Location

2014/03/20 18:41:31 -24.024 -68.986 93.2 5.1 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/20 18:41:31:0 -24.02 -68.99 93.2 5.1 Chile Stations used: C.GO01 C.GO02 C.GO03 CX.PB01 CX.PB04 CX.PB07 CX.PB09 CX.PB10 CX.PB11 CX.PB14 CX.PSGCX IU.LVC Filtering commands used: cut a -30 a 180 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 5.62e+23 dyne-cm Mw = 5.10 Z = 86 km Plane Strike Dip Rake NP1 283 61 -132 NP2 165 50 -40 Principal Axes: Axis Value Plunge Azimuth T 5.62e+23 6 42 N 0.00e+00 36 308 P -5.62e+23 53 140 Moment Tensor: (dyne-cm) Component Value Mxx 1.89e+23 Mxy 3.75e+23 Mxz 2.51e+23 Myy 1.67e+23 Myz -1.32e+23 Mzz -3.56e+23 -############# ----################## ------#################### ------##################### T -------###################### ## --------############################ --------###-########################## ---######----------------############### #########--------------------########### ##########------------------------######## ###########-------------------------###### ###########---------------------------#### ###########-----------------------------## ###########------------- ------------- ############------------ P ------------- ###########------------ ------------ ###########------------------------- ############---------------------- ###########------------------- ############---------------- ###########----------- ###########--- Global CMT Convention Moment Tensor: R T P -3.56e+23 2.51e+23 1.32e+23 2.51e+23 1.89e+23 -3.75e+23 1.32e+23 -3.75e+23 1.67e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140320184131/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 = 165
      DIP = 50
     RAKE = -40
       MW = 5.10
       HS = 86.0

The NDK file is 20140320184131.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/03/20 18:41:31:0 -24.02 -68.99 93.2 5.1 Chile Stations used: C.GO01 C.GO02 C.GO03 CX.PB01 CX.PB04 CX.PB07 CX.PB09 CX.PB10 CX.PB11 CX.PB14 CX.PSGCX IU.LVC Filtering commands used: cut a -30 a 180 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 5.62e+23 dyne-cm Mw = 5.10 Z = 86 km Plane Strike Dip Rake NP1 283 61 -132 NP2 165 50 -40 Principal Axes: Axis Value Plunge Azimuth T 5.62e+23 6 42 N 0.00e+00 36 308 P -5.62e+23 53 140 Moment Tensor: (dyne-cm) Component Value Mxx 1.89e+23 Mxy 3.75e+23 Mxz 2.51e+23 Myy 1.67e+23 Myz -1.32e+23 Mzz -3.56e+23 -############# ----################## ------#################### ------##################### T -------###################### ## --------############################ --------###-########################## ---######----------------############### #########--------------------########### ##########------------------------######## ###########-------------------------###### ###########---------------------------#### ###########-----------------------------## ###########------------- ------------- ############------------ P ------------- ###########------------ ------------ ###########------------------------- ############---------------------- ###########------------------- ############---------------- ###########----------- ###########--- Global CMT Convention Moment Tensor: R T P -3.56e+23 2.51e+23 1.32e+23 2.51e+23 1.89e+23 -3.75e+23 1.32e+23 -3.75e+23 1.67e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140320184131/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 0.06 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    2.0   125    50    70   4.33 0.2469
WVFGRD96    4.0   125    45    75   4.42 0.2763
WVFGRD96    6.0    95    85   -30   4.33 0.2528
WVFGRD96    8.0    95    85   -35   4.40 0.2708
WVFGRD96   10.0    90    80   -35   4.43 0.2895
WVFGRD96   12.0    95    85   -30   4.45 0.3020
WVFGRD96   14.0    95    85   -30   4.47 0.3087
WVFGRD96   16.0    10    55    10   4.50 0.3222
WVFGRD96   18.0    10    60    10   4.53 0.3374
WVFGRD96   20.0    10    60     5   4.55 0.3507
WVFGRD96   22.0    10    60     5   4.58 0.3620
WVFGRD96   24.0    10    60     5   4.60 0.3719
WVFGRD96   26.0    10    60     5   4.62 0.3798
WVFGRD96   28.0    10    60     5   4.65 0.3867
WVFGRD96   30.0    10    60     5   4.67 0.3921
WVFGRD96   32.0    10    60     5   4.69 0.3967
WVFGRD96   34.0    10    60     5   4.71 0.3992
WVFGRD96   36.0    10    65     5   4.74 0.4024
WVFGRD96   38.0     5    65     5   4.76 0.4093
WVFGRD96   40.0     5    55     5   4.83 0.4157
WVFGRD96   42.0     0    60   -15   4.86 0.4234
WVFGRD96   44.0     0    60   -20   4.88 0.4321
WVFGRD96   46.0     0    60   -20   4.90 0.4399
WVFGRD96   48.0     0    60   -20   4.91 0.4457
WVFGRD96   50.0   175    65   -20   4.93 0.4558
WVFGRD96   52.0   175    60   -20   4.94 0.4752
WVFGRD96   54.0   175    60   -20   4.96 0.4963
WVFGRD96   56.0   175    60   -20   4.97 0.5153
WVFGRD96   58.0   175    65   -25   4.99 0.5334
WVFGRD96   60.0   170    60   -30   5.01 0.5539
WVFGRD96   62.0   170    60   -30   5.02 0.5764
WVFGRD96   64.0   170    60   -30   5.03 0.5969
WVFGRD96   66.0   170    60   -30   5.04 0.6163
WVFGRD96   68.0   170    60   -30   5.05 0.6332
WVFGRD96   70.0   170    60   -30   5.06 0.6474
WVFGRD96   72.0   170    55   -35   5.07 0.6597
WVFGRD96   74.0   170    55   -35   5.07 0.6710
WVFGRD96   76.0   170    55   -35   5.08 0.6801
WVFGRD96   78.0   170    55   -40   5.09 0.6872
WVFGRD96   80.0   170    55   -40   5.09 0.6942
WVFGRD96   82.0   170    55   -40   5.09 0.6976
WVFGRD96   84.0   165    50   -40   5.10 0.7001
WVFGRD96   86.0   165    50   -40   5.10 0.7007
WVFGRD96   88.0   165    50   -40   5.10 0.7001
WVFGRD96   90.0   165    50   -40   5.10 0.6983
WVFGRD96   92.0   165    50   -40   5.10 0.6948
WVFGRD96   94.0   165    50   -40   5.10 0.6912
WVFGRD96   96.0   165    50   -40   5.10 0.6862
WVFGRD96   98.0   165    50   -40   5.10 0.6805
WVFGRD96  100.0   165    50   -40   5.10 0.6753
WVFGRD96  102.0   165    50   -40   5.10 0.6693
WVFGRD96  104.0   165    50   -40   5.10 0.6621
WVFGRD96  106.0   165    50   -40   5.10 0.6551
WVFGRD96  108.0   165    50   -40   5.10 0.6484

The best solution is

WVFGRD96   86.0   165    50   -40   5.10 0.7007

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 0.06 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 Fri Mar 21 08:02:53 CDT 2014