Location

2001/02/28 18:54:33 47.1530 -122.7190 52.4 6.80 Washington

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Pacific Northwest

Focal Mechanism

 SLU Moment Tensor Solution
 2001/02/28 18:54:33  47.1530  -122.7190  52.4  6.80 Washington
 
 Best Fitting Double Couple
    Mo = 1.23e+26 dyne-cm
    Mw = 6.66 
    Z  = 56 km
     Plane   Strike  Dip  Rake
      NP1        5    70   -85
      NP2      171    21   -103
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.23e+26     25      91
     N   0.00e+00      5     183
     P  -1.23e+26     65     283



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.15e+24
       Mxy     3.08e+24
       Mxz    -1.18e+25
       Myy     7.99e+25
       Myz     9.32e+25
       Mzz    -7.88e+25
                                                     
                                                     
                                                     
                                                     
                     #--------#####                  
                 ##------------########              
              ###---------------##########           
             ###----------------###########          
           ###-------------------############        
          ###--------------------#############       
         ####--------------------##############      
        ####---------------------###############     
        ####---------------------###############     
       ####---------   ----------################    
       ####--------- P ----------#########   ####    
       ####---------   ----------######### T ####    
       #####---------------------#########   ####    
        ####---------------------###############     
        ####--------------------################     
         ####-------------------###############      
          ####-----------------###############       
           ####----------------##############        
             ####-------------#############          
              #####----------#############           
                 ####-------###########              
                     ####--########                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -7.88e+25  -1.18e+25  -9.32e+25 
 -1.18e+25  -1.15e+24  -3.08e+24 
 -9.32e+25  -3.08e+24   7.99e+25 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20010228185433/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 = 5
      DIP = 70
     RAKE = -85
       MW = 6.66
       HS = 56.0

The waveform inversion is preferred. The velocity model is too simple to adequately describe the upper mantle velocities at a depth of 58 km. This is just a test of the capability to get Mw from these deep events in the Pacific northwest.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
GCMT
 SLU Moment Tensor Solution
 2001/02/28 18:54:33  47.1530  -122.7190  52.4  6.80 Washington
 
 Best Fitting Double Couple
    Mo = 1.23e+26 dyne-cm
    Mw = 6.66 
    Z  = 56 km
     Plane   Strike  Dip  Rake
      NP1        5    70   -85
      NP2      171    21   -103
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.23e+26     25      91
     N   0.00e+00      5     183
     P  -1.23e+26     65     283



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.15e+24
       Mxy     3.08e+24
       Mxz    -1.18e+25
       Myy     7.99e+25
       Myz     9.32e+25
       Mzz    -7.88e+25
                                                     
                                                     
                                                     
                                                     
                     #--------#####                  
                 ##------------########              
              ###---------------##########           
             ###----------------###########          
           ###-------------------############        
          ###--------------------#############       
         ####--------------------##############      
        ####---------------------###############     
        ####---------------------###############     
       ####---------   ----------################    
       ####--------- P ----------#########   ####    
       ####---------   ----------######### T ####    
       #####---------------------#########   ####    
        ####---------------------###############     
        ####--------------------################     
         ####-------------------###############      
          ####-----------------###############       
           ####----------------##############        
             ####-------------#############          
              #####----------#############           
                 ####-------###########              
                     ####--########                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -7.88e+25  -1.18e+25  -9.32e+25 
 -1.18e+25  -1.15e+24  -3.08e+24 
 -9.32e+25  -3.08e+24   7.99e+25 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20010228185433/index.html
	
 Global CMT

 Best Fitting Double Couple
    Mo = 1.78e+26 dyne-cm
    Mw = 6.80
    Z  = 48 km
     Plane   Strike  Dip  Rake
      NP1        2    73   -88
      NP2      176    17   -96
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.78e+26     28      91
     N   0.00e+00      2     182
     P  -1.78e+26     62     275



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -2.75e+23
       Mxy     1.50e+24
       Mxz    -7.50e+24
       Myy     9.92e+25
       Myz     1.48e+26
       Mzz    -9.89e+25




                     ##------######
                 ##-----------#########
              ###--------------###########
             ##----------------############
           ###------------------#############
          ###-------------------##############
         ###--------------------###############
        ###---------------------################
        ###---------------------################
       ####--------   ----------#################
       ####-------- P ----------##########   ####
       ####--------   ----------########## T ####
       ####---------------------##########   ####
        ###---------------------################
        ####--------------------################
         ###-------------------################
          ###------------------###############
           ###-----------------##############
             ###--------------#############
              ####------------############
                 ###---------##########
                     ###----#######




 Harvard Convention
 Moment Tensor:
      R          T          F
 -9.89e+25  -7.50e+24  -1.48e+26
 -7.50e+24  -2.75e+23  -1.50e+24
 -1.48e+26  -1.50e+24   9.92e+25
                                                                         
---------------------
Event name: 022801L

Region name: WASHINGTON
Date (y/m/d): 2001/2/28

Information on data used in inversion

Wave    nsta  nrec  cutoff
Body      68   178   45
Mantle    66   161   135
Surface    0     0   0

Timing and location information

         hr  min   sec       lat     lon    depth   mb   Ms
PDE      18   54  32.80     47.15  -122.73   51.9  6.5  6.6
CMT      18   54  37.30     47.14  -122.53   46.8
Error              0.10      0.01     0.01    0.3
Assumed half duration:  6.1


Mechanism information

Exponent for moment tensor:  26    units: dyne-cm
         Mrr     Mtt     Mpp     Mrt     Mrp     Mtp
CMT    -0.960  -0.054   1.013  -0.060  -1.453  -0.019
Error   0.006   0.004   0.005   0.009   0.013   0.004

