Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2020/12/29 11:19:54:0 45.41 16.30 10.0 6.4 Croatia Stations used: AC.BCI BS.BLKB BW.ALFT BW.BE1 BW.BGDS BW.BIB BW.GELB BW.GRMB BW.KW1 BW.MANZ BW.MGBB BW.MGS02 BW.MGS03 BW.MGS05 BW.PART BW.RMOA BW.RNON BW.ROTZ BW.SCE BW.TON BW.WETR BW.ZUGS CH.BERNI CH.BNALP CH.DAVOX CH.EMING CH.FIESA CH.FUORN CH.FUSIO CH.GRIMS CH.LIENZ CH.LLS CH.MUGIO CH.MUO CH.PANIX CH.PLONS CH.ROMAN CH.SGT05 CH.SGT18 CH.TRULL CH.VDL CH.VDR CH.WGT CH.WILA CH.ZUR GE.MATE GR.BRG GR.FUR GR.GEC2 GR.GEC7 GR.GRA4 GR.GRC1 GR.GRC3 GR.GRC4 GR.WET HU.AMBH HU.BEHE HU.BSZH HU.BUD HU.CSKK HU.KOVH HU.MORH HU.MPLH HU.SOP HU.TIH MN.BLY MN.PDG MN.TRI MN.TUE OE.ABTA OE.ARSA OE.BIOA OE.CONA OE.CSNA OE.DAVA OE.FETA OE.KBA OE.MOA OE.MOTA OE.MYKA OE.OBKA OE.RETA OE.RONA OE.SOKA OE.VIE OE.WATA OX.ACOM OX.BAD OX.BALD OX.BOO OX.CAE OX.CIMO OX.CLUD OX.DRE OX.FUSE OX.MARN OX.MLN OX.MPRI OX.PRED OX.SABO OX.VARN PL.NIE PL.OJC RO.MDVR RO.PUNG SJ.BBLS SJ.FRGS SL.BOJS SL.CADS SL.CEY SL.CRES SL.CRNS SL.DOBS SL.GBRS SL.GCIS SL.GOLS SL.GORS SL.GROS SL.JAVS SL.KNDS SL.KOGS SL.LJU SL.PDKS SL.PERS SL.ROBS SL.SKDS SL.VISS SL.VNDS SL.VOJS SL.ZAVS SX.TANN SX.TRIB SX.WERD SX.WERN TH.PLN Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 3.67e+25 dyne-cm Mw = 6.31 Z = 12 km Plane Strike Dip Rake NP1 312 80 165 NP2 45 75 10 Principal Axes: Axis Value Plunge Azimuth T 3.67e+25 18 268 N 0.00e+00 72 101 P -3.67e+25 4 359 Moment Tensor: (dyne-cm) Component Value Mxx -3.65e+25 Mxy 1.59e+24 Mxz -2.71e+24 Myy 3.33e+25 Myz -1.05e+25 Mzz 3.19e+24 ----- P ------ --------- ---------- ---------------------------- -----------------------------# #####--------------------------### #########----------------------##### #############------------------####### ################--------------########## ##################-----------########### ######################-------############# ## ###################---############### ## T ####################-################ ## ###################---############### #####################-------############ ###################-----------########## ###############---------------######## ############-------------------##### ########-----------------------### ###--------------------------- ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 3.19e+24 -2.71e+24 1.05e+25 -2.71e+24 -3.65e+25 -1.59e+24 1.05e+25 -1.59e+24 3.33e+25 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201229111954/index.html

