Location
2008/11/15 14:23:34 66.3420 -156.6280 14.0 4.60 Alaska
Arrival Times (from USGS)
Arrival time list
Felt Map
USGS Felt map for this earthquake
USGS Felt reports page for
Focal Mechanism
SLU Moment Tensor Solution
2008/11/15 14:23:34 66.3420 -156.6280 14.0 4.60 Alaska
Best Fitting Double Couple
Mo = 1.04e+23 dyne-cm
Mw = 4.61
Z = 17 km
Plane Strike Dip Rake
NP1 350 85 -20
NP2 82 70 -175
Principal Axes:
Axis Value Plunge Azimuth
T 1.04e+23 10 37
N 0.00e+00 69 157
P -1.04e+23 18 304
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.33e+22
Mxy 9.21e+22
Mxz -2.29e+21
Myy -2.72e+22
Myz 3.58e+22
Mzz -6.15e+21
----##########
--------############
------------########### T ##
-------------########### ###
- ------------##################
-- P ------------###################
--- -------------###################
--------------------####################
--------------------####################
----------------------###################-
----------------------#################---
----------------------##############------
-----------------------########-----------
####------------------#-----------------
######################------------------
######################----------------
#####################---------------
####################--------------
##################------------
#################-----------
###############-------
###########---
Harvard Convention
Moment Tensor:
R T F
-6.15e+21 -2.29e+21 -3.58e+22
-2.29e+21 3.33e+22 -9.21e+22
-3.58e+22 -9.21e+22 -2.72e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081115142334/index.html
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is
STK = 350
DIP = 85
RAKE = -20
MW = 4.61
HS = 17.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
AEIC |
SLU Moment Tensor Solution
2008/11/15 14:23:34 66.3420 -156.6280 14.0 4.60 Alaska
Best Fitting Double Couple
Mo = 1.04e+23 dyne-cm
Mw = 4.61
Z = 17 km
Plane Strike Dip Rake
NP1 350 85 -20
NP2 82 70 -175
Principal Axes:
Axis Value Plunge Azimuth
T 1.04e+23 10 37
N 0.00e+00 69 157
P -1.04e+23 18 304
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.33e+22
Mxy 9.21e+22
Mxz -2.29e+21
Myy -2.72e+22
Myz 3.58e+22
Mzz -6.15e+21
----##########
--------############
------------########### T ##
-------------########### ###
- ------------##################
-- P ------------###################
--- -------------###################
--------------------####################
--------------------####################
----------------------###################-
----------------------#################---
----------------------##############------
-----------------------########-----------
####------------------#-----------------
######################------------------
######################----------------
#####################---------------
####################--------------
##################------------
#################-----------
###############-------
###########---
Harvard Convention
Moment Tensor:
R T F
-6.15e+21 -2.29e+21 -3.58e+22
-2.29e+21 3.33e+22 -9.21e+22
-3.58e+22 -9.21e+22 -2.72e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081115142334/index.html
|
Moment tensor inversion summary for event 2008/11/15 14:23
This is a fully automatic solution. It has not yet been reviewed by a seismologist.2008/11/15 14:23
Date 2008/11/15
Region: North-central Region of Alaska
Mw=4.7
Centroid Location:
Time 14:23; Lat. 66.29N; Lon. 203.09W; Depth 20 km
Best Double Couple:
Plane 1: strike = 155; dip = 58; rake = -42
Plane 2: strike = 270; dip = 56; rake = -140
Moment Tensor:
Mo = 1.23456e+23 dyn-cm
Mxx = 835.143; Mxy = 967.439; Mxz = 107.083
Myy = -52.712; Myz = -262.199; Mzz = -782.432
Principal Axes:
T: value = 84.000; azimuth = 213; plunge = 1
N: value = 80.000; azimuth = 121; plunge = 51
P: value = 78.000; azimuth = 304; plunge = 39
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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.
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Location of broadband stations used for waveform inversion
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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 0.06 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 0.5 165 65 -25 4.29 0.4312
WVFGRD96 1.0 165 70 -30 4.32 0.4472
WVFGRD96 2.0 165 70 -30 4.39 0.5197
WVFGRD96 3.0 165 70 -30 4.42 0.5430
WVFGRD96 4.0 165 75 -30 4.43 0.5624
WVFGRD96 5.0 170 85 -25 4.44 0.5851
WVFGRD96 6.0 170 85 -25 4.46 0.6080
WVFGRD96 7.0 170 85 -20 4.48 0.6307
WVFGRD96 8.0 170 85 -25 4.51 0.6524
WVFGRD96 9.0 170 85 -25 4.53 0.6688
WVFGRD96 10.0 170 85 -20 4.54 0.6818
WVFGRD96 11.0 170 85 -20 4.55 0.6910
WVFGRD96 12.0 350 90 20 4.56 0.6939
WVFGRD96 13.0 350 90 20 4.57 0.6934
WVFGRD96 14.0 350 80 -20 4.58 0.6932
WVFGRD96 15.0 350 80 -20 4.59 0.6991
WVFGRD96 16.0 350 80 -20 4.60 0.7026
WVFGRD96 17.0 350 85 -20 4.61 0.7045
WVFGRD96 18.0 170 90 20 4.61 0.7026
WVFGRD96 19.0 170 90 15 4.63 0.7013
WVFGRD96 20.0 170 90 15 4.64 0.7001
WVFGRD96 21.0 170 90 15 4.64 0.6965
WVFGRD96 22.0 350 90 -15 4.65 0.6916
WVFGRD96 23.0 350 90 -15 4.66 0.6850
WVFGRD96 24.0 170 85 15 4.66 0.6774
WVFGRD96 25.0 350 90 -15 4.67 0.6677
WVFGRD96 26.0 350 90 -15 4.68 0.6577
WVFGRD96 27.0 170 85 10 4.69 0.6481
WVFGRD96 28.0 350 90 -10 4.69 0.6369
WVFGRD96 29.0 170 85 10 4.70 0.6255
The best solution is
WVFGRD96 17.0 350 85 -20 4.61 0.7045
The mechanism correspond to the best fit is
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Figure 1. Waveform inversion focal mechanism
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The best fit as a function of depth is given in the following figure:
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Figure 2. Depth sensitivity for waveform mechanism
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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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 bandpass filter used in the processing and for the display was
hp c 0.02 n 3
lp c 0.06 n 3
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Figure 3. Waveform comparison for selected depth
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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.
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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 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:
DATE=Sat Nov 15 12:26:36 CST 2008
Last Changed 2008/11/15