Location

2014/03/23 12:48:05 -19.740 -71.013 16.2 4.8 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/23 12:48:05:0 -19.74 -71.01 16.2 4.8 Chile Stations used: C.GO01 CX.PATCX CX.PB01 CX.PB04 CX.PB07 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 = 3.94e+22 dyne-cm Mw = 4.33 Z = 20 km Plane Strike Dip Rake NP1 165 65 75 NP2 17 29 119 Principal Axes: Axis Value Plunge Azimuth T 3.94e+22 67 48 N 0.00e+00 14 171 P -3.94e+22 19 266 Moment Tensor: (dyne-cm) Component Value Mxx 2.67e+21 Mxy 7.15e+20 Mxz 1.05e+22 Myy -3.18e+22 Myz 2.25e+22 Mzz 2.91e+22 -############# -----###############-- -------##################--- --------###################--- ---------#####################---- ----------######################---- -----------#######################---- ------------########### #########----- -------------########## T #########----- --------------########## #########------ -- ---------######################------ -- P ----------#####################------ -- ----------#####################------ ---------------###################------ ---------------###################------ ---------------#################------ ---------------###############------ ---------------############------- --------------##########------ ---------------#####-------- -------------#-------- -----######--- Global CMT Convention Moment Tensor: R T P 2.91e+22 1.05e+22 -2.25e+22 1.05e+22 2.67e+21 -7.15e+20 -2.25e+22 -7.15e+20 -3.18e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140323124805/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 = 165
      DIP = 65
     RAKE = 75
       MW = 4.33
       HS = 20.0

The NDK file is 20140323124805.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 12:48:05:0 -19.74 -71.01 16.2 4.8 Chile Stations used: C.GO01 CX.PATCX CX.PB01 CX.PB04 CX.PB07 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 = 3.94e+22 dyne-cm Mw = 4.33 Z = 20 km Plane Strike Dip Rake NP1 165 65 75 NP2 17 29 119 Principal Axes: Axis Value Plunge Azimuth T 3.94e+22 67 48 N 0.00e+00 14 171 P -3.94e+22 19 266 Moment Tensor: (dyne-cm) Component Value Mxx 2.67e+21 Mxy 7.15e+20 Mxz 1.05e+22 Myy -3.18e+22 Myz 2.25e+22 Mzz 2.91e+22 -############# -----###############-- -------##################--- --------###################--- ---------#####################---- ----------######################---- -----------#######################---- ------------########### #########----- -------------########## T #########----- --------------########## #########------ -- ---------######################------ -- P ----------#####################------ -- ----------#####################------ ---------------###################------ ---------------###################------ ---------------#################------ ---------------###############------ ---------------############------- --------------##########------ ---------------#####-------- -------------#-------- -----######--- Global CMT Convention Moment Tensor: R T P 2.91e+22 1.05e+22 -2.25e+22 1.05e+22 2.67e+21 -7.15e+20 -2.25e+22 -7.15e+20 -3.18e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140323124805/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     0    50   -90   4.07 0.4341
WVFGRD96    4.0   220    10   -45   4.17 0.3287
WVFGRD96    6.0   240    10   -25   4.16 0.4723
WVFGRD96    8.0   240    10   -25   4.24 0.5602
WVFGRD96   10.0   260    10    -5   4.24 0.6410
WVFGRD96   12.0   300    10    40   4.25 0.7005
WVFGRD96   14.0   165    75    95   4.28 0.7503
WVFGRD96   16.0   170    70    80   4.30 0.7883
WVFGRD96   18.0   165    65    90   4.32 0.8091
WVFGRD96   20.0   165    65    75   4.33 0.8157
WVFGRD96   22.0   335    25    80   4.35 0.8105
WVFGRD96   24.0   335    25    80   4.36 0.7967
WVFGRD96   26.0   160    65    75   4.37 0.7780
WVFGRD96   28.0   155    65    70   4.38 0.7548
WVFGRD96   30.0   155    65    70   4.39 0.7280
WVFGRD96   32.0   155    65    70   4.40 0.6966
WVFGRD96   34.0   155    65    70   4.41 0.6610
WVFGRD96   36.0   155    65    70   4.41 0.6226
WVFGRD96   38.0   335    25    85   4.41 0.5843
WVFGRD96   40.0   330    20    75   4.54 0.5516
WVFGRD96   42.0   330    20    75   4.54 0.5141
WVFGRD96   44.0   335    25    80   4.55 0.4792
WVFGRD96   46.0   160    65    90   4.55 0.4455
WVFGRD96   48.0   165    70    75   4.55 0.4146
WVFGRD96   50.0   165    70    70   4.55 0.3864
WVFGRD96   52.0   165    70    70   4.56 0.3603
WVFGRD96   54.0   160    70    65   4.57 0.3386
WVFGRD96   56.0   160    70    65   4.57 0.3201
WVFGRD96   58.0   160    70    65   4.58 0.3026
WVFGRD96   60.0    55    25   -15   4.56 0.2930
WVFGRD96   62.0   345    75    75   4.56 0.2919
WVFGRD96   64.0   340    75    70   4.57 0.2946
WVFGRD96   66.0   335    75    70   4.59 0.2965
WVFGRD96   68.0   345    70    75   4.59 0.2987
WVFGRD96   70.0   340    70    70   4.60 0.3014
WVFGRD96   72.0   340    70    70   4.60 0.3030
WVFGRD96   74.0   340    70    70   4.61 0.3064
WVFGRD96   76.0   345    65    75   4.61 0.3076
WVFGRD96   78.0   345    65    75   4.61 0.3120
WVFGRD96   80.0   345    65    75   4.62 0.3163
WVFGRD96   82.0   340    65    70   4.62 0.3196
WVFGRD96   84.0   340    65    70   4.63 0.3189
WVFGRD96   86.0   345    60    75   4.63 0.3223
WVFGRD96   88.0   340    65    70   4.63 0.3250
WVFGRD96   90.0   335    65    65   4.64 0.3263
WVFGRD96   92.0   340    60    70   4.64 0.3312
WVFGRD96   94.0   340    60    70   4.64 0.3336
WVFGRD96   96.0   340    60    70   4.65 0.3344
WVFGRD96   98.0   340    60    70   4.65 0.3356
WVFGRD96  100.0   335    60    65   4.65 0.3389
WVFGRD96  102.0   335    60    65   4.66 0.3365
WVFGRD96  104.0   340    55    70   4.65 0.3386
WVFGRD96  106.0   340    55    70   4.65 0.3364
WVFGRD96  108.0   340    55    70   4.65 0.3380

The best solution is

WVFGRD96   20.0   165    65    75   4.33 0.8157

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 Mar 23 17:35:31 CDT 2014