Location

2014/04/11 12:00:51 -20.068 -70.545 22.3 5.5 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/04/11 12:00:51:0 -20.07 -70.54 22.3 5.5 Chile Stations used: C.GO01 C.GO02 CX.MNMCX CX.PATCX CX.PB01 CX.PB04 CX.PB06 CX.PB07 CX.PB08 CX.PB09 CX.PB10 CX.PB11 CX.PB12 CX.PB14 CX.PB15 CX.PB16 CX.PSGCX GT.LPAZ 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 = 3.85e+23 dyne-cm Mw = 4.99 Z = 24 km Plane Strike Dip Rake NP1 350 81 -93 NP2 190 10 -70 Principal Axes: Axis Value Plunge Azimuth T 3.85e+23 36 83 N 0.00e+00 3 350 P -3.85e+23 54 256 Moment Tensor: (dyne-cm) Component Value Mxx -4.09e+21 Mxy 3.26e+20 Mxz 6.86e+22 Myy 1.28e+23 Myz 3.57e+23 Mzz -1.24e+23 ##--########## ##-------############# ###---------################ ##------------################ ##---------------################# ##----------------################## ##------------------################## ##-------------------################### ##--------------------################## ##---------------------########## ###### ##---------------------########## T ###### ##--------- ----------######### ###### ##--------- P ----------################## ##-------- ----------################# ##---------------------################# ##---------------------############### ##--------------------############## ##-------------------############# #------------------########### ##----------------########## #--------------####### -----------### Global CMT Convention Moment Tensor: R T P -1.24e+23 6.86e+22 -3.57e+23 6.86e+22 -4.09e+21 -3.26e+20 -3.57e+23 -3.26e+20 1.28e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140411120051/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 = 190
      DIP = 10
     RAKE = -70
       MW = 4.99
       HS = 24.0

The NDK file is 20140411120051.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/04/11 12:00:51:0 -20.07 -70.54 22.3 5.5 Chile Stations used: C.GO01 C.GO02 CX.MNMCX CX.PATCX CX.PB01 CX.PB04 CX.PB06 CX.PB07 CX.PB08 CX.PB09 CX.PB10 CX.PB11 CX.PB12 CX.PB14 CX.PB15 CX.PB16 CX.PSGCX GT.LPAZ 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 = 3.85e+23 dyne-cm Mw = 4.99 Z = 24 km Plane Strike Dip Rake NP1 350 81 -93 NP2 190 10 -70 Principal Axes: Axis Value Plunge Azimuth T 3.85e+23 36 83 N 0.00e+00 3 350 P -3.85e+23 54 256 Moment Tensor: (dyne-cm) Component Value Mxx -4.09e+21 Mxy 3.26e+20 Mxz 6.86e+22 Myy 1.28e+23 Myz 3.57e+23 Mzz -1.24e+23 ##--########## ##-------############# ###---------################ ##------------################ ##---------------################# ##----------------################## ##------------------################## ##-------------------################### ##--------------------################## ##---------------------########## ###### ##---------------------########## T ###### ##--------- ----------######### ###### ##--------- P ----------################## ##-------- ----------################# ##---------------------################# ##---------------------############### ##--------------------############## ##-------------------############# #------------------########### ##----------------########## #--------------####### -----------### Global CMT Convention Moment Tensor: R T P -1.24e+23 6.86e+22 -3.57e+23 6.86e+22 -4.09e+21 -3.26e+20 -3.57e+23 -3.26e+20 1.28e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140411120051/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   145    40    90   4.71 0.4919
WVFGRD96    4.0   135    30    75   4.76 0.3218
WVFGRD96    6.0   265    10    10   4.75 0.3936
WVFGRD96    8.0   255    10    -5   4.84 0.4864
WVFGRD96   10.0   240    10   -20   4.85 0.5967
WVFGRD96   12.0   220    10   -40   4.87 0.6821
WVFGRD96   14.0   215    10   -45   4.89 0.7476
WVFGRD96   16.0   210    10   -50   4.91 0.7963
WVFGRD96   18.0   205    10   -55   4.93 0.8308
WVFGRD96   20.0   205    15   -55   4.95 0.8532
WVFGRD96   22.0   205    10   -55   4.97 0.8651
WVFGRD96   24.0   190    10   -70   4.99 0.8689
WVFGRD96   26.0   190    10   -70   5.00 0.8640
WVFGRD96   28.0   190    10   -70   5.02 0.8507
WVFGRD96   30.0   195    15   -65   5.03 0.8292
WVFGRD96   32.0   195    15   -65   5.04 0.8009
WVFGRD96   34.0   200    15   -60   5.04 0.7677
WVFGRD96   36.0   210    15   -50   5.04 0.7330
WVFGRD96   38.0   205    10   -55   5.04 0.7016
WVFGRD96   40.0   205     5   -55   5.19 0.6773
WVFGRD96   42.0   205     5   -55   5.19 0.6344
WVFGRD96   44.0   215    10   -45   5.19 0.5937
WVFGRD96   46.0   200     5   -55   5.18 0.5556
WVFGRD96   48.0   255    -5     0   5.18 0.5219
WVFGRD96   50.0   345    10   110   5.18 0.4884
WVFGRD96   52.0   305    35    65   5.22 0.4647
WVFGRD96   54.0   315    30    80   5.23 0.4523
WVFGRD96   56.0   150    60    90   5.23 0.4410
WVFGRD96   58.0   330    30    90   5.24 0.4282
WVFGRD96   60.0   150    65    90   5.24 0.4155
WVFGRD96   62.0   325    25    85   5.25 0.4049
WVFGRD96   64.0   325    25    85   5.25 0.3936
WVFGRD96   66.0   315    25    75   5.26 0.3836
WVFGRD96   68.0   315    25    75   5.27 0.3749
WVFGRD96   70.0   310    25    70   5.27 0.3684
WVFGRD96   72.0   315    20    75   5.27 0.3647
WVFGRD96   74.0   310    20    70   5.28 0.3616
WVFGRD96   76.0   305    20    65   5.29 0.3578
WVFGRD96   78.0   305    20    65   5.29 0.3549
WVFGRD96   80.0   335    65   -85   5.23 0.3563
WVFGRD96   82.0   145    25  -100   5.23 0.3604
WVFGRD96   84.0   145    25  -100   5.24 0.3636
WVFGRD96   86.0   335    60   -85   5.24 0.3720
WVFGRD96   88.0   335    60   -85   5.25 0.3787
WVFGRD96   90.0   335    60   -85   5.25 0.3884
WVFGRD96   92.0   145    30   -95   5.26 0.4019
WVFGRD96   94.0   330    60   -90   5.27 0.4117
WVFGRD96   96.0   150    30   -90   5.27 0.4287
WVFGRD96   98.0   325    60   -90   5.28 0.4376
WVFGRD96  100.0   325    60   -90   5.28 0.4516
WVFGRD96  102.0   145    30   -90   5.29 0.4590
WVFGRD96  104.0   150    35   -90   5.30 0.4727
WVFGRD96  106.0   150    35   -90   5.30 0.4792
WVFGRD96  108.0   150    35   -90   5.31 0.4902

The best solution is

WVFGRD96   24.0   190    10   -70   4.99 0.8689

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 Apr 11 08:13:13 CDT 2014