Location
2009/04/16 05:06:03 58.7480 -137.5850 8.0 4.80 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 2009/04/16 05:06:03:0 58.75 -137.59 8.0 4.8 Alaska
Stations used:
AK.BESE AK.PNL AT.CRAG AT.SKAG CN.DAWY CN.DLBC CN.WHY
US.WRAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 6.38e+22 dyne-cm
Mw = 4.47
Z = 22 km
Plane Strike Dip Rake
NP1 309 60 87
NP2 135 30 95
Principal Axes:
Axis Value Plunge Azimuth
T 6.38e+22 75 211
N 0.00e+00 2 311
P -6.38e+22 15 41
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.03e+22
Mxy -2.75e+22
Mxz -2.59e+22
Myy -2.48e+22
Myz -1.91e+22
Mzz 5.51e+22
--------------
----------------------
------------------------ -
------------------------- P --
-##########---------------- ----
--##############--------------------
--##################------------------
---#####################----------------
---#######################--------------
----#########################-------------
----###########################-----------
-----############# ###########----------
------############ T #############--------
-----############ ##############------
------#############################-----
-------###########################----
-------###########################--
--------#########################-
---------#####################
------------##############--
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
5.51e+22 -2.59e+22 1.91e+22
-2.59e+22 -3.03e+22 2.75e+22
1.91e+22 2.75e+22 -2.48e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090416050603/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 = 135
DIP = 30
RAKE = 95
MW = 4.47
HS = 22.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
AEIC |
USGS/SLU Moment Tensor Solution
ENS 2009/04/16 05:06:03:0 58.75 -137.59 8.0 4.8 Alaska
Stations used:
AK.BESE AK.PNL AT.CRAG AT.SKAG CN.DAWY CN.DLBC CN.WHY
US.WRAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 6.38e+22 dyne-cm
Mw = 4.47
Z = 22 km
Plane Strike Dip Rake
NP1 309 60 87
NP2 135 30 95
Principal Axes:
Axis Value Plunge Azimuth
T 6.38e+22 75 211
N 0.00e+00 2 311
P -6.38e+22 15 41
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.03e+22
Mxy -2.75e+22
Mxz -2.59e+22
Myy -2.48e+22
Myz -1.91e+22
Mzz 5.51e+22
--------------
----------------------
------------------------ -
------------------------- P --
-##########---------------- ----
--##############--------------------
--##################------------------
---#####################----------------
---#######################--------------
----#########################-------------
----###########################-----------
-----############# ###########----------
------############ T #############--------
-----############ ##############------
------#############################-----
-------###########################----
-------###########################--
--------#########################-
---------#####################
------------##############--
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
5.51e+22 -2.59e+22 1.91e+22
-2.59e+22 -3.03e+22 2.75e+22
1.91e+22 2.75e+22 -2.48e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090416050603/index.html
|
|
Moment tensor inversion summary for event 2009/04/16 05:06
This is a fully automatic solution. It has not yet been reviewed by a seismologist.2009/04/16 05:06
Date 2009/04/16
Region: Southeastern Alaska
Mw=4.5
Centroid Location:
Time 05:06; Lat. 58.73N; Lon. 222.33W; Depth 20 km
Best Double Couple:
Plane 1: strike = 312; dip = 47; rake = 95
Plane 2: strike = 126; dip = 43; rake = 85
Moment Tensor:
Mo = 5.54533e+22 dyn-cm
Mxx = -335.204; Mxy = -269.245; Mxz = -5.077
Myy = -217.913; Myz = -45.846; Mzz = 553.117
Principal Axes:
T: value = 84.000; azimuth = 282; plunge = 86
N: value = 80.000; azimuth = 39; plunge = 2
P: value = 78.000; azimuth = 129; plunge = 3
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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 0.5 145 35 -85 4.11 0.4601
WVFGRD96 1.0 320 50 -95 4.15 0.4604
WVFGRD96 2.0 145 40 -85 4.24 0.5270
WVFGRD96 3.0 145 40 -85 4.29 0.4572
WVFGRD96 4.0 185 90 20 4.25 0.3893
WVFGRD96 5.0 175 15 -35 4.30 0.4050
WVFGRD96 6.0 175 15 -35 4.30 0.4628
WVFGRD96 7.0 180 15 -30 4.29 0.5064
WVFGRD96 8.0 170 15 -40 4.38 0.5415
WVFGRD96 9.0 170 15 -40 4.38 0.5834
WVFGRD96 10.0 175 15 -35 4.38 0.6139
WVFGRD96 11.0 185 15 -25 4.38 0.6361
WVFGRD96 12.0 175 15 -35 4.39 0.6527
WVFGRD96 13.0 180 15 -30 4.39 0.6644
WVFGRD96 14.0 285 15 75 4.39 0.6701
WVFGRD96 15.0 135 30 95 4.43 0.6914
WVFGRD96 16.0 135 30 95 4.44 0.7186
WVFGRD96 17.0 135 30 95 4.44 0.7395
WVFGRD96 18.0 135 30 95 4.45 0.7545
WVFGRD96 19.0 135 30 95 4.45 0.7648
WVFGRD96 20.0 305 60 85 4.46 0.7707
WVFGRD96 21.0 135 30 95 4.47 0.7750
WVFGRD96 22.0 135 30 95 4.47 0.7755
WVFGRD96 23.0 305 60 85 4.48 0.7730
WVFGRD96 24.0 135 30 95 4.48 0.7701
WVFGRD96 25.0 310 60 90 4.48 0.7639
WVFGRD96 26.0 135 30 95 4.49 0.7573
WVFGRD96 27.0 135 30 95 4.49 0.7488
WVFGRD96 28.0 310 60 90 4.50 0.7387
WVFGRD96 29.0 310 60 90 4.50 0.7272
The best solution is
WVFGRD96 22.0 135 30 95 4.47 0.7755
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=Thu Apr 16 11:00:08 CDT 2009
Last Changed 2009/04/16