Location

2005/06/11 11:16:10 42.27N -120.07W 5 3.4 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

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 = 360
      DIP = 45
     RAKE = -45
       MW = 3.61
       HS = 14

The waveform inversion solution if preferred. The spectral amplitude solution suffers from a lack of data. However both yield similar moment magnitudes and both would be satisfied with the preferred depth.

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 0.04 3
lp c 0.10 3
br c 0.12 0.2 n 4 p 2
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   275    40    85   3.15 0.2750
WVFGRD96    1.0   335    50   -50   3.15 0.2438
WVFGRD96    2.0   325    45   -80   3.32 0.3668
WVFGRD96    3.0   355    80     5   3.38 0.3493
WVFGRD96    4.0   235    80   -10   3.44 0.3995
WVFGRD96    5.0    40    55   -20   3.44 0.4609
WVFGRD96    6.0    35    45    -5   3.45 0.5449
WVFGRD96    7.0    30    45    -5   3.48 0.6099
WVFGRD96    8.0    25    40   -10   3.52 0.6537
WVFGRD96    9.0    30    45   -10   3.53 0.6827
WVFGRD96   10.0    10    40   -40   3.57 0.7025
WVFGRD96   11.0    10    45   -40   3.59 0.7181
WVFGRD96   12.0   360    45   -45   3.61 0.7260
WVFGRD96   13.0   360    45   -45   3.61 0.7297
WVFGRD96   14.0   360    45   -45   3.61 0.7276
WVFGRD96   15.0   360    50   -35   3.62 0.7233
WVFGRD96   16.0   360    50   -35   3.63 0.7143
WVFGRD96   17.0   360    50   -45   3.63 0.7011
WVFGRD96   18.0    25    55    30   3.65 0.6964
WVFGRD96   19.0    25    55    30   3.66 0.6906

The best solution is

WVFGRD96   13.0   360    45   -45   3.61 0.7297

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 0.04 3
lp c 0.10 3
br c 0.12 0.2 n 4 p 2
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


  NODAL PLANES 

  
  STK=     301.99
  DIP=      56.17
 RAKE=     -67.24
  
             OR
  
  STK=      85.00
  DIP=      40.00
 RAKE=    -119.99
 
 
DEPTH = 12.0 km
 
Mw = 3.63
Best Fit 0.8697 - 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
MOD       205   45 iP_C
WVOR       81  119 eP_+
YBH       256  227 eP_X
WDC       229  279 eP_X
ELK       111  438 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)   
MOD	  205	   45
WVOR	   81	  119
YBH	  256	  227
WDC	  228	  279
BMN	  130	  314
ELK	  111	  438
HOPS	  216	  444
MNV	  158	  457
CMB	  183	  471
HLID	   71	  484
TPH	  152	  525
MSO	   42	  702
NEW	   18	  705
HWUT	   93	  709
AHID	   83	  739
ISA	  169	  747
BOZ	   58	  774
MVU	  120	  787
LKWY	   69	  823
GSC	  159	  824
BW06	   83	  865
BAR	  163	 1106
GLA	  154	 1123
LAO	   61	 1204
ISCO	   98	 1245
DGMT	   56	 1418

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
br c 0.12 0.2 n 4 p 2
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 Sat Jun 11 14:04:58 CDT 2005