Location

2007/06/14 21:57:56 45.13N 120.95W 23 3.9 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
 2007/06/14 21:57:56 45.13N 120.95W 23 3.9 Oregon
 
 Best Fitting Double Couple
    Mo = 2.95e+21 dyne-cm
    Mw = 3.58 
    Z  = 18 km
     Plane   Strike  Dip  Rake
      NP1      255    85    20
      NP2      163    70   175
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.95e+21     18     121
     N   0.00e+00     69     268
     P  -2.95e+21     10      27



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.54e+21
       Mxy    -2.35e+21
       Mxz    -8.98e+20
       Myy     1.37e+21
       Myz     4.91e+20
       Mzz     1.75e+20
                                                     
                                                     
                                                     
                                                     
                     #-------------                  
                 ####-------------- P -              
              #######--------------   ----           
             ########----------------------          
           ##########------------------------        
          ###########-------------------------       
         ############--------------------------      
        #############---------------------------     
        ##############---------------------#####     
       ###############------------###############    
       ###############-----######################    
       #############---##########################    
       #######----------#########################    
        ##--------------########################     
        -----------------#################   ###     
         -----------------################ T ##      
          -----------------###############   #       
           -----------------#################        
             ----------------##############          
              ----------------############           
                 ---------------#######              
                     ------------##                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  1.75e+20  -8.98e+20  -4.91e+20 
 -8.98e+20  -1.54e+21   2.35e+21 
 -4.91e+20   2.35e+21   1.37e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070614215757/index.html
        
University of Washington First Motion Solution

        
University of Washington Automated moment Tensor

	

Preferred Solution

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

      STK = 255
      DIP = 85
     RAKE = 20
       MW = 3.58
       HS = 18

The solutions from the two techniques are in agreement

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 0.12 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    35    40   -90   3.07 0.1598
WVFGRD96    1.0   260    75     0   2.98 0.1759
WVFGRD96    2.0   255    75     5   3.15 0.2838
WVFGRD96    3.0   255    80    20   3.24 0.3574
WVFGRD96    4.0   255    80    25   3.31 0.4205
WVFGRD96    5.0    75    90   -35   3.38 0.4800
WVFGRD96    6.0    75    90   -35   3.42 0.5372
WVFGRD96    7.0   255    85    35   3.44 0.5820
WVFGRD96    8.0    75    90   -35   3.48 0.6126
WVFGRD96    9.0   255    85    35   3.49 0.6356
WVFGRD96   10.0    75    90   -30   3.50 0.6500
WVFGRD96   11.0   255    85    30   3.51 0.6609
WVFGRD96   12.0    75    90   -30   3.53 0.6681
WVFGRD96   13.0   255    85    25   3.53 0.6737
WVFGRD96   14.0    75    90   -25   3.54 0.6758
WVFGRD96   15.0   255    85    25   3.55 0.6795
WVFGRD96   16.0    75    90   -20   3.57 0.6791
WVFGRD96   17.0    75    90   -20   3.58 0.6784
WVFGRD96   18.0   255    85    20   3.58 0.6795
WVFGRD96   19.0    75    90   -20   3.61 0.6740
WVFGRD96   20.0    75    90   -20   3.62 0.6704
WVFGRD96   21.0    75    90   -25   3.65 0.6656
WVFGRD96   22.0   255    85    25   3.65 0.6669
WVFGRD96   23.0   255    85    25   3.66 0.6610
WVFGRD96   24.0   255    85    25   3.67 0.6534
WVFGRD96   25.0   255    85    25   3.68 0.6458
WVFGRD96   26.0   255    80    20   3.68 0.6364
WVFGRD96   27.0   255    80    20   3.69 0.6257
WVFGRD96   28.0   255    80    20   3.70 0.6157
WVFGRD96   29.0   255    80    20   3.71 0.6026

The best solution is

WVFGRD96   18.0   255    85    20   3.58 0.6795

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 0.12 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

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=      68.28
  DIP=      85.29
 RAKE=     -20.07
  
             OR
  
  STK=     159.99
  DIP=      70.00
 RAKE=    -174.99
 
 
DEPTH = 15.0 km
 
Mw = 3.58
Best Fit 0.9089 - 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
G06A       64   27 iP_D
G05A      294   31 iP_C
H05A      202   58 iP_D
H06A      132   66 iP_C
F06A       10   72 iP_D
F05A      335   93 iP_C
G07A       81  102 iP_C
H04A      244  110 iP_D
I05A      193  110 iP_D
F07A       43  117 iP_D
G04A      275  120 iP_C
I06A      156  144 iP_C
F04A      308  145 iP_C
E06A      359  157 iP_D
I07A      135  164 iP_C
E05A      339  171 eP_-
HAWA       38  178 iP_D
E07A       28  180 iP_D
G03A      277  184 eP_+
F08A       66  185 iP_D
I04A      219  189 iP_D
H03A      256  192 eP_X
H08A      110  193 eP_+

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 distribution

Sta Az(deg)    Dist(km)   
G07A	   81	  102
H04A	  244	  110
I05A	  193	  110
F07A	   43	  117
G04A	  275	  120
I06A	  156	  144
F04A	  308	  145
PIN	  178	  147
E06A	  359	  157
I07A	  135	  164
E05A	  339	  171
HAWA	   38	  178
E07A	   28	  180
G03A	  277	  184
F08A	   66	  185
I04A	  219	  189
H03A	  256	  192
LON	  340	  192
H08A	  110	  193
J05A	  186	  206
E08A	   44	  211
I03A	  236	  225
J04A	  204	  229
I08A	  125	  233
J07A	  146	  235
D07A	   18	  241
D05A	  341	  242
F09A	   74	  247
G09A	   85	  250
I02A	  242	  261
D04A	  328	  263
H09A	  100	  265
K06A	  168	  265
D08A	   36	  266
E09A	   54	  267
K05A	  179	  267
J08A	  134	  279
I09A	  116	  286
J02A	  228	  287
K04A	  193	  287
C05A	  349	  291
C07A	   13	  293
D09A	   43	  296
G10A	   85	  301
K07A	  153	  304
F10A	   71	  305
C06A	    1	  310
J09A	  127	  323
C04A	  332	  327
L04A	  193	  337
NLWA	  319	  337
H10A	   99	  338
L05A	  178	  343
WVOR	  147	  353
D10A	   52	  356
B05A	  346	  359
G11A	   84	  369
L07A	  159	  369
K09A	  135	  374
B07A	    9	  376
B06A	  354	  379
B08A	   18	  380
F11A	   76	  384
E11A	   68	  393
H11A	   95	  393
NEW	   39	  455
MSO	   68	  575
BW06	  102	  950
TPNV	  155	  990
DGMT	   68	 1329

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 n 3
lp c 0.10 n 3
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. 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

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 Jun 15 11:02:38 CDT 2007