Location
2008/09/25 19:34:07 63.8270 -148.7600 10.0 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
2008/09/25 19:34:07 63.8270 -148.7600 10.0 4.20 Alaska
Best Fitting Double Couple
Mo = 1.06e+22 dyne-cm
Mw = 3.95
Z = 15 km
Plane Strike Dip Rake
NP1 70 65 85
NP2 262 25 101
Principal Axes:
Axis Value Plunge Azimuth
T 1.06e+22 70 330
N 0.00e+00 5 72
P -1.06e+22 20 164
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.68e+21
Mxy 1.96e+21
Mxz 6.24e+21
Myy -4.08e+20
Myz -2.69e+21
Mzz 8.08e+21
--------------
----------------------
--------------##------------
-------##################-----
------########################----
----##############################--
---#################################--
---############# #####################
--############## T ###################--
--############### #################-----
-##################################-------
-################################---------
##############################------------
#########################---------------
#####################-------------------
----#######---------------------------
------------------------------------
----------------------------------
------------------ ---------
----------------- P --------
-------------- -----
--------------
Harvard Convention
Moment Tensor:
R T F
8.08e+21 6.24e+21 2.69e+21
6.24e+21 -7.68e+21 -1.96e+21
2.69e+21 -1.96e+21 -4.08e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080925193407/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 = 70
DIP = 65
RAKE = 85
MW = 3.95
HS = 15.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
SLU Moment Tensor Solution
2008/09/25 19:34:07 63.8270 -148.7600 10.0 4.20 Alaska
Best Fitting Double Couple
Mo = 1.06e+22 dyne-cm
Mw = 3.95
Z = 15 km
Plane Strike Dip Rake
NP1 70 65 85
NP2 262 25 101
Principal Axes:
Axis Value Plunge Azimuth
T 1.06e+22 70 330
N 0.00e+00 5 72
P -1.06e+22 20 164
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.68e+21
Mxy 1.96e+21
Mxz 6.24e+21
Myy -4.08e+20
Myz -2.69e+21
Mzz 8.08e+21
--------------
----------------------
--------------##------------
-------##################-----
------########################----
----##############################--
---#################################--
---############# #####################
--############## T ###################--
--############### #################-----
-##################################-------
-################################---------
##############################------------
#########################---------------
#####################-------------------
----#######---------------------------
------------------------------------
----------------------------------
------------------ ---------
----------------- P --------
-------------- -----
--------------
Harvard Convention
Moment Tensor:
R T F
8.08e+21 6.24e+21 2.69e+21
6.24e+21 -7.68e+21 -1.96e+21
2.69e+21 -1.96e+21 -4.08e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080925193407/index.html
|
|
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Moment tensor inversion summary for event 2008/09/25 19:34
Date: 2008/09/25
Time: 19:34 (UTC)
Region: Central Region of Alaska
Mw=4.0
Location:
Lat. 63.8273; Lon. -148.7596; Depth 15 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 3
Best Double Couple:
strike dip rake
Plane 1: 69.3 63.6 91.1
Plane 2: 246.9 26.4 87.9
Moment Tensor Parameters:
Mo = 1.03529e+22 dyn-cm
Mxx = -0.69; Mxy = 0.25; Mxz = 0.59
Myy = -0.17; Myz = -0.21; Mzz = 0.86
Principal Axes:
value azimuth plunge
T: 1.07 341.63 71.38
N: -0.07 248.83 0.94
P: -1.00 158.51 18.59
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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 0.5 255 65 -10 3.82 0.4392
WVFGRD96 1.0 255 55 -5 3.88 0.4413
WVFGRD96 2.0 250 45 -5 3.95 0.4198
WVFGRD96 3.0 235 35 -5 3.96 0.4074
WVFGRD96 4.0 215 20 -20 3.93 0.4488
WVFGRD96 5.0 210 20 -20 3.90 0.4902
WVFGRD96 6.0 185 20 15 3.95 0.5285
WVFGRD96 7.0 195 20 25 3.92 0.5589
WVFGRD96 8.0 205 20 40 3.92 0.5801
WVFGRD96 9.0 230 20 65 3.91 0.6020
WVFGRD96 10.0 230 20 70 3.95 0.6155
WVFGRD96 11.0 240 20 80 3.95 0.6271
WVFGRD96 12.0 250 25 90 3.95 0.6358
WVFGRD96 13.0 70 65 90 3.95 0.6441
WVFGRD96 14.0 70 65 85 3.96 0.6448
WVFGRD96 15.0 70 65 85 3.95 0.6475
WVFGRD96 16.0 255 25 95 3.94 0.6459
WVFGRD96 17.0 70 65 85 3.95 0.6443
WVFGRD96 18.0 70 65 90 3.94 0.6425
WVFGRD96 19.0 260 25 100 3.95 0.6375
WVFGRD96 20.0 70 65 90 3.97 0.6342
WVFGRD96 21.0 255 25 95 3.97 0.6301
WVFGRD96 22.0 70 65 90 3.97 0.6264
WVFGRD96 23.0 70 65 95 3.97 0.6205
WVFGRD96 24.0 245 25 85 3.97 0.6150
WVFGRD96 25.0 70 65 95 3.98 0.6089
WVFGRD96 26.0 240 25 80 3.98 0.6031
WVFGRD96 27.0 235 30 80 3.98 0.5973
WVFGRD96 28.0 225 30 70 3.98 0.5913
WVFGRD96 29.0 235 30 80 3.99 0.5849
WVFGRD96 30.0 225 30 70 3.99 0.5786
WVFGRD96 31.0 235 30 75 3.99 0.5721
WVFGRD96 32.0 235 30 75 4.00 0.5656
WVFGRD96 33.0 235 30 75 4.00 0.5594
WVFGRD96 34.0 235 30 75 4.01 0.5525
WVFGRD96 35.0 235 30 75 4.02 0.5460
WVFGRD96 36.0 235 30 75 4.02 0.5384
WVFGRD96 37.0 230 35 75 4.03 0.5313
WVFGRD96 38.0 240 35 80 4.04 0.5232
WVFGRD96 39.0 230 35 75 4.05 0.5147
WVFGRD96 40.0 240 30 80 4.17 0.5070
WVFGRD96 41.0 240 30 80 4.17 0.4996
WVFGRD96 42.0 240 30 80 4.18 0.4926
WVFGRD96 43.0 240 30 80 4.18 0.4847
WVFGRD96 44.0 240 30 80 4.18 0.4771
WVFGRD96 45.0 240 30 80 4.19 0.4693
WVFGRD96 46.0 245 30 85 4.20 0.4613
WVFGRD96 47.0 245 30 85 4.20 0.4538
WVFGRD96 48.0 245 30 85 4.20 0.4462
WVFGRD96 49.0 245 35 85 4.21 0.4385
WVFGRD96 50.0 245 35 85 4.21 0.4313
The best solution is
WVFGRD96 15.0 70 65 85 3.95 0.6475
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=Fri Sep 26 17:25:41 CDT 2008
Last Changed 2008/09/25