Location

SLU Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2020/10/24 16:18:47:0 62.25 -124.43 6.5 4.7 Canada, NWT Stations used: 1E.MONT2 1E.MONT7 1E.MONT9 AK.BESE AK.JIS AK.LOGN AK.PIN AK.PNL AK.R32K AK.S31K AK.S32K AK.U33K AK.V35K AT.SIT CN.BRWY CN.DAWY CN.DLBC CN.FNSB CN.FSJB CN.HYT CN.INK CN.KUKN CN.NAB1 CN.NAB2 CN.NAHA CN.NBC1 CN.NBC5 CN.NBC7 CN.NBC8 CN.PLBC CN.WHY CN.YUK2 CN.YUK3 CN.YUK4 CN.YUK5 CN.YUK6 CN.YUK7 CN.YUK8 EO.FSJ2 NY.MMPY NY.WGLY NY.WTLY RV.DEDWA TA.EPYK TA.F30M TA.F31M TA.G29M TA.G30M TA.H29M TA.H31M TA.I28M TA.I30M TA.J29N TA.J30M TA.K29M TA.L27K TA.L29M TA.M29M TA.M30M TA.M31M TA.N30M TA.N31M TA.O28M TA.O29M TA.O30N TA.P29M TA.P30M TA.P32M TA.P33M TA.Q32M TA.R31K TA.R33M TA.S34M TA.T33K TA.T35M US.WRAK Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.025 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 6.61e+22 dyne-cm Mw = 4.48 Z = 4 km Plane Strike Dip Rake NP1 325 65 80 NP2 168 27 110 Principal Axes: Axis Value Plunge Azimuth T 6.61e+22 68 215 N 0.00e+00 9 329 P -6.61e+22 19 62 Moment Tensor: (dyne-cm) Component Value Mxx -6.63e+21 Mxy -1.99e+22 Mxz -2.80e+22 Myy -4.32e+22 Myz -3.15e+22 Mzz 4.98e+22 #------------- ###------------------- ----##---------------------- ---#######-------------------- ----###########------------------- ----##############------------- -- ----################------------ P --- -----##################---------- ---- -----####################--------------- -----######################--------------- -----#######################-------------- ------#######################------------- ------########## ###########------------ -----########## T ############---------- ------######### ############---------- ------########################-------- ------########################------ ------#######################----- ------#####################--- -------###################-- ------################ ------######## Global CMT Convention Moment Tensor: R T P 4.98e+22 -2.80e+22 3.15e+22 -2.80e+22 -6.63e+21 1.99e+22 3.15e+22 1.99e+22 -4.32e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201024161847/index.html

Preferred Solution

      STK = 325
      DIP = 65
     RAKE = 80
       MW = 4.48
       HS = 4.0

The NDK file is 20201024161847.ndk

Although not very scientific, I like this one because it agrees with expected direction of maximum compressive stress axis of previous events.I had problems with the other two events this month, on 10/14 and 1/18I will review those again. Although not very scientific, I like this one because it agrees with expected direction of maximum compressive stress axis of previous events.I had problems with the other two events this month, on 10/14 and 1/18I will review those again

As I did before, I looked at the CMPINC and CMPAZ of the four NY network stations. The two that were HHZ had CMPINC and CMPAZ for E and their HHE had CMPINC CMPAZ for Z. The reason that I changed them is that the rotation with the given angles did not separate the Rayleigh and Love at low frequencies.

Since this event was larger I used the 0.025 - 0.06 Hz frequency band to avoid problems of velocity model. I used the CUS (craton) model since the long period waveforms were simple pulses in general and since many paths were on the continental side of the Rockies.

This mechanism is very dip-slippish. Perhaps this explains the lack of depth control on the earlier events this month.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

