Location

2014/04/11 08:39:45 -20.593 -70.874 27.6 4.9 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/04/11 08:39:45:0 -20.59 -70.87 27.6 4.9 Chile Stations used: C.GO01 CX.MNMCX CX.PATCX CX.PB01 CX.PB06 CX.PB07 CX.PB08 CX.PB09 CX.PB11 CX.PB12 CX.PSGCX 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 = 1.04e+23 dyne-cm Mw = 4.61 Z = 22 km Plane Strike Dip Rake NP1 335 50 65 NP2 191 46 117 Principal Axes: Axis Value Plunge Azimuth T 1.04e+23 71 178 N 0.00e+00 19 352 P -1.04e+23 2 82 Moment Tensor: (dyne-cm) Component Value Mxx 9.17e+21 Mxy -1.38e+22 Mxz -3.24e+22 Myy -1.02e+23 Myz -2.88e+21 Mzz 9.24e+22 ########------ -------##------------- ----------####-------------- ----------#######------------- ----------###########------------- ----------#############------------- ----------################------------ ----------##################------------ ----------###################---------- ----------#####################--------- P ----------#####################--------- ----------######################---------- ----------########## #########---------- ---------########## T ##########-------- ---------########## ##########-------- --------#######################------- --------######################------ -------######################----- ------####################---- ------##################---- ----#################- --############ Global CMT Convention Moment Tensor: R T P 9.24e+22 -3.24e+22 2.88e+21 -3.24e+22 9.17e+21 1.38e+22 2.88e+21 1.38e+22 -1.02e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140411083945/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 = 335
      DIP = 50
     RAKE = 65
       MW = 4.61
       HS = 22.0

The NDK file is 20140411083945.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/04/11 08:39:45:0 -20.59 -70.87 27.6 4.9 Chile Stations used: C.GO01 CX.MNMCX CX.PATCX CX.PB01 CX.PB06 CX.PB07 CX.PB08 CX.PB09 CX.PB11 CX.PB12 CX.PSGCX 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 = 1.04e+23 dyne-cm Mw = 4.61 Z = 22 km Plane Strike Dip Rake NP1 335 50 65 NP2 191 46 117 Principal Axes: Axis Value Plunge Azimuth T 1.04e+23 71 178 N 0.00e+00 19 352 P -1.04e+23 2 82 Moment Tensor: (dyne-cm) Component Value Mxx 9.17e+21 Mxy -1.38e+22 Mxz -3.24e+22 Myy -1.02e+23 Myz -2.88e+21 Mzz 9.24e+22 ########------ -------##------------- ----------####-------------- ----------#######------------- ----------###########------------- ----------#############------------- ----------################------------ ----------##################------------ ----------###################---------- ----------#####################--------- P ----------#####################--------- ----------######################---------- ----------########## #########---------- ---------########## T ##########-------- ---------########## ##########-------- --------#######################------- --------######################------ -------######################----- ------####################---- ------##################---- ----#################- --############ Global CMT Convention Moment Tensor: R T P 9.24e+22 -3.24e+22 2.88e+21 -3.24e+22 9.17e+21 1.38e+22 2.88e+21 1.38e+22 -1.02e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140411083945/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   295    45   -90   4.41 0.1523
WVFGRD96    4.0   160    65   -20   4.40 0.1233
WVFGRD96    6.0   315    75   -60   4.43 0.1372
WVFGRD96    8.0   315    75   -65   4.51 0.1612
WVFGRD96   10.0   310    75   -60   4.52 0.1845
WVFGRD96   12.0   300    65   -60   4.55 0.2029
WVFGRD96   14.0   305    65   -55   4.57 0.2164
WVFGRD96   16.0   305    65   -55   4.59 0.2248
WVFGRD96   18.0   305    65   -50   4.60 0.2289
WVFGRD96   20.0   155    55    65   4.60 0.2316
WVFGRD96   22.0   335    50    65   4.61 0.2338
WVFGRD96   24.0   335    50    65   4.62 0.2336
WVFGRD96   26.0   335    50    65   4.63 0.2311
WVFGRD96   28.0   330    50    60   4.65 0.2270
WVFGRD96   30.0   255    55    65   4.70 0.2226
WVFGRD96   32.0   255    55    65   4.72 0.2172
WVFGRD96   34.0   255    55    65   4.73 0.2103
WVFGRD96   36.0   255    55    65   4.74 0.2022
WVFGRD96   38.0   255    55    65   4.76 0.1940
WVFGRD96   40.0   255    60    70   4.87 0.1753
WVFGRD96   42.0   255    60    70   4.88 0.1696
WVFGRD96   44.0   255    60    70   4.89 0.1624
WVFGRD96   46.0   340    35    75   4.84 0.1553
WVFGRD96   48.0   330    40    60   4.85 0.1488
WVFGRD96   50.0   330    40    60   4.86 0.1422
WVFGRD96   52.0   330    40    60   4.87 0.1351
WVFGRD96   54.0   330    35    60   4.87 0.1289
WVFGRD96   56.0   330    35    60   4.87 0.1228
WVFGRD96   58.0   325    35    55   4.88 0.1171
WVFGRD96   60.0   320    35    50   4.88 0.1118
WVFGRD96   62.0   155    65    55   4.87 0.1081
WVFGRD96   64.0   155    65    55   4.87 0.1060
WVFGRD96   66.0   160    65    60   4.88 0.1040
WVFGRD96   68.0   160    65    60   4.89 0.1026
WVFGRD96   70.0   155    70    55   4.88 0.1016
WVFGRD96   72.0    95    60    65   4.91 0.1011
WVFGRD96   74.0    95    60    65   4.91 0.1005
WVFGRD96   76.0   100    60    75   4.92 0.0999
WVFGRD96   78.0   100    60    75   4.93 0.1015
WVFGRD96   80.0   100    60    75   4.93 0.1007
WVFGRD96   82.0   100    65    85   4.96 0.1025
WVFGRD96   84.0   100    65    90   4.97 0.1022
WVFGRD96   86.0   100    65    90   4.97 0.1009
WVFGRD96   88.0   100    65    90   4.97 0.1025
WVFGRD96   90.0   275    25    85   4.98 0.1019
WVFGRD96   92.0   105    65    95   4.99 0.1009
WVFGRD96   94.0   105    65    95   4.99 0.1005
WVFGRD96   96.0   275    25    85   4.98 0.1013
WVFGRD96   98.0   280    30    80   4.99 0.1008
WVFGRD96  100.0   280    30    80   4.99 0.1009
WVFGRD96  102.0   275    30    75   5.00 0.1026
WVFGRD96  104.0   270    30    70   5.01 0.1021
WVFGRD96  106.0   265    30    65   5.02 0.1027
WVFGRD96  108.0   265    30    65   5.02 0.1030

The best solution is

WVFGRD96   22.0   335    50    65   4.61 0.2338

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 Fri Apr 11 06:22:08 CDT 2014