Location

Location ANSS

The ANSS event ID is us2000dz9t and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us2000dz9t/executive.

2018/04/12 23:44:32 60.867 -138.266 10.0 4.1 Yukon

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2018/04/12 23:44:32:0  60.87 -138.27  10.0 4.1 Yukon
 
 Stations used:
   AK.BARN AK.BCP AK.BERG AK.CRQ AK.CTG AK.DIV AK.GLB AK.GRNC 
   AK.ISLE AK.JIS AK.KAI AK.KLU AK.LOGN AK.MCAR AK.MESA AK.SSP 
   AK.TABL AK.TGL AK.VRDI AK.WAX AT.SKAG CN.DAWY CN.HYT CN.WHY 
   NY.FARO NY.MAYO NY.MMPY TA.J29N TA.J30M TA.K29M TA.L27K 
   TA.L29M TA.M27K TA.M29M TA.M31M TA.N25K TA.N30M TA.N31M 
   TA.N32M TA.O29M TA.O30N TA.P29M TA.P32M TA.P33M TA.S31K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.60e+22 dyne-cm
  Mw = 4.07 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      136    62   112
   NP2      275    35    55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.60e+22     66      85
    N   0.00e+00     19     305
    P  -1.60e+22     14     210

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.13e+22
       Mxy    -6.27e+21
       Mxz     3.82e+21
       Myy    -1.01e+21
       Myz     7.90e+21
       Mzz     1.23e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------------------------           
             ------------------------------          
           ###----##################---------        
          ####-#########################------       
         ###--############################-----      
        ##-----#############################----     
        #-------##############################--     
       #---------################   ###########--    
       -----------############### T ############-    
       ------------##############   ############-    
       --------------############################    
        ---------------#########################     
        ----------------########################     
         ------------------####################      
          -------------------#################       
           ---------------------#############        
             -----   ----------------######          
              ---- P ---------------------           
                 -   ------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.23e+22   3.82e+21  -7.90e+21 
  3.82e+21  -1.13e+22   6.27e+21 
 -7.90e+21   6.27e+21  -1.01e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180412234432/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 = 275
      DIP = 35
     RAKE = 55
       MW = 4.07
       HS = 8.0

The NDK file is 20180412234432.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.

mLg Magnitude


Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.

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 o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 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   250    20     5   4.15 0.4569
WVFGRD96    2.0   250    25     5   4.06 0.5131
WVFGRD96    3.0   255    30    15   4.03 0.5618
WVFGRD96    4.0   265    30    35   4.03 0.6062
WVFGRD96    5.0   275    30    55   4.06 0.6482
WVFGRD96    6.0   275    35    55   4.07 0.6789
WVFGRD96    7.0   275    35    55   4.07 0.6948
WVFGRD96    8.0   275    35    55   4.07 0.6964
WVFGRD96    9.0   280    35    60   4.07 0.6904
WVFGRD96   10.0   275    35    55   4.10 0.6833
WVFGRD96   11.0   270    40    45   4.09 0.6674
WVFGRD96   12.0   270    40    45   4.09 0.6500
WVFGRD96   13.0   270    40    45   4.10 0.6301
WVFGRD96   14.0   265    40    35   4.09 0.6102
WVFGRD96   15.0   265    45    35   4.10 0.5896
WVFGRD96   16.0   265    45    35   4.11 0.5694
WVFGRD96   17.0   265    45    35   4.11 0.5490
WVFGRD96   18.0   265    45    30   4.11 0.5290
WVFGRD96   19.0   260    45    25   4.12 0.5095
WVFGRD96   20.0   265    40    30   4.14 0.4924
WVFGRD96   21.0   265    40    30   4.15 0.4746
WVFGRD96   22.0   265    45    30   4.16 0.4575
WVFGRD96   23.0   265    40    30   4.16 0.4411
WVFGRD96   24.0   265    40    30   4.17 0.4254
WVFGRD96   25.0   260    40    20   4.17 0.4106
WVFGRD96   26.0   260    40    20   4.18 0.3977
WVFGRD96   27.0   260    40    20   4.18 0.3859
WVFGRD96   28.0   260    40    20   4.19 0.3745
WVFGRD96   29.0   260    40    20   4.20 0.3631

The best solution is

WVFGRD96    8.0   275    35    55   4.07 0.6964

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 o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 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 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 Thu Apr 25 10:19:16 PM CDT 2024