Location

Location ANSS

The ANSS event ID is ak008dvotc4t and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak008dvotc4t/executive.

2008/10/28 14:30:12 66.412 -157.727 15.3 4.9 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2008/10/28 14:30:12:0  66.41 -157.73  15.3 4.9 Alaska
 
 Stations used:
   AK.CAST AK.COLD AK.MCK AK.PPLA AK.TNA AK.TRF AT.PMR AT.SVW2 
   IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.05 n 3
 
 Best Fitting Double Couple
  Mo = 1.46e+23 dyne-cm
  Mw = 4.71 
  Z  = 16 km
  Plane   Strike  Dip  Rake
   NP1      345    75   -25
   NP2       82    66   -164
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.46e+23      6      35
    N   0.00e+00     61     136
    P  -1.46e+23     28     302

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.61e+22
       Mxy     1.19e+23
       Mxz    -1.93e+22
       Myy    -3.52e+22
       Myz     6.06e+22
       Mzz    -3.09e+22
                                                     
                                                     
                                                     
                                                     
                     --############                  
                 --------#############               
              ------------############ T #           
             --------------###########   ##          
           -----------------#################        
          ----   ------------#################       
         ----- P ------------##################      
        ------   -------------##################     
        -----------------------#################     
       ------------------------#################-    
       -------------------------###############--    
       -------------------------############-----    
       --------------------------########--------    
        ###----------------------###------------     
        ############-----########---------------     
         #########################-------------      
          ########################------------       
           #######################-----------        
             #####################---------          
              ####################--------           
                 #################-----              
                     #############-                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.09e+22  -1.93e+22  -6.06e+22 
 -1.93e+22   6.61e+22  -1.19e+23 
 -6.06e+22  -1.19e+23  -3.52e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081028143012/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 = 345
      DIP = 75
     RAKE = -25
       MW = 4.71
       HS = 16.0

The NDK file is 20081028143012.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
        
SLU
AEIC
 USGS/SLU Moment Tensor Solution
 ENS  2008/10/28 14:30:12:0  66.41 -157.73  15.3 4.9 Alaska
 
 Stations used:
   AK.CAST AK.COLD AK.MCK AK.PPLA AK.TNA AK.TRF AT.PMR AT.SVW2 
   IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.05 n 3
 
 Best Fitting Double Couple
  Mo = 1.46e+23 dyne-cm
  Mw = 4.71 
  Z  = 16 km
  Plane   Strike  Dip  Rake
   NP1      345    75   -25
   NP2       82    66   -164
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.46e+23      6      35
    N   0.00e+00     61     136
    P  -1.46e+23     28     302

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.61e+22
       Mxy     1.19e+23
       Mxz    -1.93e+22
       Myy    -3.52e+22
       Myz     6.06e+22
       Mzz    -3.09e+22
                                                     
                                                     
                                                     
                                                     
                     --############                  
                 --------#############               
              ------------############ T #           
             --------------###########   ##          
           -----------------#################        
          ----   ------------#################       
         ----- P ------------##################      
        ------   -------------##################     
        -----------------------#################     
       ------------------------#################-    
       -------------------------###############--    
       -------------------------############-----    
       --------------------------########--------    
        ###----------------------###------------     
        ############-----########---------------     
         #########################-------------      
          ########################------------       
           #######################-----------        
             #####################---------          
              ####################--------           
                 #################-----              
                     #############-                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.09e+22  -1.93e+22  -6.06e+22 
 -1.93e+22   6.61e+22  -1.19e+23 
 -6.06e+22  -1.19e+23  -3.52e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081028143012/index.html
	

Moment tensor inversion summary for event 2008/10/28 14:30

Date: 2008/10/28
Time: 14:30 (UTC)
Region: North-central Region of Alaska
Mw=4.8

Location:

Lat.  66.4123;  Lon.  -157.7271; Depth    5 km 
(Best-fitting depth from moment tensor inversion)

Solution quality: good;
Number of stations = 8

Best Double Couple:

         strike    dip    rake 
Plane 1:  265.5   74.1  -135.6
Plane 2:  160.6   47.7   -21.7

Moment Tensor Parameters:

