Location

2011/05/06 15:12:35 37.782 14.960 22.2 4.0 Italy

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2011/05/06 15:12:35:0 37.78 14.96 22.2 4.0 Italy Stations used: IV.ECNV IV.GALF IV.HAGA IV.HAVL IV.HCRL IV.HLNI IV.HMDC IV.ILLI IV.IVPL IV.JOPP IV.MILZ IV.MMME IV.MPAZ IV.MSRU IV.NOV IV.SSY Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.60e+22 dyne-cm Mw = 4.07 Z = 23 km Plane Strike Dip Rake NP1 269 79 134 NP2 10 45 15 Principal Axes: Axis Value Plunge Azimuth T 1.60e+22 39 218 N 0.00e+00 43 79 P -1.60e+22 22 327 Moment Tensor: (dyne-cm) Component Value Mxx -3.87e+21 Mxy 1.10e+22 Mxz -1.08e+22 Myy -2.79e+20 Myz -1.90e+21 Mzz 4.15e+21 ------------## ------------------#### ---- ---------------###### ----- P ----------------###### ------- -----------------####### ----------------------------######## ------------------------------######## -------------------------------######### -------------------------------######### --------------------------------########## ----############################--------## ################################---------- ################################---------- ##############################---------- ##############################---------- ########## ###############---------- ######### T ##############---------- ######## #############---------- #####################--------- ##################---------- #############--------- ######-------- Global CMT Convention Moment Tensor: R T P 4.15e+21 -1.08e+22 1.90e+21 -1.08e+22 -3.87e+21 -1.10e+22 1.90e+21 -1.10e+22 -2.79e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.IT/20110506151235/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 = 10
      DIP = 45
     RAKE = 15
       MW = 4.07
       HS = 23.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2011/05/06 15:12:35:0 37.78 14.96 22.2 4.0 Italy Stations used: IV.ECNV IV.GALF IV.HAGA IV.HAVL IV.HCRL IV.HLNI IV.HMDC IV.ILLI IV.IVPL IV.JOPP IV.MILZ IV.MMME IV.MPAZ IV.MSRU IV.NOV IV.SSY Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.60e+22 dyne-cm Mw = 4.07 Z = 23 km Plane Strike Dip Rake NP1 269 79 134 NP2 10 45 15 Principal Axes: Axis Value Plunge Azimuth T 1.60e+22 39 218 N 0.00e+00 43 79 P -1.60e+22 22 327 Moment Tensor: (dyne-cm) Component Value Mxx -3.87e+21 Mxy 1.10e+22 Mxz -1.08e+22 Myy -2.79e+20 Myz -1.90e+21 Mzz 4.15e+21 ------------## ------------------#### ---- ---------------###### ----- P ----------------###### ------- -----------------####### ----------------------------######## ------------------------------######## -------------------------------######### -------------------------------######### --------------------------------########## ----############################--------## ################################---------- ################################---------- ##############################---------- ##############################---------- ########## ###############---------- ######### T ##############---------- ######## #############---------- #####################--------- ##################---------- #############--------- ######-------- Global CMT Convention Moment Tensor: R T P 4.15e+21 -1.08e+22 1.90e+21 -1.08e+22 -3.87e+21 -1.10e+22 1.90e+21 -1.10e+22 -2.79e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.IT/20110506151235/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    50   -65   3.65 0.2051
WVFGRD96    2.0   140    50   -60   3.74 0.2266
WVFGRD96    3.0   160    65   -25   3.72 0.2182
WVFGRD96    4.0   350    70    10   3.75 0.2373
WVFGRD96    5.0   350    65    10   3.81 0.2590
WVFGRD96    6.0   350    65    15   3.84 0.2772
WVFGRD96    7.0   355    60    20   3.86 0.2950
WVFGRD96    8.0   350    65    15   3.87 0.3127
WVFGRD96    9.0   355    65    20   3.88 0.3270
WVFGRD96   10.0    -5    65    20   3.90 0.3396
WVFGRD96   11.0    10    60     0   3.90 0.3507
WVFGRD96   12.0     5    60    -5   3.92 0.3625
WVFGRD96   13.0     5    60    -5   3.93 0.3726
WVFGRD96   14.0     5    60    -5   3.95 0.3808
WVFGRD96   15.0    10    55     5   3.97 0.3878
WVFGRD96   16.0    10    50    10   3.99 0.3946
WVFGRD96   17.0    10    50    10   4.00 0.4012
WVFGRD96   18.0    10    50    10   4.01 0.4070
WVFGRD96   19.0    10    50    10   4.02 0.4118
WVFGRD96   20.0    10    45    10   4.04 0.4159
WVFGRD96   21.0    10    45    10   4.05 0.4189
WVFGRD96   22.0    10    45    10   4.06 0.4204
WVFGRD96   23.0    10    45    15   4.07 0.4206
WVFGRD96   24.0    10    45    10   4.08 0.4197
WVFGRD96   25.0    10    45    10   4.09 0.4177
WVFGRD96   26.0    10    45    10   4.10 0.4148
WVFGRD96   27.0    10    45    10   4.11 0.4113
WVFGRD96   28.0    10    45    10   4.11 0.4078
WVFGRD96   29.0    10    50    10   4.13 0.4063

The best solution is

WVFGRD96   23.0    10    45    15   4.07 0.4206

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 May 7 09:29:50 CDT 2011

Last Changed 2011/05/06