Location

2014/03/23 10:10:53 -20.089 -68.829 118.3 4.5 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/23 10:10:53:0 -20.09 -68.83 118.3 4.5 Chile Stations used: 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 = 1.66e+22 dyne-cm Mw = 4.08 Z = 100 km Plane Strike Dip Rake NP1 328 67 -101 NP2 175 25 -65 Principal Axes: Axis Value Plunge Azimuth T 1.66e+22 22 66 N 0.00e+00 10 332 P -1.66e+22 66 218 Moment Tensor: (dyne-cm) Component Value Mxx 6.02e+20 Mxy 3.92e+21 Mxz 7.18e+21 Myy 1.09e+22 Myz 9.08e+21 Mzz -1.15e+22 --############ ---################### ####---##################### ####------#################### ####-----------################### ####-------------################### #####---------------############# ## #####-----------------############ T ### #####-------------------########## ### #####---------------------################ #####----------------------############### #####-----------------------############## #####----------- ----------############# #####---------- P -----------########### #####---------- ------------########## #####------------------------######### #####------------------------####### #####-----------------------###### #####---------------------#### #####--------------------### #####----------------- ####---------- Global CMT Convention Moment Tensor: R T P -1.15e+22 7.18e+21 -9.08e+21 7.18e+21 6.02e+20 -3.92e+21 -9.08e+21 -3.92e+21 1.09e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140323101053/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 = 175
      DIP = 25
     RAKE = -65
       MW = 4.08
       HS = 100.0

The NDK file is 20140323101053.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/03/23 10:10:53:0 -20.09 -68.83 118.3 4.5 Chile Stations used: 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 = 1.66e+22 dyne-cm Mw = 4.08 Z = 100 km Plane Strike Dip Rake NP1 328 67 -101 NP2 175 25 -65 Principal Axes: Axis Value Plunge Azimuth T 1.66e+22 22 66 N 0.00e+00 10 332 P -1.66e+22 66 218 Moment Tensor: (dyne-cm) Component Value Mxx 6.02e+20 Mxy 3.92e+21 Mxz 7.18e+21 Myy 1.09e+22 Myz 9.08e+21 Mzz -1.15e+22 --############ ---################### ####---##################### ####------#################### ####-----------################### ####-------------################### #####---------------############# ## #####-----------------############ T ### #####-------------------########## ### #####---------------------################ #####----------------------############### #####-----------------------############## #####----------- ----------############# #####---------- P -----------########### #####---------- ------------########## #####------------------------######### #####------------------------####### #####-----------------------###### #####---------------------#### #####--------------------### #####----------------- ####---------- Global CMT Convention Moment Tensor: R T P -1.15e+22 7.18e+21 -9.08e+21 7.18e+21 6.02e+20 -3.92e+21 -9.08e+21 -3.92e+21 1.09e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140323101053/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   325    65    20   3.21 0.2351
WVFGRD96    4.0   320    60     0   3.31 0.2794
WVFGRD96    6.0   130    60   -30   3.41 0.3175
WVFGRD96    8.0   130    60   -30   3.50 0.3365
WVFGRD96   10.0   130    65   -35   3.55 0.3397
WVFGRD96   12.0    60    55    20   3.62 0.3474
WVFGRD96   14.0    60    55    20   3.66 0.3663
WVFGRD96   16.0    55    60    15   3.67 0.3838
WVFGRD96   18.0    55    60    15   3.70 0.3980
WVFGRD96   20.0    50    65    15   3.70 0.4108
WVFGRD96   22.0    50    65    15   3.72 0.4238
WVFGRD96   24.0    50    65    15   3.74 0.4344
WVFGRD96   26.0    45    80    15   3.72 0.4448
WVFGRD96   28.0   225    65    25   3.76 0.4571
WVFGRD96   30.0   220    70     0   3.74 0.4662
WVFGRD96   32.0   220    70     0   3.75 0.4758
WVFGRD96   34.0   220    70     5   3.77 0.4850
WVFGRD96   36.0   225    50    35   3.83 0.4916
WVFGRD96   38.0   220    50    30   3.85 0.4955
WVFGRD96   40.0    40    90     0   3.84 0.4925
WVFGRD96   42.0   220    80     5   3.86 0.4934
WVFGRD96   44.0   220    85     5   3.88 0.4898
WVFGRD96   46.0   220    80     5   3.89 0.4878
WVFGRD96   48.0   220    80     5   3.91 0.4830
WVFGRD96   50.0   220    75     5   3.91 0.4794
WVFGRD96   52.0   220    65     5   3.92 0.4804
WVFGRD96   54.0    45    80    20   3.99 0.4811
WVFGRD96   56.0   215    65   -25   3.96 0.4846
WVFGRD96   58.0   220    75   -25   3.97 0.4907
WVFGRD96   60.0   220    75   -25   3.98 0.4953
WVFGRD96   62.0   220    70   -25   3.98 0.5005
WVFGRD96   64.0   220    70   -25   3.99 0.5052
WVFGRD96   66.0   220    70   -25   4.00 0.5079
WVFGRD96   68.0   220    70   -30   4.01 0.5118
WVFGRD96   70.0   220    70   -30   4.02 0.5146
WVFGRD96   72.0   220    65   -30   4.02 0.5159
WVFGRD96   74.0   220    65   -30   4.02 0.5189
WVFGRD96   76.0   220    65   -30   4.03 0.5208
WVFGRD96   78.0   225    70   -25   4.04 0.5220
WVFGRD96   80.0   220    60   -30   4.03 0.5216
WVFGRD96   82.0   195    35   -45   4.04 0.5238
WVFGRD96   84.0   190    30   -50   4.05 0.5270
WVFGRD96   86.0   190    30   -50   4.05 0.5274
WVFGRD96   88.0   190    30   -50   4.05 0.5274
WVFGRD96   90.0   190    30   -50   4.05 0.5292
WVFGRD96   92.0   180    25   -60   4.07 0.5293
WVFGRD96   94.0   180    25   -60   4.07 0.5304
WVFGRD96   96.0   180    25   -60   4.07 0.5321
WVFGRD96   98.0   180    25   -60   4.07 0.5308
WVFGRD96  100.0   190    25   -55   4.07 0.5330
WVFGRD96  102.0   190    25   -55   4.07 0.5314
WVFGRD96  104.0   175    20   -70   4.08 0.5311
WVFGRD96  106.0   175    20   -70   4.09 0.5312
WVFGRD96  108.0   175    20   -70   4.09 0.5307

The best solution is

WVFGRD96  100.0   190    25   -55   4.07 0.5330

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 Sun Mar 23 05:58:29 CDT 2014