Location

2014/04/18 23:04:06 63.406 -144.972 12.5 4.1 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  2014/04/18 23:04:06:0  63.41 -144.97  12.5 4.1 Alaska
 
 Stations used:
   AK.BARN AK.BPAW AK.BWN AK.CCB AK.CNP AK.CRQ AK.CTG AK.DHY 
   AK.EYAK AK.FID AK.FYU AK.GLB AK.HARP AK.HDA AK.HIN AK.HMT 
   AK.KNK AK.KTH AK.MCAR AK.MCK AK.MESA AK.MLY AK.NEA AK.PPD 
   AK.PPLA AK.RAG AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN 
   AK.SWD AK.TGL AK.TRF AK.WAX AK.WRH AT.MENT AT.MID AT.PMR 
   CN.DAWY CN.HYT IU.COLA TA.EPYK US.EGAK 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.05 n 3 
 
 Best Fitting Double Couple
  Mo = 1.22e+22 dyne-cm
  Mw = 3.99 
  Z  = 13 km
  Plane   Strike  Dip  Rake
   NP1      275    55    65
   NP2      134    42   121
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.22e+22     69     130
    N   0.00e+00     20     290
    P  -1.22e+22      7      23

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.55e+21
       Mxy    -5.05e+21
       Mxz    -4.01e+21
       Myy    -8.10e+20
       Myz     2.61e+21
       Mzz     1.04e+22
                                                     
                                                     
                                                     
                                                     
                     -------------                   
                 ----------------- P --              
              --------------------   -----           
             ------------------------------          
           #---------------------------------        
          ##----------------------------------       
         ###-------#############---------------      
        ####--########################----------     
        ##--##############################------     
       #-----################################----    
       ------##################################--    
       -------################   ###############-    
       --------############### T ################    
        --------##############   ###############     
        ----------##############################     
         ----------############################      
          ------------########################       
           -------------#####################        
             ---------------###############          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.04e+22  -4.01e+21  -2.61e+21 
 -4.01e+21  -9.55e+21   5.05e+21 
 -2.61e+21   5.05e+21  -8.10e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140418230406/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 = 275
      DIP = 55
     RAKE = 65
       MW = 3.99
       HS = 13.0

The NDK file is 20140418230406.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  2014/04/18 23:04:06:0  63.41 -144.97  12.5 4.1 Alaska
 
 Stations used:
   AK.BARN AK.BPAW AK.BWN AK.CCB AK.CNP AK.CRQ AK.CTG AK.DHY 
   AK.EYAK AK.FID AK.FYU AK.GLB AK.HARP AK.HDA AK.HIN AK.HMT 
   AK.KNK AK.KTH AK.MCAR AK.MCK AK.MESA AK.MLY AK.NEA AK.PPD 
   AK.PPLA AK.RAG AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN 
   AK.SWD AK.TGL AK.TRF AK.WAX AK.WRH AT.MENT AT.MID AT.PMR 
   CN.DAWY CN.HYT IU.COLA TA.EPYK US.EGAK 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.05 n 3 
 
 Best Fitting Double Couple
  Mo = 1.22e+22 dyne-cm
  Mw = 3.99 
  Z  = 13 km
  Plane   Strike  Dip  Rake
   NP1      275    55    65
   NP2      134    42   121
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.22e+22     69     130
    N   0.00e+00     20     290
    P  -1.22e+22      7      23

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.55e+21
       Mxy    -5.05e+21
       Mxz    -4.01e+21
       Myy    -8.10e+20
       Myz     2.61e+21
       Mzz     1.04e+22
                                                     
                                                     
                                                     
                                                     
                     -------------                   
                 ----------------- P --              
              --------------------   -----           
             ------------------------------          
           #---------------------------------        
          ##----------------------------------       
         ###-------#############---------------      
        ####--########################----------     
        ##--##############################------     
       #-----################################----    
       ------##################################--    
       -------################   ###############-    
       --------############### T ################    
        --------##############   ###############     
        ----------##############################     
         ----------############################      
          ------------########################       
           -------------#####################        
             ---------------###############          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.04e+22  -4.01e+21  -2.61e+21 
 -4.01e+21  -9.55e+21   5.05e+21 
 -2.61e+21   5.05e+21  -8.10e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140418230406/index.html
	
Moment magnitude derived from a moment tensor inversion 
of complete waveforms at regional distances (less than ~8 
degrees), generally used for the analysis of small to 
moderate size earthquakes (typically Mw 3.5-6.0) crust 
or upper mantle earthquakes.

Moment
    1.47e+15 N-m
Magnitude
    4.0
Percent DC
    61%
Depth
    13.0 km
Updated
    2014-04-18 23:47:29 UTC
Author
    us
Catalog
    us
Contributor
    us
Code
    us_b000pqte_mwr

Principal Axes
Axis	Value	Plunge	Azimuth
T	1.331	74	113
N	0.253	16	290
P	-1.584	1	20
Nodal Planes
Plane	Strike	Dip	Rake
NP1	275°	48	69
NP2	125°	46	112

        

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 0.05 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    1.0   245    80   -10   3.64 0.3569
WVFGRD96    2.0   245    85   -15   3.71 0.4013
WVFGRD96    3.0    65    90    40   3.79 0.4049
WVFGRD96    4.0    65    90    50   3.84 0.4208
WVFGRD96    5.0    70    85    45   3.84 0.4366
WVFGRD96    6.0    70    80    45   3.85 0.4548
WVFGRD96    7.0    75    50    15   3.87 0.4713
WVFGRD96    8.0    75    40     5   3.92 0.4807
WVFGRD96    9.0   265    65    60   3.95 0.4979
WVFGRD96   10.0   275    60    70   3.99 0.5240
WVFGRD96   11.0   280    55    75   4.00 0.5463
WVFGRD96   12.0   280    55    75   4.00 0.5583
WVFGRD96   13.0   275    55    65   3.99 0.5615
WVFGRD96   14.0   275    55    65   3.99 0.5599
WVFGRD96   15.0   270    55    60   3.99 0.5547
WVFGRD96   16.0   265    60    50   3.98 0.5470
WVFGRD96   17.0   265    60    50   3.98 0.5386
WVFGRD96   18.0    60    65   -30   3.98 0.5333
WVFGRD96   19.0    60    65   -30   3.98 0.5285
WVFGRD96   20.0    65    65   -25   3.99 0.5223
WVFGRD96   21.0    65    65   -25   4.00 0.5161
WVFGRD96   22.0    65    65   -25   4.00 0.5097
WVFGRD96   23.0    65    65   -25   4.01 0.5020
WVFGRD96   24.0    65    65   -25   4.01 0.4933
WVFGRD96   25.0    65    65   -20   4.02 0.4853
WVFGRD96   26.0    70    70   -20   4.03 0.4770
WVFGRD96   27.0    70    70   -20   4.03 0.4681
WVFGRD96   28.0    70    70   -15   4.04 0.4590
WVFGRD96   29.0    70    70   -15   4.05 0.4500

The best solution is

WVFGRD96   13.0   275    55    65   3.99 0.5615

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 0.05 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.
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 00:11:22 CST 2015