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
Natural Resources Canada Location
NEIC Location
2010/09/13 10:07:57 62.645 -125.525 5.0 4.00 NWT, Canada
Arrival Times (from USGS)
Arrival time list
Felt Map
USGS Felt map for this earthquake
USGS Felt reports main page
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.
|
Discussion
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