Location

SLU Location

2010/02/19 07:18:04 66.198 -142.278 5.1 4.60 Alaska

Arrival times were picked and the evented with a fixed depth of 15 km using the program elocate and the WUS model, listed below. The first arrival picks were difficult to make, perhaps because of a smaller event 46 seconds earlier, e.g.,

 MAG      DATE     TIME      LAT       LON     H(km)   Location
4.2   2010/02/19 07:17:14    66.233   -142.207   11.9   NORTHERN ALASKA
This could be a problem in the analysis, since we might expect some long period signal distortion if the magnitude of the two events were equal.

The arrival time picks used for the preferred location for this event were lost, and the current output of elocate is slightly different. Out location uses many Lg phase picks to overcome the very poor in quality P-wave first arrivals.

The mechanism and depth may be OK, the location is a problems.

AEIC Location

 MAG      DATE     TIME      LAT       LON     H(km)   Location
 4.6  2010/02/19 07:18:00   66.366   -141.551   5.1   NORTHERN ALASKA

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2010/02/19 07:18:04:0  66.20 -142.28   5.1 4.6 Alaska
 
 Stations used:
   AK.CCB AK.COLD AK.DDM AK.DHY AK.FYU AK.HDA AK.KTH AK.MCK 
   AK.MDM AK.MLY AK.RND AK.SCM AK.WRH CN.DAWY IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 1.97e+22 dyne-cm
  Mw = 4.13 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1      156    85   -170
   NP2       65    80    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.97e+22      4     290
    N   0.00e+00     79     182
    P  -1.97e+22     11      21

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.43e+22
       Mxy    -1.27e+22
       Mxz    -2.91e+21
       Myy     1.49e+22
       Myz    -2.41e+21
       Mzz    -5.88e+20
                                                     
                                                     
                                                     
                                                     
                     ------------                    
                 ###------------- P ---              
              ######-------------   ------           
             ########----------------------          
           ###########-----------------------        
          ############------------------------       
           ############-----------------------#      
         T #############--------------------####     
           ##############-----------------######     
       ##################--------------##########    
       ###################---------##############    
       ####################-----#################    
       ##########################################    
        ################-----###################     
        ###########-----------##################     
         ###-------------------################      
          ---------------------###############       
           ---------------------#############        
             --------------------##########          
              --------------------########           
                 -------------------###              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -5.88e+20  -2.91e+21   2.41e+21 
 -2.91e+21  -1.43e+22   1.27e+22 
  2.41e+21   1.27e+22   1.49e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100219071804/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 = 65
      DIP = 80
     RAKE = -5
       MW = 4.13
       HS = 15.0

The waveform inversion is preferred. Although the CUS model has been used previously for earthquakes in this area, the WUS model actually gives a much better waveform fit, perhaps because this better characterized the majority of paths to the south and southwest.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
AEIC
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2010/02/19 07:18:04:0  66.20 -142.28   5.1 4.6 Alaska
 
 Stations used:
   AK.CCB AK.COLD AK.DDM AK.DHY AK.FYU AK.HDA AK.KTH AK.MCK 
   AK.MDM AK.MLY AK.RND AK.SCM AK.WRH CN.DAWY IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 1.97e+22 dyne-cm
  Mw = 4.13 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1      156    85   -170
   NP2       65    80    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.97e+22      4     290
    N   0.00e+00     79     182
    P  -1.97e+22     11      21

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.43e+22
       Mxy    -1.27e+22
       Mxz    -2.91e+21
       Myy     1.49e+22
       Myz    -2.41e+21
       Mzz    -5.88e+20
                                                     
                                                     
                                                     
                                                     
                     ------------                    
                 ###------------- P ---              
              ######-------------   ------           
             ########----------------------          
           ###########-----------------------        
          ############------------------------       
           ############-----------------------#      
         T #############--------------------####     
           ##############-----------------######     
       ##################--------------##########    
       ###################---------##############    
       ####################-----#################    
       ##########################################    
        ################-----###################     
        ###########-----------##################     
         ###-------------------################      
          ---------------------###############       
           ---------------------#############        
             --------------------##########          
              --------------------########           
                 -------------------###              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -5.88e+20  -2.91e+21   2.41e+21 
 -2.91e+21  -1.43e+22   1.27e+22 
  2.41e+21   1.27e+22   1.49e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100219071804/index.html
	
