Location

2014/04/06 14:06:07 -20.436 -71.057 8.7 5.6 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/04/06 14:06:07:0 -20.44 -71.06 8.7 5.6 Chile Stations used: CX.MNMCX CX.PATCX CX.PB01 CX.PB04 CX.PB07 CX.PB08 CX.PB09 CX.PB10 CX.PB11 CX.PB12 CX.PB14 CX.PB15 CX.PB16 CX.PSGCX 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 0.06 n 3 Best Fitting Double Couple Mo = 1.16e+24 dyne-cm Mw = 5.31 Z = 18 km Plane Strike Dip Rake NP1 330 65 75 NP2 182 29 119 Principal Axes: Axis Value Plunge Azimuth T 1.16e+24 67 213 N 0.00e+00 14 336 P -1.16e+24 19 71 Moment Tensor: (dyne-cm) Component Value Mxx 2.11e+22 Mxy -2.36e+23 Mxz -4.71e+23 Myy -8.80e+23 Myz -5.61e+23 Mzz 8.59e+23 ####---------- --###----------------- ------##-------------------- -----#######------------------ ------##########------------------ ------#############----------------- ------###############----------------- ------#################------------ -- ------###################---------- P -- ------#####################--------- --- ------######################-------------- ------#######################------------- ------########## ##########------------- -----########## T ###########----------- ------######### ############---------- -----########################--------- -----#######################-------- -----#######################------ -----#####################---- -----####################--- ----#################- ---########### Global CMT Convention Moment Tensor: R T P 8.59e+23 -4.71e+23 5.61e+23 -4.71e+23 2.11e+22 2.36e+23 5.61e+23 2.36e+23 -8.80e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140406140607/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 = 330
      DIP = 65
     RAKE = 75
       MW = 5.31
       HS = 18.0

The NDK file is 20140406140607.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/04/06 14:06:07:0 -20.44 -71.06 8.7 5.6 Chile Stations used: CX.MNMCX CX.PATCX CX.PB01 CX.PB04 CX.PB07 CX.PB08 CX.PB09 CX.PB10 CX.PB11 CX.PB12 CX.PB14 CX.PB15 CX.PB16 CX.PSGCX 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 0.06 n 3 Best Fitting Double Couple Mo = 1.16e+24 dyne-cm Mw = 5.31 Z = 18 km Plane Strike Dip Rake NP1 330 65 75 NP2 182 29 119 Principal Axes: Axis Value Plunge Azimuth T 1.16e+24 67 213 N 0.00e+00 14 336 P -1.16e+24 19 71 Moment Tensor: (dyne-cm) Component Value Mxx 2.11e+22 Mxy -2.36e+23 Mxz -4.71e+23 Myy -8.80e+23 Myz -5.61e+23 Mzz 8.59e+23 ####---------- --###----------------- ------##-------------------- -----#######------------------ ------##########------------------ ------#############----------------- ------###############----------------- ------#################------------ -- ------###################---------- P -- ------#####################--------- --- ------######################-------------- ------#######################------------- ------########## ##########------------- -----########## T ###########----------- ------######### ############---------- -----########################--------- -----#######################-------- -----#######################------ -----#####################---- -----####################--- ----#################- ---########### Global CMT Convention Moment Tensor: R T P 8.59e+23 -4.71e+23 5.61e+23 -4.71e+23 2.11e+22 2.36e+23 5.61e+23 2.36e+23 -8.80e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140406140607/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   160    45   -90   5.06 0.4339
WVFGRD96    4.0   345    75   -70   5.16 0.3285
WVFGRD96    6.0   335    85   -80   5.17 0.4350
WVFGRD96    8.0   335    80   -80   5.25 0.5040
WVFGRD96   10.0   335    80   -75   5.24 0.5507
WVFGRD96   12.0   335    70    85   5.26 0.5984
WVFGRD96   14.0   335    70    80   5.28 0.6430
WVFGRD96   16.0   330    65    75   5.30 0.6705
WVFGRD96   18.0   330    65    75   5.31 0.6807
WVFGRD96   20.0   330    65    75   5.32 0.6765
WVFGRD96   22.0   330    65    75   5.33 0.6630
WVFGRD96   24.0   330    65    70   5.34 0.6412
WVFGRD96   26.0   325    65    70   5.35 0.6150
WVFGRD96   28.0   325    65    70   5.36 0.5841
WVFGRD96   30.0   320    65    65   5.37 0.5496
WVFGRD96   32.0   320    65    65   5.37 0.5128
WVFGRD96   34.0   320    65    65   5.38 0.4737
WVFGRD96   36.0   320    60    65   5.38 0.4331
WVFGRD96   38.0   320    60    65   5.39 0.3916
WVFGRD96   40.0   325    65    75   5.50 0.3593
WVFGRD96   42.0   320    65    70   5.51 0.3271
WVFGRD96   44.0   320    60    65   5.50 0.2958
WVFGRD96   46.0   320    60    65   5.50 0.2667
WVFGRD96   48.0   315    60    60   5.50 0.2403
WVFGRD96   50.0   310    60    55   5.50 0.2168
WVFGRD96   52.0   140    45    50   5.47 0.1980
WVFGRD96   54.0   140    50    50   5.47 0.1839
WVFGRD96   56.0   135    50    40   5.47 0.1734
WVFGRD96   58.0   140    50    45   5.47 0.1668
WVFGRD96   60.0   140    50    45   5.47 0.1601
WVFGRD96   62.0   140    55    40   5.48 0.1569
WVFGRD96   64.0   140    55    40   5.48 0.1556
WVFGRD96   66.0   140    55    40   5.48 0.1525
WVFGRD96   68.0   140    60    40   5.49 0.1530
WVFGRD96   70.0   140    60    40   5.49 0.1544
WVFGRD96   72.0   140    60    40   5.50 0.1544
WVFGRD96   74.0   140    60    40   5.50 0.1564
WVFGRD96   76.0   145    60    45   5.51 0.1569
WVFGRD96   78.0   145    60    45   5.51 0.1597
WVFGRD96   80.0   145    60    45   5.52 0.1609
WVFGRD96   82.0   145    65    45   5.53 0.1637
WVFGRD96   84.0   145    65    50   5.53 0.1650
WVFGRD96   86.0   145    65    50   5.53 0.1671
WVFGRD96   88.0   145    65    50   5.54 0.1682
WVFGRD96   90.0   145    65    50   5.54 0.1696
WVFGRD96   92.0   145    65    55   5.55 0.1709
WVFGRD96   94.0   145    65    55   5.55 0.1722
WVFGRD96   96.0   145    65    55   5.55 0.1732
WVFGRD96   98.0   145    70    60   5.56 0.1739
WVFGRD96  100.0   145    70    60   5.57 0.1754
WVFGRD96  102.0   275    60   -75   5.57 0.1766
WVFGRD96  104.0   275    60   -75   5.57 0.1796
WVFGRD96  106.0   275    60   -75   5.57 0.1825
WVFGRD96  108.0   275    60   -75   5.58 0.1853

The best solution is

WVFGRD96   18.0   330    65    75   5.31 0.6807

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 Sun Apr 6 09:49:53 CDT 2014