Location

2011/04/04 13:11:51 41.675 14.918 19.5 3.3 Italy

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2011/04/04 13:11:51:0 41.67 14.92 19.5 3.3 Italy Stations used: IV.BSSO IV.CERA IV.FRES IV.MRB1 IV.MRVN IV.PALZ IV.PSB1 IV.RNI2 IV.SGTA IV.TRIV IV.VAGA Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.33e+21 dyne-cm Mw = 3.35 Z = 14 km Plane Strike Dip Rake NP1 267 71 -159 NP2 170 70 -20 Principal Axes: Axis Value Plunge Azimuth T 1.33e+21 1 38 N 0.00e+00 62 307 P -1.33e+21 28 129 Moment Tensor: (dyne-cm) Component Value Mxx 4.12e+20 Mxy 1.16e+21 Mxz 3.61e+20 Myy -1.18e+20 Myz -4.19e+20 Mzz -2.93e+20 ---########### ------################ ---------################# T ----------################# -----------####################### ------------######################## -------------######################### --------------########################## --------------####----------############ --------#######-----------------------#### ----############-------------------------- ################-------------------------- ################-------------------------- ################------------------------ ################------------------------ ################------------- ------ ###############------------- P ----- ###############------------ ---- ##############---------------- ##############-------------- ############---------- ##########---- Global CMT Convention Moment Tensor: R T P -2.93e+20 3.61e+20 4.19e+20 3.61e+20 4.12e+20 -1.16e+21 4.19e+20 -1.16e+21 -1.18e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.IT/20110404131151/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 = 170
      DIP = 70
     RAKE = -20
       MW = 3.35
       HS = 14.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2011/04/04 13:11:51:0 41.67 14.92 19.5 3.3 Italy Stations used: IV.BSSO IV.CERA IV.FRES IV.MRB1 IV.MRVN IV.PALZ IV.PSB1 IV.RNI2 IV.SGTA IV.TRIV IV.VAGA Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.33e+21 dyne-cm Mw = 3.35 Z = 14 km Plane Strike Dip Rake NP1 267 71 -159 NP2 170 70 -20 Principal Axes: Axis Value Plunge Azimuth T 1.33e+21 1 38 N 0.00e+00 62 307 P -1.33e+21 28 129 Moment Tensor: (dyne-cm) Component Value Mxx 4.12e+20 Mxy 1.16e+21 Mxz 3.61e+20 Myy -1.18e+20 Myz -4.19e+20 Mzz -2.93e+20 ---########### ------################ ---------################# T ----------################# -----------####################### ------------######################## -------------######################### --------------########################## --------------####----------############ --------#######-----------------------#### ----############-------------------------- ################-------------------------- ################-------------------------- ################------------------------ ################------------------------ ################------------- ------ ###############------------- P ----- ###############------------ ---- ##############---------------- ##############-------------- ############---------- ##########---- Global CMT Convention Moment Tensor: R T P -2.93e+20 3.61e+20 4.19e+20 3.61e+20 4.12e+20 -1.16e+21 4.19e+20 -1.16e+21 -1.18e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.IT/20110404131151/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
br c 0.12 0.25 n 4 p 2

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    15    55    55   3.10 0.2231
WVFGRD96    2.0     0    70    35   3.15 0.2647
WVFGRD96    3.0   170    75   -20   3.18 0.3014
WVFGRD96    4.0   165    65   -25   3.24 0.3537
WVFGRD96    5.0   165    65   -30   3.30 0.3785
WVFGRD96    6.0   165    65   -30   3.31 0.4019
WVFGRD96    7.0   165    60   -25   3.33 0.4160
WVFGRD96    8.0   165    65   -25   3.31 0.4290
WVFGRD96    9.0   165    65   -25   3.32 0.4345
WVFGRD96   10.0   165    65   -25   3.32 0.4373
WVFGRD96   11.0   165    65   -25   3.33 0.4388
WVFGRD96   12.0   170    70   -20   3.34 0.4401
WVFGRD96   13.0   170    70   -20   3.35 0.4407
WVFGRD96   14.0   170    70   -20   3.35 0.4421
WVFGRD96   15.0   165    65   -25   3.37 0.4386
WVFGRD96   16.0   165    65   -25   3.37 0.4377
WVFGRD96   17.0   170    70   -25   3.39 0.4358
WVFGRD96   18.0   170    70   -25   3.40 0.4332
WVFGRD96   19.0   170    70   -25   3.40 0.4299
WVFGRD96   20.0   170    70   -25   3.41 0.4267
WVFGRD96   21.0   125    60    -5   3.40 0.4247
WVFGRD96   22.0   125    60    -5   3.41 0.4226
WVFGRD96   23.0   120    55   -10   3.42 0.4200
WVFGRD96   24.0   120    55   -10   3.43 0.4168
WVFGRD96   25.0   120    55   -10   3.44 0.4129
WVFGRD96   26.0   120    55   -10   3.45 0.4081
WVFGRD96   27.0   120    55   -10   3.46 0.4021
WVFGRD96   28.0   120    55   -10   3.47 0.3963
WVFGRD96   29.0   170    60   -30   3.49 0.3914

The best solution is

WVFGRD96   14.0   170    70   -20   3.35 0.4421

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
br c 0.12 0.25 n 4 p 2

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 9 15:52:01 CDT 2011

Last Changed 2011/04/04