Location

Location SLU

First arrivals were picked and elocate was use to locate this event using the CUS model (given below). The epicenter and origin time agree with the ANSS solution and the depth agrees with the regional moment tensor depth. The first motion plot is compatible with the regional moment tensor solution.

Location ANSS

2016/09/10 16:16:31 49.170 -119.252 5.0 4.1 BC, Canada

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/09/10 16:16:31:0  49.17 -119.25   5.0 4.1 BC, Canada
 
 Stations used:
   CC.JRO CC.STD CC.SWF2 CN.LLLB CN.PNT CN.WALA MB.JTMT 
   TA.G05D TA.H04D TA.I04A TA.I05D TD.TD008 TD.TD012 TD.TD013 
   TD.TD028 TD.TD029 US.HAWA US.MSO US.NEW US.NLWA UW.BRAN 
   UW.DAVN UW.DDRF UW.FISH UW.HOOD UW.IZEE UW.KENT UW.LCCR 
   UW.LTY UW.MRBL UW.OMAK UW.PASS UW.PHIN UW.RADR UW.RATT 
   UW.SP2 UW.STOR UW.TOLT UW.TUCA UW.UMAT UW.WOLL 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      105    67   136
   NP2      215    50    30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21     46      62
    N   0.00e+00     42     263
    P  -7.50e+21     11     164

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.89e+21
       Mxy     3.44e+21
       Mxz     3.05e+21
       Myy     2.20e+21
       Myz     2.93e+21
       Mzz     3.69e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ------------------##########           
             ---------------###############          
           ---------------###################        
          --------------######################       
         -------------#########################      
        ------------################   #########     
        -----------################# T #########     
       ###--------##################   ##########    
       #####-----################################    
       ########-#################################    
       #########---##############################    
        #######--------#########################     
        #######----------------#############----     
         ######--------------------------------      
          #####-------------------------------       
           ####------------------------------        
             ##----------------------------          
              ##----------------   -------           
                 --------------- P ----              
                     -----------                     
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.69e+21   3.05e+21  -2.93e+21 
  3.05e+21  -5.89e+21  -3.44e+21 
 -2.93e+21  -3.44e+21   2.20e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160910161631/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 = 215
      DIP = 50
     RAKE = 30
       MW = 3.85
       HS = 9.0

The NDK file is 20160910161631.ndk The CUS model is preferred becuase surface wave tomography indicates that this is the appropriate model for this area..

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2016/09/10 16:16:31:0  49.17 -119.25   5.0 4.1 BC, Canada
 
 Stations used:
   CC.JRO CC.STD CC.SWF2 CN.LLLB CN.PNT CN.WALA MB.JTMT 
   TA.G05D TA.H04D TA.I04A TA.I05D TD.TD008 TD.TD012 TD.TD013 
   TD.TD028 TD.TD029 US.HAWA US.MSO US.NEW US.NLWA UW.BRAN 
   UW.DAVN UW.DDRF UW.FISH UW.HOOD UW.IZEE UW.KENT UW.LCCR 
   UW.LTY UW.MRBL UW.OMAK UW.PASS UW.PHIN UW.RADR UW.RATT 
   UW.SP2 UW.STOR UW.TOLT UW.TUCA UW.UMAT UW.WOLL 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      105    67   136
   NP2      215    50    30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21     46      62
    N   0.00e+00     42     263
    P  -7.50e+21     11     164

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.89e+21
       Mxy     3.44e+21
       Mxz     3.05e+21
       Myy     2.20e+21
       Myz     2.93e+21
       Mzz     3.69e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ------------------##########           
             ---------------###############          
           ---------------###################        
          --------------######################       
         -------------#########################      
        ------------################   #########     
        -----------################# T #########     
       ###--------##################   ##########    
       #####-----################################    
       ########-#################################    
       #########---##############################    
        #######--------#########################     
        #######----------------#############----     
         ######--------------------------------      
          #####-------------------------------       
           ####------------------------------        
             ##----------------------------          
              ##----------------   -------           
                 --------------- P ----              
                     -----------                     
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.69e+21   3.05e+21  -2.93e+21 
  3.05e+21  -5.89e+21  -3.44e+21 
 -2.93e+21  -3.44e+21   2.20e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160910161631/index.html
	


First motions and takeoff angles from an elocate run.

Magnitudes

mLg Magnitude


(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

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 using wvfgrd96

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 o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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    20    85   -20   3.73 0.4904
WVFGRD96    2.0   200    35    -5   3.86 0.4997
WVFGRD96    3.0   205    40     5   3.83 0.5223
WVFGRD96    4.0   210    40    15   3.83 0.5467
WVFGRD96    5.0   215    45    25   3.84 0.5681
WVFGRD96    6.0   220    45    35   3.85 0.5978
WVFGRD96    7.0   215    50    30   3.85 0.6159
WVFGRD96    8.0   215    50    30   3.85 0.6254
WVFGRD96    9.0   215    50    30   3.85 0.6282
WVFGRD96   10.0   215    50    30   3.87 0.6264
WVFGRD96   11.0   215    50    30   3.87 0.6207
WVFGRD96   12.0   215    50    30   3.87 0.6112
WVFGRD96   13.0   215    50    30   3.87 0.6000
WVFGRD96   14.0   210    55    20   3.88 0.5892
WVFGRD96   15.0   210    55    20   3.88 0.5779
WVFGRD96   16.0   210    55    20   3.89 0.5657
WVFGRD96   17.0   215    55    20   3.89 0.5538
WVFGRD96   18.0   215    55    20   3.90 0.5431
WVFGRD96   19.0   215    55    20   3.90 0.5317
WVFGRD96   20.0   215    55    20   3.92 0.5200
WVFGRD96   21.0   215    55    20   3.93 0.5077
WVFGRD96   22.0   215    55    20   3.93 0.4948
WVFGRD96   23.0   215    55    20   3.93 0.4820
WVFGRD96   24.0   215    55    20   3.94 0.4689
WVFGRD96   25.0   215    55    20   3.94 0.4560
WVFGRD96   26.0   215    55    20   3.94 0.4430
WVFGRD96   27.0   215    55    20   3.95 0.4296
WVFGRD96   28.0   215    55    20   3.95 0.4164
WVFGRD96   29.0   215    55    15   3.96 0.4036

The best solution is

WVFGRD96    9.0   215    50    30   3.85 0.6282

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 o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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 CUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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 

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 Sat Sep 10 14:21:30 CDT 2016