Preferred Solution

      STK = 45
      DIP = 75
     RAKE = 10
       MW = 6.31
       HS = 12.0

The NDK file is 20201229111954.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU	USGSW	OTHER
USGS/SLU Moment Tensor Solution ENS 2020/12/29 11:19:54:0 45.41 16.30 10.0 6.4 Croatia Stations used: AC.BCI BS.BLKB BW.ALFT BW.BE1 BW.BGDS BW.BIB BW.GELB BW.GRMB BW.KW1 BW.MANZ BW.MGBB BW.MGS02 BW.MGS03 BW.MGS05 BW.PART BW.RMOA BW.RNON BW.ROTZ BW.SCE BW.TON BW.WETR BW.ZUGS CH.BERNI CH.BNALP CH.DAVOX CH.EMING CH.FIESA CH.FUORN CH.FUSIO CH.GRIMS CH.LIENZ CH.LLS CH.MUGIO CH.MUO CH.PANIX CH.PLONS CH.ROMAN CH.SGT05 CH.SGT18 CH.TRULL CH.VDL CH.VDR CH.WGT CH.WILA CH.ZUR GE.MATE GR.BRG GR.FUR GR.GEC2 GR.GEC7 GR.GRA4 GR.GRC1 GR.GRC3 GR.GRC4 GR.WET HU.AMBH HU.BEHE HU.BSZH HU.BUD HU.CSKK HU.KOVH HU.MORH HU.MPLH HU.SOP HU.TIH MN.BLY MN.PDG MN.TRI MN.TUE OE.ABTA OE.ARSA OE.BIOA OE.CONA OE.CSNA OE.DAVA OE.FETA OE.KBA OE.MOA OE.MOTA OE.MYKA OE.OBKA OE.RETA OE.RONA OE.SOKA OE.VIE OE.WATA OX.ACOM OX.BAD OX.BALD OX.BOO OX.CAE OX.CIMO OX.CLUD OX.DRE OX.FUSE OX.MARN OX.MLN OX.MPRI OX.PRED OX.SABO OX.VARN PL.NIE PL.OJC RO.MDVR RO.PUNG SJ.BBLS SJ.FRGS SL.BOJS SL.CADS SL.CEY SL.CRES SL.CRNS SL.DOBS SL.GBRS SL.GCIS SL.GOLS SL.GORS SL.GROS SL.JAVS SL.KNDS SL.KOGS SL.LJU SL.PDKS SL.PERS SL.ROBS SL.SKDS SL.VISS SL.VNDS SL.VOJS SL.ZAVS SX.TANN SX.TRIB SX.WERD SX.WERN TH.PLN Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 3.67e+25 dyne-cm Mw = 6.31 Z = 12 km Plane Strike Dip Rake NP1 312 80 165 NP2 45 75 10 Principal Axes: Axis Value Plunge Azimuth T 3.67e+25 18 268 N 0.00e+00 72 101 P -3.67e+25 4 359 Moment Tensor: (dyne-cm) Component Value Mxx -3.65e+25 Mxy 1.59e+24 Mxz -2.71e+24 Myy 3.33e+25 Myz -1.05e+25 Mzz 3.19e+24 ----- P ------ --------- ---------- ---------------------------- -----------------------------# #####--------------------------### #########----------------------##### #############------------------####### ################--------------########## ##################-----------########### ######################-------############# ## ###################---############### ## T ####################-################ ## ###################---############### #####################-------############ ###################-----------########## ###############---------------######## ############-------------------##### ########-----------------------### ###--------------------------- ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 3.19e+24 -2.71e+24 1.05e+25 -2.71e+24 -3.65e+25 -1.59e+24 1.05e+25 -1.59e+24 3.33e+25 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201229111954/index.html	W-phase Moment Tensor (Mww) Moment 4.411e+18 N-m Magnitude 6.36 Mww Depth 13.5 km Percent DC 97% Half Duration 3.93 s Catalog US Data Source US 1 Contributor US 1 Nodal Planes Plane Strike Dip Rake NP1 224 89 14 NP2 134 76 179 Principal Axes Axis Value Plunge Azimuth T 4.444e+18 N-m 11 90 N -0.067e+18 N-m 76 228 P -4.377e+18 N-m 9 358	F-E Region NW Balkan Region Mw Beach Ball Time 2020-12-29 11:19:55.6 UTC Magnitude 6.4 (Mw) Epicenter 16.21E 45.48N Depth 10 km Status M - manually revised Event Map for gfz2020zocb Moment tensor solutions GEOFON standard1 Beachball Time 2020-12-29 11:19:55 Magnitude 6.4 Latitude 45.47N Longitude 16.20E Depth 11 km Nodal planes Strike Dip Rake 131 83 -167 40 77 -6

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 0.02 n 3 
lp c 0.06 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   220    90     0   5.97 0.4634
WVFGRD96    2.0    40    85    -5   6.07 0.5974
WVFGRD96    3.0    40    90     0   6.11 0.6527
WVFGRD96    4.0    40    90     5   6.14 0.6888
WVFGRD96    5.0    40    85     5   6.17 0.7146
WVFGRD96    6.0   220    90   -10   6.19 0.7353
WVFGRD96    7.0    40    85    10   6.22 0.7571
WVFGRD96    8.0    45    75    10   6.26 0.7786
WVFGRD96    9.0    45    75    10   6.27 0.7862
WVFGRD96   10.0    45    70    10   6.29 0.7912
WVFGRD96   11.0    45    70    10   6.31 0.7942
WVFGRD96   12.0    45    75    10   6.31 0.7949
WVFGRD96   13.0    45    75    10   6.33 0.7936
WVFGRD96   14.0    45    75    10   6.34 0.7904
WVFGRD96   15.0    45    75    10   6.35 0.7859
WVFGRD96   16.0    45    75    10   6.35 0.7797
WVFGRD96   17.0    45    75    10   6.36 0.7722
WVFGRD96   18.0    45    75    10   6.37 0.7634
WVFGRD96   19.0    45    75    10   6.38 0.7535
WVFGRD96   20.0    40    80    10   6.38 0.7429
WVFGRD96   21.0    40    80    10   6.38 0.7315
WVFGRD96   22.0    40    80    10   6.39 0.7192
WVFGRD96   23.0    40    80    10   6.40 0.7062
WVFGRD96   24.0    40    80    10   6.40 0.6928
WVFGRD96   25.0    40    80    10   6.41 0.6791
WVFGRD96   26.0    40    80    10   6.41 0.6649
WVFGRD96   27.0    40    80   -10   6.42 0.6506
WVFGRD96   28.0    40    80   -10   6.43 0.6368
WVFGRD96   29.0    40    80   -10   6.43 0.6227

The best solution is

WVFGRD96   12.0    45    75    10   6.31 0.7949

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 0.02 n 3 
lp c 0.06 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: