Location

2007/06/10 18:06:06 66.27N 142.30W 10 4.4 Alaska

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Alaska

Focal Mechanism

 SLU Moment Tensor Solution
 2007/06/10 18:06:06 66.27N 142.30W 10 4.4 Alaska
 
 Best Fitting Double Couple
    Mo = 3.20e+22 dyne-cm
    Mw = 4.27 
    Z  = 12 km
     Plane   Strike  Dip  Rake
      NP1      145    85   -175
      NP2       55    85    -5
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.20e+22      0     280
     N   0.00e+00     83     190
     P  -3.20e+22      7      10



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -2.95e+22
       Mxy    -1.11e+22
       Mxz    -3.84e+21
       Myy     3.00e+22
       Myz    -7.00e+20
       Mzz    -4.84e+20
                                                     
                                                     
                                                     
                                                     
                     --------- P --                  
                 -------------   ------              
              ###-------------------------           
             #####-------------------------          
           ########--------------------------        
          ##########-----------------------###       
         ############--------------------######      
        ##############-----------------#########     
         ##############-------------############     
       T ################---------###############    
         #################------#################    
       ####################--####################    
       ###################--#####################    
        ###############-------##################     
        #############----------#################     
         #########--------------###############      
          ####--------------------############       
           ------------------------##########        
             ------------------------######          
              ------------------------####           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -4.84e+20  -3.84e+21   7.00e+20 
 -3.84e+21  -2.95e+22   1.11e+22 
  7.00e+20   1.11e+22   3.00e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070610180606/index.html
        
Alaska Earthquake Information Center Moment Tensor
http://www.giseis.alaska.edu/cgi-bin/moment_tensors.pl

Preferred Solution

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

      STK = 235
      DIP = 95
     RAKE = 5
       MW = 4.27
       HS = 12

The solution is well determined and agrees with the AEIC solution

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    50    65   -25   4.20 0.6185
WVFGRD96    1.0    50    65   -25   4.21 0.6248
WVFGRD96    2.0    50    60   -20   4.22 0.6299
WVFGRD96    3.0    55    80     0   4.17 0.6433
WVFGRD96    4.0    55    80     5   4.19 0.6545
WVFGRD96    5.0    55    80    -5   4.20 0.6637
WVFGRD96    6.0    55    80    -5   4.21 0.6720
WVFGRD96    7.0    55    80    -5   4.22 0.6800
WVFGRD96    8.0   235   100     5   4.23 0.6879
WVFGRD96    9.0   235   100     5   4.24 0.6949
WVFGRD96   10.0   235   100     5   4.25 0.7005
WVFGRD96   11.0   235   100     5   4.26 0.7034
WVFGRD96   12.0   235    95     5   4.27 0.7046
WVFGRD96   13.0   240    80    10   4.27 0.7043
WVFGRD96   14.0   240    80     5   4.28 0.7042
WVFGRD96   15.0   240    80     5   4.29 0.7033
WVFGRD96   16.0   240    80     5   4.29 0.7010
WVFGRD96   17.0   240    80     5   4.30 0.6971
WVFGRD96   18.0   240    80     5   4.31 0.6921
WVFGRD96   19.0   240    80     5   4.32 0.6858

The best solution is

WVFGRD96   12.0   235    95     5   4.27 0.7046

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=     326.03
  DIP=      75.52
 RAKE=     164.50
  
             OR
  
  STK=      59.99
  DIP=      75.00
 RAKE=      15.00
 
 
DEPTH = 12.0 km
 
Mw = 4.37
Best Fit 0.7638 - 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
EGAK      162  175 iP_D
DAWY      150  281 iP_D
MCK       231  422 iP_C
INK        55  441 iP_C
DIV       198  598 -12345
EYAK      197  660 -12345
WHY       146  726 -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)   
EGAK	  162	  175
DAWY	  150	  281
MCK	  231	  422
INK	   55	  441
DIV	  198	  598
EYAK	  197	  660
WHY	  146	  726
DLBC	  138	 1076
TNA	  278	 1162
WRAK	  150	 1217
FNBB	  121	 1279
MOBC	  154	 1565
BBB	  146	 1756
UNV	  235	 1908
PHC	  147	 1927
RES	   41	 1928
EDB	  147	 2021
CBB	  144	 2050
NIKO	  238	 2077
LLLB	  136	 2092
SHB	  141	 2136
EDM	  118	 2147
OZB	  145	 2157
SLEB	  129	 2160
PNT	  134	 2292
NLWA	  144	 2363
NEW	  132	 2479
WALA	  126	 2514
FCC	   86	 2545
HAWA	  137	 2590
MSO	  128	 2730
BMO	  135	 2816
DGMT	  113	 2949
WVOR	  140	 3025
LAO	  118	 3034
HLID	  132	 3041
FLWY	  127	 3118
ULM	  102	 3146
AHID	  128	 3235
FRB	   60	 3287
DUG	  133	 3437
EYMN	  100	 3544
TPNV	  140	 3667
ECSD	  110	 3710
KAPO	   89	 3813
COWI	   99	 3815
MVCO	  129	 3888
CBKS	  117	 4056
JFWS	  104	 4060
GLMI	   96	 4132
AAM	   97	 4409
WMOK	  119	 4499
ERPA	   94	 4590

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

Appendix A

The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the theoretical predictions as a function of period for the mechanism given above. The CUS model earth model was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.

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 Mon Jun 11 10:54:09 CDT 2007