Mw = 6.8   Scalar Moment = 1.76e+26
Fault plane:  strike=176    dip=17   slip=-96
Fault plane:  strike=2    dip=73   slip=-88
Eigenvector:  eigenvalue:  1.78   plunge: 28   azimuth:  90
Eigenvector:  eigenvalue: -0.05   plunge:  2   azimuth: 181
Eigenvector:  eigenvalue: -1.73   plunge: 62   azimuth: 275

        

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 0.01 n 3
lp c 0.050 n 3
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   205    40    95   5.93 0.2841
WVFGRD96    1.0    20    45    85   5.99 0.3139
WVFGRD96    2.0    25    45    95   6.07 0.3778
WVFGRD96    3.0    25    45    95   6.16 0.4230
WVFGRD96    4.0    20    50    95   6.21 0.4086
WVFGRD96    5.0   185    45    75   6.23 0.3538
WVFGRD96    6.0   330    90     5   6.23 0.3192
WVFGRD96    7.0   330    90     5   6.26 0.3148
WVFGRD96    8.0   330    90    10   6.29 0.3063
WVFGRD96    9.0   145    80   -15   6.29 0.2910
WVFGRD96   10.0   145    80   -15   6.30 0.2817
WVFGRD96   11.0    15    75    70   6.17 0.2737
WVFGRD96   12.0    10    80    70   6.17 0.2904
WVFGRD96   13.0    10    80    70   6.18 0.3070
WVFGRD96   14.0    10    80    70   6.18 0.3224
WVFGRD96   15.0    10    80    70   6.19 0.3367
WVFGRD96   16.0    10    80    70   6.20 0.3510
WVFGRD96   17.0    10    80    70   6.20 0.3636
WVFGRD96   18.0    10    80    70   6.21 0.3765
WVFGRD96   19.0    10    80    70   6.22 0.3885
WVFGRD96   20.0     5    80    70   6.23 0.3995
WVFGRD96   21.0     5    80    70   6.25 0.4114
WVFGRD96   22.0     5    80    70   6.26 0.4223
WVFGRD96   23.0     5    80    70   6.27 0.4328
WVFGRD96   24.0     5    80    70   6.28 0.4431
WVFGRD96   25.0     0    75    65   6.30 0.4531
WVFGRD96   26.0   270    10   -15   6.27 0.4643
WVFGRD96   27.0   265    10   -20   6.28 0.4759
WVFGRD96   28.0   265    10   -20   6.29 0.4870
WVFGRD96   29.0   255    10   -25   6.30 0.4974
WVFGRD96   30.0   250    10   -25   6.32 0.5076
WVFGRD96   31.0   250    10   -25   6.33 0.5173
WVFGRD96   32.0   235    10   -40   6.34 0.5265
WVFGRD96   33.0   235    10   -40   6.35 0.5351
WVFGRD96   34.0   230    10   -45   6.35 0.5431
WVFGRD96   35.0   225    10   -50   6.36 0.5505
WVFGRD96   36.0     5    80   -85   6.37 0.5568
WVFGRD96   37.0     5    80   -95   6.37 0.5634
WVFGRD96   38.0   190    10   -85   6.38 0.5691
WVFGRD96   39.0     5    80   -95   6.38 0.5741
WVFGRD96   40.0   165    10  -110   6.53 0.5729
WVFGRD96   41.0   160    10  -110   6.55 0.5832
WVFGRD96   42.0     0    80   -90   6.56 0.5923
WVFGRD96   43.0     0    80   -90   6.56 0.6009
WVFGRD96   44.0     0    80   -90   6.57 0.6084
WVFGRD96   45.0     0    75   -85   6.59 0.6169
WVFGRD96   46.0     0    75   -85   6.59 0.6244
WVFGRD96   47.0   210    15   -65   6.59 0.6287
WVFGRD96   48.0   210    15   -65   6.60 0.6337
WVFGRD96   49.0   210    15   -65   6.61 0.6376
WVFGRD96   50.0   210    15   -65   6.61 0.6408
WVFGRD96   51.0   210    15   -65   6.62 0.6431
WVFGRD96   52.0     0    75   -90   6.63 0.6485
WVFGRD96   53.0     5    70   -85   6.65 0.6502
WVFGRD96   54.0     5    70   -85   6.65 0.6519
WVFGRD96   55.0     5    70   -85   6.66 0.6528
WVFGRD96   56.0     5    70   -85   6.66 0.6529
WVFGRD96   57.0     5    70   -85   6.67 0.6523
WVFGRD96   58.0     5    70   -85   6.67 0.6510
WVFGRD96   59.0     5    70   -85   6.68 0.6491
WVFGRD96   60.0     5    70   -85   6.68 0.6464
WVFGRD96   61.0     5    70   -80   6.69 0.6435
WVFGRD96   62.0     5    70   -80   6.69 0.6401
WVFGRD96   63.0     5    70   -80   6.69 0.6362
WVFGRD96   64.0     5    70   -80   6.70 0.6317
WVFGRD96   65.0     5    70   -80   6.70 0.6267
WVFGRD96   66.0    10    70   -80   6.70 0.6216
WVFGRD96   67.0    10    70   -80   6.70 0.6165
WVFGRD96   68.0    10    70   -80   6.71 0.6109
WVFGRD96   69.0    10    70   -75   6.71 0.6054

The best solution is

WVFGRD96   56.0     5    70   -85   6.66 0.6529

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 0.01 n 3
lp c 0.050 n 3
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.

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=Sun Dec 21 13:15:36 CST 2008

Last Changed 2001/02/28