Location

2014/03/24 15:45:31 -19.605 -70.820 19.6 5.5 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/24 15:45:31:0 -19.60 -70.82 19.6 5.5 Chile Stations used: C.GO01 CX.PATCX CX.PB01 CX.PB04 CX.PB07 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 = 2.57e+24 dyne-cm Mw = 5.54 Z = 24 km Plane Strike Dip Rake NP1 166 71 95 NP2 330 20 75 Principal Axes: Axis Value Plunge Azimuth T 2.57e+24 64 85 N 0.00e+00 5 344 P -2.57e+24 26 252 Moment Tensor: (dyne-cm) Component Value Mxx -2.02e+23 Mxy -5.77e+23 Mxz 4.10e+23 Myy -1.39e+24 Myz 1.96e+24 Mzz 1.60e+24 ##------------ -----##########------- --------##############------ ---------################----- -----------##################----- ------------###################----- -------------#####################---- --------------######################---- --------------######################---- ---------------########### #########---- ----------------########## T #########---- ----------------########## #########---- ----- ---------#####################---- ---- P ---------#####################--- ---- ----------####################--- -----------------###################-- ----------------##################-- ----------------################-- ---------------##############- ---------------############- --------------######## -----------### Global CMT Convention Moment Tensor: R T P 1.60e+24 4.10e+23 -1.96e+24 4.10e+23 -2.02e+23 5.77e+23 -1.96e+24 5.77e+23 -1.39e+24 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140324154531/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 = 330
      DIP = 20
     RAKE = 75
       MW = 5.54
       HS = 24.0

The NDK file is 20140324154531.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 15:45:31:0 -19.60 -70.82 19.6 5.5 Chile Stations used: C.GO01 CX.PATCX CX.PB01 CX.PB04 CX.PB07 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 = 2.57e+24 dyne-cm Mw = 5.54 Z = 24 km Plane Strike Dip Rake NP1 166 71 95 NP2 330 20 75 Principal Axes: Axis Value Plunge Azimuth T 2.57e+24 64 85 N 0.00e+00 5 344 P -2.57e+24 26 252 Moment Tensor: (dyne-cm) Component Value Mxx -2.02e+23 Mxy -5.77e+23 Mxz 4.10e+23 Myy -1.39e+24 Myz 1.96e+24 Mzz 1.60e+24 ##------------ -----##########------- --------##############------ ---------################----- -----------##################----- ------------###################----- -------------#####################---- --------------######################---- --------------######################---- ---------------########### #########---- ----------------########## T #########---- ----------------########## #########---- ----- ---------#####################---- ---- P ---------#####################--- ---- ----------####################--- -----------------###################-- ----------------##################-- ----------------################-- ---------------##############- ---------------############- --------------######## -----------### Global CMT Convention Moment Tensor: R T P 1.60e+24 4.10e+23 -1.96e+24 4.10e+23 -2.02e+23 5.77e+23 -1.96e+24 5.77e+23 -1.39e+24 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140324154531/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    -5    50    90   5.24 0.3930
WVFGRD96    4.0   265    10     0   5.31 0.2653
WVFGRD96    6.0   265    10     0   5.32 0.4147
WVFGRD96    8.0   265    10     0   5.40 0.5090
WVFGRD96   10.0   275    10    10   5.41 0.6089
WVFGRD96   12.0   285    10    25   5.43 0.6836
WVFGRD96   14.0   305    15    45   5.45 0.7423
WVFGRD96   16.0   315    15    55   5.47 0.7882
WVFGRD96   18.0   330    20    75   5.49 0.8257
WVFGRD96   20.0   330    20    75   5.51 0.8519
WVFGRD96   22.0   335    20    80   5.53 0.8661
WVFGRD96   24.0   330    20    75   5.54 0.8728
WVFGRD96   26.0   330    20    75   5.56 0.8712
WVFGRD96   28.0   330    20    75   5.57 0.8619
WVFGRD96   30.0   330    20    75   5.58 0.8454
WVFGRD96   32.0   325    20    70   5.59 0.8214
WVFGRD96   34.0   325    20    70   5.60 0.7929
WVFGRD96   36.0   325    20    70   5.61 0.7612
WVFGRD96   38.0   325    20    70   5.61 0.7310
WVFGRD96   40.0   325    15    65   5.74 0.7006
WVFGRD96   42.0   330    15    75   5.75 0.6699
WVFGRD96   44.0   325    15    65   5.76 0.6395
WVFGRD96   46.0   325    15    65   5.76 0.6111
WVFGRD96   48.0   320    15    60   5.77 0.5838
WVFGRD96   50.0   315    15    55   5.77 0.5578
WVFGRD96   52.0   310    15    50   5.78 0.5337
WVFGRD96   54.0   305    15    45   5.78 0.5111
WVFGRD96   56.0   310    10    50   5.79 0.4911
WVFGRD96   58.0   300    10    40   5.79 0.4734
WVFGRD96   60.0   295    10    35   5.80 0.4578
WVFGRD96   62.0   280    10    20   5.80 0.4437
WVFGRD96   64.0   275    10    15   5.80 0.4309
WVFGRD96   66.0   265    10     5   5.81 0.4199
WVFGRD96   68.0   255    10    -5   5.81 0.4097
WVFGRD96   70.0   260     5     5   5.82 0.4010
WVFGRD96   72.0   235     5   -20   5.83 0.3944
WVFGRD96   74.0   220     5   -35   5.83 0.3881
WVFGRD96   76.0   225    15   -35   5.82 0.3845
WVFGRD96   78.0   225    20   -40   5.82 0.3824
WVFGRD96   80.0   220    20   -45   5.83 0.3820
WVFGRD96   82.0   220    20   -45   5.83 0.3810
WVFGRD96   84.0   215    20   -50   5.83 0.3789
WVFGRD96   86.0   215    25   -55   5.83 0.3818
WVFGRD96   88.0   210    25   -55   5.84 0.3851
WVFGRD96   90.0   340    65   -80   5.84 0.3910
WVFGRD96   92.0   335    60   -85   5.84 0.3972
WVFGRD96   94.0   335    60   -85   5.85 0.4064
WVFGRD96   96.0   335    60   -85   5.86 0.4143
WVFGRD96   98.0   340    60   -85   5.86 0.4188
WVFGRD96  100.0   340    60   -85   5.87 0.4253
WVFGRD96  102.0   150    30  -100   5.87 0.4283
WVFGRD96  104.0   200    35   -65   5.87 0.4344
WVFGRD96  106.0   200    35   -65   5.87 0.4389
WVFGRD96  108.0   200    35   -65   5.88 0.4405

The best solution is

WVFGRD96   24.0   330    20    75   5.54 0.8728

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 11:26:31 CDT 2014