2009/04/21 10:25:44 33.011 -87.145 5.0 3.80 Alabama
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution
ENS 2009/04/21 10:25:44:0 33.01 -87.14 5.0 3.8 Alabama
Stations used:
ET.SWET IU.WCI IU.WVT NM.PLAL NM.USIN TA.KMSC US.GOGA
US.LRAL US.OXF
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.33e+21 dyne-cm
Mw = 3.35
Z = 6 km
Plane Strike Dip Rake
NP1 275 85 -10
NP2 6 80 -175
Principal Axes:
Axis Value Plunge Azimuth
T 1.33e+21 3 321
N 0.00e+00 79 69
P -1.33e+21 11 230
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.67e+20
Mxy -1.28e+21
Mxz 2.17e+20
Myy -2.27e+20
Myz 1.34e+20
Mzz -4.02e+19
#########-----
##############--------
T ##############-----------
# ##############------------
####################--------------
#####################---------------
######################----------------
#######################-----------------
#######################-----------------
########################------------------
------------------------#-----------------
-----------------------###############----
-----------------------###################
----------------------##################
----------------------##################
--------------------##################
-- --------------#################
- P --------------################
-------------###############
--------------##############
----------############
-----#########
Global CMT Convention Moment Tensor:
R T P
-4.02e+19 2.17e+20 -1.34e+20
2.17e+20 2.67e+20 1.28e+21
-1.34e+20 1.28e+21 -2.27e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090421102544/index.html
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STK = 275
DIP = 85
RAKE = -10
MW = 3.35
HS = 6.0
The waveform inversion is preferred.
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution
ENS 2009/04/21 10:25:44:0 33.01 -87.14 5.0 3.8 Alabama
Stations used:
ET.SWET IU.WCI IU.WVT NM.PLAL NM.USIN TA.KMSC US.GOGA
US.LRAL US.OXF
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.33e+21 dyne-cm
Mw = 3.35
Z = 6 km
Plane Strike Dip Rake
NP1 275 85 -10
NP2 6 80 -175
Principal Axes:
Axis Value Plunge Azimuth
T 1.33e+21 3 321
N 0.00e+00 79 69
P -1.33e+21 11 230
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.67e+20
Mxy -1.28e+21
Mxz 2.17e+20
Myy -2.27e+20
Myz 1.34e+20
Mzz -4.02e+19
#########-----
##############--------
T ##############-----------
# ##############------------
####################--------------
#####################---------------
######################----------------
#######################-----------------
#######################-----------------
########################------------------
------------------------#-----------------
-----------------------###############----
-----------------------###################
----------------------##################
----------------------##################
--------------------##################
-- --------------#################
- P --------------################
-------------###############
--------------##############
----------############
-----#########
Global CMT Convention Moment Tensor:
R T P
-4.02e+19 2.17e+20 -1.34e+20
2.17e+20 2.67e+20 1.28e+21
-1.34e+20 1.28e+21 -2.27e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090421102544/index.html
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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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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 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 280 45 10 3.32 0.3344
WVFGRD96 1.0 280 55 15 3.30 0.3503
WVFGRD96 2.0 280 70 20 3.30 0.3629
WVFGRD96 3.0 280 80 15 3.31 0.3622
WVFGRD96 4.0 95 90 15 3.33 0.3627
WVFGRD96 5.0 95 90 15 3.35 0.3621
WVFGRD96 6.0 275 85 -10 3.35 0.3660
WVFGRD96 7.0 100 90 10 3.36 0.3599
WVFGRD96 8.0 100 90 10 3.38 0.3561
WVFGRD96 9.0 100 90 10 3.40 0.3507
WVFGRD96 10.0 275 80 -10 3.40 0.3538
WVFGRD96 11.0 275 80 -10 3.41 0.3462
WVFGRD96 12.0 275 80 -10 3.43 0.3391
WVFGRD96 13.0 275 80 -5 3.44 0.3308
WVFGRD96 14.0 275 75 -5 3.44 0.3227
WVFGRD96 15.0 270 75 -10 3.45 0.3155
WVFGRD96 16.0 270 75 -10 3.46 0.3086
WVFGRD96 17.0 270 70 -10 3.46 0.3025
WVFGRD96 18.0 270 70 -10 3.48 0.2976
WVFGRD96 19.0 270 70 -10 3.49 0.2926
WVFGRD96 20.0 270 70 -15 3.51 0.2888
WVFGRD96 21.0 270 70 -15 3.52 0.2837
WVFGRD96 22.0 270 70 -15 3.53 0.2786
WVFGRD96 23.0 270 70 -15 3.54 0.2741
WVFGRD96 24.0 270 70 -15 3.55 0.2694
WVFGRD96 25.0 270 70 -15 3.56 0.2646
WVFGRD96 26.0 270 70 -20 3.56 0.2597
WVFGRD96 27.0 270 70 -20 3.57 0.2544
WVFGRD96 28.0 270 70 -20 3.58 0.2498
WVFGRD96 29.0 270 70 -20 3.58 0.2457
The best solution is
WVFGRD96 6.0 275 85 -10 3.35 0.3660
The mechanism correspond to the best fit is
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The best fit as a function of depth is given in the following figure:
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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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| 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. |
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.
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.
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
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Tue Apr 21 08:09:28 CDT 2009