Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2020/11/11 03:54:15:0 41.69 20.80 8.0 5.0 N. Macedonia Stations used: HL.ATH HL.DION HL.JAN HL.KEK HL.KZN HL.NEO HL.NVR HL.PENT HL.PTL HL.RLS HL.SMTH HL.THL HL.VLS HT.ALN HT.EVGI HT.GRG HT.IGT HT.KAVA HT.KNT HT.KPRO HT.NEST HT.OUR HT.PSDA HT.SOH HT.SRS HT.THAS HT.THE HT.TYRN HT.XOR MN.BLY MN.PDG RO.BAIL RO.BZS RO.DEV RO.GZR RO.HERR RO.LOT RO.MDVR RO.SIRR SJ.BBLS SJ.FRGS Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.025 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 3.35e+23 dyne-cm Mw = 4.95 Z = 12 km Plane Strike Dip Rake NP1 203 69 -148 NP2 100 60 -25 Principal Axes: Axis Value Plunge Azimuth T 3.35e+23 5 330 N 0.00e+00 52 233 P -3.35e+23 38 64 Moment Tensor: (dyne-cm) Component Value Mxx 2.09e+23 Mxy -2.26e+23 Mxz -4.33e+22 Myy -8.62e+22 Myz -1.62e+23 Mzz -1.23e+23 ############## T ##############----- ### ############---------- #################------------- ##################---------------- #################------------------- #################------------ ------ #################------------- P ------- ################-------------- ------- -###############-------------------------- ---############--------------------------- -----##########--------------------------- -------#######---------------------------- ---------####--------------------------# ------------##---------------------##### -----------########################### ---------########################### --------########################## ------######################## -----####################### ---################### ############## Global CMT Convention Moment Tensor: R T P -1.23e+23 -4.33e+22 1.62e+23 -4.33e+22 2.09e+23 2.26e+23 1.62e+23 2.26e+23 -8.62e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201111035415/index.html

Preferred Solution

      STK = 100
      DIP = 60
     RAKE = -25
       MW = 4.95
       HS = 12.0

The NDK file is 20201111035415.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

OTHER

USGS/SLU Moment Tensor Solution ENS 2020/11/11 03:54:15:0 41.69 20.80 8.0 5.0 N. Macedonia Stations used: HL.ATH HL.DION HL.JAN HL.KEK HL.KZN HL.NEO HL.NVR HL.PENT HL.PTL HL.RLS HL.SMTH HL.THL HL.VLS HT.ALN HT.EVGI HT.GRG HT.IGT HT.KAVA HT.KNT HT.KPRO HT.NEST HT.OUR HT.PSDA HT.SOH HT.SRS HT.THAS HT.THE HT.TYRN HT.XOR MN.BLY MN.PDG RO.BAIL RO.BZS RO.DEV RO.GZR RO.HERR RO.LOT RO.MDVR RO.SIRR SJ.BBLS SJ.FRGS Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.025 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 3.35e+23 dyne-cm Mw = 4.95 Z = 12 km Plane Strike Dip Rake NP1 203 69 -148 NP2 100 60 -25 Principal Axes: Axis Value Plunge Azimuth T 3.35e+23 5 330 N 0.00e+00 52 233 P -3.35e+23 38 64 Moment Tensor: (dyne-cm) Component Value Mxx 2.09e+23 Mxy -2.26e+23 Mxz -4.33e+22 Myy -8.62e+22 Myz -1.62e+23 Mzz -1.23e+23 ############## T ##############----- ### ############---------- #################------------- ##################---------------- #################------------------- #################------------ ------ #################------------- P ------- ################-------------- ------- -###############-------------------------- ---############--------------------------- -----##########--------------------------- -------#######---------------------------- ---------####--------------------------# ------------##---------------------##### -----------########################### ---------########################### --------########################## ------######################## -----####################### ---################### ############## Global CMT Convention Moment Tensor: R T P -1.23e+23 -4.33e+22 1.62e+23 -4.33e+22 2.09e+23 2.26e+23 1.62e+23 2.26e+23 -8.62e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201111035415/index.html

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

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 o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.025 n 3 
lp c 0.05 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   100    70   -10   4.65 0.3521
WVFGRD96    2.0   100    65   -10   4.73 0.4117
WVFGRD96    3.0   100    55   -10   4.78 0.4300
WVFGRD96    4.0   100    50   -10   4.82 0.4492
WVFGRD96    5.0   100    50   -15   4.84 0.4727
WVFGRD96    6.0   100    50   -20   4.86 0.4964
WVFGRD96    7.0   100    55   -25   4.87 0.5197
WVFGRD96    8.0    90    45   -40   4.94 0.5466
WVFGRD96    9.0    95    50   -35   4.94 0.5666
WVFGRD96   10.0    95    50   -35   4.95 0.5765
WVFGRD96   11.0   100    55   -30   4.95 0.5814
WVFGRD96   12.0   100    60   -25   4.95 0.5823
WVFGRD96   13.0   100    60   -25   4.96 0.5809
WVFGRD96   14.0   100    60   -20   4.96 0.5771
WVFGRD96   15.0   105    65   -15   4.96 0.5736
WVFGRD96   16.0   105    65   -15   4.97 0.5677
WVFGRD96   17.0   105    65   -15   4.97 0.5601
WVFGRD96   18.0   105    70   -10   4.98 0.5532
WVFGRD96   19.0   105    70   -10   4.98 0.5455
WVFGRD96   20.0   105    70    -5   4.99 0.5376
WVFGRD96   21.0   105    70    -5   5.00 0.5295
WVFGRD96   22.0   105    70     5   5.00 0.5211
WVFGRD96   23.0   105    70     5   5.01 0.5124
WVFGRD96   24.0   105    70     5   5.01 0.5038
WVFGRD96   25.0   105    70     5   5.02 0.4949
WVFGRD96   26.0   105    70     5   5.03 0.4856
WVFGRD96   27.0   105    70     5   5.03 0.4763
WVFGRD96   28.0   295    70    15   5.05 0.4641
WVFGRD96   29.0   295    70    15   5.05 0.4565

The best solution is

WVFGRD96   12.0   100    60   -25   4.95 0.5823

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 0.025 n 3 
lp c 0.05 n 3 

Figure 3. Waveform comparison for selected depth

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 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: