   2.55e+21   5.52e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20050812205348/index.html
        

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 = 55
      DIP = 35
     RAKE = -10
       MW = 3.95
       HS = 13

Both techniques give the same solution. 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 0.02 3
lp c 0.10 3
br c 0.12 0.2 n 8 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   155    40   -85   3.57 0.1745
WVFGRD96    1.0   155    40   -85   3.61 0.1720
WVFGRD96    2.0   190    40   -80   3.73 0.2071
WVFGRD96    3.0   220    75    -5   3.77 0.1840
WVFGRD96    4.0   225    70    10   3.81 0.1795
WVFGRD96    5.0    65    25     0   3.83 0.2325
WVFGRD96    6.0    60    30    -5   3.86 0.2816
WVFGRD96    7.0    60    30   -10   3.88 0.3176
WVFGRD96    8.0    50    30   -25   3.94 0.3414
WVFGRD96    9.0    45    30   -35   3.95 0.3593
WVFGRD96   10.0    40    35   -40   3.97 0.3715
WVFGRD96   11.0    40    40   -40   3.98 0.3771
WVFGRD96   12.0    45    40   -35   3.97 0.3790
WVFGRD96   13.0    45    40   -35   3.97 0.3781
WVFGRD96   14.0    50    40   -25   3.97 0.3759
WVFGRD96   15.0    50    40   -25   3.97 0.3722
WVFGRD96   16.0    55    40   -15   3.97 0.3685
WVFGRD96   17.0    55    45   -20   3.99 0.3645
WVFGRD96   18.0    55    45   -20   3.99 0.3602
WVFGRD96   19.0    60    45   -15   4.00 0.3560

The best solution is

WVFGRD96   12.0    45    40   -35   3.97 0.3790

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 3
lp c 0.10 3
br c 0.12 0.2 n 8 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


  NODAL PLANES 

  
  STK=     149.99
  DIP=      74.99
 RAKE=    -130.00
  
             OR
  
  STK=      42.85
  DIP=      42.27
 RAKE=     -22.64
 
 
DEPTH = 13.0 km
 
Mw = 3.89
Best Fit 0.8813 - 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
WVOR       77  119 iP_C
HUMO      282  244 eP_X
WDC       230  276 eP_C
HLID       70  484 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)   
WVOR	   77	  119
HUMO	  282	  244
WDC	  230	  276
HLID	   70	  484
TPH	  151	  517
LON	  345	  525
SAO	  192	  615
TTW	  348	  625
GNW	  341	  636
DAC	  161	  691
HWUT	   92	  706
MSO	   41	  707
AHID	   82	  738
ISA	  169	  738
REDW	   77	  763
GSC	  159	  816
BW06	   82	  864
MWC	  168	  902
GLA	  154	 1114
LAO	   61	 1206
TUC	  140	 1369
MNTX	  127	 1746
LTX	  129	 2050
SLM	   89	 2548
PVMO	   94	 2680
USIN	   89	 2785

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 3
lp c 0.10 3

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 Aug 12 22:25:44 CDT 2005