Mo = 1.96293e+23 dyn-cm
Mxx =  0.99; Mxy =  1.30; Mxz =  1.11
Myy = -0.24; Myz = -0.37; Mzz = -0.75


Principal Axes:

     value   azimuth   plunge
T:    2.03    27.33   16.42
N:   -0.14   281.13   43.42
P:   -1.89   132.71   41.99



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:

hp c 0.02 n 3
lp c 0.05 n 3
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   160    70   -40   4.65 0.5142
WVFGRD96    1.0   160    70   -40   4.65 0.5199
WVFGRD96    2.0   165    70   -30   4.63 0.5243
WVFGRD96    3.0   330    35   -40   4.75 0.5301
WVFGRD96    4.0   335    45   -40   4.71 0.5318
WVFGRD96    5.0   335    65   -45   4.68 0.5493
WVFGRD96    6.0   335    65   -40   4.67 0.5640
WVFGRD96    7.0   335    65   -40   4.67 0.5749
WVFGRD96    8.0   340    65   -35   4.67 0.5845
WVFGRD96    9.0   340    65   -35   4.67 0.5922
WVFGRD96   10.0   340    65   -35   4.69 0.6004
WVFGRD96   11.0   340    65   -35   4.69 0.6071
WVFGRD96   12.0   340    65   -30   4.69 0.6111
WVFGRD96   13.0   340    70   -30   4.69 0.6152
WVFGRD96   14.0   345    70   -25   4.70 0.6175
WVFGRD96   15.0   345    75   -25   4.70 0.6190
WVFGRD96   16.0   345    75   -25   4.71 0.6200
WVFGRD96   17.0   345    75   -25   4.71 0.6193
WVFGRD96   18.0   345    75   -25   4.72 0.6175
WVFGRD96   19.0   345    75   -20   4.73 0.6150
WVFGRD96   20.0   345    75   -25   4.74 0.6130
WVFGRD96   21.0   345    75   -25   4.75 0.6094
WVFGRD96   22.0   345    75   -25   4.75 0.6049
WVFGRD96   23.0   345    75   -20   4.76 0.6004
WVFGRD96   24.0   345    75   -20   4.77 0.5954
WVFGRD96   25.0   345    75   -20   4.77 0.5896
WVFGRD96   26.0   345    75   -20   4.78 0.5832
WVFGRD96   27.0   345    75   -20   4.78 0.5762
WVFGRD96   28.0   345    80   -20   4.79 0.5694
WVFGRD96   29.0   345    80   -20   4.80 0.5622
WVFGRD96   30.0   345    80   -20   4.80 0.5546
WVFGRD96   31.0   170    85    20   4.81 0.5449
WVFGRD96   32.0   170    85    20   4.82 0.5379
WVFGRD96   33.0   170    85    15   4.84 0.5315
WVFGRD96   34.0   170    85    15   4.85 0.5254
WVFGRD96   35.0   170    85    15   4.86 0.5192
WVFGRD96   36.0   345    85   -15   4.86 0.5116
WVFGRD96   37.0   170    85    15   4.88 0.5057
WVFGRD96   38.0   350    85   -10   4.90 0.4966
WVFGRD96   39.0   350    85   -10   4.91 0.4885
WVFGRD96   40.0   170    80    20   4.94 0.4797
WVFGRD96   41.0   170    80    20   4.95 0.4746
WVFGRD96   42.0   170    80    20   4.96 0.4687
WVFGRD96   43.0   170    80    20   4.96 0.4623
WVFGRD96   44.0   170    80    20   4.97 0.4553
WVFGRD96   45.0   170    80    20   4.98 0.4481
WVFGRD96   46.0   170    80    20   4.98 0.4405
WVFGRD96   47.0   170    80    15   4.99 0.4330
WVFGRD96   48.0   170    80    15   5.00 0.4254
WVFGRD96   49.0   170    80    15   5.00 0.4175
WVFGRD96   50.0   170    80    15   5.01 0.4093

The best solution is

WVFGRD96   16.0   345    75   -25   4.71 0.6200

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

hp c 0.02 n 3
lp c 0.05 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 CUS.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
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 0.00  0.00  1.00  1.00 
Last Changed Sun Apr 28 01:02:34 PM CDT 2024