Location

Location ANSS

The ANSS event ID is us10008r4v and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us10008r4v/executive.

2017/05/10 15:14:26 65.030 -134.219 12.5 4.4 Yukon, Canada

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/05/10 15:14:26:0 65.03 -134.22 12.5 4.4 Yukon, Canada Stations used: AK.BAL AK.BARN AK.BCP AK.CCB AK.CTG AK.DOT AK.FYU AK.GLB AK.HDA AK.KIAG AK.LOGN AK.MCAR AK.MDM AK.PAX AK.PIN AK.PNL AK.PPD AK.PTPK AK.RIDG AK.SAMH AK.SCRK AK.VRDI AK.WRH AT.MENT AT.SKAG CN.DAWY CN.HYT CN.INK CN.PLBC CN.YUK2 CN.YUK3 CN.YUK4 CN.YUK5 CN.YUK6 CN.YUK7 CN.YUK8 IM.IL31 IU.COLA NY.MAYO NY.MMPY NY.WGLY TA.C36M TA.E27K TA.EPYK TA.F31M TA.G24K TA.G26K TA.G27K TA.G30M TA.H24K TA.H25L TA.H27K TA.HARP TA.I27K TA.I29M TA.J25K TA.J26L TA.J29M TA.K24K TA.K29M TA.L26K TA.L27K TA.L29M TA.M26K TA.M27K TA.M29M TA.M30M TA.M31M TA.N25K TA.N30M TA.N31M TA.O28M TA.O29M TA.O30N TA.P29M TA.P30M TA.P32M TA.P33M TA.POKR TA.R33M US.EGAK Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 4.52e+22 dyne-cm Mw = 4.37 Z = 12 km Plane Strike Dip Rake NP1 275 65 70 NP2 136 32 126 Principal Axes: Axis Value Plunge Azimuth T 4.52e+22 64 151 N 0.00e+00 18 284 P -4.52e+22 18 20 Moment Tensor: (dyne-cm) Component Value Mxx -2.98e+22 Mxy -1.66e+22 Mxz -2.78e+22 Myy -2.68e+21 Myz 4.13e+21 Mzz 3.25e+22 -------------- --------------- ---- ------------------ P ------- ------------------- -------- #--------------------------------- ##---------------------------------- ##------------------------------------ ###------------###---------------------- ###--#########################---------- ##--###############################------- -----##################################--- -----####################################- ------#################################### ------################ ############### -------############### T ############### -------############## ############## --------############################ ---------######################### ----------#################### -------------##############- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 3.25e+22 -2.78e+22 -4.13e+21 -2.78e+22 -2.98e+22 1.66e+22 -4.13e+21 1.66e+22 -2.68e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170510151426/index.html

Preferred Solution

      STK = 275
      DIP = 65
     RAKE = 70
       MW = 4.37
       HS = 12.0

The NDK file is 20170510151426.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

