Location
2008/10/28 14:30:12 66.3810 -157.7640 5.0 4.70 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
2008/10/28 14:30:12 66.3810 -157.7640 5.0 4.70 Alaska
Best Fitting Double Couple
Mo = 1.46e+23 dyne-cm
Mw = 4.71
Z = 16 km
Plane Strike Dip Rake
NP1 345 75 -25
NP2 82 66 -164
Principal Axes:
Axis Value Plunge Azimuth
T 1.46e+23 6 35
N 0.00e+00 61 136
P -1.46e+23 28 302
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.61e+22
Mxy 1.19e+23
Mxz -1.93e+22
Myy -3.52e+22
Myz 6.06e+22
Mzz -3.09e+22
--############
--------#############
------------############ T #
--------------########### ##
-----------------#################
---- ------------#################
----- P ------------##################
------ -------------##################
-----------------------#################
------------------------#################-
-------------------------###############--
-------------------------############-----
--------------------------########--------
###----------------------###------------
############-----########---------------
#########################-------------
########################------------
#######################-----------
#####################---------
####################--------
#################-----
#############-
Harvard Convention
Moment Tensor:
R T F
-3.09e+22 -1.93e+22 -6.06e+22
-1.93e+22 6.61e+22 -1.19e+23
-6.06e+22 -1.19e+23 -3.52e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081028143012/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 = 345
DIP = 75
RAKE = -25
MW = 4.71
HS = 16.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
AEIC |
SLU Moment Tensor Solution
2008/10/28 14:30:12 66.3810 -157.7640 5.0 4.70 Alaska
Best Fitting Double Couple
Mo = 1.46e+23 dyne-cm
Mw = 4.71
Z = 16 km
Plane Strike Dip Rake
NP1 345 75 -25
NP2 82 66 -164
Principal Axes:
Axis Value Plunge Azimuth
T 1.46e+23 6 35
N 0.00e+00 61 136
P -1.46e+23 28 302
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.61e+22
Mxy 1.19e+23
Mxz -1.93e+22
Myy -3.52e+22
Myz 6.06e+22
Mzz -3.09e+22
--############
--------#############
------------############ T #
--------------########### ##
-----------------#################
---- ------------#################
----- P ------------##################
------ -------------##################
-----------------------#################
------------------------#################-
-------------------------###############--
-------------------------############-----
--------------------------########--------
###----------------------###------------
############-----########---------------
#########################-------------
########################------------
#######################-----------
#####################---------
####################--------
#################-----
#############-
Harvard Convention
Moment Tensor:
R T F
-3.09e+22 -1.93e+22 -6.06e+22
-1.93e+22 6.61e+22 -1.19e+23
-6.06e+22 -1.19e+23 -3.52e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081028143012/index.html
|
|
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Moment tensor inversion summary for event 2008/10/28 14:30
Date: 2008/10/28
Time: 14:30 (UTC)
Region: North-central Region of Alaska
Mw=4.8
Location:
Lat. 66.4123; Lon. -157.7271; Depth 5 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 8
Best Double Couple:
strike dip rake
Plane 1: 265.5 74.1 -135.6
Plane 2: 160.6 47.7 -21.7
Moment Tensor Parameters:
Mo = 1.96293e+23 dyn-cm
Mxx = 0.99; Mxy = 1.30; Mxz = 1.11
Myy = -0.24; Myz = -0.37; Mzz = -0.75
Principal Axes:
value azimuth plunge
T: 2.03 27.33 16.42
N: -0.14 281.13 43.42
P: -1.89 132.71 41.99
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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 0.05 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 0.5 160 70 -40 4.65 0.5142
WVFGRD96 1.0 160 70 -40 4.65 0.5199
WVFGRD96 2.0 165 70 -30 4.63 0.5243
WVFGRD96 3.0 330 35 -40 4.75 0.5301
WVFGRD96 4.0 335 45 -40 4.71 0.5318
WVFGRD96 5.0 335 65 -45 4.68 0.5493
WVFGRD96 6.0 335 65 -40 4.67 0.5640
WVFGRD96 7.0 335 65 -40 4.67 0.5749
WVFGRD96 8.0 340 65 -35 4.67 0.5845
WVFGRD96 9.0 340 65 -35 4.67 0.5922
WVFGRD96 10.0 340 65 -35 4.69 0.6004
WVFGRD96 11.0 340 65 -35 4.69 0.6071
WVFGRD96 12.0 340 65 -30 4.69 0.6111
WVFGRD96 13.0 340 70 -30 4.69 0.6152
WVFGRD96 14.0 345 70 -25 4.70 0.6175
WVFGRD96 15.0 345 75 -25 4.70 0.6190
WVFGRD96 16.0 345 75 -25 4.71 0.6200
WVFGRD96 17.0 345 75 -25 4.71 0.6193
WVFGRD96 18.0 345 75 -25 4.72 0.6175
WVFGRD96 19.0 345 75 -20 4.73 0.6150
WVFGRD96 20.0 345 75 -25 4.74 0.6130
WVFGRD96 21.0 345 75 -25 4.75 0.6094
WVFGRD96 22.0 345 75 -25 4.75 0.6049
WVFGRD96 23.0 345 75 -20 4.76 0.6004
WVFGRD96 24.0 345 75 -20 4.77 0.5954
WVFGRD96 25.0 345 75 -20 4.77 0.5896
WVFGRD96 26.0 345 75 -20 4.78 0.5832
WVFGRD96 27.0 345 75 -20 4.78 0.5762
WVFGRD96 28.0 345 80 -20 4.79 0.5694
WVFGRD96 29.0 345 80 -20 4.80 0.5622
WVFGRD96 30.0 345 80 -20 4.80 0.5546
WVFGRD96 31.0 170 85 20 4.81 0.5449
WVFGRD96 32.0 170 85 20 4.82 0.5379
WVFGRD96 33.0 170 85 15 4.84 0.5315
WVFGRD96 34.0 170 85 15 4.85 0.5254
WVFGRD96 35.0 170 85 15 4.86 0.5192
WVFGRD96 36.0 345 85 -15 4.86 0.5116
WVFGRD96 37.0 170 85 15 4.88 0.5057
WVFGRD96 38.0 350 85 -10 4.90 0.4966
WVFGRD96 39.0 170 85 15 4.91 0.4888
WVFGRD96 40.0 170 80 20 4.94 0.4797
WVFGRD96 41.0 170 80 20 4.95 0.4746
WVFGRD96 42.0 170 80 20 4.96 0.4687
WVFGRD96 43.0 170 80 20 4.96 0.4623
WVFGRD96 44.0 170 80 20 4.97 0.4553
WVFGRD96 45.0 170 80 20 4.98 0.4481
WVFGRD96 46.0 170 80 20 4.98 0.4405
WVFGRD96 47.0 170 80 15 4.99 0.4330
WVFGRD96 48.0 170 80 15 5.00 0.4254
WVFGRD96 49.0 170 80 15 5.00 0.4175
WVFGRD96 50.0 170 80 15 5.01 0.4093
The best solution is
WVFGRD96 16.0 345 75 -25 4.71 0.6200
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 0.05 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 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=Mon Nov 3 17:53:06 CST 2008
Last Changed 2008/10/28