Location

Location ANSS

2018/02/17 14:31:07 51.76 -3.84 16 4.7 Wales-England

The location given here was made using the Comtuer Programs in Seismology program elocate. This was done because the initial plot of time delays indicated a locaiton about 10 km north and 2 seconds later than the EMSC location. The results of this location are given in elocate.txt. The takeoff angles from this solution were used to compare first motion data to the waveform inversion mechanism.

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/02/17 14:31:07:0 51.76 -3.84 16.0 4.7 Wales-England Stations used: BN.LLW EI.DSB EI.IWEX GB.CCA1 GB.CWF GB.DRUM GB.DYA GB.EDI GB.EDMD GB.ESK GB.HMNX GB.HPK GB.HTL GB.KESW GB.LBWR GB.LMK GB.MCH1 GB.WACR GB.WLF1 Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 8.91e+21 dyne-cm Mw = 3.90 Z = 11 km Plane Strike Dip Rake NP1 15 75 -20 NP2 110 71 -164 Principal Axes: Axis Value Plunge Azimuth T 8.91e+21 3 63 N 0.00e+00 65 160 P -8.91e+21 25 332 Moment Tensor: (dyne-cm) Component Value Mxx -3.94e+21 Mxy 6.62e+21 Mxz -2.78e+21 Myy 5.47e+21 Myz 1.99e+21 Mzz -1.52e+21 -------------# -----------------##### ----- ------------######## ------ P ------------######### -------- ------------########### ------------------------########### -------------------------########### T #------------------------############ ###----------------------############### ######--------------------################ ########-----------------################# ###########--------------################# ###############---------################## ##################-----################# ######################--################ ####################------------------ ###################----------------- #################----------------- ##############---------------- ############---------------- ########-------------- ##------------ Global CMT Convention Moment Tensor: R T P -1.52e+21 -2.78e+21 -1.99e+21 -2.78e+21 -3.94e+21 -6.62e+21 -1.99e+21 -6.62e+21 5.47e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180217143107/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 = 15
      DIP = 75
     RAKE = -20
       MW = 3.90
       HS = 11.0

The NDK file is 20180217143107.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU SLUFM

USGS/SLU Moment Tensor Solution ENS 2018/02/17 14:31:07:0 51.76 -3.84 16.0 4.7 Wales-England Stations used: BN.LLW EI.DSB EI.IWEX GB.CCA1 GB.CWF GB.DRUM GB.DYA GB.EDI GB.EDMD GB.ESK GB.HMNX GB.HPK GB.HTL GB.KESW GB.LBWR GB.LMK GB.MCH1 GB.WACR GB.WLF1 Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 8.91e+21 dyne-cm Mw = 3.90 Z = 11 km Plane Strike Dip Rake NP1 15 75 -20 NP2 110 71 -164 Principal Axes: Axis Value Plunge Azimuth T 8.91e+21 3 63 N 0.00e+00 65 160 P -8.91e+21 25 332 Moment Tensor: (dyne-cm) Component Value Mxx -3.94e+21 Mxy 6.62e+21 Mxz -2.78e+21 Myy 5.47e+21 Myz 1.99e+21 Mzz -1.52e+21 -------------# -----------------##### ----- ------------######## ------ P ------------######### -------- ------------########### ------------------------########### -------------------------########### T #------------------------############ ###----------------------############### ######--------------------################ ########-----------------################# ###########--------------################# ###############---------################## ##################-----################# ######################--################ ####################------------------ ###################----------------- #################----------------- ##############---------------- ############---------------- ########-------------- ##------------ Global CMT Convention Moment Tensor: R T P -1.52e+21 -2.78e+21 -1.99e+21 -2.78e+21 -3.94e+21 -6.62e+21 -1.99e+21 -6.62e+21 5.47e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180217143107/index.html

First motions and takeoff angles from an elocate run.

Magnitudes

mLg Magnitude

(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

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 o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2

The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   280    60   -25   3.79 0.3726
WVFGRD96    2.0   290    75   -15   3.78 0.3947
WVFGRD96    3.0   290    80   -10   3.79 0.4088
WVFGRD96    4.0   115    75    10   3.82 0.4216
WVFGRD96    5.0   115    75    15   3.84 0.4305
WVFGRD96    6.0    15    75   -25   3.86 0.4352
WVFGRD96    7.0    15    75   -25   3.87 0.4466
WVFGRD96    8.0    15    75   -25   3.88 0.4548
WVFGRD96    9.0    15    75   -20   3.88 0.4605
WVFGRD96   10.0    15    75   -20   3.90 0.4631
WVFGRD96   11.0    15    75   -20   3.90 0.4646
WVFGRD96   12.0    15    75   -20   3.91 0.4636
WVFGRD96   13.0    15    80   -20   3.91 0.4611
WVFGRD96   14.0    15    80   -20   3.92 0.4578
WVFGRD96   15.0    20    90   -15   3.92 0.4536
WVFGRD96   16.0    20    90   -15   3.93 0.4496
WVFGRD96   17.0   205    85    15   3.94 0.4446
WVFGRD96   18.0   205    85    15   3.95 0.4387
WVFGRD96   19.0   205    85    15   3.96 0.4322
WVFGRD96   20.0   205    80    20   3.97 0.4260
WVFGRD96   21.0   205    80    20   3.98 0.4195
WVFGRD96   22.0   205    80    15   3.98 0.4123
WVFGRD96   23.0   205    80    20   3.99 0.4047
WVFGRD96   24.0   205    80    20   4.00 0.3963
WVFGRD96   25.0   205    80    20   4.00 0.3871
WVFGRD96   26.0   205    80    20   4.00 0.3774
WVFGRD96   27.0   205    80    15   4.00 0.3670
WVFGRD96   28.0   205    80    20   4.01 0.3558
WVFGRD96   29.0   205    85    20   4.02 0.3442

The best solution is

WVFGRD96   11.0    15    75   -20   3.90 0.4646

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 o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2

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

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.model 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 Sun Feb 18 10:14:23 CST 2018