USGSMT

USGS/SLU Moment Tensor Solution ENS 2017/05/10 15:14:26:0 65.03 -134.22 12.5 4.4 Yukon, Canada Stations used: AK.BAL AK.BARN AK.BCP AK.CCB AK.CTG AK.DOT AK.FYU AK.GLB AK.HDA AK.KIAG AK.LOGN AK.MCAR AK.MDM AK.PAX AK.PIN AK.PNL AK.PPD AK.PTPK AK.RIDG AK.SAMH AK.SCRK AK.VRDI AK.WRH AT.MENT AT.SKAG CN.DAWY CN.HYT CN.INK CN.PLBC CN.YUK2 CN.YUK3 CN.YUK4 CN.YUK5 CN.YUK6 CN.YUK7 CN.YUK8 IM.IL31 IU.COLA NY.MAYO NY.MMPY NY.WGLY TA.C36M TA.E27K TA.EPYK TA.F31M TA.G24K TA.G26K TA.G27K TA.G30M TA.H24K TA.H25L TA.H27K TA.HARP TA.I27K TA.I29M TA.J25K TA.J26L TA.J29M TA.K24K TA.K29M TA.L26K TA.L27K TA.L29M TA.M26K TA.M27K TA.M29M TA.M30M TA.M31M TA.N25K TA.N30M TA.N31M TA.O28M TA.O29M TA.O30N TA.P29M TA.P30M TA.P32M TA.P33M TA.POKR TA.R33M US.EGAK Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 4.52e+22 dyne-cm Mw = 4.37 Z = 12 km Plane Strike Dip Rake NP1 275 65 70 NP2 136 32 126 Principal Axes: Axis Value Plunge Azimuth T 4.52e+22 64 151 N 0.00e+00 18 284 P -4.52e+22 18 20 Moment Tensor: (dyne-cm) Component Value Mxx -2.98e+22 Mxy -1.66e+22 Mxz -2.78e+22 Myy -2.68e+21 Myz 4.13e+21 Mzz 3.25e+22 -------------- --------------- ---- ------------------ P ------- ------------------- -------- #--------------------------------- ##---------------------------------- ##------------------------------------ ###------------###---------------------- ###--#########################---------- ##--###############################------- -----##################################--- -----####################################- ------#################################### ------################ ############### -------############### T ############### -------############## ############## --------############################ ---------######################### ----------#################### -------------##############- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 3.25e+22 -2.78e+22 -4.13e+21 -2.78e+22 -2.98e+22 1.66e+22 -4.13e+21 1.66e+22 -2.68e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170510151426/index.html

Regional Moment Tensor (Mwr) Moment 4.222e+15 N-m Magnitude 4.4 Mwr Depth 16.0 km Percent DC 87 % Half Duration – Catalog AK Data Source US2 Contributor US2 Nodal Planes Plane Strike Dip Rake NP1 256 58 37 NP2 144 60 142 Principal Axes Axis Value Plunge Azimuth T 4.074e+15 N-m 47 109 N 0.284e+15 N-m 43 291 P -4.357e+15 N-m 1 200

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 M_L by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for M_L is adjusted to remove a linear distance trend in residuals to give a regionally defined M_L. The defined M_L 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

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 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   115    55    95   4.24 0.5534
WVFGRD96    2.0   290    30    90   4.34 0.5349
WVFGRD96    3.0    60    45   -20   4.28 0.5191
WVFGRD96    4.0    60    45   -20   4.28 0.5475
WVFGRD96    5.0    55    45   -25   4.29 0.5761
WVFGRD96    6.0    55    50   -30   4.30 0.6054
WVFGRD96    7.0    55    55   -35   4.32 0.6303
WVFGRD96    8.0    55    55   -35   4.32 0.6502
WVFGRD96    9.0    55    55   -35   4.33 0.6649
WVFGRD96   10.0   275    65    70   4.36 0.6654
WVFGRD96   11.0    55    55   -35   4.37 0.6709
WVFGRD96   12.0   275    65    70   4.37 0.6720
WVFGRD96   13.0   270    65    65   4.38 0.6699
WVFGRD96   14.0   270    70    60   4.38 0.6660
WVFGRD96   15.0   270    70    60   4.39 0.6588
WVFGRD96   16.0   265    70    55   4.41 0.6498
WVFGRD96   17.0   265    70    55   4.41 0.6387
WVFGRD96   18.0   265    70    55   4.42 0.6249
WVFGRD96   19.0   265    70    55   4.43 0.6098
WVFGRD96   20.0   265    70    60   4.46 0.5962
WVFGRD96   21.0   265    70    55   4.47 0.5783
WVFGRD96   22.0   265    70    55   4.47 0.5596
WVFGRD96   23.0   265    70    55   4.48 0.5401
WVFGRD96   24.0   260    75    55   4.48 0.5202
WVFGRD96   25.0   260    75    55   4.49 0.5001
WVFGRD96   26.0   260    75    55   4.50 0.4799
WVFGRD96   27.0   260    75    55   4.50 0.4600
WVFGRD96   28.0   260    75    55   4.50 0.4403
WVFGRD96   29.0   260    75    55   4.51 0.4209

The best solution is

WVFGRD96   12.0   275    65    70   4.37 0.6720

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 +60
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:

• The origin time and epicentral distance are incorrect
• The velocity model used for the inversion is incorrect
• The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

 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