Location

2008/01/30 10:07:07 62.433 -137.052 10.0 4.4 Yukon, Canada

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
 2008/01/30 10:07:07 62.433 -137.052 10.0 4.4 Yukon, Canada
 
 Best Fitting Double Couple
    Mo = 3.43e+22 dyne-cm
    Mw = 4.29 
    Z  = 12 km
     Plane   Strike  Dip  Rake
      NP1      302    56   113
      NP2       85    40    60
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.43e+22     69     263
     N   0.00e+00     19     109
     P  -3.43e+22      9      16



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -3.09e+22
       Mxy    -8.31e+21
       Mxz    -6.28e+21
       Myy     1.69e+21
       Myz    -1.26e+22
       Mzz     2.92e+22
                                                     
                                                     
                                                     
                                                     
                     -----------                     
                 --------------- P ----              
              ------------------   -------           
             ------------------------------          
           ----####--------------------------        
          #################-------------------       
         ######################----------------      
        ##########################--------------     
        ############################------------     
       ###############################-----------    
       ##############   ################--------#    
       ############## T ##################------#    
       -#############   ###################---###    
        -###################################-###     
        ---################################--###     
         ----############################----##      
          ------#####################---------       
           -----------##########-------------        
             ------------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  2.92e+22  -6.28e+21   1.26e+22 
 -6.28e+21  -3.09e+22   8.31e+21 
  1.26e+22   8.31e+21   1.69e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080130100707/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 = 85
      DIP = 40
     RAKE = 60
       MW = 4.29
       HS = 12

The waveform inversion solution is preferred. The surface-wave amplitude spectrum result is simliar.

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 0.25 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    80    85   -10   4.09 0.5057
WVFGRD96    1.0    80    80   -10   4.11 0.5075
WVFGRD96    2.0    80    75   -20   4.15 0.4834
WVFGRD96    3.0   260    90    45   4.20 0.4727
WVFGRD96    4.0    80    90   -45   4.20 0.4873
WVFGRD96    5.0   260    90    45   4.20 0.5037
WVFGRD96    6.0    75    85   -45   4.19 0.5186
WVFGRD96    7.0   120    30    80   4.33 0.5457
WVFGRD96    8.0   115    35    75   4.33 0.5716
WVFGRD96    9.0   110    35    70   4.31 0.5827
WVFGRD96   10.0   115    35    75   4.34 0.5861
WVFGRD96   11.0    85    40    60   4.30 0.5888
WVFGRD96   12.0    85    40    60   4.29 0.5914
WVFGRD96   13.0    85    40    60   4.29 0.5897
WVFGRD96   14.0    85    40    55   4.29 0.5863
WVFGRD96   15.0    80    40    50   4.29 0.5816
WVFGRD96   16.0    80    40    50   4.30 0.5755
WVFGRD96   17.0    80    40    50   4.30 0.5683
WVFGRD96   18.0    80    40    50   4.31 0.5606
WVFGRD96   19.0    75    45    45   4.31 0.5518
WVFGRD96   20.0    75    40    45   4.34 0.5413
WVFGRD96   21.0    75    40    40   4.35 0.5328
WVFGRD96   22.0    75    40    40   4.36 0.5232
WVFGRD96   23.0    75    40    40   4.36 0.5122
WVFGRD96   24.0    75    40    40   4.37 0.5029
WVFGRD96   25.0    70    40    35   4.38 0.4910
WVFGRD96   26.0    70    40    35   4.38 0.4788
WVFGRD96   27.0    70    40    35   4.39 0.4678
WVFGRD96   28.0    70    40    35   4.40 0.4545
WVFGRD96   29.0    70    40    35   4.40 0.4427

The best solution is

WVFGRD96   12.0    85    40    60   4.29 0.5914

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 0.25 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=     329.30
  DIP=      58.23
 RAKE=     137.57
  
             OR
  
  STK=      84.99
  DIP=      55.00
 RAKE=      40.00
 
 
DEPTH = 12.0 km
 
Mw = 4.35
Best Fit 0.8116 - 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
DAWY      328  217 -12345
WHY       149  229 -12345
EGAK      324  331 -12345
PNL       203  333 -12345
SKAG      164  344 -12345
DOT       294  379 -12345
PAX       282  435 -12345
EYAK      249  508 -12345
DLBC      136  589 -12345
COLA      302  599 -12345
MCK       289  617 -12345
PMR       267  639 -12345

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.

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

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
DAWY	  328	  217
WHY	  149	  229
EGAK	  324	  331
PNL	  203	  333
SKAG	  164	  344
DOT	  294	  379
PAX	  282	  435
BESE	  163	  446
EYAK	  250	  508
DLBC	  136	  589
SAW	  268	  592
COLA	  302	  599
SIT	  170	  607
MCK	  289	  617
PMR	  267	  639
WRAK	  156	  721
BPAW	  291	  722
COLD	  316	  819
FNBB	  111	  861
CTLN	   69	 1068
BMBC	  123	 1105
COWN	   65	 1301
EDM	  116	 1726
PGC	  145	 1750
PNT	  136	 1810
NLWA	  148	 1867
WALA	  126	 2059
HAWA	  140	 2102
ARVN	   74	 2232
FCC	   80	 2344
WVOR	  143	 2533
LAO	  118	 2610
LOHW	  128	 2706
SNOW	  129	 2714
ELK	  139	 2816
SMCO	  128	 3284
JFWS	  104	 3714
KSU1	  115	 3749
HDIL	  105	 3971
WMOK	  121	 4065
AAM	   98	 4102

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 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 0.25 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

Appendix A


Spectra fit plots to each station

Velocity Model

The CUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 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=Wed Jan 30 10:35:17 CST 2008

Last Changed 2008/01/30