Location

2006/09/01 23:44:49 66.43N 142.44W 10 4.7 Alaska

2006/09/01 23:44:44 66.780 -141.746  9.7 4.7 AEIC
2006/09/01 23:44:49 66.43  -142.44  10*      SLU Elocate JB Tables


  CHOOSE VELOCITY MODEL
 TELE     = Model( 1) P       S
 BEAM     = Model( 2) P
 HALF     = Model( 3) P       S
 CUS      = Model( 4) P       S
 UPL      = Model( 5) P       S       Lg
 EMBN     = Model( 6) P       Ps      Sp      S
  WHICH MODEL ? 1 -  6
1
STA   IWT       ARRIVAL TIME   PhIDQL PHASE  FM CHAN
BMR     0 20060901234614.197    1 0 i P        D  Z
BMR     2 20060901234742.054    0 0 e Lg       X  Z      NOT USED
BPAW    2 20060901234555.202    1 0 e P        +  Z
BPAW    2 20060901234701.957    0 0 e Lg       X  Z      NOT USED
CHUM    2 20060901234604.196    1 0 e P        X  Z
CHUM    2 20060901234728.579    0 0 e Lg       X  Z      NOT USED
COLA    2 20060901234534.607    1 0 e P        +  Z
COLA    2 20060901234617.742    0 0 e Lg       X  Z      NOT USED
COLD    2 20060901234542.717    1 0 e P        +  Z
COLD    2 20060901234634.680    0 0 e Lg       X  Z      NOT USED
DIV     0 20060901234613.857    1 0 i P        D  Z
DIV     2 20060901234735.431    0 0 e Lg       X  Z      NOT USED
EYAK    0 20060901234621.403    1 0 i P        D  Z
FIB     2 20060901234623.135    1 0 e P        X  Z
KTH     0 20060901234600.190    1 0 i P        C  Z
KTH     2 20060901234713.043    0 0 e Lg       X  Z      NOT USED
MCK     2 20060901234549.915    1 0 e P        +  Z
MCK     2 20060901234648.036    0 0 e Lg       X  Z      NOT USED
PPLA    2 20060901234612.005    1 0 e P        X  Z
RC01    2 20060901234625.755    1 0 e P        X  Z
TRF     2 20060901234558.729    1 0 e P        +  Z
TRF     2 20060901234708.793    0 0 e Lg       X  Z      NOT USED
DAWY    2 20060901234532.030    1 0 e P        -  Z
DAWY    2 20060901234608.713    0 0 e Lg       X  Z      NOT USED
INK     2 20060901234552.108    1 0 e P        X  Z
INK     2 20060901234637.508    2 0 e S        X  Z
WHY     2 20060901234627.156    1 0 e P        X  Z
 Enter Teleseism Test Latitude and Longitude
66 -142
  enter depth, depth < 0 is fixed at abs(depth)
-10
    66.0000 -142.0000     10.00 20060901234451.582      124.58
    66.3470 -142.3040     10.00 20060901234449.630       15.84
    66.3593 -142.3154     10.00 20060901234449.556       15.72
    66.3593 -142.3151     10.00 20060901234449.555       15.72
    66.3593 -142.3151     10.00 20060901234449.545        9.49
    66.3759 -142.3413     10.00 20060901234449.446        8.71
    66.3900 -142.3649     10.00 20060901234449.372        7.93
    66.4013 -142.3852     10.00 20060901234449.319        7.27
    66.4100 -142.4014     10.00 20060901234449.282        6.76
    66.4162 -142.4133     10.00 20060901234449.255        6.39
    66.4204 -142.4215     10.00 20060901234449.238        6.15
    66.4231 -142.4268     10.00 20060901234449.226        6.02
    66.4248 -142.4301     10.00 20060901234449.218        5.96
    66.4258 -142.4322     10.00 20060901234449.214        5.92
    66.4264 -142.4334     10.00 20060901234449.211        5.89
    66.4268 -142.4342     10.00 20060901234449.209        5.88
    66.4270 -142.4347     10.00 20060901234449.208        5.87
    66.4272 -142.4349     10.00 20060901234449.207        5.87
    66.4273 -142.4351     10.00 20060901234449.207        5.86
 17 phases used

