Location

2008/01/17 19:46:45 68.015 -136.149 10.0 5.2 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
 2008/01/17 19:46:45 68.015 -136.149 10.0 5.2 Yukon, Canada
 
 Best Fitting Double Couple
    Mo = 3.02e+23 dyne-cm
    Mw = 4.92 
    Z  = 18 km
     Plane   Strike  Dip  Rake
      NP1      155    76   159
      NP2      250    70    15
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.02e+23     24     111
     N   0.00e+00     65     302
     P  -3.02e+23      4     203



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -2.21e+23
       Mxy    -1.94e+23
       Mxz    -2.21e+22
       Myy     1.70e+23
       Myz     1.14e+23
       Mzz     5.02e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ##--------------------              
              #####-----------------------           
             #######-----------------------          
           #########-------------------------        
          ##########--------------------------       
         ############--------------------------      
        #############-----------################     
        ##############----######################     
       ##############--##########################    
       ###########------#########################    
       ########---------#########################    
       ######------------########################    
        ###---------------###############   ####     
        #------------------############## T ####     
         -------------------#############   ###      
          --------------------################       
           --------------------##############        
             -------------------###########          
              -----   ------------########           
                 -- P -------------####              
                      -------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  5.02e+22  -2.21e+22  -1.14e+23 
 -2.21e+22  -2.21e+23   1.94e+23 
 -1.14e+23   1.94e+23   1.70e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080117194645/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 = 250
      DIP = 70
     RAKE = 15
       MW = 4.92
       HS = 18

Both techniques provide the same solution, even though there is poor depth control. The presence of strong S-pulses at MCK and BPAW and PNL supports the depth, since the higher modes forming the S-pulse stand out from the fundamental mode spectrum when the source is deep in the crust.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 SLU Moment Tensor Solution
 2008/01/17 19:46:45 68.015 -136.149 10.0 5.2 Yukon, Canada
 
 Best Fitting Double Couple
    Mo = 3.02e+23 dyne-cm
    Mw = 4.92 
    Z  = 18 km
     Plane   Strike  Dip  Rake
      NP1      155    76   159
      NP2      250    70    15
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.02e+23     24     111
     N   0.00e+00     65     302
     P  -3.02e+23      4     203



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -2.21e+23
       Mxy    -1.94e+23
       Mxz    -2.21e+22
       Myy     1.70e+23
       Myz     1.14e+23
       Mzz     5.02e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ##--------------------              
              #####-----------------------           
             #######-----------------------          
           #########-------------------------        
          ##########--------------------------       
         ############--------------------------      
        #############-----------################     
        ##############----######################     
       ##############--##########################    
       ###########------#########################    
       ########---------#########################    
       ######------------########################    
        ###---------------###############   ####     
        #------------------############## T ####     
         -------------------#############   ###      
          --------------------################       
           --------------------##############        
             -------------------###########          
              -----   ------------########           
                 -- P -------------####              
                      -------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  5.02e+22  -2.21e+22  -1.14e+23 
 -2.21e+22  -2.21e+23   1.94e+23 
 -1.14e+23   1.94e+23   1.70e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080117194645/index.html
	

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 0.06 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   245    75   -45   4.85 0.7883
WVFGRD96    1.0    60    70   -30   4.81 0.7998
WVFGRD96    2.0    60    70   -40   4.85 0.8105
WVFGRD96    3.0    55    70   -30   4.85 0.8070
WVFGRD96    4.0   245    65     0   4.82 0.8142
WVFGRD96    5.0   250    65     0   4.82 0.8307
WVFGRD96    6.0   250    65     0   4.83 0.8451
WVFGRD96    7.0   250    65     0   4.83 0.8577
WVFGRD96    8.0   250    65     5   4.84 0.8685
WVFGRD96    9.0   250    70     5   4.85 0.8784
WVFGRD96   10.0   250    65     5   4.86 0.8869
WVFGRD96   11.0   250    65    10   4.87 0.8933
WVFGRD96   12.0   250    65    10   4.87 0.8984
WVFGRD96   13.0   250    65    10   4.88 0.9022
WVFGRD96   14.0   250    70    10   4.89 0.9052
WVFGRD96   15.0   250    70    15   4.90 0.9076
WVFGRD96   16.0   250    70    15   4.91 0.9092
WVFGRD96   17.0   250    70    15   4.91 0.9100
WVFGRD96   18.0   250    70    15   4.92 0.9101
WVFGRD96   19.0   250    70    15   4.92 0.9095
WVFGRD96   20.0   250    65    15   4.94 0.9080
WVFGRD96   21.0   250    65    15   4.94 0.9054
WVFGRD96   22.0   250    65    15   4.95 0.9019
WVFGRD96   23.0   250    65    15   4.95 0.8976
WVFGRD96   24.0   255    65    25   4.97 0.8930
WVFGRD96   25.0   255    65    25   4.97 0.8881
WVFGRD96   26.0   255    65    25   4.98 0.8823
WVFGRD96   27.0   250    70    25   4.99 0.8759
WVFGRD96   28.0   250    70    25   5.00 0.8692
WVFGRD96   29.0   250    70    25   5.01 0.8620

