Location

2013/04/20 07:57:41 43.457 12.289 6.7 3.6 Italy

Arrival Times (from USGS)

Felt Map

Focal Mechanism

SLU Moment Tensor Solution ENS 2013/04/20 07:57:41:0 43.46 12.29 6.7 3.6 Italy Stations used: IV.ASQU IV.ASSB IV.CAFI IV.CASP IV.CERT IV.CING IV.CRMI IV.CSNT IV.FAGN IV.FIAM IV.FSSB IV.MGAB IV.MTRZ IV.MURB IV.PESA IV.SACS IV.SSFR IV.TERO Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.64e+21 dyne-cm Mw = 3.41 Z = 2 km Plane Strike Dip Rake NP1 288 48 -109 NP2 135 45 -70 Principal Axes: Axis Value Plunge Azimuth T 1.64e+21 2 31 N 0.00e+00 14 301 P -1.64e+21 76 128 Moment Tensor: (dyne-cm) Component Value Mxx 1.17e+21 Mxy 7.71e+20 Mxz 2.81e+20 Myy 3.74e+20 Myz -2.81e+20 Mzz -1.54e+21 ############## #################### T ####################### ## ############################## --################################ --###---------------################ --#-----------------------############ #####-------------------------########## #####----------------------------####### ######------------------------------###### #######------------------------------##### ########--------------- -------------### #########-------------- P --------------## #########------------- --------------# ###########----------------------------- ###########--------------------------- #############----------------------- ##############-------------------- ################-------------- ############################ ###################### ############## Global CMT Convention Moment Tensor: R T P -1.54e+21 2.81e+20 2.81e+20 2.81e+20 1.17e+21 -7.71e+20 2.81e+20 -7.71e+20 3.74e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.IT/20130420075741/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 = 135
      DIP = 45
     RAKE = -70
       MW = 3.41
       HS = 2.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

SLU Moment Tensor Solution ENS 2013/04/20 07:57:41:0 43.46 12.29 6.7 3.6 Italy Stations used: IV.ASQU IV.ASSB IV.CAFI IV.CASP IV.CERT IV.CING IV.CRMI IV.CSNT IV.FAGN IV.FIAM IV.FSSB IV.MGAB IV.MTRZ IV.MURB IV.PESA IV.SACS IV.SSFR IV.TERO Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.64e+21 dyne-cm Mw = 3.41 Z = 2 km Plane Strike Dip Rake NP1 288 48 -109 NP2 135 45 -70 Principal Axes: Axis Value Plunge Azimuth T 1.64e+21 2 31 N 0.00e+00 14 301 P -1.64e+21 76 128 Moment Tensor: (dyne-cm) Component Value Mxx 1.17e+21 Mxy 7.71e+20 Mxz 2.81e+20 Myy 3.74e+20 Myz -2.81e+20 Mzz -1.54e+21 ############## #################### T ####################### ## ############################## --################################ --###---------------################ --#-----------------------############ #####-------------------------########## #####----------------------------####### ######------------------------------###### #######------------------------------##### ########--------------- -------------### #########-------------- P --------------## #########------------- --------------# ###########----------------------------- ###########--------------------------- #############----------------------- ##############-------------------- ################-------------- ############################ ###################### ############## Global CMT Convention Moment Tensor: R T P -1.54e+21 2.81e+20 2.81e+20 2.81e+20 1.17e+21 -7.71e+20 2.81e+20 -7.71e+20 3.74e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.IT/20130420075741/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 0.10 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    1.0   140    45   -60   3.32 0.4415
WVFGRD96    2.0   135    45   -70   3.41 0.4910
WVFGRD96    3.0   140    45   -60   3.42 0.4505
WVFGRD96    4.0   340    70   -30   3.37 0.4222
WVFGRD96    5.0   335    60   -45   3.46 0.4237
WVFGRD96    6.0   355    65    35   3.44 0.4017
WVFGRD96    7.0   175    75    45   3.47 0.4083
WVFGRD96    8.0   170    80    35   3.45 0.4129
WVFGRD96    9.0   170    80    35   3.47 0.4130
WVFGRD96   10.0   170    80    35   3.48 0.4098
WVFGRD96   11.0   170    85    35   3.49 0.4033
WVFGRD96   12.0   170    85    35   3.50 0.3961
WVFGRD96   13.0   170    75    30   3.50 0.3875
WVFGRD96   14.0   175    70    30   3.52 0.3798
WVFGRD96   15.0   175    70    35   3.54 0.3669
WVFGRD96   16.0   170    80    35   3.55 0.3575
WVFGRD96   17.0   170    80    35   3.55 0.3492
WVFGRD96   18.0   170    80    35   3.56 0.3407
WVFGRD96   19.0   170    80    35   3.57 0.3336
WVFGRD96   20.0   340    75   -40   3.58 0.3316
WVFGRD96   21.0   340    75   -40   3.59 0.3294
WVFGRD96   22.0   340    75   -40   3.60 0.3273
WVFGRD96   23.0   340    75   -40   3.61 0.3261
WVFGRD96   24.0   340    75   -40   3.61 0.3247
WVFGRD96   25.0   340    75   -35   3.61 0.3223
WVFGRD96   26.0   340    75   -35   3.62 0.3190
WVFGRD96   27.0   330    55   -45   3.64 0.3140
WVFGRD96   28.0   330    55   -45   3.65 0.3107
WVFGRD96   29.0   170    80    10   3.65 0.3040

The best solution is

WVFGRD96    2.0   135    45   -70   3.41 0.4910

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 0.10 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

Velocity Model

The nnCIA used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
C.It. A. Di Luzio et al Earth Plan Lettrs 280 (2009) 1-12 Fig 5. 7-8 MODEL/SURF3
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
     1.5000     3.7497     2.1436     2.2753  0.500E-02  0.100E-01   0.00       0.00       1.00       1.00    
     3.0000     4.9399     2.8210     2.4858  0.500E-02  0.100E-01   0.00       0.00       1.00       1.00    
     3.0000     6.0129     3.4336     2.7058  0.500E-02  0.100E-01   0.00       0.00       1.00       1.00    
     7.0000     5.5516     3.1475     2.6093  0.167E-02  0.333E-02   0.00       0.00       1.00       1.00    
    15.0000     5.8805     3.3583     2.6770  0.167E-02  0.333E-02   0.00       0.00       1.00       1.00    
     6.0000     7.1059     4.0081     3.0002  0.167E-02  0.333E-02   0.00       0.00       1.00       1.00    
     8.0000     7.1000     3.9864     3.0120  0.167E-02  0.333E-02   0.00       0.00       1.00       1.00    
     0.0000     7.9000     4.4036     3.2760  0.167E-02  0.333E-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:

DATE=Sat Apr 20 10:12:12 CDT 2013

Last Changed 2013/04/20