Location

2013/12/24 20:56:47 60.241 -141.675 6.2 4.0 Alaska

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2013/12/24 20:56:47:0  60.24 -141.68   6.2 4.0 Alaska
 
 Stations used:
   AK.BAL AK.BARN AK.BGLC AK.BRLK AK.CCB AK.CRQ AK.CTG AK.DHY 
   AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.GOAT AK.GRIN 
   AK.GRNC AK.HIN AK.KHIT AK.KIAG AK.KTH AK.MCAR AK.PAX AK.PPD 
   AK.PTPK AK.PWL AK.RIDG AK.RKAV AK.RND AK.SAMH AK.SCM 
   AK.SCRK AK.SGA AK.SKN AK.SSP AK.SWD AK.TGL AK.VRDI AK.WAT6 
   AK.WAX AT.MENT AT.PMR IM.IL31 IU.COLA TA.HDA TA.POKR 
   US.EGAK 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.40e+22 dyne-cm
  Mw = 4.03 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1      103    71   107
   NP2      240    25    50
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.40e+22     60      37
    N   0.00e+00     16     277
    P  -1.40e+22     24     180

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.43e+21
       Mxy     1.65e+21
       Mxz     1.00e+22
       Myy     1.24e+21
       Myz     3.61e+21
       Mzz     8.19e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -----------############-----           
             -------######################-          
           -------##########################-        
          ------##############################       
         -----#################   #############      
        -----################## T ##############     
        ---####################   ##############     
       ###-######################################    
       ###--#####################################    
       ###------#################################    
       ##-----------#############################    
        #------------------#####################     
        #---------------------------------------     
         #-------------------------------------      
          ------------------------------------       
           ----------------------------------        
             --------------   -------------          
              ------------- P ------------           
                 ----------   ---------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  8.19e+21   1.00e+22  -3.61e+21 
  1.00e+22  -9.43e+21  -1.65e+21 
 -3.61e+21  -1.65e+21   1.24e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20131224205647/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 = 240
      DIP = 25
     RAKE = 50
       MW = 4.03
       HS = 11.0

The NDK file is 20131224205647.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2013/12/24 20:56:47:0  60.24 -141.68   6.2 4.0 Alaska
 
 Stations used:
   AK.BAL AK.BARN AK.BGLC AK.BRLK AK.CCB AK.CRQ AK.CTG AK.DHY 
   AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.GOAT AK.GRIN 
   AK.GRNC AK.HIN AK.KHIT AK.KIAG AK.KTH AK.MCAR AK.PAX AK.PPD 
   AK.PTPK AK.PWL AK.RIDG AK.RKAV AK.RND AK.SAMH AK.SCM 
   AK.SCRK AK.SGA AK.SKN AK.SSP AK.SWD AK.TGL AK.VRDI AK.WAT6 
   AK.WAX AT.MENT AT.PMR IM.IL31 IU.COLA TA.HDA TA.POKR 
   US.EGAK 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.40e+22 dyne-cm
  Mw = 4.03 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1      103    71   107
   NP2      240    25    50
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.40e+22     60      37
    N   0.00e+00     16     277
    P  -1.40e+22     24     180

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.43e+21
       Mxy     1.65e+21
       Mxz     1.00e+22
       Myy     1.24e+21
       Myz     3.61e+21
       Mzz     8.19e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -----------############-----           
             -------######################-          
           -------##########################-        
          ------##############################       
         -----#################   #############      
        -----################## T ##############     
        ---####################   ##############     
       ###-######################################    
       ###--#####################################    
       ###------#################################    
       ##-----------#############################    
        #------------------#####################     
        #---------------------------------------     
         #-------------------------------------      
          ------------------------------------       
           ----------------------------------        
             --------------   -------------          
              ------------- P ------------           
                 ----------   ---------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  8.19e+21   1.00e+22  -3.61e+21 
  1.00e+22  -9.43e+21  -1.65e+21 
 -3.61e+21  -1.65e+21   1.24e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20131224205647/index.html
	
Moment
    1.13e+15 N-m
Magnitude
    4.0
Percent DC
    94%
Depth
    10.0 km
Updated
    2013-12-24 21:29:47 UTC
Author
    us
Catalog
    ak
Contributor
    us
Code
    us_c000lr2t_mwr

Principal Axes
Axis	Value	Plunge	Azimuth
T	1.112	64	12
N	0.032	5	273
P	-1.144	25	180
Nodal Planes
Plane	Strike	Dip	Rake
NP1	94	70	95
NP2	260	20	77



        

Magnitudes

ML Magnitude


(a) ML computed using the IASPEI formula for Horizontal components; (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.


(a) ML computed using the IASPEI formula for Vertical components (research); (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

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:

cut a -30 a 180
rtr
taper w 0.1
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   255    45    90   3.64 0.3073
WVFGRD96    1.0    75    45    95   3.66 0.2750
WVFGRD96    2.0   260    45   100   3.76 0.3165
WVFGRD96    3.0   220    80    50   3.80 0.2834
WVFGRD96    4.0   220    10    35   3.93 0.3526
WVFGRD96    5.0   220    15    25   3.94 0.4147
WVFGRD96    6.0   230    15    40   3.94 0.4610
WVFGRD96    7.0   230    20    40   3.94 0.4989
WVFGRD96    8.0   235    20    45   4.01 0.5183
WVFGRD96    9.0   240    20    50   4.02 0.5416
WVFGRD96   10.0   240    25    55   4.02 0.5571
WVFGRD96   11.0   240    25    50   4.03 0.5644
WVFGRD96   12.0   235    30    50   4.03 0.5642
WVFGRD96   13.0   240    30    50   4.03 0.5593
WVFGRD96   14.0   235    30    45   4.03 0.5503
WVFGRD96   15.0   235    30    45   4.03 0.5378
WVFGRD96   16.0   235    30    45   4.03 0.5228
WVFGRD96   17.0   235    30    40   4.03 0.5067
WVFGRD96   18.0   235    30    40   4.03 0.4902
WVFGRD96   19.0   230    35    35   4.03 0.4730
WVFGRD96   20.0   230    35    35   4.03 0.4563
WVFGRD96   21.0   240    30    40   4.04 0.4408
WVFGRD96   22.0   230    35    30   4.04 0.4246
WVFGRD96   23.0   230    35    30   4.04 0.4081
WVFGRD96   24.0   230    35    25   4.04 0.3936
WVFGRD96   25.0   230    35    25   4.05 0.3793
WVFGRD96   26.0   200    45   -25   4.03 0.3673
WVFGRD96   27.0   205    50   -20   4.03 0.3556
WVFGRD96   28.0   205    50   -20   4.04 0.3442
WVFGRD96   29.0   205    50   -20   4.04 0.3328

The best solution is

WVFGRD96   11.0   240    25    50   4.03 0.5644

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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

The bandpass filter used in the processing and for the display was

cut a -30 a 180
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.06 n 3 
Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.
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.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

Assuming only a mislocation, the time shifts are fit to a functional form:

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS model 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:

Last Changed Mon Dec 7 00:24:01 CST 2015