 STA  COMP DIS(D) AZM  AIN    ARR TIME             RES(SEC) WT QFM PHASE    WGT
 DAWY   Z    2.69 150.  68. 20060901234532.030        -1.18  2 e- P        0.10
 COLA   Z    2.72 238.  68. 20060901234534.607         0.86  2 e+ P        0.14
 COLD   Z    3.16 288.  66. 20060901234542.717         2.59  2 e+ P        0.04
 MCK    Z    3.85 228.  64. 20060901234549.915         0.02  2 e+ P        0.48
 INK    Z    3.91  57.  64. 20060901234552.108         1.23  2 eX P        0.10
 INK    Z    3.91  57.  66. 20060901234637.508        -0.18  2 eX S        0.36
 BPAW   Z    4.27 241.  64. 20060901234555.202        -0.73  2 e+ P        0.17
 TRF    Z    4.46 232.  64. 20060901234558.729         0.03  2 e+ P        0.47
 KTH    Z    4.60 235.  64. 20060901234600.190        -0.40  0 iC P        0.51
 CHUM   Z    4.87 243.  63. 20060901234604.196        -0.22  2 eX P        0.33
 PPLA   Z    5.47 234.  63. 20060901234612.005        -0.81  2 eX P        0.15
 DIV    Z    5.50 197.  63. 20060901234613.857         0.48  0 iD P        0.45
 BMR    Z    5.55 191.  63. 20060901234614.197         0.19  0 iD P        0.70
 EYAK   Z    6.07 196.  62. 20060901234621.403         0.13  0 iD P        0.78
 RC01   Z    6.24 214.  62. 20060901234625.755         2.09  2 eX P        0.05
 FIB    Z    6.27 217.  62. 20060901234623.135        -1.02  2 eX P        0.12
 WHY    Z    6.68 146.  62. 20060901234627.156        -2.66  2 eX P        0.04
 RETURN FOR NEXT SCREEN


 Error Ellipse  X=   3.3440 km  Y= 4.3176 km  Theta = 100.0470 deg

 RMS Error        :               0.451              sec
 Travel_Time_Table:          TELE
 Latitude         :             66.4273 +-    0.0384 N         4.2912 km
 Longitude        :           -142.4352 +-    0.0774 E         3.3777 km
 Depth            :               10.00 +-     22.46 km
 Epoch Time       :      1157154289.207 +-      2.26 sec
 Event Time       :  20060901234449.207 +-      2.26 sec
 HYPO71 Quality   :                  DD
 Gap              :                 262              deg

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Alaska US

Focal Mechanism

 SLU Moment Tensor Solution
 2006/09/01 23:44:49 66.43N 142.44W 10 4.7 Alaska
 
 Best Fitting Double Couple
    Mo = 4.62e+22 dyne-cm
    Mw = 4.41 
    Z  = 3 km
     Plane   Strike  Dip  Rake
      NP1      316    76   164
      NP2       50    75    15
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   4.62e+22     21     273
     N   0.00e+00     69      94
     P  -4.62e+22      0       3



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -4.60e+22
       Mxy    -4.54e+21
       Mxz     5.09e+20
       Myy     4.00e+22
       Myz    -1.55e+22
       Mzz     5.98e+21
                                                     
                                                     
                                                     
                                                     
                     ------- P ----                  
                 -----------   --------              
              ----------------------------           
             ###---------------------------          
           ########------------------------##        
          ############--------------------####       
         ###############-----------------######      
        ###################-------------########     
        #####################---------##########     
       ###   #################-------############    
       ### T ###################---##############    
       ###   ####################-###############    
       ########################----##############    
        ####################---------###########     
        ##################------------##########     
         ##############----------------########      
          ##########--------------------######       
           #####-------------------------####        
             -----------------------------#          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  5.98e+21   5.09e+20   1.55e+22 
  5.09e+20  -4.60e+22   4.54e+21 
  1.55e+22   4.54e+21   4.00e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060901234449/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 = 50
      DIP = 75
     RAKE = 15
       MW = 4.41
       HS = 3

the surface-wave solution is preferred because of its sensitivity to the shallow source depth. : A slightly lower passband is used for the waveform inversion because of undertainty of velocity model to be used. The CUS model does an adequate job interms of wave form shape and timing. In the comparison figure below, the apparent poor fit to the Rayleigh wave on the Z and R components is due to the fact that the Rayleigh waves are much smaller than the Love wave on the T component.

