Location
2008/09/24 12:19:52 63.411 -150.068 7.0 4.20 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
2008/09/24 12:19:52 63.411 -150.068 7.0 4.20 Alaska
Best Fitting Double Couple
Mo = 1.97e+22 dyne-cm
Mw = 4.13
Z = 9 km
Plane Strike Dip Rake
NP1 210 55 45
NP2 90 55 135
Principal Axes:
Axis Value Plunge Azimuth
T 1.97e+22 55 60
N 0.00e+00 35 240
P -1.97e+22 0 330
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.32e+22
Mxy 1.14e+22
Mxz 4.54e+21
Myy 6.47e+19
Myz 8.13e+21
Mzz 1.31e+22
--------------
P -----------------###
-- ------------###########
----------------##############
---------------###################
---------------#####################
--------------########################
--------------############ ###########
------------############## T ###########
------------############### ############
------------##############################
#----------##############################-
###-------#############################---
####-----###########################----
################################--------
#######------############-------------
######------------------------------
#####-----------------------------
###---------------------------
###-------------------------
----------------------
--------------
Harvard Convention
Moment Tensor:
R T F
1.31e+22 4.54e+21 -8.13e+21
4.54e+21 -1.32e+22 -1.14e+22
-8.13e+21 -1.14e+22 6.47e+19
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080924121952/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 = 210
DIP = 55
RAKE = 45
MW = 4.13
HS = 9.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
AEIC |
SLU Moment Tensor Solution
2008/09/24 12:19:52 63.411 -150.068 7.0 4.20 Alaska
Best Fitting Double Couple
Mo = 1.97e+22 dyne-cm
Mw = 4.13
Z = 9 km
Plane Strike Dip Rake
NP1 210 55 45
NP2 90 55 135
Principal Axes:
Axis Value Plunge Azimuth
T 1.97e+22 55 60
N 0.00e+00 35 240
P -1.97e+22 0 330
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.32e+22
Mxy 1.14e+22
Mxz 4.54e+21
Myy 6.47e+19
Myz 8.13e+21
Mzz 1.31e+22
--------------
P -----------------###
-- ------------###########
----------------##############
---------------###################
---------------#####################
--------------########################
--------------############ ###########
------------############## T ###########
------------############### ############
------------##############################
#----------##############################-
###-------#############################---
####-----###########################----
################################--------
#######------############-------------
######------------------------------
#####-----------------------------
###---------------------------
###-------------------------
----------------------
--------------
Harvard Convention
Moment Tensor:
R T F
1.31e+22 4.54e+21 -8.13e+21
4.54e+21 -1.32e+22 -1.14e+22
-8.13e+21 -1.14e+22 6.47e+19
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080924121952/index.html
|
|
Moment tensor inversion summary for event 2008/09/24 12:19
This is a fully automatic solution. It has not yet been reviewed by a seismologist.2008/09/24 12:19
Date 2008/09/24
Region: Central Region of Alaska
Mw=4.1
Centroid Location:
Time 12:19; Lat. 63.46N; Lon. 209.83W; Depth 10 km
Best Double Couple:
Plane 1: strike = 211; dip = 69; rake = 40
Plane 2: strike = 104; dip = 53; rake = 154
Moment Tensor:
Mo = 1.42074e+22 dyn-cm
Mxx = -109.997; Mxy = 83.630; Mxz = -12.066
Myy = 43.179; Myz = 64.976; Mzz = 66.818
Principal Axes:
T: value = 80.000; azimuth = 334; plunge = 10
N: value = 84.000; azimuth = 73; plunge = 43
P: value = 78.000; azimuth = 233; plunge = 46
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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 0.5 185 35 -15 4.02 0.4421
WVFGRD96 1.0 185 30 -15 4.07 0.4476
WVFGRD96 2.0 190 35 -15 4.07 0.4494
WVFGRD96 3.0 195 35 -10 4.08 0.4550
WVFGRD96 4.0 200 40 5 4.07 0.4714
WVFGRD96 5.0 205 50 25 4.08 0.5027
WVFGRD96 6.0 205 55 35 4.10 0.5451
WVFGRD96 7.0 205 55 35 4.11 0.5766
WVFGRD96 8.0 210 55 40 4.12 0.5908
WVFGRD96 9.0 210 55 45 4.13 0.5953
WVFGRD96 10.0 210 55 40 4.14 0.5938
WVFGRD96 11.0 210 55 35 4.13 0.5839
WVFGRD96 12.0 205 60 35 4.13 0.5710
WVFGRD96 13.0 205 60 35 4.12 0.5559
WVFGRD96 14.0 205 60 35 4.12 0.5384
WVFGRD96 15.0 205 60 30 4.11 0.5192
WVFGRD96 16.0 205 60 30 4.11 0.5017
WVFGRD96 17.0 205 60 30 4.11 0.4844
WVFGRD96 18.0 200 70 40 4.11 0.4736
WVFGRD96 19.0 200 70 40 4.11 0.4659
WVFGRD96 20.0 200 70 40 4.13 0.4552
WVFGRD96 21.0 200 70 40 4.13 0.4462
WVFGRD96 22.0 200 70 40 4.14 0.4363
WVFGRD96 23.0 200 75 40 4.14 0.4264
WVFGRD96 24.0 200 75 40 4.14 0.4169
WVFGRD96 25.0 200 75 40 4.14 0.4071
WVFGRD96 26.0 195 80 40 4.14 0.3971
WVFGRD96 27.0 195 80 40 4.14 0.3881
WVFGRD96 28.0 195 80 40 4.15 0.3788
WVFGRD96 29.0 195 80 35 4.15 0.3702
WVFGRD96 30.0 195 80 35 4.15 0.3616
WVFGRD96 31.0 195 85 35 4.16 0.3536
WVFGRD96 32.0 195 85 35 4.16 0.3459
WVFGRD96 33.0 195 85 35 4.17 0.3379
WVFGRD96 34.0 195 85 35 4.17 0.3300
WVFGRD96 35.0 15 90 -35 4.17 0.3185
WVFGRD96 36.0 10 90 -30 4.18 0.3117
WVFGRD96 37.0 195 85 30 4.19 0.3079
WVFGRD96 38.0 195 80 40 4.17 0.3016
WVFGRD96 39.0 195 80 40 4.17 0.2960
WVFGRD96 40.0 195 80 45 4.26 0.2890
WVFGRD96 41.0 95 60 -20 4.26 0.2889
WVFGRD96 42.0 95 60 -20 4.27 0.2876
WVFGRD96 43.0 95 60 -15 4.27 0.2863
WVFGRD96 44.0 95 60 -15 4.28 0.2847
WVFGRD96 45.0 95 60 -15 4.28 0.2834
WVFGRD96 46.0 95 65 -15 4.28 0.2819
WVFGRD96 47.0 95 65 -15 4.29 0.2811
WVFGRD96 48.0 95 65 -15 4.29 0.2803
WVFGRD96 49.0 95 65 -15 4.30 0.2786
WVFGRD96 50.0 95 65 -15 4.30 0.2783
The best solution is
WVFGRD96 9.0 210 55 45 4.13 0.5953
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 model 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=Wed Sep 24 11:35:04 CDT 2008
Last Changed 2008/09/24