Location

2014/03/23 22:04:26 -19.819 -70.728 18.2 4.9 Chile

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/03/23 22:04:26:0 -19.82 -70.73 18.2 4.9 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 = 3.85e+23 dyne-cm Mw = 4.99 Z = 24 km Plane Strike Dip Rake NP1 146 83 103 NP2 265 15 30 Principal Axes: Axis Value Plunge Azimuth T 3.85e+23 51 70 N 0.00e+00 13 324 P -3.85e+23 36 224 Moment Tensor: (dyne-cm) Component Value Mxx -1.10e+23 Mxy -7.65e+22 Mxz 1.94e+23 Myy 1.42e+22 Myz 3.06e+23 Mzz 9.61e+22 -------------- --------#######------- ###--###################---- ###--#######################-- ###-----########################-- ##--------#########################- ##----------#########################- ##------------#########################- #--------------############# ######### #----------------############ T #########- #-----------------########### ########## -------------------####################### ---------------------##################### ---------------------################### ----------------------################## -------- ------------############### ------- P -------------############# ------ ---------------########## -----------------------####### ------------------------#### ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 9.61e+22 1.94e+23 -3.06e+23 1.94e+23 -1.10e+23 7.65e+22 -3.06e+23 7.65e+22 1.42e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140323220426/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 = 265
      DIP = 15
     RAKE = 30
       MW = 4.99
       HS = 24.0

The NDK file is 20140323220426.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 22:04:26:0 -19.82 -70.73 18.2 4.9 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 = 3.85e+23 dyne-cm Mw = 4.99 Z = 24 km Plane Strike Dip Rake NP1 146 83 103 NP2 265 15 30 Principal Axes: Axis Value Plunge Azimuth T 3.85e+23 51 70 N 0.00e+00 13 324 P -3.85e+23 36 224 Moment Tensor: (dyne-cm) Component Value Mxx -1.10e+23 Mxy -7.65e+22 Mxz 1.94e+23 Myy 1.42e+22 Myz 3.06e+23 Mzz 9.61e+22 -------------- --------#######------- ###--###################---- ###--#######################-- ###-----########################-- ##--------#########################- ##----------#########################- ##------------#########################- #--------------############# ######### #----------------############ T #########- #-----------------########### ########## -------------------####################### ---------------------##################### ---------------------################### ----------------------################## -------- ------------############### ------- P -------------############# ------ ---------------########## -----------------------####### ------------------------#### ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 9.61e+22 1.94e+23 -3.06e+23 1.94e+23 -1.10e+23 7.65e+22 -3.06e+23 7.65e+22 1.42e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140323220426/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    45    90   4.69 0.4273
WVFGRD96    4.0   340    60    85   4.76 0.2986
WVFGRD96    6.0   270    10    30   4.77 0.4250
WVFGRD96    8.0   270    10    30   4.85 0.5199
WVFGRD96   10.0   270    10    35   4.87 0.6200
WVFGRD96   12.0   260    15    25   4.88 0.6962
WVFGRD96   14.0   265    15    30   4.90 0.7541
WVFGRD96   16.0   265    15    30   4.91 0.7981
WVFGRD96   18.0   265    15    30   4.93 0.8293
WVFGRD96   20.0   260    15    25   4.95 0.8498
WVFGRD96   22.0   265    15    30   4.97 0.8613
WVFGRD96   24.0   265    15    30   4.99 0.8653
WVFGRD96   26.0   260    15    25   5.00 0.8619
WVFGRD96   28.0   265    10    30   5.01 0.8517
WVFGRD96   30.0   265    10    30   5.02 0.8356
WVFGRD96   32.0   265    10    30   5.03 0.8137
WVFGRD96   34.0   270    10    35   5.04 0.7881
WVFGRD96   36.0   270    10    35   5.04 0.7613
WVFGRD96   38.0   270    10    40   5.05 0.7360
WVFGRD96   40.0   285     5    50   5.19 0.7131
WVFGRD96   42.0   270    10    35   5.20 0.6786
WVFGRD96   44.0   275    10    40   5.20 0.6462
WVFGRD96   46.0   275    10    40   5.20 0.6155
WVFGRD96   48.0   270    10    35   5.21 0.5862
WVFGRD96   50.0   270    10    35   5.21 0.5577
WVFGRD96   52.0   270    10    35   5.22 0.5315
WVFGRD96   54.0   260    10    25   5.22 0.5058
WVFGRD96   56.0   260    10    25   5.23 0.4822
WVFGRD96   58.0   325    90   -70   5.22 0.4605
WVFGRD96   60.0   145    90    70   5.23 0.4438
WVFGRD96   62.0   145    90    70   5.23 0.4281
WVFGRD96   64.0   145    90    70   5.24 0.4132
WVFGRD96   66.0   160    65    85   5.29 0.4011
WVFGRD96   68.0   145    70    75   5.29 0.3931
WVFGRD96   70.0   150    70    80   5.29 0.3866
WVFGRD96   72.0   150    70    85   5.30 0.3806
WVFGRD96   74.0   150    70    85   5.30 0.3746
WVFGRD96   76.0   165    20   -60   5.27 0.3724
WVFGRD96   78.0   115    30  -100   5.26 0.3777
WVFGRD96   80.0   305    60   -85   5.27 0.3842
WVFGRD96   82.0   305    60   -90   5.28 0.3902
WVFGRD96   84.0   305    60   -90   5.28 0.3960
WVFGRD96   86.0   305    65  -100   5.30 0.4013
WVFGRD96   88.0   150    25   -70   5.30 0.4071
WVFGRD96   90.0   150    25   -70   5.31 0.4124
WVFGRD96   92.0   150    25   -70   5.32 0.4164
WVFGRD96   94.0   155    30   -65   5.32 0.4209
WVFGRD96   96.0   155    30   -65   5.32 0.4248
WVFGRD96   98.0   155    30   -65   5.33 0.4323
WVFGRD96  100.0   155    30   -65   5.33 0.4361
WVFGRD96  102.0   155    30   -65   5.34 0.4435
WVFGRD96  104.0   155    30   -65   5.34 0.4505
WVFGRD96  106.0   155    30   -65   5.35 0.4528
WVFGRD96  108.0   155    30   -65   5.35 0.4582

The best solution is

WVFGRD96   24.0   265    15    30   4.99 0.8653

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 12:45:29 CDT 2014