Location

2014/04/07 10:41:35 -20.509 -70.983 14.9 4.8 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/04/07 10:41:35:0 -20.51 -70.98 14.9 4.8 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 = 7.33e+22 dyne-cm Mw = 4.51 Z = 18 km Plane Strike Dip Rake NP1 155 70 75 NP2 13 25 125 Principal Axes: Axis Value Plunge Azimuth T 7.33e+22 62 42 N 0.00e+00 14 160 P -7.33e+22 24 257 Moment Tensor: (dyne-cm) Component Value Mxx 5.53e+21 Mxy -5.97e+21 Mxz 2.88e+22 Myy -5.10e+22 Myz 4.64e+22 Mzz 4.55e+22 #############- --##################-- -----####################--- ------#####################--- --------######################---- ---------#######################---- -----------#######################---- ------------########### #########----- -------------########## T ##########---- --------------########## ##########----- ---------------######################----- ----------------#####################----- ---- ----------####################----- --- P -----------##################----- --- ------------#################----- ------------------###############----- ------------------#############----- ------------------##########------ ------------------#######----- -------------------###------ ----------------##---- --------###### Global CMT Convention Moment Tensor: R T P 4.55e+22 2.88e+22 -4.64e+22 2.88e+22 5.53e+21 5.97e+21 -4.64e+22 5.97e+21 -5.10e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140407104135/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 = 155
      DIP = 70
     RAKE = 75
       MW = 4.51
       HS = 18.0

The NDK file is 20140407104135.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/04/07 10:41:35:0 -20.51 -70.98 14.9 4.8 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 = 7.33e+22 dyne-cm Mw = 4.51 Z = 18 km Plane Strike Dip Rake NP1 155 70 75 NP2 13 25 125 Principal Axes: Axis Value Plunge Azimuth T 7.33e+22 62 42 N 0.00e+00 14 160 P -7.33e+22 24 257 Moment Tensor: (dyne-cm) Component Value Mxx 5.53e+21 Mxy -5.97e+21 Mxz 2.88e+22 Myy -5.10e+22 Myz 4.64e+22 Mzz 4.55e+22 #############- --##################-- -----####################--- ------#####################--- --------######################---- ---------#######################---- -----------#######################---- ------------########### #########----- -------------########## T ##########---- --------------########## ##########----- ---------------######################----- ----------------#####################----- ---- ----------####################----- --- P -----------##################----- --- ------------#################----- ------------------###############----- ------------------#############----- ------------------##########------ ------------------#######----- -------------------###------ ----------------##---- --------###### Global CMT Convention Moment Tensor: R T P 4.55e+22 2.88e+22 -4.64e+22 2.88e+22 5.53e+21 5.97e+21 -4.64e+22 5.97e+21 -5.10e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140407104135/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   4.27 0.4432
WVFGRD96    4.0   340    80   -70   4.39 0.3797
WVFGRD96    6.0   335    85   -75   4.39 0.5172
WVFGRD96    8.0   335    85   -75   4.46 0.5992
WVFGRD96   10.0   155    85    75   4.46 0.6668
WVFGRD96   12.0   155    75    75   4.48 0.7185
WVFGRD96   14.0   155    75    75   4.49 0.7553
WVFGRD96   16.0   155    70    75   4.50 0.7748
WVFGRD96   18.0   155    70    75   4.51 0.7779
WVFGRD96   20.0   155    70    70   4.52 0.7686
WVFGRD96   22.0   150    70    70   4.54 0.7523
WVFGRD96   24.0   150    70    70   4.55 0.7302
WVFGRD96   26.0   150    70    70   4.56 0.7036
WVFGRD96   28.0   150    70    70   4.57 0.6733
WVFGRD96   30.0   150    70    65   4.58 0.6401
WVFGRD96   32.0   150    70    70   4.58 0.6048
WVFGRD96   34.0   150    70    65   4.59 0.5679
WVFGRD96   36.0   150    70    65   4.60 0.5299
WVFGRD96   38.0   150    75    65   4.60 0.4937
WVFGRD96   40.0   150    75    75   4.72 0.4646
WVFGRD96   42.0   150    75    75   4.73 0.4278
WVFGRD96   44.0   150    75    75   4.73 0.3924
WVFGRD96   46.0   150    70    65   4.73 0.3615
WVFGRD96   48.0   155    70    70   4.73 0.3336
WVFGRD96   50.0   155    70    70   4.73 0.3076
WVFGRD96   52.0   155    70    70   4.73 0.2834
WVFGRD96   54.0   150    75    60   4.73 0.2625
WVFGRD96   56.0   320    70    50   4.75 0.2506
WVFGRD96   58.0   320    70    50   4.76 0.2484
WVFGRD96   60.0   105    45    10   4.74 0.2475
WVFGRD96   62.0   115    40    20   4.74 0.2497
WVFGRD96   64.0   120    40    25   4.74 0.2524
WVFGRD96   66.0   120    40    25   4.75 0.2535
WVFGRD96   68.0   130    35    40   4.76 0.2580
WVFGRD96   70.0   135    35    45   4.77 0.2622
WVFGRD96   72.0   130    40    40   4.77 0.2664
WVFGRD96   74.0   135    40    45   4.78 0.2692
WVFGRD96   76.0   135    40    45   4.78 0.2732
WVFGRD96   78.0   140    40    50   4.79 0.2767
WVFGRD96   80.0   140    40    50   4.79 0.2796
WVFGRD96   82.0   140    40    50   4.80 0.2841
WVFGRD96   84.0   140    40    50   4.80 0.2864
WVFGRD96   86.0   145    40    55   4.81 0.2914
WVFGRD96   88.0   145    40    55   4.81 0.2955
WVFGRD96   90.0   145    40    55   4.81 0.2967
WVFGRD96   92.0   145    45    55   4.82 0.3012
WVFGRD96   94.0   145    45    55   4.82 0.3044
WVFGRD96   96.0   145    45    55   4.82 0.3057
WVFGRD96   98.0   145    45    55   4.83 0.3082
WVFGRD96  100.0   145    45    55   4.83 0.3107
WVFGRD96  102.0   150    45    60   4.83 0.3106
WVFGRD96  104.0   150    45    60   4.83 0.3122
WVFGRD96  106.0   145    50    55   4.83 0.3137
WVFGRD96  108.0   145    50    55   4.84 0.3134

The best solution is

WVFGRD96   18.0   155    70    75   4.51 0.7779

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 Mon Apr 7 08:30:06 CDT 2014