Moment tensor inversion summary for event 2010/02/19 07:18

Date: 2010/02/19
Time: 07:18 (UTC)
Region: North-central Region of Alaska
Mw=4.3

Location:

Lat.  66.3660;  Lon.  -141.5513; Depth    5 km 
(Best-fitting depth from moment tensor inversion)

Solution quality: good;
Number of stations = 6

Best Double Couple:

         strike    dip    rake 
Plane 1:  242.2   86.8   -42.4
Plane 2:  335.1   47.7  -175.6

Moment Tensor Parameters:

Mo = 2.84232e+22 dyn-cm
Mxx = -1.41; Mxy = -1.29; Mxz =  1.56
Myy =  1.70; Myz = -0.91; Mzz = -0.29


Principal Axes:

     value   azimuth   plunge
T:    2.84   296.68   25.84
N:   -0.25    58.63   47.54
P:   -2.59   189.74   31.04

	

        

        

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    70    55    25   3.81 0.3513
WVFGRD96    1.0    65    60    20   3.81 0.3732
WVFGRD96    2.0    70    60    25   3.90 0.4706
WVFGRD96    3.0    65    65    20   3.92 0.5104
WVFGRD96    4.0    65    75    10   3.92 0.5440
WVFGRD96    5.0    65    75    10   3.95 0.5733
WVFGRD96    6.0    65    75    10   3.97 0.6002
WVFGRD96    7.0    65    80    10   4.00 0.6253
WVFGRD96    8.0    65    75    10   4.03 0.6521
WVFGRD96    9.0    65    75    10   4.05 0.6697
WVFGRD96   10.0    60    70   -10   4.07 0.6862
WVFGRD96   11.0    65    80   -10   4.08 0.7023
WVFGRD96   12.0    65    80   -10   4.10 0.7145
WVFGRD96   13.0    65    80   -10   4.11 0.7225
WVFGRD96   14.0    65    80   -10   4.12 0.7266
WVFGRD96   15.0    65    80    -5   4.13 0.7275
WVFGRD96   16.0    65    80    -5   4.14 0.7255
WVFGRD96   17.0    65    80    -5   4.15 0.7205
WVFGRD96   18.0    65    80    -5   4.16 0.7128
WVFGRD96   19.0    65    80    -5   4.17 0.7030
WVFGRD96   20.0    65    80    -5   4.18 0.6914
WVFGRD96   21.0    65    80    -5   4.19 0.6780
WVFGRD96   22.0    65    85     5   4.19 0.6629
WVFGRD96   23.0    65    85     5   4.20 0.6475
WVFGRD96   24.0    65    85     5   4.21 0.6313
WVFGRD96   25.0    65    85     5   4.21 0.6141
WVFGRD96   26.0    65    85     5   4.22 0.5964
WVFGRD96   27.0   245    90    -5   4.22 0.5764
WVFGRD96   28.0   245    90    -5   4.23 0.5588
WVFGRD96   29.0    65    85     5   4.23 0.5428

The best solution is

WVFGRD96   15.0    65    80    -5   4.13 0.7275

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 selected depth
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.

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.

Velocity Model

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

MODEL.01
Model after     8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
      H(KM)   VP(KM/S)   VS(KM/S) RHO(GM/CC)         QP         QS       ETAP       ETAS      FREFP      FREFS
     1.9000     3.4065     2.0089     2.2150  0.302E-02  0.679E-02   0.00       0.00       1.00       1.00    
     6.1000     5.5445     3.2953     2.6089  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
    13.0000     6.2708     3.7396     2.7812  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
    19.0000     6.4075     3.7680     2.8223  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
     0.0000     7.9000     4.6200     3.2760  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00    

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=Sun Feb 21 14:21:10 CST 2010

Last Changed 2010/02/19