Location
2009/08/19 18:19:27 61.21 -150.81 58.0 5.00 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 2009/08/19 18:19:27:0 61.21 -150.81 58.0 5.0 Alaska
Stations used:
AK.BMR AK.CAST AK.CHUM AK.DIV AK.EYAK AK.MCK AK.PPLA AK.SAW
AK.SSN AK.TRF AT.PMR AT.SVW2 IU.COLA
Filtering commands used:
hp c 0.015 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 3.02e+23 dyne-cm
Mw = 4.92
Z = 69 km
Plane Strike Dip Rake
NP1 332 80 113
NP2 85 25 25
Principal Axes:
Axis Value Plunge Azimuth
T 3.02e+23 50 268
N 0.00e+00 23 148
P -3.02e+23 31 43
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.17e+23
Mxy -1.05e+23
Mxz -1.03e+23
Myy 1.93e+22
Myz -2.40e+23
Mzz 9.78e+22
--------------
##--------------------
#######---------------------
#########---------------------
#############------------ ------
###############----------- P -------
#################---------- --------
###################---------------------
####################--------------------
######################--------------------
######### ###########-------------------
######### T ############-----------------#
-######## #############---------------##
#########################--------------#
--########################-----------###
--########################---------###
---######################-------####
----#####################----#####
-----#########################
------------###--------#####
---------------------#
--------------
Global CMT Convention Moment Tensor:
R T P
9.78e+22 -1.03e+23 2.40e+23
-1.03e+23 -1.17e+23 1.05e+23
2.40e+23 1.05e+23 1.93e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090819181927/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 = 85
DIP = 25
RAKE = 25
MW = 4.92
HS = 69.0
The waveform inversion is preferred. The AEIC Mw is slightly larger, which is related to differences in the velocity model. Out WUS model probably has slightly smaller velocities at this source depth than the AEIC model. When we can this using the 0.02 - 0.10 Hz band one nodal plane was vertical and essentially striking NS. Upon using the 0.015 - 0.05 Hz band, this nodal plane strikes a bit west of north. The plane dipping slightly to the south is not in the direction for shallow subduction in this region.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
AEIC |
USGS/SLU Moment Tensor Solution
ENS 2009/08/19 18:19:27:0 61.21 -150.81 58.0 5.0 Alaska
Stations used:
AK.BMR AK.CAST AK.CHUM AK.DIV AK.EYAK AK.MCK AK.PPLA AK.SAW
AK.SSN AK.TRF AT.PMR AT.SVW2 IU.COLA
Filtering commands used:
hp c 0.015 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 3.02e+23 dyne-cm
Mw = 4.92
Z = 69 km
Plane Strike Dip Rake
NP1 332 80 113
NP2 85 25 25
Principal Axes:
Axis Value Plunge Azimuth
T 3.02e+23 50 268
N 0.00e+00 23 148
P -3.02e+23 31 43
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.17e+23
Mxy -1.05e+23
Mxz -1.03e+23
Myy 1.93e+22
Myz -2.40e+23
Mzz 9.78e+22
--------------
##--------------------
#######---------------------
#########---------------------
#############------------ ------
###############----------- P -------
#################---------- --------
###################---------------------
####################--------------------
######################--------------------
######### ###########-------------------
######### T ############-----------------#
-######## #############---------------##
#########################--------------#
--########################-----------###
--########################---------###
---######################-------####
----#####################----#####
-----#########################
------------###--------#####
---------------------#
--------------
Global CMT Convention Moment Tensor:
R T P
9.78e+22 -1.03e+23 2.40e+23
-1.03e+23 -1.17e+23 1.05e+23
2.40e+23 1.05e+23 1.93e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090819181927/index.html
|
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Moment tensor inversion summary for event 2009/08/19 18:19
Date: 2009/08/19
Time: 18:19 (UTC)
Region: Cook Inlet Region of Alaska
Mw=5.1
Location:
Lat. 61.2286; Lon. -150.8254; Depth 70 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 7
Best Double Couple:
