Location

2007/04/28 05:20:30 69.65N 144.79W 11 4.9 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
 2007/04/28 05:20:30 69.65N 144.79W 11 4.9 Alaska
 
 Best Fitting Double Couple
    Mo = 2.54e+23 dyne-cm
    Mw = 4.87 
    Z  = 21 km
     Plane   Strike  Dip  Rake
      NP1      295    90   -140
      NP2      205    50     0
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.54e+23     27      62
     N   0.00e+00     50     295
     P  -2.54e+23     27     168



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.49e+23
       Mxy     1.25e+23
       Mxz     1.48e+23
       Myy     1.49e+23
       Myz     6.90e+22
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------------#######              
              --------------##############           
             -------------#################          
           -------------#####################        
          -------------#######################       
         #-----------###################   ####      
        ######------#################### T #####     
        ##########-#####################   #####     
       ###########----###########################    
       ###########--------#######################    
       ##########-------------###################    
       ##########-----------------###############    
        ########----------------------##########     
        ########--------------------------######     
         #######------------------------------#      
          ######------------------------------       
           #####-----------------------------        
             ####-------------   ----------          
              ####------------ P ---------           
                 #------------   ------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  0.00e+00   1.48e+23  -6.90e+22 
  1.48e+23  -1.49e+23  -1.25e+23 
 -6.90e+22  -1.25e+23   1.49e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070428052030/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 = 50
     RAKE = 0
       MW = 4.87
       HS = 21

There is little depth control for this data set. The surface-wave and waveform inversion results are similar. The waveform inversion is used.

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 0.01 n 3
lp c 0.04 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   195    85   -50   4.83 0.4998
WVFGRD96    1.0    15    65   -15   4.75 0.5040
WVFGRD96    2.0   195    80   -30   4.78 0.5065
WVFGRD96    3.0    15   100    30   4.79 0.4989
WVFGRD96    4.0    20    80    25   4.79 0.4883
WVFGRD96    5.0   205    50    10   4.81 0.4879
WVFGRD96    6.0   210    50    20   4.82 0.4924
WVFGRD96    7.0   210    50    20   4.82 0.4959
WVFGRD96    8.0   210    50    20   4.82 0.4977
WVFGRD96    9.0   210    50    15   4.82 0.4989
WVFGRD96   10.0   210    50    15   4.83 0.5002
WVFGRD96   11.0   205    50    10   4.83 0.5016
WVFGRD96   12.0   205    50    10   4.83 0.5036
WVFGRD96   13.0   205    50     5   4.83 0.5053
WVFGRD96   14.0   205    50     5   4.83 0.5071
WVFGRD96   15.0   205    50     5   4.83 0.5082
WVFGRD96   16.0   205    50     5   4.84 0.5095
WVFGRD96   17.0   205    50     0   4.84 0.5108
WVFGRD96   18.0   205    50     0   4.84 0.5112
WVFGRD96   19.0   205    55     0   4.85 0.5124
WVFGRD96   20.0    20    55    -5   4.87 0.5116
WVFGRD96   21.0   205    50     0   4.87 0.5132
WVFGRD96   22.0   205    50     0   4.87 0.5131
WVFGRD96   23.0   205    50    -5   4.88 0.5131
WVFGRD96   24.0   200    55   -10   4.89 0.5126
WVFGRD96   25.0   200    55   -10   4.89 0.5130
WVFGRD96   26.0   200    55   -10   4.90 0.5128
WVFGRD96   27.0   200    55   -10   4.90 0.5123
WVFGRD96   28.0   200    55   -10   4.91 0.5116
WVFGRD96   29.0   200    55   -10   4.91 0.5105

The best solution is

WVFGRD96   21.0   205    50     0   4.87 0.5132

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 0.01 n 3
lp c 0.04 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=     202.88
  DIP=      71.24
 RAKE=     -21.17
  
             OR
  
  STK=     299.98
  DIP=      70.00
 RAKE=    -159.99
 
 
DEPTH = 19.0 km
 
Mw = 4.94
Best Fit 0.8234 - 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      222  349 eP_+
INK       103  475 eP_X
EGAK      162  565 eP_-
DAWY      157  666 eP_D
DOT       177  670 -12345
BPAW      207  675 eP_X
MCK       197  684 -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 distributiuon

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
COLD	  222	  349
INK	  103	  475
EGAK	  162	  565
DAWY	  157	  666
DOT	  177	  670
BPAW	  207	  675
CHUM	  211	  722
KTH	  205	  731
TRF	  202	  732
PAX	  183	  745
PPLA	  207	  822
SAW	  192	  889
DIV	  183	  951
BMR	  179	  968
RC01	  196	  981
EYAK	  183	 1016
TNA	  256	 1076
SWD	  194	 1086
WHY	  150	 1102
PNL	  164	 1141
DLBC	  143	 1434
WRAK	  151	 1596
BMBC	  132	 1882
FCC	   92	 2641
NLWA	  144	 2730
NEW	  134	 2811
WALA	  128	 2824
HAWA	  138	 2940
BMO	  136	 3160
DGMT	  116	 3202
DLMT	  130	 3240
ULM	  105	 3333
HLID	  133	 3374
WVOR	  140	 3380
FLWY	  128	 3429
MOOW	  128	 3462
RRI2	  129	 3486
SNOW	  129	 3491
AHID	  130	 3553
EYMN	  102	 3718
DUG	  133	 3771
ECSD	  111	 3940
COWI	  102	 3985
ISCO	  125	 4029
JFWS	  105	 4254
KSU1	  115	 4404
AAM	   99	 4565
PKME	   83	 4832
BINY	   92	 4872
TZTN	  102	 5142
BLA	   99	 5194

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 0.01 n 3
lp c 0.04 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 Apr 30 08:45:58 CDT 2007