Location
2009/09/21 10:41:26 60.920 -147.119 19.1 4.5 Alaska
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 2009/09/21 10:41:26:0 60.92 -147.12 19.1 4.5 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CHUM AK.DDM AK.DIV AK.EYAK
AK.KLU AK.MCK AK.PPLA AK.RND AK.SCM AK.SSN AT.PMR IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.55e+22 dyne-cm
Mw = 4.30
Z = 27 km
Plane Strike Dip Rake
NP1 195 70 -70
NP2 328 28 -133
Principal Axes:
Axis Value Plunge Azimuth
T 3.55e+22 22 270
N 0.00e+00 19 8
P -3.55e+22 60 134
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.27e+21
Mxy 4.52e+21
Mxz 1.06e+22
Myy 2.57e+22
Myz -2.36e+22
Mzz -2.14e+22
------------##
############--########
###############----#########
###############--------#######
################-----------#######
################-------------#######
################----------------######
#################-----------------######
################-------------------#####
################--------------------######
### ##########---------------------#####
### T ##########---------------------#####
### #########---------- ---------#####
##############---------- P ---------####
##############---------- ---------####
############-----------------------###
###########----------------------###
##########----------------------##
########---------------------#
########-------------------#
#####-----------------
#-------------
Global CMT Convention Moment Tensor:
R T P
-2.14e+22 1.06e+22 2.36e+22
1.06e+22 -4.27e+21 -4.52e+21
2.36e+22 -4.52e+21 2.57e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090921104126/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 = 195
DIP = 70
RAKE = -70
MW = 4.30
HS = 27.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
AEIC |
USGS/SLU Moment Tensor Solution
ENS 2009/09/21 10:41:26:0 60.92 -147.12 19.1 4.5 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CHUM AK.DDM AK.DIV AK.EYAK
AK.KLU AK.MCK AK.PPLA AK.RND AK.SCM AK.SSN AT.PMR IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.55e+22 dyne-cm
Mw = 4.30
Z = 27 km
Plane Strike Dip Rake
NP1 195 70 -70
NP2 328 28 -133
Principal Axes:
Axis Value Plunge Azimuth
T 3.55e+22 22 270
N 0.00e+00 19 8
P -3.55e+22 60 134
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.27e+21
Mxy 4.52e+21
Mxz 1.06e+22
Myy 2.57e+22
Myz -2.36e+22
Mzz -2.14e+22
------------##
############--########
###############----#########
###############--------#######
################-----------#######
################-------------#######
################----------------######
#################-----------------######
################-------------------#####
################--------------------######
### ##########---------------------#####
### T ##########---------------------#####
### #########---------- ---------#####
##############---------- P ---------####
##############---------- ---------####
############-----------------------###
###########----------------------###
##########----------------------##
########---------------------#
########-------------------#
#####-----------------
#-------------
Global CMT Convention Moment Tensor:
R T P
-2.14e+22 1.06e+22 2.36e+22
1.06e+22 -4.27e+21 -4.52e+21
2.36e+22 -4.52e+21 2.57e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090921104126/index.html
|
|
Moment tensor inversion summary for event 2009/09/21 10:41
Date: 2009/09/21
Time: 10:41 (UTC)
Region: Prince William Sound Region of Alaska
Mw=4.4
Location:
Lat. 60.9199; Lon. -147.1196; Depth 20 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 7
Best Double Couple:
strike dip rake
Plane 1: 190.5 76.5 -84.1
Plane 2: 346.5 14.7 -113.2
Moment Tensor Parameters:
Mo = 5.08063e+22 dyn-cm
Mxx = 0.20; Mxy = 0.14; Mxz = 0.96
Myy = 2.17; Myz = -4.26; Mzz = -2.38
Principal Axes:
value azimuth plunge
T: 4.76 275.58 31.26
N: 0.32 9.08 5.75
P: -5.08 108.39 58.10
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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 0.10 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 25 55 -60 3.90 0.3022
WVFGRD96 1.0 20 55 -70 3.96 0.3230
WVFGRD96 2.0 5 45 -85 4.06 0.3468
WVFGRD96 3.0 185 40 -90 4.09 0.2992
WVFGRD96 4.0 40 75 40 4.02 0.3079
WVFGRD96 5.0 40 75 40 4.02 0.3249
WVFGRD96 6.0 40 75 40 4.02 0.3401
WVFGRD96 7.0 200 90 -45 4.01 0.3555
WVFGRD96 8.0 25 85 45 4.02 0.3743
WVFGRD96 9.0 200 90 -45 4.03 0.3884
WVFGRD96 10.0 25 85 45 4.07 0.4025
WVFGRD96 11.0 20 90 50 4.07 0.4173
WVFGRD96 12.0 20 90 50 4.08 0.4314
WVFGRD96 13.0 20 90 50 4.10 0.4447
WVFGRD96 14.0 200 90 -50 4.11 0.4573
WVFGRD96 15.0 20 90 50 4.12 0.4688
WVFGRD96 16.0 205 75 -55 4.14 0.4885
WVFGRD96 17.0 200 75 -60 4.15 0.5053
WVFGRD96 18.0 200 70 -60 4.17 0.5228
WVFGRD96 19.0 200 70 -60 4.18 0.5402
WVFGRD96 20.0 200 70 -60 4.22 0.5563
WVFGRD96 21.0 200 70 -65 4.23 0.5727
WVFGRD96 22.0 200 70 -65 4.25 0.5872
WVFGRD96 23.0 200 70 -65 4.26 0.5993
WVFGRD96 24.0 195 70 -70 4.27 0.6095
WVFGRD96 25.0 195 70 -70 4.28 0.6178
WVFGRD96 26.0 195 70 -70 4.29 0.6229
WVFGRD96 27.0 195 70 -70 4.30 0.6245
WVFGRD96 28.0 195 70 -70 4.31 0.6234
WVFGRD96 29.0 190 70 -75 4.32 0.6193
WVFGRD96 30.0 195 75 -75 4.33 0.6151
WVFGRD96 31.0 195 75 -75 4.34 0.6089
WVFGRD96 32.0 195 75 -75 4.34 0.6004
WVFGRD96 33.0 195 75 -75 4.35 0.5902
WVFGRD96 34.0 195 75 -75 4.35 0.5793
WVFGRD96 35.0 195 75 -75 4.36 0.5676
WVFGRD96 36.0 195 75 -75 4.36 0.5550
WVFGRD96 37.0 190 75 -75 4.36 0.5430
WVFGRD96 38.0 190 75 -75 4.36 0.5309
WVFGRD96 39.0 190 75 -75 4.36 0.5187
The best solution is
WVFGRD96 27.0 195 70 -70 4.30 0.6245
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 0.10 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 Sep 21 13:08:53 CDT 2009
Last Changed 2009/09/21