Location

2014/03/23 20:23:03 -19.905 -70.934 23.4 5.3 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/23 20:23:03:0 -19.91 -70.93 23.4 5.3 Chile Stations used: C.GO01 C.GO02 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 = 3.47e+23 dyne-cm Mw = 4.96 Z = 20 km Plane Strike Dip Rake NP1 160 60 85 NP2 350 30 99 Principal Axes: Axis Value Plunge Azimuth T 3.47e+23 74 57 N 0.00e+00 4 162 P -3.47e+23 15 254 Moment Tensor: (dyne-cm) Component Value Mxx -1.82e+22 Mxy -7.61e+22 Mxz 7.33e+22 Myy -2.81e+23 Myz 1.57e+23 Mzz 2.99e+23 ########------ ---#############------ -----################------- ------##################------ --------###################------- ---------#####################------ ----------######################------ -----------######################------- -----------############ ########------ ------------############ T ########------- -------------########### #########------ -------------#######################------ -- ---------######################------ - P ----------#####################----- - -----------###################------ --------------###################----- ---------------################----- ---------------##############----- --------------############---- ---------------#########---- --------------#####--- ------------## Global CMT Convention Moment Tensor: R T P 2.99e+23 7.33e+22 -1.57e+23 7.33e+22 -1.82e+22 7.61e+22 -1.57e+23 7.61e+22 -2.81e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140323202303/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 = 160
      DIP = 60
     RAKE = 85
       MW = 4.96
       HS = 20.0

The NDK file is 20140323202303.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/03/23 20:23:03:0 -19.91 -70.93 23.4 5.3 Chile Stations used: C.GO01 C.GO02 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 = 3.47e+23 dyne-cm Mw = 4.96 Z = 20 km Plane Strike Dip Rake NP1 160 60 85 NP2 350 30 99 Principal Axes: Axis Value Plunge Azimuth T 3.47e+23 74 57 N 0.00e+00 4 162 P -3.47e+23 15 254 Moment Tensor: (dyne-cm) Component Value Mxx -1.82e+22 Mxy -7.61e+22 Mxz 7.33e+22 Myy -2.81e+23 Myz 1.57e+23 Mzz 2.99e+23 ########------ ---#############------ -----################------- ------##################------ --------###################------- ---------#####################------ ----------######################------ -----------######################------- -----------############ ########------ ------------############ T ########------- -------------########### #########------ -------------#######################------ -- ---------######################------ - P ----------#####################----- - -----------###################------ --------------###################----- ---------------################----- ---------------##############----- --------------############---- ---------------#########---- --------------#####--- ------------## Global CMT Convention Moment Tensor: R T P 2.99e+23 7.33e+22 -1.57e+23 7.33e+22 -1.82e+22 7.61e+22 -1.57e+23 7.61e+22 -2.81e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140323202303/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   165    40   -90   4.70 0.4409
WVFGRD96    4.0   180    15   -85   4.77 0.3052
WVFGRD96    6.0   215    10   -50   4.77 0.4322
WVFGRD96    8.0   210     5   -50   4.85 0.5154
WVFGRD96   10.0   235     5   -25   4.85 0.5947
WVFGRD96   12.0   170    80    85   4.87 0.6553
WVFGRD96   14.0   350    25    95   4.91 0.7206
WVFGRD96   16.0   160    60    85   4.94 0.7767
WVFGRD96   18.0   160    60    85   4.95 0.8103
WVFGRD96   20.0   160    60    85   4.96 0.8243
WVFGRD96   22.0   160    60    85   4.97 0.8238
WVFGRD96   24.0   160    60    80   4.98 0.8129
WVFGRD96   26.0   160    60    80   4.99 0.7947
WVFGRD96   28.0   160    60    80   5.00 0.7701
WVFGRD96   30.0   160    60    85   5.00 0.7401
WVFGRD96   32.0   160    60    85   5.01 0.7049
WVFGRD96   34.0   160    60    85   5.02 0.6648
WVFGRD96   36.0   160    60    80   5.03 0.6220
WVFGRD96   38.0   160    65    85   5.03 0.5813
WVFGRD96   40.0   345    20    90   5.15 0.5429
WVFGRD96   42.0   165    65    90   5.16 0.5080
WVFGRD96   44.0   160    65    85   5.16 0.4725
WVFGRD96   46.0   160    65    85   5.17 0.4380
WVFGRD96   48.0   165    65    90   5.17 0.4047
WVFGRD96   50.0   165    65    90   5.17 0.3736
WVFGRD96   52.0   160    70    85   5.17 0.3461
WVFGRD96   54.0    10    20   -65   5.17 0.3203
WVFGRD96   56.0    15    20   -60   5.17 0.3081
WVFGRD96   58.0    10    15   -65   5.17 0.2971
WVFGRD96   60.0    20    15   -55   5.18 0.2893
WVFGRD96   62.0    35    15   -35   5.18 0.2822
WVFGRD96   64.0   345    75    80   5.18 0.2788
WVFGRD96   66.0   345    70    80   5.19 0.2815
WVFGRD96   68.0   345    70    80   5.20 0.2848
WVFGRD96   70.0   345    70    80   5.20 0.2897
WVFGRD96   72.0   345    70    80   5.21 0.2944
WVFGRD96   74.0   345    65    80   5.22 0.2988
WVFGRD96   76.0   345    65    80   5.22 0.3009
WVFGRD96   78.0   185    25   105   5.23 0.3037
WVFGRD96   80.0   185    30   105   5.23 0.3067
WVFGRD96   82.0   185    30   105   5.23 0.3099
WVFGRD96   84.0   180    30   100   5.24 0.3095
WVFGRD96   86.0   350    60    85   5.24 0.3115
WVFGRD96   88.0   345    60    80   5.24 0.3129
WVFGRD96   90.0   350    55    85   5.24 0.3104
WVFGRD96   92.0   350    55    85   5.24 0.3123
WVFGRD96   94.0   145    25    80   5.26 0.3138
WVFGRD96   96.0   145    25    85   5.26 0.3117
WVFGRD96   98.0   330    65    85   5.27 0.3170
WVFGRD96  100.0   330    65    85   5.27 0.3155
WVFGRD96  102.0   350    55    85   5.26 0.3208
WVFGRD96  104.0   180    35   100   5.26 0.3218
WVFGRD96  106.0   180    35   100   5.26 0.3198
WVFGRD96  108.0   180    35   100   5.26 0.3239

The best solution is

WVFGRD96   20.0   160    60    85   4.96 0.8243

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 12:56:15 CDT 2014