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 0.016 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    85    15   4.31 0.5627
WVFGRD96    1.0    50    85    15   4.32 0.5720
WVFGRD96    2.0    50    90    20   4.34 0.5837
WVFGRD96    3.0   225    80   -25   4.36 0.5910
WVFGRD96    4.0   250    90   -25   4.38 0.5985
WVFGRD96    5.0   250    90   -20   4.38 0.6047
WVFGRD96    6.0   250    85   -20   4.39 0.6104
WVFGRD96    7.0   250    85   -20   4.40 0.6154
WVFGRD96    8.0   250    85   -20   4.41 0.6209
WVFGRD96    9.0   250    85   -20   4.42 0.6255
WVFGRD96   10.0   250    85   -20   4.43 0.6287
WVFGRD96   11.0   250    85   -20   4.43 0.6317
WVFGRD96   12.0   235    75    20   4.41 0.6342
WVFGRD96   13.0   235    75    20   4.42 0.6383
WVFGRD96   14.0   235    75    15   4.42 0.6406
WVFGRD96   15.0   235    75    15   4.43 0.6425
WVFGRD96   16.0   235    75    15   4.44 0.6452
WVFGRD96   17.0   235    75    15   4.44 0.6454
WVFGRD96   18.0   235    75    15   4.45 0.6449
WVFGRD96   19.0   235    75    15   4.46 0.6444
WVFGRD96   20.0   235    75    15   4.47 0.6434
WVFGRD96   21.0   235    75    10   4.48 0.6418
WVFGRD96   22.0   235    75    10   4.49 0.6391
WVFGRD96   23.0   235    75    10   4.49 0.6355
WVFGRD96   24.0   230    75     0   4.51 0.6323
WVFGRD96   25.0   230    75     0   4.51 0.6274
WVFGRD96   26.0   230    75     0   4.52 0.6214
WVFGRD96   27.0   250    75    -5   4.55 0.6178
WVFGRD96   28.0   250    75    -5   4.55 0.6099
WVFGRD96   29.0   250    80    -5   4.56 0.6017

The best solution is

WVFGRD96   17.0   235    75    15   4.44 0.6454

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 0.016 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=     316.03
  DIP=      75.52
 RAKE=     164.50
  
             OR
  
  STK=      49.99
  DIP=      75.00
 RAKE=      15.00
 
 
DEPTH = 3.0 km
 
Mw = 4.41
Best Fit 0.8479 - 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      150  300 eP_-
COLA      238  304 eP_+
COLD      288  353 eP_+
MCK       228  429 eP_+
INK        57  437 eP_X
BPAW      241  476 eP_+
TRF       232  498 eP_+
KTH       235  513 iP_C
CHUM      243  543 eP_X

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 distributiuon

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
DAWY	  150	  300
COLA	  238	  304
COLD	  288	  353
MCK	  228	  429
INK	   57	  437
BPAW	  241	  476
TRF	  232	  498
KTH	  235	  513
CHUM	  243	  543
PPLA	  234	  610
DIV	  197	  614
BMR	  191	  619
EYAK	  196	  676
RC01	  214	  696
FIB	  217	  700
WHY	  146	  745
PNL	  167	  770
SWD	  210	  788
SKAG	  152	  856
DLBC	  139	 1095
SIT	  157	 1110
OHAK	  214	 1172
FNBB	  121	 1295
CRAG	  153	 1320
YKW1	   95	 1404
RES	   41	 1923
EDM	  119	 2164
WALA	  126	 2532
FCC	   86	 2554
ULM	  102	 3160
FRB	   60	 3289
KAPO	   90	 3824

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 0.016 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 Sat Sep 2 15:43:14 CDT 2006