Location

2008/05/18 22:19:55 46.168 -119.546 19.9 3.7 Washington

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
 2008/05/18 22:19:55 46.168 -119.546 19.9 3.7 Washington
 
 Best Fitting Double Couple
    Mo = 1.01e+21 dyne-cm
    Mw = 3.27 
    Z  = 21 km
     Plane   Strike  Dip  Rake
      NP1      308    62   139
      NP2       60    55    35
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.01e+21     48     271
     N   0.00e+00     42      99
     P  -1.01e+21      4       6



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -9.97e+20
       Mxy    -1.03e+20
       Mxz    -6.58e+19
       Myy     4.52e+20
       Myz    -5.11e+20
       Mzz     5.45e+20
                                                     
                                                     
                                                     
                                                     
                     ------- P ----                  
                 -----------   --------              
              ----------------------------           
             ------------------------------          
           #########-------------------------        
          ###############---------------------       
         ###################------------------#      
        #######################--------------###     
        ##########################----------####     
       #########   #################-------######    
       ######### T ##################-----#######    
       #########   ####################--########    
       ################################-#########    
        #############################-----######     
        ##########################---------#####     
         ######################------------####      
          #################-----------------##       
           ---------------------------------#        
             ------------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  5.45e+20  -6.58e+19   5.11e+20 
 -6.58e+19  -9.97e+20   1.03e+20 
  5.11e+20   1.03e+20   4.52e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080518221955/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 = 60
      DIP = 55
     RAKE = 35
       MW = 3.27
       HS = 21.0

This is a small earthquake. The surface-wave analysis could not be performed. The waveforms are fit well using the WUS velocity model (listed below). The waveform inversion mechanism is preferred over the P-wave forst motion mechanism despite the apparent first-motion inconsistencies. This inversion wave made possible because of the nearby Transportable Array stations of USArray.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
UW
 SLU Moment Tensor Solution
 2008/05/18 22:19:55 46.168 -119.546 19.9 3.7 Washington
 
 Best Fitting Double Couple
    Mo = 1.01e+21 dyne-cm
    Mw = 3.27 
    Z  = 21 km
     Plane   Strike  Dip  Rake
      NP1      308    62   139
      NP2       60    55    35
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.01e+21     48     271
     N   0.00e+00     42      99
     P  -1.01e+21      4       6



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -9.97e+20
       Mxy    -1.03e+20
       Mxz    -6.58e+19
       Myy     4.52e+20
       Myz    -5.11e+20
       Mzz     5.45e+20
                                                     
                                                     
                                                     
                                                     
                     ------- P ----                  
                 -----------   --------              
              ----------------------------           
             ------------------------------          
           #########-------------------------        
          ###############---------------------       
         ###################------------------#      
        #######################--------------###     
        ##########################----------####     
       #########   #################-------######    
       ######### T ##################-----#######    
       #########   ####################--########    
       ################################-#########    
        #############################-----######     
        ##########################---------#####     
         ######################------------####      
          #################-----------------##       
           ---------------------------------#        
             ------------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  5.45e+20  -6.58e+19   5.11e+20 
 -6.58e+19  -9.97e+20   1.03e+20 
  5.11e+20   1.03e+20   4.52e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080518221955/index.html
	

P-wave First Motion Fault Plane Solution for 08051822195e

Fault Plane Parameters for 08051822195e
Fault Choice 1Fault Choice 2
Strike(deg)60.0254.0
Dip(deg)45.046.0
Rake(deg)80.099.8
Fault Typereversereverse

        

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 0.05 n 4
lp c 0.30 n 4
br c 0.16 0.25 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   300    45   -90   2.31 0.1003
WVFGRD96    1.0   305    35   -90   2.26 0.0871
WVFGRD96    2.0   305    40   -85   2.53 0.1073
WVFGRD96    3.0    70    60    85   2.54 0.0983
WVFGRD96    4.0    60    70    65   2.61 0.1036
WVFGRD96    5.0    60    85    75   2.70 0.1102
WVFGRD96    6.0   230    65    45   2.82 0.1410
WVFGRD96    7.0   225    65    40   2.92 0.1769
WVFGRD96    8.0   225    60    45   3.03 0.2087
WVFGRD96    9.0    45    50    30   3.06 0.2529
WVFGRD96   10.0    50    50    40   3.12 0.3017
WVFGRD96   11.0    55    50    50   3.16 0.3466
WVFGRD96   12.0    55    50    50   3.18 0.3745
WVFGRD96   13.0    55    50    45   3.20 0.4008
WVFGRD96   14.0    55    50    45   3.21 0.4224
WVFGRD96   15.0    55    50    40   3.22 0.4413
WVFGRD96   16.0    55    50    40   3.23 0.4579
WVFGRD96   17.0    55    50    40   3.23 0.4729
WVFGRD96   18.0    55    55    30   3.25 0.4899
WVFGRD96   19.0    55    55    30   3.26 0.5022
WVFGRD96   20.0    55    55    30   3.26 0.5035
WVFGRD96   21.0    60    55    35   3.27 0.5083
WVFGRD96   22.0    60    55    40   3.27 0.5027
WVFGRD96   23.0    60    50    40   3.26 0.4919
WVFGRD96   24.0    60    50    40   3.26 0.4883
WVFGRD96   25.0    60    50    40   3.26 0.4823
WVFGRD96   26.0    60    50    35   3.27 0.4806
WVFGRD96   27.0    60    55    35   3.27 0.4771
WVFGRD96   28.0    60    55    35   3.27 0.4698
WVFGRD96   29.0    60    55    40   3.26 0.4718

The best solution is

WVFGRD96   21.0    60    55    35   3.27 0.5083

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 0.05 n 4
lp c 0.30 n 4
br c 0.16 0.25 n 4 p 2
Figure 3. Waveform comparison for selected depth
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.

UW P-Wave First Motion Mechanism

The Pacific Northwest Network published the first motion focal mechanism shown above. The URL is

http://www.pnsn.org/SEIS/EQ_Special/WEBDIR_08051822195e/foc.html

The source parameters from that page are Strike 60, Dip 45 and Rake 80.0

Waveform comparison for this mechanism

To check that mechanism against the observed waveforms, synthetics were made using the PNW focal mechanism parameters. In addition the forward predictions assume Mw=3.4 and a source depth of HS=20.0 km.

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 0.05 n 3
lp c 0.30 n 3
br c 0.16 0.25 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.

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

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=Mon May 19 09:34:56 CDT 2008

Last Changed 2008/05/18