The best solution is

WVFGRD96   18.0   250    70    15   4.92 0.9101

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 0.06 n 3
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=     157.89
  DIP=      71.25
 RAKE=     158.83
  
             OR
  
  STK=     254.98
  DIP=      70.00
 RAKE=      20.00
 
 
DEPTH = 16.0 km
 
Mw = 4.98
Best Fit 0.7911 - 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
INK        72  114 -12345
EGAK      214  425 -12345
DAWY      200  464 -12345
COLD      268  602 -12345
COLA      242  627 -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)   
INK	   72	  114
EGAK	  214	  425
DAWY	  200	  464
COLD	  268	  602
DOT	  220	  606
COLA	  242	  627
PAX	  222	  707
MCK	  237	  751
BPAW	  244	  798
TRF	  238	  821
WHY	  175	  822
KTH	  241	  836
SAW	  225	  898
PPLA	  240	  933
PMR	  227	  941
PNL	  191	  944
EYAK	  214	  951
SKAG	  177	  954
RC01	  226	 1006
FIB	  228	 1012
BESE	  176	 1054
DLBC	  161	 1110
FNBB	  141	 1204
WRAK	  170	 1307
CRAG	  172	 1407
TNA	  274	 1408
RES	   45	 1605
MOBC	  170	 1672
BBB	  162	 1816
PHC	  162	 1986
EDM	  132	 2042
EDB	  162	 2081
CBB	  158	 2090
LLLB	  151	 2090
SHB	  156	 2161
PNT	  148	 2276
FCC	   96	 2282
BILL	  297	 2310
NLWA	  157	 2400
NEW	  144	 2445
WALA	  138	 2446
HAWA	  150	 2589
EGMT	  133	 2673
MSO	  140	 2676
BMO	  117	 2802
DGMT	  124	 2810
BOZ	  138	 2862
DLMT	  140	 2865
LAO	  129	 2919
HUMO	  158	 2935
ULM	  112	 2946
RLMT	  135	 2984
HLID	  144	 3007
WVOR	  151	 3032
MOOW	  138	 3084
DCID1	  139	 3088
LOHW	  138	 3102
RRI2	  139	 3110
SNOW	  138	 3114
AGMN	  114	 3130
AHID	  139	 3177
BW06	  137	 3220
RSSD	  128	 3250
ELK	  147	 3287
EYMN	  109	 3333
DUG	  144	 3404
ECSD	  119	 3546
OGNE	  128	 3639
ISCO	  134	 3644
MVCO	  139	 3832
GSC	  152	 3839
SCIA	  117	 3853
JFWS	  112	 3866
OSI	  154	 3877
GLMI	  105	 3905
MWC	  154	 3932
CBKS	  127	 3933
WUAZ	  144	 3937
KSU1	  123	 4011
BAR	  152	 4126
ANMO	  138	 4128
HDIL	  113	 4135
AAM	  106	 4186
AMTX	  132	 4277
SLM	  116	 4291
TUC	  144	 4295
ERPA	  102	 4353
BLO	  111	 4380
WMOK	  128	 4385
SIUC	  115	 4421
USIN	  113	 4455
PBMO	  117	 4470
LBNH	   92	 4484
ACCN	   95	 4493
PKME	   89	 4497
BINY	   98	 4510
SSPA	  101	 4582
MIAR	  122	 4583
MCWV	  104	 4598
WVT	  115	 4633
MVL	  100	 4708
PAL	   97	 4711
SDMD	  101	 4745
TZTN	  110	 4757
JCT	  132	 4796
BLA	  106	 4815
CBN	  102	 4841
HKT	  127	 4978
GOGA	  112	 5075
NHSC	  108	 5234

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 0.06 n 3

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.

Appendix A


Spectra fit plots to each station

Velocity Model

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


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=Fri Jan 18 10:21:21 CST 2008

Last Changed 2008/01/17