Location
2009/04/07 20:12:55 61.4690 -149.7580 25.0 4.60 Southern 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 2009/04/07 20:12:55:0 61.47 -149.76 25.0 4.6 Southern Alaska
Stations used:
AK.BMR AK.DIV AK.EYAK AK.PAX AK.SAW AK.SWD AK.TRF AT.PMR
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 8.71e+22 dyne-cm
Mw = 4.56
Z = 50 km
Plane Strike Dip Rake
NP1 216 55 -87
NP2 30 35 -95
Principal Axes:
Axis Value Plunge Azimuth
T 8.71e+22 10 304
N 0.00e+00 3 34
P -8.71e+22 80 140
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.42e+22
Mxy -3.75e+22
Mxz 2.02e+22
Myy 5.74e+22
Myz -2.26e+22
Mzz -8.15e+22
##############
######################
####################------##
#################-----------##
##############--------------###
# T ###########-----------------####
## #########--------------------####
##############---------------------#####
############-----------------------#####
############------------------------######
###########-------------------------######
##########------------ ----------#######
##########------------ P ----------#######
########------------- ---------#######
#######-------------------------########
######------------------------########
#####----------------------#########
####---------------------#########
##-------------------#########
#----------------###########
---------#############
##############
Global CMT Convention Moment Tensor:
R T P
-8.15e+22 2.02e+22 2.26e+22
2.02e+22 2.42e+22 3.75e+22
2.26e+22 3.75e+22 5.74e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090407201255/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is
STK = 30
DIP = 35
RAKE = -95
MW = 4.56
HS = 50.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
AEIC |
USGS/SLU Moment Tensor Solution
ENS 2009/04/07 20:12:55:0 61.47 -149.76 25.0 4.6 Southern Alaska
Stations used:
AK.BMR AK.DIV AK.EYAK AK.PAX AK.SAW AK.SWD AK.TRF AT.PMR
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 8.71e+22 dyne-cm
Mw = 4.56
Z = 50 km
Plane Strike Dip Rake
NP1 216 55 -87
NP2 30 35 -95
Principal Axes:
Axis Value Plunge Azimuth
T 8.71e+22 10 304
N 0.00e+00 3 34
P -8.71e+22 80 140
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.42e+22
Mxy -3.75e+22
Mxz 2.02e+22
Myy 5.74e+22
Myz -2.26e+22
Mzz -8.15e+22
##############
######################
####################------##
#################-----------##
##############--------------###
# T ###########-----------------####
## #########--------------------####
##############---------------------#####
############-----------------------#####
############------------------------######
###########-------------------------######
##########------------ ----------#######
##########------------ P ----------#######
########------------- ---------#######
#######-------------------------########
######------------------------########
#####----------------------#########
####---------------------#########
##-------------------#########
#----------------###########
---------#############
##############
Global CMT Convention Moment Tensor:
R T P
-8.15e+22 2.02e+22 2.26e+22
2.02e+22 2.42e+22 3.75e+22
2.26e+22 3.75e+22 5.74e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090407201255/index.html
|
|
Date: 2009/04/07
Time: 20:12 (UTC)
Region: Cook Inlet Region of Alaska
Mw=4.6
Location:
Lat. 61.4686; Lon. -149.7585; Depth 45 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 8
Best Double Couple:
strike dip rake
Plane 1: 7.8 48.2 -106.5
Plane 2: 211.7 44.3 -72.4
Moment Tensor Parameters:
Mo = 8.83406e+22 dyn-cm
Mxx = 0.51; Mxy = -3.01; Mxz = 1.47
Myy = 7.83; Myz = 1.15; Mzz = -8.34
Principal Axes:
value azimuth plunge
T: 8.93 109.35 1.98
N: -0.18 18.92 12.19
P: -8.74 208.44 77.64
|
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 30.0 235 55 -65 4.27 0.6456
WVFGRD96 31.0 235 55 -70 4.28 0.6538
WVFGRD96 32.0 230 55 -70 4.29 0.6635
WVFGRD96 33.0 230 55 -70 4.30 0.6723
WVFGRD96 34.0 230 55 -70 4.31 0.6799
WVFGRD96 35.0 230 55 -75 4.32 0.6874
WVFGRD96 36.0 230 55 -75 4.33 0.6935
WVFGRD96 37.0 225 55 -75 4.35 0.7003
WVFGRD96 38.0 225 55 -75 4.36 0.7064
WVFGRD96 39.0 225 55 -75 4.37 0.7112
WVFGRD96 40.0 220 50 -80 4.47 0.7049
WVFGRD96 41.0 215 50 -85 4.49 0.7136
WVFGRD96 42.0 215 50 -85 4.50 0.7223
WVFGRD96 43.0 215 50 -85 4.50 0.7283
WVFGRD96 44.0 30 40 -95 4.51 0.7329
WVFGRD96 45.0 215 50 -85 4.52 0.7362
WVFGRD96 46.0 25 40 -95 4.52 0.7388
WVFGRD96 47.0 220 55 -85 4.54 0.7419
WVFGRD96 48.0 35 35 -95 4.55 0.7424
WVFGRD96 49.0 220 55 -85 4.55 0.7438
WVFGRD96 50.0 30 35 -95 4.56 0.7440
WVFGRD96 51.0 215 55 -90 4.57 0.7432
WVFGRD96 52.0 35 35 -90 4.57 0.7418
WVFGRD96 53.0 215 55 -90 4.57 0.7390
WVFGRD96 54.0 35 35 -90 4.58 0.7359
WVFGRD96 55.0 215 55 -90 4.58 0.7308
WVFGRD96 56.0 40 35 -85 4.59 0.7258
WVFGRD96 57.0 215 55 -90 4.59 0.7193
WVFGRD96 58.0 210 55 -90 4.59 0.7119
WVFGRD96 59.0 215 55 -95 4.60 0.7044
The best solution is
WVFGRD96 50.0 30 35 -95 4.56 0.7440
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 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=Wed Apr 8 09:00:39 CDT 2009
Last Changed 2009/04/07