SLUFM

USGS/SLU Moment Tensor Solution ENS 2020/10/24 16:18:47:0 62.25 -124.43 6.5 4.7 Canada, NWT Stations used: 1E.MONT2 1E.MONT7 1E.MONT9 AK.BESE AK.JIS AK.LOGN AK.PIN AK.PNL AK.R32K AK.S31K AK.S32K AK.U33K AK.V35K AT.SIT CN.BRWY CN.DAWY CN.DLBC CN.FNSB CN.FSJB CN.HYT CN.INK CN.KUKN CN.NAB1 CN.NAB2 CN.NAHA CN.NBC1 CN.NBC5 CN.NBC7 CN.NBC8 CN.PLBC CN.WHY CN.YUK2 CN.YUK3 CN.YUK4 CN.YUK5 CN.YUK6 CN.YUK7 CN.YUK8 EO.FSJ2 NY.MMPY NY.WGLY NY.WTLY RV.DEDWA TA.EPYK TA.F30M TA.F31M TA.G29M TA.G30M TA.H29M TA.H31M TA.I28M TA.I30M TA.J29N TA.J30M TA.K29M TA.L27K TA.L29M TA.M29M TA.M30M TA.M31M TA.N30M TA.N31M TA.O28M TA.O29M TA.O30N TA.P29M TA.P30M TA.P32M TA.P33M TA.Q32M TA.R31K TA.R33M TA.S34M TA.T33K TA.T35M US.WRAK Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.025 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 6.61e+22 dyne-cm Mw = 4.48 Z = 4 km Plane Strike Dip Rake NP1 325 65 80 NP2 168 27 110 Principal Axes: Axis Value Plunge Azimuth T 6.61e+22 68 215 N 0.00e+00 9 329 P -6.61e+22 19 62 Moment Tensor: (dyne-cm) Component Value Mxx -6.63e+21 Mxy -1.99e+22 Mxz -2.80e+22 Myy -4.32e+22 Myz -3.15e+22 Mzz 4.98e+22 #------------- ###------------------- ----##---------------------- ---#######-------------------- ----###########------------------- ----##############------------- -- ----################------------ P --- -----##################---------- ---- -----####################--------------- -----######################--------------- -----#######################-------------- ------#######################------------- ------########## ###########------------ -----########## T ############---------- ------######### ############---------- ------########################-------- ------########################------ ------#######################----- ------#####################--- -------###################-- ------################ ------######## Global CMT Convention Moment Tensor: R T P 4.98e+22 -2.80e+22 3.15e+22 -2.80e+22 -6.63e+21 1.99e+22 3.15e+22 1.99e+22 -4.32e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201024161847/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

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.025 n 3 
lp c 0.06 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   305    80    75   4.55 0.6777
WVFGRD96    2.0   310    75    75   4.50 0.6808
WVFGRD96    3.0   315    70    75   4.47 0.6868
WVFGRD96    4.0   325    65    80   4.48 0.6891
WVFGRD96    5.0   155    25    95   4.48 0.6835
WVFGRD96    6.0   155    30    95   4.48 0.6678
WVFGRD96    7.0   330    60    90   4.47 0.6475
WVFGRD96    8.0   330    60    90   4.45 0.6244
WVFGRD96    9.0   265    50   -55   4.44 0.6282
WVFGRD96   10.0   265    50   -55   4.46 0.6277
WVFGRD96   11.0   265    50   -55   4.46 0.6355
WVFGRD96   12.0   265    50   -55   4.46 0.6363
WVFGRD96   13.0   265    50   -55   4.45 0.6316
WVFGRD96   14.0   270    50   -50   4.45 0.6233
WVFGRD96   15.0   270    50   -50   4.45 0.6136
WVFGRD96   16.0   270    50   -50   4.45 0.6024
WVFGRD96   17.0   270    50   -50   4.45 0.5906
WVFGRD96   18.0   270    50   -50   4.45 0.5788
WVFGRD96   19.0   270    50   -50   4.45 0.5671
WVFGRD96   20.0   270    50   -50   4.47 0.5508
WVFGRD96   21.0   270    50   -50   4.47 0.5360
WVFGRD96   22.0   335    55   -80   4.46 0.5252
WVFGRD96   23.0   335    55   -80   4.46 0.5166
WVFGRD96   24.0   335    55   -80   4.46 0.5072
WVFGRD96   25.0   335    55   -80   4.47 0.4976
WVFGRD96   26.0   340    55   -75   4.47 0.4880
WVFGRD96   27.0   340    55   -75   4.47 0.4780
WVFGRD96   28.0   340    55   -75   4.47 0.4679
WVFGRD96   29.0   340    55   -75   4.48 0.4577

The best solution is

WVFGRD96    4.0   325    65    80   4.48 0.6891

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.025 n 3 
lp c 0.06 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:

• 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.

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 Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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: