2010/09/05 10:41:26 63.359 -145.100 4.0 4.20 Alaska
The original location was
2010/09/05 10:41:27 63.465 -145.041 14.1
and the revised location is given above.
the waveform inversion performed using the original location required significat time shifts for alignment. Such time shifts are indicative of the use of an incorrect velocity model, or a poor location. To examine this the time shifts can be decomposed into a common offset and an azimuthal component. For this event, the time shifts on the vertical and radial compoents were converted to a kilometer difference in epicentral distance by assuming a constant group velocity of 3.1 km/s for the Rayleigh wave pulse; a group velocity of 3.5 km/s was assumed for the Love wave pulse on the transverse component. An obvious azimuthal effect was observed, as shown in the next figure.
A positive time shift in the comparison of observed and predicted waveforms, as seen below, means that the predicted trace must be moved later in time which would occur if the actual velocity model is slower than assumed, if the origin time is actually later, or if the epicentral distance is greater than assumed. The previous figure assumes that the shift is due to a mislocation. Thus a positive time shift means that the assumed epicentral distance is too small, ro that the epicenter must be moved away from the station.
After noticing this revised source coordinates were published that move the epicenter 12 km in a direction of 194 degrees and made the origin time 1 second earlier. We reran the source inversion using the new source coordinates. The correspongind time shift plot is show in the next figure.
h
It is obvious that the change in source coordinate reduced the time shifts and inferred epicentral distance shifts. The time shift are small at short distance, but increase for the distant stations to the south. The shape of the waveforms and the large time delay required may indicate the need for a sloweer velocity model for paths to the southern station.
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution
ENS 2010/09/05 10:41:26:0 63.36 -145.10 4.0 4.2 Alaska
Stations used:
AK.BAL AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CHUM AK.CNP
AK.COLD AK.CRQ AK.CTG AK.DHY AK.DOT AK.EYAK AK.FID AK.FYU
AK.GLI AK.HDA AK.MCK AK.MLY AK.PAX AK.PIN AK.PPLA AK.RAG
AK.RC01 AK.RIDG AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AK.TGL
AK.TRF AK.WRH AT.PMR CN.DAWY IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.04e+22 dyne-cm
Mw = 4.14
Z = 13 km
Plane Strike Dip Rake
NP1 137 57 130
NP2 260 50 45
Principal Axes:
Axis Value Plunge Azimuth
T 2.04e+22 57 104
N 0.00e+00 33 293
P -2.04e+22 4 200
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.76e+22
Mxy -7.96e+21
Mxz -8.57e+20
Myy 3.35e+21
Myz 9.57e+21
Mzz 1.42e+22
--------------
----------------------
----------------------------
#-----------------------------
###-------------------------------
####--------------#######-----------
#####-------#####################-----
#######--############################---
######-################################-
#####----#################################
###-------################################
##---------################# ###########
#------------############### T ###########
-------------############## ##########
---------------#########################
----------------######################
-----------------###################
-------------------###############
---------------------#########
----------------------------
--- ----------------
P ------------
Global CMT Convention Moment Tensor:
R T P
1.42e+22 -8.57e+20 -9.57e+21
-8.57e+20 -1.76e+22 7.96e+21
-9.57e+21 7.96e+21 3.35e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100905104126/index.html
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STK = 260
DIP = 50
RAKE = 45
MW = 4.14
HS = 13.0
The waveform inversion is preferred.
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution
ENS 2010/09/05 10:41:26:0 63.36 -145.10 4.0 4.2 Alaska
Stations used:
AK.BAL AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CHUM AK.CNP
AK.COLD AK.CRQ AK.CTG AK.DHY AK.DOT AK.EYAK AK.FID AK.FYU
AK.GLI AK.HDA AK.MCK AK.MLY AK.PAX AK.PIN AK.PPLA AK.RAG
AK.RC01 AK.RIDG AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AK.TGL
AK.TRF AK.WRH AT.PMR CN.DAWY IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.04e+22 dyne-cm
Mw = 4.14
Z = 13 km
Plane Strike Dip Rake
NP1 137 57 130
NP2 260 50 45
Principal Axes:
Axis Value Plunge Azimuth
T 2.04e+22 57 104
N 0.00e+00 33 293
P -2.04e+22 4 200
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.76e+22
Mxy -7.96e+21
Mxz -8.57e+20
Myy 3.35e+21
Myz 9.57e+21
Mzz 1.42e+22
--------------
----------------------
----------------------------
#-----------------------------
###-------------------------------
####--------------#######-----------
#####-------#####################-----
#######--############################---
######-################################-
#####----#################################
###-------################################
##---------################# ###########
#------------############### T ###########
-------------############## ##########
---------------#########################
----------------######################
-----------------###################
-------------------###############
---------------------#########
----------------------------
--- ----------------
P ------------
Global CMT Convention Moment Tensor:
R T P
1.42e+22 -8.57e+20 -9.57e+21
-8.57e+20 -1.76e+22 7.96e+21
-9.57e+21 7.96e+21 3.35e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100905104126/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 13.0 260 50 45 4.14 0.5483
The best solution is
WVFGRD96 13.0 260 50 45 4.14 0.5483
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 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 0.10 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 Sep 7 11:04:20 CDT 2010