Location

Location ANSS

The ANSS event ID is ak013ft6lwl0 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak013ft6lwl0/executive.

2013/12/10 16:35:38 63.150 -151.510 12.5 3.7 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2013/12/10 16:35:38:0  63.15 -151.51  12.5 3.7 Alaska
 
 Stations used:
   AK.BWN AK.CCB AK.CHUM AK.CRQ AK.DHY AK.DOT AK.EYAK AK.FID 
   AK.GHO AK.HIN AK.KIAG AK.KNK AK.KTH AK.MCK AK.PAX AK.PPD 
   AK.PPLA AK.PTPK AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM 
   AK.SCRK AK.SKN AK.SSN AK.SWD AK.TGL AK.TRF AK.WAT1 AK.WAT2 
   AK.WAT3 AK.WAT4 AK.WAT5 AK.WAT6 AK.WAT7 AK.WRH AT.MENT 
   AT.PMR AT.SVW2 AT.TTA IU.COLA TA.HDA TA.POKR TA.TCOL 
   US.EGAK YE.PIC1 YE.PIC2 YE.PIC3 YE.PIC4 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 3.76e+21 dyne-cm
  Mw = 3.65 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1      180    65    65
   NP2       48    35   132
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.76e+21     62      51
    N   0.00e+00     23     191
    P  -3.76e+21     16     288

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.52e+14
       Mxy     1.44e+21
       Mxz     6.71e+20
       Myy    -2.61e+21
       Myz     2.19e+21
       Mzz     2.61e+21
                                                     
                                                     
                                                     
                                                     
                     ------########                  
                 ---------#############              
              -----------#################           
             -----------###################          
           ------------#####################-        
          -------------######################-       
         -   ---------#######################--      
        -- P ---------##########   ##########---     
        --   ---------########## T ##########---     
       ---------------##########   ##########----    
       ---------------######################-----    
       ---------------#####################------    
       ---------------#####################------    
        --------------####################------     
        --------------##################--------     
         -------------#################--------      
          -------------##############---------       
           ------------############----------        
             -----------########-----------          
              ###--------#----------------           
                 #########-------------              
                     ######--------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.61e+21   6.71e+20  -2.19e+21 
  6.71e+20  -2.52e+14  -1.44e+21 
 -2.19e+21  -1.44e+21  -2.61e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20131210163538/index.html
        

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is

      STK = 180
      DIP = 65
     RAKE = 65
       MW = 3.65
       HS = 14.0

The NDK file is 20131210163538.ndk The waveform inversion is preferred.

Magnitudes

Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.

ML Magnitude


Left: ML computed using the IASPEI formula for Horizontal components. Center: 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. Right: Residuals from new relation as a function of distance and azimuth.


Left: ML computed using the IASPEI formula for Vertical components (research). Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. 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). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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 0.10 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   175    40    90   3.21 0.2172
WVFGRD96    2.0   180    40    95   3.36 0.2814
WVFGRD96    3.0   160    80   -50   3.37 0.2216
WVFGRD96    4.0   340    90   -60   3.39 0.2563
WVFGRD96    5.0   340    90   -65   3.41 0.3100
WVFGRD96    6.0   170    80    65   3.43 0.3583
WVFGRD96    7.0   170    80    65   3.45 0.3951
WVFGRD96    8.0   175    80    70   3.53 0.4200
WVFGRD96    9.0   175    75    70   3.55 0.4497
WVFGRD96   10.0   180    70    70   3.58 0.4750
WVFGRD96   11.0   180    70    70   3.59 0.4934
WVFGRD96   12.0   180    65    70   3.62 0.5068
WVFGRD96   13.0   180    65    70   3.63 0.5153
WVFGRD96   14.0   180    65    65   3.65 0.5183
WVFGRD96   15.0   180    65    65   3.66 0.5169
WVFGRD96   16.0   180    65    65   3.67 0.5110
WVFGRD96   17.0   180    65    65   3.68 0.5014
WVFGRD96   18.0   180    65    65   3.69 0.4889
WVFGRD96   19.0   175    70    65   3.69 0.4740
WVFGRD96   20.0   180    70    65   3.70 0.4594
WVFGRD96   21.0   180    70    65   3.72 0.4440
WVFGRD96   22.0   180    70    65   3.72 0.4281
WVFGRD96   23.0   180    70    65   3.73 0.4125
WVFGRD96   24.0   175    75    65   3.73 0.3966
WVFGRD96   25.0   180    70    65   3.74 0.3818
WVFGRD96   26.0   175    70    60   3.74 0.3678
WVFGRD96   27.0   175    70    60   3.75 0.3542
WVFGRD96   28.0   175    70    60   3.75 0.3412
WVFGRD96   29.0   175    70    60   3.75 0.3283

The best solution is

WVFGRD96   14.0   180    65    65   3.65 0.5183

The mechanism corresponding 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 0.10 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. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

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    
Last Changed Fri Apr 26 10:58:09 PM CDT 2024