Location

The initial IGN location led to large time shifts in the waveform fit. This is not a problem since this earthquake occurred outside the IGN network. The program elocate was used to relocate the earthquake, and these source parameters are used for this inversion. The details of the location are given in the file elocate.txt. The first motion data are compared to the waveform inversion nodal planes in the figure below.

2011/02/14 06:02:55 31.9759 -5.7860 16 4.90 MOROCCO

Focal Mechanism

SLU Moment Tensor Solution ENS 2011/02/14 06:02:55:0 31.98 -5.79 16.0 4.9 MOROCCO Stations used: IB.M019 IB.M201 IB.M206 IB.M211 IB.M214 IB.M215 IB.M216 WM.AVE Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 5.96e+22 dyne-cm Mw = 4.45 Z = 11 km Plane Strike Dip Rake NP1 307 80 -170 NP2 215 80 -10 Principal Axes: Axis Value Plunge Azimuth T 5.96e+22 0 81 N 0.00e+00 76 350 P -5.96e+22 14 171 Moment Tensor: (dyne-cm) Component Value Mxx -5.31e+22 Mxy 1.81e+22 Mxz 1.39e+22 Myy 5.67e+22 Myz -2.12e+21 Mzz -3.54e+21 -------------- ---------------------- ------------------------#### -----------------------####### ##---------------------########### #######----------------############# ###########-----------################ ###############-------################## ##################--################### ####################-################### T ###################-----################ #################---------################ ################------------############## ##############----------------########## #############-------------------######## ###########---------------------###### #########------------------------### #######--------------------------# ####-------------------------- ##------------- ---------- ------------ P ------- -------- --- Global CMT Convention Moment Tensor: R T P -3.54e+21 1.39e+22 2.12e+21 1.39e+22 -5.31e+22 -1.81e+22 2.12e+21 -1.81e+22 5.67e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110214060255/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 = 215
      DIP = 80
     RAKE = -10
       MW = 4.45
       HS = 11.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU SLUFM

SLU Moment Tensor Solution ENS 2011/02/14 06:02:54:0 31.95 -6.19 3.0 4.9 MOROCCO Stations used: IB.M019 IB.M201 IB.M206 IB.M211 IB.M214 IB.M215 IB.M216 WM.AVE Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 5.96e+22 dyne-cm Mw = 4.45 Z = 11 km Plane Strike Dip Rake NP1 307 80 -170 NP2 215 80 -10 Principal Axes: Axis Value Plunge Azimuth T 5.96e+22 0 81 N 0.00e+00 76 350 P -5.96e+22 14 171 Moment Tensor: (dyne-cm) Component Value Mxx -5.31e+22 Mxy 1.81e+22 Mxz 1.39e+22 Myy 5.67e+22 Myz -2.12e+21 Mzz -3.54e+21 -------------- ---------------------- ------------------------#### -----------------------####### ##---------------------########### #######----------------############# ###########-----------################ ###############-------################## ##################--################### ####################-################### T ###################-----################ #################---------################ ################------------############## ##############----------------########## #############-------------------######## ###########---------------------###### #########------------------------### #######--------------------------# ####-------------------------- ##------------- ---------- ------------ P ------- -------- --- Global CMT Convention Moment Tensor: R T P -3.54e+21 1.39e+22 2.12e+21 1.39e+22 -5.31e+22 -1.81e+22 2.12e+21 -1.81e+22 5.67e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110214060254/index.html SLU Moment Tensor Solution ENS 2011/02/14 06:02:55:0 31.98 -5.79 16.0 4.9 MOROCCO Stations used: IB.M019 IB.M201 IB.M206 IB.M211 IB.M214 IB.M215 IB.M216 WM.AVE Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 5.96e+22 dyne-cm Mw = 4.45 Z = 11 km Plane Strike Dip Rake NP1 307 80 -170 NP2 215 80 -10 Principal Axes: Axis Value Plunge Azimuth T 5.96e+22 0 81 N 0.00e+00 76 350 P -5.96e+22 14 171 Moment Tensor: (dyne-cm) Component Value Mxx -5.31e+22 Mxy 1.81e+22 Mxz 1.39e+22 Myy 5.67e+22 Myz -2.12e+21 Mzz -3.54e+21 -------------- ---------------------- ------------------------#### -----------------------####### ##---------------------########### #######----------------############# ###########-----------################ ###############-------################## ##################--################### ####################-################### T ###################-----################ #################---------################ ################------------############## ##############----------------########## #############-------------------######## ###########---------------------###### #########------------------------### #######--------------------------# ####-------------------------- ##------------- ---------- ------------ P ------- -------- --- Global CMT Convention Moment Tensor: R T P -3.54e+21 1.39e+22 2.12e+21 1.39e+22 -5.31e+22 -1.81e+22 2.12e+21 -1.81e+22 5.67e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110214060255/index.html

First motions and takeoff angles from an elocate run.

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:

hp c 0.02 n 3
lp c 0.10 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    0.5   300    65   -35   3.99 0.1840
WVFGRD96    1.0   305    75   -15   3.96 0.1876
WVFGRD96    2.0   220    70    20   4.12 0.2526
WVFGRD96    3.0    35    90     0   4.16 0.2826
WVFGRD96    4.0    40    80    20   4.21 0.3079
WVFGRD96    5.0   220    90   -20   4.24 0.3327
WVFGRD96    6.0   220    80    15   4.29 0.3589
WVFGRD96    7.0   220    85    15   4.32 0.3845
WVFGRD96    8.0   220    85    15   4.37 0.4104
WVFGRD96    9.0   215    75   -10   4.41 0.4258
WVFGRD96   10.0   215    80   -10   4.43 0.4347
WVFGRD96   11.0   215    80   -10   4.45 0.4394
WVFGRD96   12.0    40    90   -10   4.46 0.4373
WVFGRD96   13.0    40    90   -10   4.48 0.4338
WVFGRD96   14.0   220    90    10   4.49 0.4275
WVFGRD96   15.0   220    90    10   4.51 0.4187
WVFGRD96   16.0    40    90   -10   4.52 0.4076
WVFGRD96   17.0   220    90    10   4.53 0.3957
WVFGRD96   18.0   220    90    10   4.54 0.3827
WVFGRD96   19.0   220    90    10   4.54 0.3682
WVFGRD96   20.0   220    90    10   4.55 0.3533
WVFGRD96   21.0    40    90   -10   4.56 0.3378
WVFGRD96   22.0   220    90    10   4.56 0.3214
WVFGRD96   23.0    40    90   -10   4.56 0.3051
WVFGRD96   24.0    40    90   -10   4.56 0.2883
WVFGRD96   25.0    40    90   -10   4.56 0.2728
WVFGRD96   26.0    40    90   -10   4.56 0.2576
WVFGRD96   27.0   220    85    10   4.56 0.2432
WVFGRD96   28.0   310    80    10   4.56 0.2373
WVFGRD96   29.0   310    80    10   4.57 0.2372

The best solution is

WVFGRD96   11.0   215    80   -10   4.45 0.4394

The mechanism corresponding 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

hp c 0.02 n 3
lp c 0.10 n 3

Figure 3. Waveform comparison for selected depth

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

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 Wed May 23 03:24:49 CDT 2012