Location

2014/04/05 06:32:09 -20.090 -70.910 9.9 4.8 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/04/05 06:32:09:0 -20.09 -70.91 9.9 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 = 4.52e+22 dyne-cm Mw = 4.37 Z = 18 km Plane Strike Dip Rake NP1 170 60 90 NP2 350 30 90 Principal Axes: Axis Value Plunge Azimuth T 4.52e+22 75 80 N 0.00e+00 -0 170 P -4.52e+22 15 260 Moment Tensor: (dyne-cm) Component Value Mxx -1.18e+21 Mxy -6.69e+21 Mxz 3.92e+21 Myy -3.80e+22 Myz 2.22e+22 Mzz 3.91e+22 ---#####------ ------##########------ --------#############------- --------################------ ----------#################------- ----------####################------ -----------#####################------ ------------#####################------- ------------######################------ -------------########### ########------- -------------########### T #########------ -- --------########### #########------ -- P ---------######################------ - ---------######################----- --------------####################------ -------------####################----- -------------##################----- -------------################----- ------------##############---- ------------############---- -----------########--- ---------###-- Global CMT Convention Moment Tensor: R T P 3.91e+22 3.92e+21 -2.22e+22 3.92e+21 -1.18e+21 6.69e+21 -2.22e+22 6.69e+21 -3.80e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140405063209/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 = 170
      DIP = 60
     RAKE = 90
       MW = 4.37
       HS = 18.0

The NDK file is 20140405063209.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/04/05 06:32:09:0 -20.09 -70.91 9.9 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 = 4.52e+22 dyne-cm Mw = 4.37 Z = 18 km Plane Strike Dip Rake NP1 170 60 90 NP2 350 30 90 Principal Axes: Axis Value Plunge Azimuth T 4.52e+22 75 80 N 0.00e+00 -0 170 P -4.52e+22 15 260 Moment Tensor: (dyne-cm) Component Value Mxx -1.18e+21 Mxy -6.69e+21 Mxz 3.92e+21 Myy -3.80e+22 Myz 2.22e+22 Mzz 3.91e+22 ---#####------ ------##########------ --------#############------- --------################------ ----------#################------- ----------####################------ -----------#####################------ ------------#####################------- ------------######################------ -------------########### ########------- -------------########### T #########------ -- --------########### #########------ -- P ---------######################------ - ---------######################----- --------------####################------ -------------####################----- -------------##################----- -------------################----- ------------##############---- ------------############---- -----------########--- ---------###-- Global CMT Convention Moment Tensor: R T P 3.91e+22 3.92e+21 -2.22e+22 3.92e+21 -1.18e+21 6.69e+21 -2.22e+22 6.69e+21 -3.80e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140405063209/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    40   -90   4.11 0.4277
WVFGRD96    4.0   350    80   -80   4.21 0.3448
WVFGRD96    6.0   170    90    80   4.20 0.4693
WVFGRD96    8.0   170    90    80   4.28 0.5523
WVFGRD96   10.0   170    85    80   4.28 0.6248
WVFGRD96   12.0   170    70    85   4.31 0.6938
WVFGRD96   14.0   350    30    90   4.35 0.7593
WVFGRD96   16.0   165    60    85   4.36 0.8028
WVFGRD96   18.0   170    60    90   4.37 0.8199
WVFGRD96   20.0   355    30    95   4.37 0.8185
WVFGRD96   22.0   165    60    85   4.39 0.8053
WVFGRD96   24.0   165    60    85   4.39 0.7819
WVFGRD96   26.0   165    60    85   4.40 0.7523
WVFGRD96   28.0   165    60    85   4.41 0.7176
WVFGRD96   30.0   355    30    95   4.41 0.6785
WVFGRD96   32.0   365    30   110   4.42 0.6358
WVFGRD96   34.0   165    65    80   4.42 0.5906
WVFGRD96   36.0   165    65    80   4.42 0.5456
WVFGRD96   38.0   165    65    80   4.43 0.5024
WVFGRD96   40.0   365    20   110   4.55 0.4661
WVFGRD96   42.0   165    70    80   4.55 0.4265
WVFGRD96   44.0   165    70    80   4.55 0.3903
WVFGRD96   46.0   155    65    75   4.56 0.3580
WVFGRD96   48.0   155    65    70   4.56 0.3300
WVFGRD96   50.0     5    30   -65   4.57 0.3114
WVFGRD96   52.0     5    25   -70   4.57 0.2993
WVFGRD96   54.0   165    70   -95   4.57 0.2842
WVFGRD96   56.0     0    20   -75   4.58 0.2738
WVFGRD96   58.0     5    20   -70   4.58 0.2645
WVFGRD96   60.0    10    20   -65   4.59 0.2552
WVFGRD96   62.0   335    75    65   4.58 0.2536
WVFGRD96   64.0   335    75    65   4.59 0.2606
WVFGRD96   66.0   340    70    70   4.60 0.2654
WVFGRD96   68.0   340    70    70   4.61 0.2752
WVFGRD96   70.0   340    70    70   4.62 0.2836
WVFGRD96   72.0   345    65    75   4.63 0.2946
WVFGRD96   74.0   340    65    70   4.64 0.3013
WVFGRD96   76.0   340    65    70   4.65 0.3066
WVFGRD96   78.0   345    60    75   4.65 0.3143
WVFGRD96   80.0   340    60    75   4.66 0.3227
WVFGRD96   82.0   340    60    75   4.66 0.3256
WVFGRD96   84.0   340    60    75   4.67 0.3287
WVFGRD96   86.0   345    55    80   4.67 0.3328
WVFGRD96   88.0   340    55    75   4.67 0.3349
WVFGRD96   90.0   340    55    75   4.68 0.3366
WVFGRD96   92.0   180    35   105   4.68 0.3401
WVFGRD96   94.0   180    40   105   4.68 0.3405
WVFGRD96   96.0   340    55    75   4.68 0.3444
WVFGRD96   98.0   345    50    80   4.69 0.3455
WVFGRD96  100.0   345    50    80   4.69 0.3485
WVFGRD96  102.0   345    50    80   4.69 0.3491
WVFGRD96  104.0   345    50    80   4.69 0.3482
WVFGRD96  106.0   345    50    80   4.69 0.3501
WVFGRD96  108.0   350    50    85   4.69 0.3530

The best solution is

WVFGRD96   18.0   170    60    90   4.37 0.8199

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 Sat Apr 5 15:10:18 CDT 2014