Location

2014/03/17 08:32:36 -20.030 -70.706 10.2 5.3 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/17 08:32:36:0 -20.03 -70.71 10.2 5.3 Chile Stations used: C.GO01 C.GO02 CX.PB01 CX.PB04 CX.PB10 CX.PB11 GT.LPAZ IU.LVC Filtering commands used: cut a -30 a 180 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.02e+23 dyne-cm Mw = 4.92 Z = 13 km Plane Strike Dip Rake NP1 225 75 -70 NP2 350 25 -142 Principal Axes: Axis Value Plunge Azimuth T 3.02e+23 27 299 N 0.00e+00 19 40 P -3.02e+23 56 160 Moment Tensor: (dyne-cm) Component Value Mxx -2.88e+22 Mxy -7.09e+22 Mxz 1.93e+23 Myy 1.71e+23 Myz -1.55e+23 Mzz -1.42e+23 #########----- #################----- ######################------ #########################--### ##########################---##### ########################-------##### #### ################----------##### ##### T #############--------------##### ##### ###########----------------##### ##################-------------------##### #################--------------------##### ###############----------------------##### #############------------------------##### ###########-------------------------#### ##########----------- -----------##### #######------------- P -----------#### #####-------------- ----------#### ###---------------------------#### #--------------------------### ------------------------#### -------------------### -------------# Global CMT Convention Moment Tensor: R T P -1.42e+23 1.93e+23 1.55e+23 1.93e+23 -2.88e+22 7.09e+22 1.55e+23 7.09e+22 1.71e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140317083236/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 = 225
      DIP = 75
     RAKE = -70
       MW = 4.92
       HS = 13.0

The NDK file is 20140317083236.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/03/17 08:32:36:0 -20.03 -70.71 10.2 5.3 Chile Stations used: C.GO01 C.GO02 CX.PB01 CX.PB04 CX.PB10 CX.PB11 GT.LPAZ IU.LVC Filtering commands used: cut a -30 a 180 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.02e+23 dyne-cm Mw = 4.92 Z = 13 km Plane Strike Dip Rake NP1 225 75 -70 NP2 350 25 -142 Principal Axes: Axis Value Plunge Azimuth T 3.02e+23 27 299 N 0.00e+00 19 40 P -3.02e+23 56 160 Moment Tensor: (dyne-cm) Component Value Mxx -2.88e+22 Mxy -7.09e+22 Mxz 1.93e+23 Myy 1.71e+23 Myz -1.55e+23 Mzz -1.42e+23 #########----- #################----- ######################------ #########################--### ##########################---##### ########################-------##### #### ################----------##### ##### T #############--------------##### ##### ###########----------------##### ##################-------------------##### #################--------------------##### ###############----------------------##### #############------------------------##### ###########-------------------------#### ##########----------- -----------##### #######------------- P -----------#### #####-------------- ----------#### ###---------------------------#### #--------------------------### ------------------------#### -------------------### -------------# Global CMT Convention Moment Tensor: R T P -1.42e+23 1.93e+23 1.55e+23 1.93e+23 -2.88e+22 7.09e+22 1.55e+23 7.09e+22 1.71e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140317083236/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:

cut a -30 a 180
rtr
taper w 0.1
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    45    50   -95   4.55 0.3234
WVFGRD96    2.0    45    50   -95   4.69 0.4225
WVFGRD96    3.0    95    65   -35   4.68 0.3653
WVFGRD96    4.0   100    40    -5   4.68 0.3758
WVFGRD96    5.0   225    85   -80   4.73 0.4448
WVFGRD96    6.0   225    85   -75   4.74 0.5076
WVFGRD96    7.0   225    80   -75   4.75 0.5557
WVFGRD96    8.0   225    80   -80   4.84 0.5916
WVFGRD96    9.0   225    80   -75   4.86 0.6280
WVFGRD96   10.0   225    80   -75   4.87 0.6530
WVFGRD96   11.0   225    75   -75   4.89 0.6707
WVFGRD96   12.0   225    75   -70   4.90 0.6803
WVFGRD96   13.0   225    75   -70   4.92 0.6844
WVFGRD96   14.0   230    80   -65   4.94 0.6841
WVFGRD96   15.0   230    80   -65   4.95 0.6786
WVFGRD96   16.0   230    80   -65   4.96 0.6691
WVFGRD96   17.0   230    80   -65   4.98 0.6564
WVFGRD96   18.0   230    80   -65   4.99 0.6412
WVFGRD96   19.0   230    80   -65   5.00 0.6238
WVFGRD96   20.0   230    80   -65   5.01 0.6041
WVFGRD96   21.0   230    85   -65   5.03 0.5840
WVFGRD96   22.0   150    10    15   5.02 0.5647
WVFGRD96   23.0   150    10    15   5.03 0.5466
WVFGRD96   24.0   155    10    20   5.03 0.5266
WVFGRD96   25.0   155    10    20   5.04 0.5070
WVFGRD96   26.0   155    10    20   5.05 0.4859
WVFGRD96   27.0   155    10    20   5.06 0.4637
WVFGRD96   28.0   155    10    20   5.06 0.4412
WVFGRD96   29.0   150    15    15   5.07 0.4173

The best solution is

WVFGRD96   13.0   225    75   -70   4.92 0.6844

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

cut a -30 a 180
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 n 3

Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.

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

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, 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:

Last Changed Mon Mar 17 13:09:15 CDT 2014