Location

2011/09/08 22:55:05 44.620 10.219 18.2 3.1 Italy

Arrival Times (from USGS)

Felt Map

Focal Mechanism

SLU Moment Tensor Solution ENS 2011/09/08 22:55:05:0 44.62 10.22 18.2 3.1 Italy Stations used: GU.SC2M IV.ASQU IV.BDI IV.CRE IV.FROS IV.GROG IV.MSSA IV.PRMA IV.SASS MN.VLC Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.82e+21 dyne-cm Mw = 3.44 Z = 17 km Plane Strike Dip Rake NP1 125 80 85 NP2 332 11 116 Principal Axes: Axis Value Plunge Azimuth T 1.82e+21 55 29 N 0.00e+00 5 126 P -1.82e+21 35 219 Moment Tensor: (dyne-cm) Component Value Mxx -2.69e+20 Mxy -3.45e+20 Mxz 1.41e+21 Myy -3.51e+20 Myz 9.55e+20 Mzz 6.20e+20 -########----- ###################--- ########################---- ############################-- ###############################--- --################### ##########-- -----################# T ###########-- --------############### ############-- ----------############################-- -------------###########################-- ---------------#########################-- ------------------######################-- ---------------------###################-- ----------------------#################- -------------------------#############-- --------- ----------------#########- -------- P -------------------#####- ------- -----------------------# ------------------------------ ---------------------------# ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 6.20e+20 1.41e+21 -9.55e+20 1.41e+21 -2.69e+20 3.45e+20 -9.55e+20 3.45e+20 -3.51e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.IT/20110908225505/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 = 125
      DIP = 80
     RAKE = 85
       MW = 3.44
       HS = 17.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

SLU Moment Tensor Solution ENS 2011/09/08 22:55:05:0 44.62 10.22 18.2 3.1 Italy Stations used: GU.SC2M IV.ASQU IV.BDI IV.CRE IV.FROS IV.GROG IV.MSSA IV.PRMA IV.SASS MN.VLC Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.82e+21 dyne-cm Mw = 3.44 Z = 17 km Plane Strike Dip Rake NP1 125 80 85 NP2 332 11 116 Principal Axes: Axis Value Plunge Azimuth T 1.82e+21 55 29 N 0.00e+00 5 126 P -1.82e+21 35 219 Moment Tensor: (dyne-cm) Component Value Mxx -2.69e+20 Mxy -3.45e+20 Mxz 1.41e+21 Myy -3.51e+20 Myz 9.55e+20 Mzz 6.20e+20 -########----- ###################--- ########################---- ############################-- ###############################--- --################### ##########-- -----################# T ###########-- --------############### ############-- ----------############################-- -------------###########################-- ---------------#########################-- ------------------######################-- ---------------------###################-- ----------------------#################- -------------------------#############-- --------- ----------------#########- -------- P -------------------#####- ------- -----------------------# ------------------------------ ---------------------------# ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 6.20e+20 1.41e+21 -9.55e+20 1.41e+21 -2.69e+20 3.45e+20 -9.55e+20 3.45e+20 -3.51e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.IT/20110908225505/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   275    35    85   3.11 0.3342
WVFGRD96    2.0   275    35    90   3.19 0.3651
WVFGRD96    3.0   230    40    30   3.19 0.3390
WVFGRD96    4.0   220    50     5   3.19 0.3536
WVFGRD96    5.0   225    35    15   3.26 0.3856
WVFGRD96    6.0   220    40     0   3.27 0.4162
WVFGRD96    7.0   220    35     0   3.28 0.4486
WVFGRD96    8.0   220    40     0   3.26 0.4789
WVFGRD96    9.0   225    30    15   3.26 0.5078
WVFGRD96   10.0   235    25    25   3.27 0.5368
WVFGRD96   11.0   230    25    20   3.29 0.5614
WVFGRD96   12.0   240    20    35   3.30 0.5871
WVFGRD96   13.0   240    20    35   3.32 0.6054
WVFGRD96   14.0   240    20    35   3.34 0.6213
WVFGRD96   15.0   125    80    85   3.40 0.6341
WVFGRD96   16.0   125    80    85   3.42 0.6431
WVFGRD96   17.0   125    80    85   3.44 0.6486
WVFGRD96   18.0   125    80    85   3.45 0.6450
WVFGRD96   19.0   125    80    85   3.47 0.6343
WVFGRD96   20.0   215    20    10   3.47 0.6194
WVFGRD96   21.0   300    90   -75   3.47 0.6049
WVFGRD96   22.0   185    20   -20   3.48 0.5895
WVFGRD96   23.0   175    25   -35   3.49 0.5811
WVFGRD96   24.0   175    25   -40   3.50 0.5748
WVFGRD96   25.0   175    25   -40   3.51 0.5656
WVFGRD96   26.0   175    30   -45   3.52 0.5546
WVFGRD96   27.0   175    30   -45   3.53 0.5426
WVFGRD96   28.0   185    35   -45   3.55 0.5286
WVFGRD96   29.0   175    35   -50   3.55 0.5150

The best solution is

WVFGRD96   17.0   125    80    85   3.44 0.6486

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=Fri Sep 9 08:01:28 CDT 2011

Last Changed 2011/09/08