Location

2014/04/05 05:44:55 -20.131 -70.445 19.0 5.3 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/04/05 05:44:55:0 -20.13 -70.44 19.0 5.3 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 = 4.57e+23 dyne-cm Mw = 5.04 Z = 26 km Plane Strike Dip Rake NP1 165 70 85 NP2 359 21 103 Principal Axes: Axis Value Plunge Azimuth T 4.57e+23 65 67 N 0.00e+00 5 167 P -4.57e+23 25 259 Moment Tensor: (dyne-cm) Component Value Mxx -8.89e+20 Mxy -4.08e+22 Mxz 1.03e+23 Myy -2.92e+23 Myz 3.33e+23 Mzz 2.93e+23 -##########--- -----##############--- --------################---- --------###################--- ----------####################---- -----------#####################---- ------------######################---- --------------######################---- --------------########### ########---- ---------------########### T #########---- ----------------########## #########---- ---- ---------######################---- ---- P ---------######################---- --- ----------####################---- -----------------###################---- ----------------##################---- ----------------################---- ----------------##############---- ---------------############--- ---------------#########---- --------------#####--- -----------#-- Global CMT Convention Moment Tensor: R T P 2.93e+23 1.03e+23 -3.33e+23 1.03e+23 -8.89e+20 4.08e+22 -3.33e+23 4.08e+22 -2.92e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140405054455/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 = 70
     RAKE = 85
       MW = 5.04
       HS = 26.0

The NDK file is 20140405054455.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 05:44:55:0 -20.13 -70.44 19.0 5.3 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 = 4.57e+23 dyne-cm Mw = 5.04 Z = 26 km Plane Strike Dip Rake NP1 165 70 85 NP2 359 21 103 Principal Axes: Axis Value Plunge Azimuth T 4.57e+23 65 67 N 0.00e+00 5 167 P -4.57e+23 25 259 Moment Tensor: (dyne-cm) Component Value Mxx -8.89e+20 Mxy -4.08e+22 Mxz 1.03e+23 Myy -2.92e+23 Myz 3.33e+23 Mzz 2.93e+23 -##########--- -----##############--- --------################---- --------###################--- ----------####################---- -----------#####################---- ------------######################---- --------------######################---- --------------########### ########---- ---------------########### T #########---- ----------------########## #########---- ---- ---------######################---- ---- P ---------######################---- --- ----------####################---- -----------------###################---- ----------------##################---- ----------------################---- ----------------##############---- ---------------############--- ---------------#########---- --------------#####--- -----------#-- Global CMT Convention Moment Tensor: R T P 2.93e+23 1.03e+23 -3.33e+23 1.03e+23 -8.89e+20 4.08e+22 -3.33e+23 4.08e+22 -2.92e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140405054455/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   150    45    90   4.68 0.3396
WVFGRD96    4.0   295    80   -60   4.69 0.2345
WVFGRD96    6.0   260    10    -5   4.77 0.3281
WVFGRD96    8.0   265    10     0   4.86 0.4106
WVFGRD96   10.0   295    10    35   4.88 0.5078
WVFGRD96   12.0   155    70    80   4.90 0.6027
WVFGRD96   14.0   155    70    80   4.92 0.6859
WVFGRD96   16.0   160    65    85   4.96 0.7529
WVFGRD96   18.0   160    65    85   4.97 0.8010
WVFGRD96   20.0   165    65    85   4.99 0.8328
WVFGRD96   22.0   165    65    85   5.01 0.8497
WVFGRD96   24.0   165    70    85   5.02 0.8580
WVFGRD96   26.0   165    70    85   5.04 0.8585
WVFGRD96   28.0   165    70    85   5.05 0.8508
WVFGRD96   30.0   165    70    85   5.06 0.8358
WVFGRD96   32.0   165    70    85   5.07 0.8138
WVFGRD96   34.0   165    70    85   5.08 0.7865
WVFGRD96   36.0   165    70    85   5.08 0.7557
WVFGRD96   38.0   165    70    85   5.09 0.7246
WVFGRD96   40.0   150    75    80   5.20 0.6909
WVFGRD96   42.0   150    75    80   5.21 0.6652
WVFGRD96   44.0   150    75    80   5.22 0.6404
WVFGRD96   46.0   145    75    75   5.22 0.6175
WVFGRD96   48.0   145    75    75   5.22 0.5959
WVFGRD96   50.0   145    75    75   5.23 0.5746
WVFGRD96   52.0   145    75    75   5.24 0.5536
WVFGRD96   54.0   145    75    80   5.24 0.5340
WVFGRD96   56.0   145    75    80   5.25 0.5153
WVFGRD96   58.0   145    75    80   5.26 0.4965
WVFGRD96   60.0   145    75    85   5.26 0.4790
WVFGRD96   62.0   145    75    85   5.27 0.4622
WVFGRD96   64.0   135    80    80   5.26 0.4490
WVFGRD96   66.0   135    80    80   5.26 0.4360
WVFGRD96   68.0   135    80    80   5.27 0.4231
WVFGRD96   70.0   145    80    90   5.29 0.4109
WVFGRD96   72.0   145    80    90   5.29 0.4000
WVFGRD96   74.0   315    10    80   5.30 0.3893
WVFGRD96   76.0   310    10    75   5.30 0.3796
WVFGRD96   78.0   295    15    55   5.32 0.3703
WVFGRD96   80.0   290    15    50   5.32 0.3618
WVFGRD96   82.0   205    15   -50   5.29 0.3574
WVFGRD96   84.0   210    20   -50   5.29 0.3567
WVFGRD96   86.0   205    20   -55   5.30 0.3567
WVFGRD96   88.0   200    20   -60   5.30 0.3572
WVFGRD96   90.0   200    20   -60   5.30 0.3575
WVFGRD96   92.0   190    20   -70   5.30 0.3577
WVFGRD96   94.0   170    30   -70   5.29 0.3651
WVFGRD96   96.0   170    30   -70   5.29 0.3707
WVFGRD96   98.0   165    35   -75   5.29 0.3800
WVFGRD96  100.0   165    35   -75   5.29 0.3853
WVFGRD96  102.0   165    35   -75   5.30 0.3939
WVFGRD96  104.0   165    35   -75   5.31 0.3985
WVFGRD96  106.0   165    35   -75   5.31 0.4041
WVFGRD96  108.0   165    35   -75   5.31 0.4081

The best solution is

WVFGRD96   26.0   165    70    85   5.04 0.8585

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 14:54:00 CDT 2014