Location

2014/03/24 15:33:54 -25.163 -70.706 42.7 5.0 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/24 15:33:54:0 -25.16 -70.71 42.7 5.0 Chile Stations used: C.GO02 C.GO03 C.GO04 CX.PB01 CX.PB04 CX.PB07 CX.PB09 CX.PB10 CX.PB14 CX.PB15 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 = 2.51e+22 dyne-cm Mw = 4.20 Z = 34 km Plane Strike Dip Rake NP1 325 50 80 NP2 160 41 102 Principal Axes: Axis Value Plunge Azimuth T 2.51e+22 81 182 N 0.00e+00 8 331 P -2.51e+22 5 62 Moment Tensor: (dyne-cm) Component Value Mxx -4.87e+21 Mxy -1.03e+22 Mxz -4.76e+21 Myy -1.95e+22 Myz -1.91e+21 Mzz 2.44e+22 -------------- ##-------------------- ----######------------------ ----###########--------------- -----##############-------------- ------################------------ P ------###################---------- -------#####################------------ -------######################----------- --------#######################----------- --------########################---------- ---------########## ###########--------- ---------########## T ###########--------- ---------######### ############------- ----------#######################------- ---------#######################------ ----------#####################----- ----------####################---- ----------##################-- -----------###############-- -----------########### -------------# Global CMT Convention Moment Tensor: R T P 2.44e+22 -4.76e+21 1.91e+21 -4.76e+21 -4.87e+21 1.03e+22 1.91e+21 1.03e+22 -1.95e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140324153354/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 = 325
      DIP = 50
     RAKE = 80
       MW = 4.20
       HS = 34.0

The NDK file is 20140324153354.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/03/24 15:33:54:0 -25.16 -70.71 42.7 5.0 Chile Stations used: C.GO02 C.GO03 C.GO04 CX.PB01 CX.PB04 CX.PB07 CX.PB09 CX.PB10 CX.PB14 CX.PB15 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 = 2.51e+22 dyne-cm Mw = 4.20 Z = 34 km Plane Strike Dip Rake NP1 325 50 80 NP2 160 41 102 Principal Axes: Axis Value Plunge Azimuth T 2.51e+22 81 182 N 0.00e+00 8 331 P -2.51e+22 5 62 Moment Tensor: (dyne-cm) Component Value Mxx -4.87e+21 Mxy -1.03e+22 Mxz -4.76e+21 Myy -1.95e+22 Myz -1.91e+21 Mzz 2.44e+22 -------------- ##-------------------- ----######------------------ ----###########--------------- -----##############-------------- ------################------------ P ------###################---------- -------#####################------------ -------######################----------- --------#######################----------- --------########################---------- ---------########## ###########--------- ---------########## T ###########--------- ---------######### ############------- ----------#######################------- ---------#######################------ ----------#####################----- ----------####################---- ----------##################-- -----------###############-- -----------########### -------------# Global CMT Convention Moment Tensor: R T P 2.44e+22 -4.76e+21 1.91e+21 -4.76e+21 -4.87e+21 1.03e+22 1.91e+21 1.03e+22 -1.95e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140324153354/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   3.79 0.3759
WVFGRD96    4.0   245    70    35   3.90 0.3788
WVFGRD96    6.0   240    80    25   3.94 0.3674
WVFGRD96    8.0   240    85    30   3.99 0.3641
WVFGRD96   10.0   240    90    40   4.00 0.3586
WVFGRD96   12.0    55    80   -45   4.02 0.3798
WVFGRD96   14.0   310    70    70   3.96 0.4154
WVFGRD96   16.0   315    65    75   4.00 0.4720
WVFGRD96   18.0   315    65    75   4.02 0.5246
WVFGRD96   20.0   315    65    75   4.04 0.5693
WVFGRD96   22.0   315    60    75   4.08 0.6060
WVFGRD96   24.0   320    60    80   4.10 0.6397
WVFGRD96   26.0   315    60    75   4.11 0.6659
WVFGRD96   28.0   320    55    80   4.14 0.6865
WVFGRD96   30.0   320    55    80   4.15 0.7003
WVFGRD96   32.0   320    55    75   4.17 0.7061
WVFGRD96   34.0   325    50    80   4.20 0.7086
WVFGRD96   36.0   325    50    80   4.22 0.7057
WVFGRD96   38.0   325    50    75   4.25 0.6984
WVFGRD96   40.0   330    50    75   4.38 0.6652
WVFGRD96   42.0   330    50    75   4.39 0.6671
WVFGRD96   44.0   335    45    80   4.42 0.6609
WVFGRD96   46.0   335    45    80   4.43 0.6488
WVFGRD96   48.0   335    45    80   4.44 0.6315
WVFGRD96   50.0   330    45    75   4.44 0.6110
WVFGRD96   52.0   330    45    75   4.44 0.5897
WVFGRD96   54.0   335    40    80   4.47 0.5713
WVFGRD96   56.0   335    40    80   4.47 0.5522
WVFGRD96   58.0   335    40    80   4.48 0.5321
WVFGRD96   60.0   330    40    75   4.47 0.5113
WVFGRD96   62.0   330    40    75   4.47 0.4901
WVFGRD96   64.0   340    35    85   4.50 0.4731
WVFGRD96   66.0   335    35    80   4.50 0.4556
WVFGRD96   68.0   335    35    80   4.50 0.4372
WVFGRD96   70.0   165    55    95   4.49 0.4182
WVFGRD96   72.0    65    45     0   4.50 0.4083
WVFGRD96   74.0    70    40     5   4.49 0.4024
WVFGRD96   76.0    70    40     5   4.50 0.3952
WVFGRD96   78.0    75    40    10   4.48 0.3895
WVFGRD96   80.0    75    40    10   4.49 0.3851
WVFGRD96   82.0    80    40    20   4.47 0.3806
WVFGRD96   84.0    80    40    20   4.47 0.3757
WVFGRD96   86.0    80    40    20   4.48 0.3713
WVFGRD96   88.0    85    40    25   4.47 0.3661
WVFGRD96   90.0    85    40    25   4.47 0.3607
WVFGRD96   92.0    90    40    35   4.45 0.3564
WVFGRD96   94.0    90    40    35   4.46 0.3520
WVFGRD96   96.0    95    40    40   4.45 0.3477
WVFGRD96   98.0   105    40    50   4.43 0.3436
WVFGRD96  100.0   110    40    55   4.43 0.3403
WVFGRD96  102.0   110    40    55   4.43 0.3369
WVFGRD96  104.0   115    40    60   4.43 0.3336
WVFGRD96  106.0   115    40    60   4.44 0.3305
WVFGRD96  108.0   120    40    65   4.44 0.3284

The best solution is

WVFGRD96   34.0   325    50    80   4.20 0.7086

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 Mar 24 17:19:01 CDT 2014