strike dip rake
Plane 1: 174.0 87.0 -104.0
Plane 2: 72.3 14.3 -12.1
Moment Tensor Parameters:
Mo = 4.23092e+23 dyn-cm
Mxx = -0.19; Mxy = -0.96; Mxz = -0.48
Myy = 0.62; Myz = -4.05; Mzz = -0.43
Principal Axes:
value azimuth plunge
T: 4.22 277.03 40.44
N: 0.01 174.77 14.00
P: -4.23 69.71 46.19
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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 0.015 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 1.0 0 45 -85 4.20 0.2219
WVFGRD96 2.0 0 45 -90 4.29 0.2695
WVFGRD96 3.0 0 40 -85 4.35 0.2709
WVFGRD96 4.0 10 40 -70 4.37 0.2474
WVFGRD96 5.0 35 70 0 4.34 0.2416
WVFGRD96 6.0 35 70 5 4.37 0.2458
WVFGRD96 7.0 40 70 15 4.38 0.2520
WVFGRD96 8.0 40 65 15 4.41 0.2555
WVFGRD96 9.0 40 65 15 4.43 0.2620
WVFGRD96 10.0 40 65 15 4.44 0.2668
WVFGRD96 11.0 40 65 15 4.45 0.2725
WVFGRD96 12.0 40 65 15 4.45 0.2786
WVFGRD96 13.0 40 65 20 4.47 0.2864
WVFGRD96 14.0 45 60 20 4.46 0.2956
WVFGRD96 15.0 45 60 20 4.46 0.3055
WVFGRD96 16.0 45 60 20 4.47 0.3159
WVFGRD96 17.0 45 60 20 4.48 0.3268
WVFGRD96 18.0 45 55 15 4.48 0.3383
WVFGRD96 19.0 45 55 15 4.49 0.3503
WVFGRD96 20.0 45 55 15 4.50 0.3626
WVFGRD96 21.0 45 50 10 4.50 0.3733
WVFGRD96 22.0 50 45 10 4.50 0.3869
WVFGRD96 23.0 50 45 10 4.51 0.4011
WVFGRD96 24.0 50 45 10 4.52 0.4149
WVFGRD96 25.0 50 40 5 4.52 0.4294
WVFGRD96 26.0 55 35 5 4.53 0.4445
WVFGRD96 27.0 55 35 5 4.54 0.4599
WVFGRD96 28.0 55 35 5 4.55 0.4749
WVFGRD96 29.0 55 35 5 4.56 0.4896
WVFGRD96 30.0 55 35 5 4.57 0.5038
WVFGRD96 31.0 55 35 5 4.58 0.5176
WVFGRD96 32.0 55 35 5 4.59 0.5306
WVFGRD96 33.0 60 35 5 4.60 0.5430
WVFGRD96 34.0 60 35 5 4.60 0.5546
WVFGRD96 35.0 60 35 5 4.61 0.5653
WVFGRD96 36.0 60 35 5 4.62 0.5748
WVFGRD96 37.0 60 40 10 4.63 0.5832
WVFGRD96 38.0 60 40 10 4.63 0.5918
WVFGRD96 39.0 60 40 10 4.64 0.5991
WVFGRD96 40.0 65 25 10 4.75 0.5999
WVFGRD96 41.0 65 25 10 4.76 0.6109
WVFGRD96 42.0 65 25 10 4.77 0.6211
WVFGRD96 43.0 65 25 10 4.78 0.6307
WVFGRD96 44.0 65 30 10 4.79 0.6400
WVFGRD96 45.0 65 30 10 4.79 0.6490
WVFGRD96 46.0 65 30 10 4.80 0.6574
WVFGRD96 47.0 65 30 10 4.81 0.6652
WVFGRD96 48.0 65 30 10 4.81 0.6726
WVFGRD96 49.0 65 30 10 4.82 0.6794
WVFGRD96 50.0 70 25 15 4.82 0.6860
WVFGRD96 51.0 70 25 15 4.83 0.6920
WVFGRD96 52.0 70 25 15 4.83 0.6978
WVFGRD96 53.0 75 25 15 4.84 0.7036
WVFGRD96 54.0 75 25 15 4.85 0.7093
WVFGRD96 55.0 75 25 15 4.86 0.7144
WVFGRD96 56.0 75 25 15 4.86 0.7190
WVFGRD96 57.0 75 25 15 4.87 0.7229
WVFGRD96 58.0 80 25 20 4.87 0.7264
WVFGRD96 59.0 80 25 20 4.88 0.7306
WVFGRD96 60.0 80 25 20 4.88 0.7339
WVFGRD96 61.0 80 25 20 4.89 0.7365
WVFGRD96 62.0 80 25 20 4.89 0.7384
WVFGRD96 63.0 80 25 20 4.90 0.7405
WVFGRD96 64.0 80 25 20 4.90 0.7424
WVFGRD96 65.0 80 25 20 4.90 0.7435
WVFGRD96 66.0 85 25 25 4.91 0.7443
WVFGRD96 67.0 85 25 25 4.91 0.7452
WVFGRD96 68.0 85 25 25 4.92 0.7452
WVFGRD96 69.0 85 25 25 4.92 0.7452
WVFGRD96 70.0 85 25 25 4.92 0.7444
WVFGRD96 71.0 85 25 25 4.93 0.7435
WVFGRD96 72.0 90 25 25 4.93 0.7423
WVFGRD96 73.0 90 25 25 4.94 0.7407
WVFGRD96 74.0 90 25 25 4.94 0.7390
WVFGRD96 75.0 90 25 25 4.94 0.7372
WVFGRD96 76.0 90 25 25 4.95 0.7344
WVFGRD96 77.0 90 25 25 4.95 0.7317
WVFGRD96 78.0 90 25 25 4.95 0.7291
WVFGRD96 79.0 90 25 25 4.96 0.7252
WVFGRD96 80.0 90 25 25 4.96 0.7221
WVFGRD96 81.0 90 25 25 4.96 0.7185
WVFGRD96 82.0 90 25 25 4.96 0.7143
WVFGRD96 83.0 95 25 30 4.96 0.7104
WVFGRD96 84.0 95 25 30 4.97 0.7063
WVFGRD96 85.0 95 25 30 4.97 0.7014
WVFGRD96 86.0 95 25 30 4.97 0.6973
WVFGRD96 87.0 95 25 30 4.97 0.6928
WVFGRD96 88.0 100 25 30 4.98 0.6876
WVFGRD96 89.0 100 25 30 4.98 0.6835
WVFGRD96 90.0 100 25 30 4.98 0.6781
WVFGRD96 91.0 100 25 30 4.98 0.6738
WVFGRD96 92.0 100 25 30 4.99 0.6687
WVFGRD96 93.0 100 25 30 4.99 0.6639
WVFGRD96 94.0 100 25 30 4.99 0.6587
WVFGRD96 95.0 100 25 30 4.99 0.6536
WVFGRD96 96.0 100 25 30 4.99 0.6483
WVFGRD96 97.0 100 25 30 4.99 0.6434
WVFGRD96 98.0 100 25 30 5.00 0.6381
WVFGRD96 99.0 100 25 30 5.00 0.6332
The best solution is
WVFGRD96 69.0 85 25 25 4.92 0.7452
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 0.015 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 Aug 19 14:10:13 MDT 2009
Last Changed 2009/08/19