Location
2010/09/20 21:24:24 61.127 -150.209 40.0 4.90 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 2010/09/20 21:24:24:0 61.13 -150.21 40.0 4.9 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CNP AK.CRQ AK.DHY AK.DIV AK.EYAK
AK.FID AK.GLI AK.PAX AK.RAG AK.RC01 AK.RND AK.SAW AK.SCM
AK.SKN AK.SSN AK.SWD AT.PMR AT.TTA
Filtering commands used:
hp c 0.02 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 2.14e+23 dyne-cm
Mw = 4.82
Z = 49 km
Plane Strike Dip Rake
NP1 185 80 -85
NP2 338 11 -116
Principal Axes:
Axis Value Plunge Azimuth
T 2.14e+23 35 271
N 0.00e+00 5 4
P -2.14e+23 55 101
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.63e+21
Mxy 1.17e+22
Mxz 2.07e+22
Myy 7.55e+22
Myz -1.99e+23
Mzz -7.28e+22
########--####
############-------###
##############-----------###
###############-------------##
################---------------###
#################-----------------##
#################-------------------##
##################-------------------###
##################--------------------##
###### ##########--------------------###
###### T ##########--------- ---------##
###### #########---------- P ---------##
##################---------- ---------##
#################---------------------##
#################---------------------##
################--------------------##
###############--------------------#
##############-------------------#
############-----------------#
###########----------------#
#########------------#
#####---------
Global CMT Convention Moment Tensor:
R T P
-7.28e+22 2.07e+22 1.99e+23
2.07e+22 -2.63e+21 -1.17e+22
1.99e+23 -1.17e+22 7.55e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100920212424/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 = 185
DIP = 80
RAKE = -85
MW = 4.82
HS = 49.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
USGSMT |
AEIC |
USGS/SLU Moment Tensor Solution
ENS 2010/09/20 21:24:24:0 61.13 -150.21 40.0 4.9 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CNP AK.CRQ AK.DHY AK.DIV AK.EYAK
AK.FID AK.GLI AK.PAX AK.RAG AK.RC01 AK.RND AK.SAW AK.SCM
AK.SKN AK.SSN AK.SWD AT.PMR AT.TTA
Filtering commands used:
hp c 0.02 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 2.14e+23 dyne-cm
Mw = 4.82
Z = 49 km
Plane Strike Dip Rake
NP1 185 80 -85
NP2 338 11 -116
Principal Axes:
Axis Value Plunge Azimuth
T 2.14e+23 35 271
N 0.00e+00 5 4
P -2.14e+23 55 101
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.63e+21
Mxy 1.17e+22
Mxz 2.07e+22
Myy 7.55e+22
Myz -1.99e+23
Mzz -7.28e+22
########--####
############-------###
##############-----------###
###############-------------##
################---------------###
#################-----------------##
#################-------------------##
##################-------------------###
##################--------------------##
###### ##########--------------------###
###### T ##########--------- ---------##
###### #########---------- P ---------##
##################---------- ---------##
#################---------------------##
#################---------------------##
################--------------------##
###############--------------------#
##############-------------------#
############-----------------#
###########----------------#
#########------------#
#####---------
Global CMT Convention Moment Tensor:
R T P
-7.28e+22 2.07e+22 1.99e+23
2.07e+22 -2.63e+21 -1.17e+22
1.99e+23 -1.17e+22 7.55e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100920212424/index.html
|
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Moment tensor inversion summary for event 2010/09/20 21:24
Date: 2010/09/20
Time: 21:24 (UTC)
Region: Cook Inlet Region of Alaska
Mw=4.8
Location:
Lat. 61.1293; Lon. -150.2360; Depth 30 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 9
Best Double Couple:
strike dip rake
Plane 1: 181.2 83.9 -93.7
Plane 2: 32.7 7.2 -58.8
Moment Tensor Parameters:
Mo = 1.97165e+23 dyn-cm
Mxx = 0.12; Mxy = -0.13; Mxz = 0.01
Myy = 0.34; Myz = -1.87; Mzz = -0.46
Principal Axes:
value azimuth plunge
T: 1.85 274.60 38.77
N: 0.12 181.61 3.71
P: -1.97 87.02 50.99
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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 0.08 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 20.0 25 85 50 4.41 0.4667
WVFGRD96 21.0 200 90 -50 4.43 0.4786
WVFGRD96 22.0 25 85 50 4.44 0.4931
WVFGRD96 23.0 25 85 50 4.46 0.5058
WVFGRD96 24.0 25 90 50 4.47 0.5178
WVFGRD96 25.0 25 90 55 4.48 0.5304
WVFGRD96 26.0 25 90 55 4.49 0.5421
WVFGRD96 27.0 200 85 -55 4.50 0.5532
WVFGRD96 28.0 190 90 -60 4.51 0.5645
WVFGRD96 29.0 10 90 60 4.53 0.5772
WVFGRD96 30.0 10 90 60 4.54 0.5890
WVFGRD96 31.0 10 90 65 4.55 0.5999
WVFGRD96 32.0 10 90 65 4.56 0.6098
WVFGRD96 33.0 185 85 -65 4.57 0.6247
WVFGRD96 34.0 190 85 -70 4.58 0.6341
WVFGRD96 35.0 185 80 -70 4.58 0.6434
WVFGRD96 36.0 185 80 -70 4.59 0.6524
WVFGRD96 37.0 185 80 -70 4.60 0.6599
WVFGRD96 38.0 185 80 -70 4.60 0.6667
WVFGRD96 39.0 185 80 -70 4.61 0.6703
WVFGRD96 40.0 185 80 -80 4.75 0.6652
WVFGRD96 41.0 185 80 -80 4.76 0.6773
WVFGRD96 42.0 185 80 -80 4.77 0.6868
WVFGRD96 43.0 185 80 -80 4.78 0.6952
WVFGRD96 44.0 185 80 -80 4.78 0.7011
WVFGRD96 45.0 185 80 -80 4.79 0.7071
WVFGRD96 46.0 185 80 -80 4.80 0.7105
WVFGRD96 47.0 185 80 -80 4.80 0.7139
WVFGRD96 48.0 185 80 -85 4.81 0.7159
WVFGRD96 49.0 185 80 -85 4.82 0.7163
WVFGRD96 50.0 -10 10 -105 4.83 0.7153
WVFGRD96 51.0 -10 10 -105 4.83 0.7143
WVFGRD96 52.0 -10 10 -105 4.84 0.7121
WVFGRD96 53.0 -10 10 -105 4.84 0.7092
WVFGRD96 54.0 50 15 -50 4.86 0.7049
WVFGRD96 55.0 50 15 -50 4.87 0.7032
WVFGRD96 56.0 50 15 -50 4.87 0.6999
WVFGRD96 57.0 50 15 -50 4.88 0.6959
WVFGRD96 58.0 50 15 -50 4.88 0.6916
WVFGRD96 59.0 50 15 -50 4.88 0.6856
The best solution is
WVFGRD96 49.0 185 80 -85 4.82 0.7163
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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
hp c 0.02 n 3
lp c 0.08 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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A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
- The origin time and epicentral distance are incorrect
- The velocity model used for the inversion is incorrect
- The velocity model used to define the P-arrival time is not the
same as the velocity model used for the waveform inversion
(assuming that the initial trace alignment is based on the
P arrival time)
Assuming only a mislocation, the time shifts are fit to a functional form:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
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=Mon Sep 20 19:18:40 CDT 2010
Last Changed 2010/09/20