Location

2014/04/14 05:56:18 -20.764 -70.694 15.4 5.3 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/04/14 05:56:18:0 -20.76 -70.69 15.4 5.3 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 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 = 6.03e+23 dyne-cm Mw = 5.12 Z = 18 km Plane Strike Dip Rake NP1 170 70 88 NP2 355 20 95 Principal Axes: Axis Value Plunge Azimuth T 6.03e+23 65 77 N 0.00e+00 2 170 P -6.03e+23 25 261 Moment Tensor: (dyne-cm) Component Value Mxx -6.05e+21 Mxy -5.12e+22 Mxz 8.92e+22 Myy -3.80e+23 Myz 4.54e+23 Mzz 3.86e+23 ---########--- ------############---- ---------###############---- ---------#################---- -----------###################---- ------------####################---- -------------#####################---- --------------######################---- --------------######################---- ---------------########### #########---- ----------------########## T #########---- ---- ---------########## #########---- ---- P ---------######################---- --- ----------####################---- ----------------####################---- ----------------##################---- ----------------#################--- ---------------###############---- --------------#############--- --------------##########---- ------------#######--- ----------##-- Global CMT Convention Moment Tensor: R T P 3.86e+23 8.92e+22 -4.54e+23 8.92e+22 -6.05e+21 5.12e+22 -4.54e+23 5.12e+22 -3.80e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140414055618/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 = 355
      DIP = 20
     RAKE = 95
       MW = 5.12
       HS = 18.0

The NDK file is 20140414055618.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/04/14 05:56:18:0 -20.76 -70.69 15.4 5.3 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 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 = 6.03e+23 dyne-cm Mw = 5.12 Z = 18 km Plane Strike Dip Rake NP1 170 70 88 NP2 355 20 95 Principal Axes: Axis Value Plunge Azimuth T 6.03e+23 65 77 N 0.00e+00 2 170 P -6.03e+23 25 261 Moment Tensor: (dyne-cm) Component Value Mxx -6.05e+21 Mxy -5.12e+22 Mxz 8.92e+22 Myy -3.80e+23 Myz 4.54e+23 Mzz 3.86e+23 ---########--- ------############---- ---------###############---- ---------#################---- -----------###################---- ------------####################---- -------------#####################---- --------------######################---- --------------######################---- ---------------########### #########---- ----------------########## T #########---- ---- ---------########## #########---- ---- P ---------######################---- --- ----------####################---- ----------------####################---- ----------------##################---- ----------------#################--- ---------------###############---- --------------#############--- --------------##########---- ------------#######--- ----------##-- Global CMT Convention Moment Tensor: R T P 3.86e+23 8.92e+22 -4.54e+23 8.92e+22 -6.05e+21 5.12e+22 -4.54e+23 5.12e+22 -3.80e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140414055618/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   355    50    90   4.86 0.3771
WVFGRD96    4.0   165    90    75   4.96 0.3287
WVFGRD96    6.0   350    90   -80   4.98 0.4915
WVFGRD96    8.0   170    85    80   5.06 0.5939
WVFGRD96   10.0   170    75    90   5.07 0.6907
WVFGRD96   12.0   355    20    95   5.09 0.7663
WVFGRD96   14.0   170    70    90   5.10 0.8173
WVFGRD96   16.0   170    70    90   5.11 0.8444
WVFGRD96   18.0   355    20    95   5.12 0.8550
WVFGRD96   20.0   350    20    90   5.13 0.8530
WVFGRD96   22.0   170    70    90   5.15 0.8431
WVFGRD96   24.0   350    20    90   5.16 0.8257
WVFGRD96   26.0   345    20    85   5.17 0.8033
WVFGRD96   28.0   340    15    80   5.18 0.7772
WVFGRD96   30.0   340    15    80   5.18 0.7484
WVFGRD96   32.0   335    15    75   5.19 0.7154
WVFGRD96   34.0   335    15    75   5.20 0.6807
WVFGRD96   36.0   330    15    70   5.20 0.6448
WVFGRD96   38.0   325    15    65   5.21 0.6114
WVFGRD96   40.0   335    10    75   5.35 0.5849
WVFGRD96   42.0   340    10    80   5.35 0.5466
WVFGRD96   44.0   330    10    70   5.36 0.5099
WVFGRD96   46.0   330    10    70   5.36 0.4745
WVFGRD96   48.0   315    10    55   5.36 0.4408
WVFGRD96   50.0   305    10    45   5.37 0.4095
WVFGRD96   52.0   300    10    35   5.36 0.3808
WVFGRD96   54.0   160    70    60   5.39 0.3571
WVFGRD96   56.0   160    70    60   5.39 0.3391
WVFGRD96   58.0   155    75    55   5.39 0.3235
WVFGRD96   60.0   155    75    55   5.40 0.3091
WVFGRD96   62.0   160    75    60   5.40 0.2959
WVFGRD96   64.0   160    75    60   5.40 0.2852
WVFGRD96   66.0   165    30    75   5.37 0.2776
WVFGRD96   68.0   165    35    75   5.38 0.2827
WVFGRD96   70.0   290    50   -60   5.38 0.2836
WVFGRD96   72.0   290    50   -60   5.39 0.2874
WVFGRD96   74.0   290    50   -65   5.40 0.2895
WVFGRD96   76.0   285    50   -65   5.40 0.2923
WVFGRD96   78.0   285    50   -70   5.41 0.2958
WVFGRD96   80.0   175    35   -90   5.41 0.2982
WVFGRD96   82.0   190    35   -75   5.42 0.3008
WVFGRD96   84.0   175    40   -90   5.42 0.3068
WVFGRD96   86.0   195    40   -75   5.43 0.3092
WVFGRD96   88.0   190    40   -75   5.43 0.3131
WVFGRD96   90.0   190    40   -75   5.44 0.3173
WVFGRD96   92.0   190    40   -75   5.44 0.3197
WVFGRD96   94.0   190    40   -75   5.45 0.3226
WVFGRD96   96.0   190    40   -75   5.45 0.3237
WVFGRD96   98.0   190    40   -75   5.45 0.3262
WVFGRD96  100.0   190    45   -75   5.46 0.3309
WVFGRD96  102.0   190    45   -75   5.46 0.3372
WVFGRD96  104.0   190    45   -75   5.46 0.3382
WVFGRD96  106.0   190    45   -75   5.47 0.3419
WVFGRD96  108.0   190    45   -75   5.47 0.3439

The best solution is

WVFGRD96   18.0   355    20    95   5.12 0.8550

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 14 07:36:51 CDT 2014