Location

2011/07/14 21:46:12 43.626 20.83 2.0 4.10 Serbia

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2011/07/14 21:46:12:0 43.63 20.83 2.0 4.1 Serbia Stations used: GE.TIRR MN.BLY MN.DIVS MN.PDG MN.TIR MN.TRI MN.VTS PL.OJC RO.BUR31 RO.MLR SL.BOJS SL.KOGS SL.LJU Filtering commands used: hp c 0.02 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 1.45e+22 dyne-cm Mw = 4.04 Z = 12 km Plane Strike Dip Rake NP1 15 85 -175 NP2 285 85 -5 Principal Axes: Axis Value Plunge Azimuth T 1.45e+22 0 150 N 0.00e+00 83 60 P -1.45e+22 7 240 Moment Tensor: (dyne-cm) Component Value Mxx 7.38e+21 Mxy -1.24e+22 Mxz 8.74e+20 Myy -7.16e+21 Myz 1.53e+21 Mzz -2.19e+20 ############-- ################------ ###################--------- ####################---------- #####################------------- ######################-------------- ######################---------------- #######################----------------- -----##################----------------- ---------------########------------------- ----------------------#------------------- ----------------------#######------------- ----------------------#############------- --------------------##################-- -------------------##################### - --------------#################### P -------------#################### -------------################### ------------################## ----------############# ## -------############# T --############ Global CMT Convention Moment Tensor: R T P -2.19e+20 8.74e+20 -1.53e+21 8.74e+20 7.38e+21 1.24e+22 -1.53e+21 1.24e+22 -7.16e+21 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20110714214612/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 285
      DIP = 85
     RAKE = -5
       MW = 4.04
       HS = 12.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2011/07/14 21:46:12:0 43.63 20.83 2.0 4.1 Serbia Stations used: GE.TIRR MN.BLY MN.DIVS MN.PDG MN.TIR MN.TRI MN.VTS PL.OJC RO.BUR31 RO.MLR SL.BOJS SL.KOGS SL.LJU Filtering commands used: hp c 0.02 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 1.45e+22 dyne-cm Mw = 4.04 Z = 12 km Plane Strike Dip Rake NP1 15 85 -175 NP2 285 85 -5 Principal Axes: Axis Value Plunge Azimuth T 1.45e+22 0 150 N 0.00e+00 83 60 P -1.45e+22 7 240 Moment Tensor: (dyne-cm) Component Value Mxx 7.38e+21 Mxy -1.24e+22 Mxz 8.74e+20 Myy -7.16e+21 Myz 1.53e+21 Mzz -2.19e+20 ############-- ################------ ###################--------- ####################---------- #####################------------- ######################-------------- ######################---------------- #######################----------------- -----##################----------------- ---------------########------------------- ----------------------#------------------- ----------------------#######------------- ----------------------#############------- --------------------##################-- -------------------##################### - --------------#################### P -------------#################### -------------################### ------------################## ----------############# ## -------############# T --############ Global CMT Convention Moment Tensor: R T P -2.19e+20 8.74e+20 -1.53e+21 8.74e+20 7.38e+21 1.24e+22 -1.53e+21 1.24e+22 -7.16e+21 Details of the solution is found at http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20110714214612/index.html

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:

hp c 0.02 n 3
lp c 0.05 n 3

The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   115    65    30   3.71 0.3152
WVFGRD96    1.0   115    65    25   3.71 0.3338
WVFGRD96    2.0   295    65    30   3.82 0.4278
WVFGRD96    3.0   290    70    20   3.84 0.4717
WVFGRD96    4.0   110    75    15   3.85 0.5172
WVFGRD96    5.0   105    85     5   3.89 0.5664
WVFGRD96    6.0   105    85     5   3.92 0.6101
WVFGRD96    7.0   105    90     5   3.95 0.6506
WVFGRD96    8.0   285    85    -5   3.97 0.6899
WVFGRD96    9.0   285    85    -5   4.00 0.7230
WVFGRD96   10.0   285    85    -5   4.01 0.7461
WVFGRD96   11.0   285    85    -5   4.03 0.7594
WVFGRD96   12.0   285    85    -5   4.04 0.7649
WVFGRD96   13.0   285    85    -5   4.05 0.7642
WVFGRD96   14.0   285    85    -5   4.06 0.7572
WVFGRD96   15.0   285    85    -5   4.06 0.7469
WVFGRD96   16.0   105    85     5   4.07 0.7329
WVFGRD96   17.0   105    85     5   4.08 0.7176
WVFGRD96   18.0   105    85     5   4.08 0.7012
WVFGRD96   19.0   105    85     5   4.08 0.6830
WVFGRD96   20.0   105    85     5   4.09 0.6640
WVFGRD96   21.0   105    85    10   4.09 0.6452
WVFGRD96   22.0   105    85    10   4.10 0.6253
WVFGRD96   23.0   110    80    15   4.09 0.6100
WVFGRD96   24.0   110    80    15   4.09 0.5966
WVFGRD96   25.0   110    80    15   4.10 0.5829
WVFGRD96   26.0   110    80    15   4.10 0.5695
WVFGRD96   27.0   110    80    20   4.11 0.5564
WVFGRD96   28.0   110    80    20   4.12 0.5433
WVFGRD96   29.0   110    75    15   4.12 0.5304

The best solution is

WVFGRD96   12.0   285    85    -5   4.04 0.7649

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

hp c 0.02 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)

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.

Discussion

The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS 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:

DATE=Sat Jul 16 10:15:05 CDT 2011

Last Changed 2011/07/14