Location

2014/03/24 20:18:55 -27.874 -71.161 17.5 4.8 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/24 20:18:55:0 -27.87 -71.16 17.5 4.8 Chile Stations used: C.GO02 C.GO03 C.GO04 CX.PB15 G.PEL 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 = 2.34e+22 dyne-cm Mw = 4.18 Z = 22 km Plane Strike Dip Rake NP1 180 60 85 NP2 10 30 99 Principal Axes: Axis Value Plunge Azimuth T 2.34e+22 74 77 N 0.00e+00 4 182 P -2.34e+22 15 274 Moment Tensor: (dyne-cm) Component Value Mxx -3.09e+14 Mxy 1.77e+21 Mxz 1.02e+21 Myy -2.02e+22 Myz 1.17e+22 Mzz 2.02e+22 ------######-- --------###########--- ----------##############---- ----------################---- -----------##################----- ------------###################----- ------------#####################----- -------------#####################------ -------------######################----- -- --------########## ##########------ -- P --------########## T ##########------ -- --------########## ##########------ -------------######################------- -------------#####################------ -------------####################------- ------------####################------ ------------##################------ -----------################------- ----------##############------ ----------###########------- --------########------ ------##------ Global CMT Convention Moment Tensor: R T P 2.02e+22 1.02e+21 -1.17e+22 1.02e+21 -3.09e+14 -1.77e+21 -1.17e+22 -1.77e+21 -2.02e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140324201855/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 = 180
      DIP = 60
     RAKE = 85
       MW = 4.18
       HS = 22.0

The NDK file is 20140324201855.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/03/24 20:18:55:0 -27.87 -71.16 17.5 4.8 Chile Stations used: C.GO02 C.GO03 C.GO04 CX.PB15 G.PEL 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 = 2.34e+22 dyne-cm Mw = 4.18 Z = 22 km Plane Strike Dip Rake NP1 180 60 85 NP2 10 30 99 Principal Axes: Axis Value Plunge Azimuth T 2.34e+22 74 77 N 0.00e+00 4 182 P -2.34e+22 15 274 Moment Tensor: (dyne-cm) Component Value Mxx -3.09e+14 Mxy 1.77e+21 Mxz 1.02e+21 Myy -2.02e+22 Myz 1.17e+22 Mzz 2.02e+22 ------######-- --------###########--- ----------##############---- ----------################---- -----------##################----- ------------###################----- ------------#####################----- -------------#####################------ -------------######################----- -- --------########## ##########------ -- P --------########## T ##########------ -- --------########## ##########------ -------------######################------- -------------#####################------ -------------####################------- ------------####################------ ------------##################------ -----------################------- ----------##############------ ----------###########------- --------########------ ------##------ Global CMT Convention Moment Tensor: R T P 2.02e+22 1.02e+21 -1.17e+22 1.02e+21 -3.09e+14 -1.77e+21 -1.17e+22 -1.77e+21 -2.02e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140324201855/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    2.0   150    50    90   3.80 0.3853
WVFGRD96    4.0   105    65   -20   3.85 0.2579
WVFGRD96    6.0   275    50   -30   3.89 0.3341
WVFGRD96    8.0   260    25   -20   3.92 0.4125
WVFGRD96   10.0   250    25   -40   3.98 0.5038
WVFGRD96   12.0   250    25   -40   4.02 0.5746
WVFGRD96   14.0   220    20   -65   4.06 0.6257
WVFGRD96   16.0   215    20   -70   4.09 0.6591
WVFGRD96   18.0    15    70   -90   4.13 0.6753
WVFGRD96   20.0   180    60    85   4.15 0.6965
WVFGRD96   22.0   180    60    85   4.18 0.7138
WVFGRD96   24.0   190    65    90   4.22 0.7076
WVFGRD96   26.0   185    65    85   4.23 0.6856
WVFGRD96   28.0    10    25    95   4.25 0.6468
WVFGRD96   30.0   180    65    85   4.25 0.5942
WVFGRD96   32.0     5    30    90   4.25 0.5534
WVFGRD96   34.0     5    30    90   4.25 0.5165
WVFGRD96   36.0     5    30    85   4.25 0.4780
WVFGRD96   38.0   175    80   -65   4.26 0.4859
WVFGRD96   40.0   160    80   -75   4.38 0.4915
WVFGRD96   42.0   160    75   -75   4.39 0.4916
WVFGRD96   44.0   160    75   -75   4.40 0.4993
WVFGRD96   46.0   160    75   -75   4.41 0.5053
WVFGRD96   48.0   160    75   -75   4.43 0.5086
WVFGRD96   50.0   175    80   -65   4.45 0.5092
WVFGRD96   52.0   170    80   -65   4.45 0.4962
WVFGRD96   54.0   165    80   -70   4.46 0.4954
WVFGRD96   56.0   165    80   -70   4.47 0.4943
WVFGRD96   58.0   165    80   -70   4.48 0.4932
WVFGRD96   60.0   165    80   -70   4.49 0.4738
WVFGRD96   62.0   165    80   -70   4.51 0.4749
WVFGRD96   64.0   175    85   -65   4.52 0.4753
WVFGRD96   66.0   175    85   -65   4.53 0.4573
WVFGRD96   68.0     0    90    60   4.53 0.4565
WVFGRD96   70.0   175    85   -65   4.56 0.4439
WVFGRD96   72.0     0    90    60   4.55 0.4430
WVFGRD96   74.0     0    90    60   4.56 0.4273
WVFGRD96   76.0     0    90    60   4.58 0.4294
WVFGRD96   78.0   210    90    50   4.58 0.4344
WVFGRD96   80.0    -5    75   -70   4.45 0.4402
WVFGRD96   82.0   355    75   -70   4.46 0.4446
WVFGRD96   84.0   360    75   -70   4.47 0.4480
WVFGRD96   86.0   360    75   -70   4.48 0.4505
WVFGRD96   88.0   370    75   -65   4.49 0.4524
WVFGRD96   90.0   370    75   -65   4.50 0.4540
WVFGRD96   92.0     0    70   -65   4.47 0.4551
WVFGRD96   94.0     5    70   -65   4.48 0.4562
WVFGRD96   96.0     5    70   -65   4.49 0.4572
WVFGRD96   98.0     5    70   -65   4.49 0.4578
WVFGRD96  100.0     5    70   -65   4.49 0.4578
WVFGRD96  102.0    10    70   -65   4.51 0.4574
WVFGRD96  104.0    15    70   -60   4.51 0.4572
WVFGRD96  106.0    15    70   -60   4.52 0.4570
WVFGRD96  108.0    10    70   -65   4.52 0.4564

The best solution is

WVFGRD96   22.0   180    60    85   4.18 0.7138

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 24 17:23:08 CDT 2014