## Location

The initial location from the USGS led to the following estimate of a mislocation error.

This was computed by fitting the pattern of time shifts between observed and predicted seismograms by an equation of the form
TIME_SHIFT = A + B cos AZ + C sin AZ
The T component (Love wave) were corrected to be pseudo-Rayleigh wave delays by assuming a ratio of Rayleigh Wave group velocity to Love wave group velocity of 0.92. The time offset is converted to a distance using a group velocity of 3.15 km/s for the Rayleigh wave. This exercise wants the NEIC solution to move 11 km in an eastware direction (119 degrees) and to occur about 0.4 seconds later. The NRCAN solution is 11 km east (104 degrees) and 1 second earlier than the USGS solution.

### NRCAN Location

2010/09/13 10:07:56 62.62 -125.32 1.0g 4.1ML 231 km WNW of Fort Simpson

### NEIC Location

2010/09/13 10:07:57 62.645 -125.525 5.0 4.00 NWT, Canada

## Focal Mechanism

 ``` USGS/SLU Moment Tensor Solution ENS 2010/09/13 10:07:57:3 62.65 -125.53 5.0 4.0 NWT, Canada Stations used: AK.BAL AK.TGL AT.CRAG CN.DHRN CN.DLBC CN.KUKN CN.WHY CN.YKW3 US.EGAK Filtering commands used: hp c 0.02 n 4 lp c 0.10 n 4 Best Fitting Double Couple Mo = 3.16e+21 dyne-cm Mw = 3.60 Z = 11 km Plane Strike Dip Rake NP1 260 75 -40 NP2 2 52 -161 Principal Axes: Axis Value Plunge Azimuth T 3.16e+21 15 316 N 0.00e+00 48 63 P -3.16e+21 38 214 Moment Tensor: (dyne-cm) Component Value Mxx 1.85e+20 Mxy -2.37e+21 Mxz 1.84e+21 Myy 8.31e+20 Myz 3.12e+20 Mzz -1.02e+21 #########----- ###############------- ####################-------- # ##################-------- ### T ###################--------- #### ###################---------- ############################---------- #############################----------- ##########################---######----- ################---------------########### #########----------------------########### #####--------------------------########### ##----------------------------############ -----------------------------########### -----------------------------########### ---------- --------------########### --------- P --------------########## -------- -------------########## ---------------------######### -------------------######### --------------######## --------###### Global CMT Convention Moment Tensor: R T P -1.02e+21 1.84e+21 -3.12e+20 1.84e+21 1.85e+20 2.37e+21 -3.12e+20 2.37e+21 8.31e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100913100757/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 = 260
DIP = 75
RAKE = -40
MW = 3.60
HS = 11.0
```

The source inversion is marginal because of the small size and the low S/N.

## Moment Tensor Comparison

The following compares this source inversion to others
 SLU ``` USGS/SLU Moment Tensor Solution ENS 2010/09/13 10:07:57:3 62.65 -125.53 5.0 4.0 NWT, Canada Stations used: AK.BAL AK.TGL AT.CRAG CN.DHRN CN.DLBC CN.KUKN CN.WHY CN.YKW3 US.EGAK Filtering commands used: hp c 0.02 n 4 lp c 0.10 n 4 Best Fitting Double Couple Mo = 3.16e+21 dyne-cm Mw = 3.60 Z = 11 km Plane Strike Dip Rake NP1 260 75 -40 NP2 2 52 -161 Principal Axes: Axis Value Plunge Azimuth T 3.16e+21 15 316 N 0.00e+00 48 63 P -3.16e+21 38 214 Moment Tensor: (dyne-cm) Component Value Mxx 1.85e+20 Mxy -2.37e+21 Mxz 1.84e+21 Myy 8.31e+20 Myz 3.12e+20 Mzz -1.02e+21 #########----- ###############------- ####################-------- # ##################-------- ### T ###################--------- #### ###################---------- ############################---------- #############################----------- ##########################---######----- ################---------------########### #########----------------------########### #####--------------------------########### ##----------------------------############ -----------------------------########### -----------------------------########### ---------- --------------########### --------- P --------------########## -------- -------------########## ---------------------######### -------------------######### --------------######## --------###### Global CMT Convention Moment Tensor: R T P -1.02e+21 1.84e+21 -3.12e+20 1.84e+21 1.85e+20 2.37e+21 -3.12e+20 2.37e+21 8.31e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100913100757/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:

```hp c 0.02 n 4
lp c 0.10 n 4
```
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    85    70    -5   3.45 0.5076
WVFGRD96    1.0    90    65    15   3.45 0.5133
WVFGRD96    2.0   275    70    45   3.53 0.5257
WVFGRD96    3.0   290    60    65   3.61 0.5298
WVFGRD96    4.0   265    85   -50   3.57 0.5082
WVFGRD96    5.0   260    75   -45   3.56 0.5168
WVFGRD96    6.0   260    75   -40   3.55 0.5252
WVFGRD96    7.0   260    75   -40   3.56 0.5316
WVFGRD96    8.0   260    75   -40   3.56 0.5364
WVFGRD96    9.0   260    75   -35   3.56 0.5405
WVFGRD96   10.0   260    75   -40   3.59 0.5428
WVFGRD96   11.0   260    75   -40   3.60 0.5438
WVFGRD96   12.0   260    75   -40   3.60 0.5434
WVFGRD96   13.0   260    75   -40   3.61 0.5414
WVFGRD96   14.0   260    75   -40   3.62 0.5382
WVFGRD96   15.0   260    75   -35   3.63 0.5332
WVFGRD96   16.0   260    75   -40   3.64 0.5275
WVFGRD96   17.0   260    75   -40   3.64 0.5200
WVFGRD96   18.0   260    75   -40   3.65 0.5110
WVFGRD96   19.0   260    75   -40   3.66 0.5007
WVFGRD96   20.0   255    70   -50   3.69 0.4925
WVFGRD96   21.0   255    70   -50   3.70 0.4817
WVFGRD96   22.0   255    70   -50   3.71 0.4702
WVFGRD96   23.0   250    70   -55   3.73 0.4582
WVFGRD96   24.0   160    50   -25   3.71 0.4522
WVFGRD96   25.0   160    50   -25   3.71 0.4535
WVFGRD96   26.0   155    50   -30   3.73 0.4537
WVFGRD96   27.0   155    50   -30   3.73 0.4537
WVFGRD96   28.0   155    50   -30   3.74 0.4532
WVFGRD96   29.0   155    55   -30   3.75 0.4516
```

The best solution is

```WVFGRD96   11.0   260    75   -40   3.60 0.5438
```

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

```hp c 0.02 n 4
lp c 0.10 n 4
```
 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.

## The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

## Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

## Velocity Model

The CUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

```MODEL.01
CUS Model with Q from simple gamma values
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
1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00
9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00
10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00
20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00
0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 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:

DATE=Tue Sep 14 08:30:14 CDT 2010

Last Changed 2010/09/13