Location

2007/04/10 16:34:21 69.73N 144.68W 5 4.5 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
 2007/04/10 16:34:21 69.73N 144.68W 5 4.5 Alaska
 
 Best Fitting Double Couple
    Mo = 6.84e+22 dyne-cm
    Mw = 4.49 
    Z  = 18 km
     Plane   Strike  Dip  Rake
      NP1      104    67   153
      NP2      205    65    25
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   6.84e+22     35      64
     N   0.00e+00     55     247
     P  -6.84e+22      2     155



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -4.70e+22
       Mxy     4.46e+22
       Mxz     1.59e+22
       Myy     2.48e+22
       Myz     2.79e+22
       Mzz     2.21e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -----------------#####              
              ------------------##########           
             -----------------#############          
           -----------------#################        
          -----------------###################       
         ----------------##############   #####      
        ----------------############### T ######     
        ---------------################   ######     
       ###------------###########################    
       #####---------############################    
       ########------############################    
       ###########--#############################    
        ############---#########################     
        ###########------------############-----     
         ##########----------------------------      
          #########---------------------------       
           #######---------------------------        
             #####-------------------------          
              #####------------------   --           
                 ##------------------ P              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  2.21e+22   1.59e+22  -2.79e+22 
  1.59e+22  -4.70e+22  -4.46e+22 
 -2.79e+22  -4.46e+22   2.48e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070410163425/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 = 205
      DIP = 65
     RAKE = 25
       MW = 4.49
       HS = 18

The waveform inversion is preferred slightly. There is little depth control with this data set.

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
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    10    60   -25   4.24 0.1978
WVFGRD96    1.0    10    50   -25   4.29 0.2044
WVFGRD96    2.0     0    50   -45   4.34 0.2172
WVFGRD96    3.0     5    50   -35   4.34 0.2183
WVFGRD96    4.0    10    55   -25   4.32 0.2183
WVFGRD96    5.0    10    55   -25   4.32 0.2198
WVFGRD96    6.0    15    60   -15   4.32 0.2215
WVFGRD96    7.0    15    60   -10   4.32 0.2242
WVFGRD96    8.0    15    65   -10   4.34 0.2268
WVFGRD96    9.0    20    65    10   4.34 0.2319
WVFGRD96   10.0   205    60    20   4.37 0.2379
WVFGRD96   11.0   205    60    20   4.39 0.2439
WVFGRD96   12.0   205    65    20   4.41 0.2494
WVFGRD96   13.0   205    65    20   4.42 0.2567
WVFGRD96   14.0   205    65    25   4.44 0.2597
WVFGRD96   15.0   205    65    25   4.45 0.2660
WVFGRD96   16.0   205    65    20   4.46 0.2665
WVFGRD96   17.0   205    65    20   4.47 0.2662
WVFGRD96   18.0   205    65    25   4.49 0.2682
WVFGRD96   19.0   205    65    25   4.50 0.2662
WVFGRD96   20.0   205    65    30   4.52 0.2636
WVFGRD96   21.0   205    65    30   4.53 0.2610
WVFGRD96   22.0   205    65    30   4.54 0.2560
WVFGRD96   23.0   205    65    30   4.55 0.2504
WVFGRD96   24.0   110    65    30   4.55 0.2514
WVFGRD96   25.0   110    65    30   4.55 0.2492
WVFGRD96   26.0   115    60    35   4.56 0.2469
WVFGRD96   27.0   115    60    35   4.57 0.2498
WVFGRD96   28.0   115    60    35   4.58 0.2469
WVFGRD96   29.0   115    60    35   4.59 0.2437

The best solution is

WVFGRD96   18.0   205    65    25   4.49 0.2682

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
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=     294.99
  DIP=      75.00
 RAKE=    -155.00
  
             OR
  
  STK=     198.10
  DIP=      65.90
 RAKE=     -16.48
 
 
DEPTH = 20.0 km
 
Mw = 4.64
Best Fit 0.7899 - 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
COLD      224  373 eP_X
INK       106  455 eP_X
COLA      198  566 eP_X
COLA      198  566 eP_X
DAWY      159  667 eP_X
DOT       179  680 eP_D
BPAW      208  696 eP_+

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)   
COLD	  224	  373
INK	  106	  455
COLA	  198	  566
DAWY	  159	  667
DOT	  180	  680
BPAW	  208	  696
CHUM	  212	  744
KTH	  206	  751
PAX	  185	  758
HARP	  184	  820
PPLA	  209	  842
FIB	  199	  995
RC01	  198	  998
EYAK	  185	 1028
WHY	  152	 1100
TNA	  256	 1101
SWD	  195	 1103
DLBC	  145	 1430
FNBB	  130	 1569
WRAK	  153	 1595
BMBC	  133	 1872
FCC	   92	 2618
PNT	  137	 2626
NLWA	  145	 2726
NEW	  134	 2802
HAWA	  140	 2933
MSO	  131	 3040
BMO	  137	 3152
DGMT	  117	 3186
BOZ	  129	 3229
HLID	  134	 3365
WVOR	  141	 3373
AGMN	  108	 3501
AHID	  130	 3542
BW06	  128	 3588
HWUT	  132	 3649
EYMN	  103	 3697
DUG	  134	 3762
ECSD	  112	 3922
COWI	  102	 3965
ISCO	  126	 4016
JFWS	  106	 4235
CBKS	  119	 4310
KSU1	  115	 4388
LONY	   89	 4667
FVM	  109	 4722
BLO	  105	 4746

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

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 Tue Apr 10 13:59:16 CDT 2007