2008/08/05 19:48:10 61.4460 -141.5240 5.0 3.60 Alaska
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2008/08/05 19:48:10 61.4460 -141.5240 5.0 3.60 Alaska
Best Fitting Double Couple
Mo = 8.91e+21 dyne-cm
Mw = 3.90
Z = 10 km
Plane Strike Dip Rake
NP1 120 70 142
NP2 225 55 25
Principal Axes:
Axis Value Plunge Azimuth
T 8.91e+21 41 77
N 0.00e+00 48 276
P -8.91e+21 9 175
Moment Tensor: (dyne-cm)
Component Value
Mxx -8.39e+21
Mxy 1.77e+21
Mxz 2.37e+21
Myy 4.85e+21
Myz 4.19e+21
Mzz 3.54e+21
--------------
----------------------
----------------------------
---------------------#########
------------------################
----------------####################
##-------------#######################
####----------##########################
######------################### ######
#########---#################### T #######
##########-##################### #######
#########----#############################
########--------##########################
#######-----------######################
######---------------###################
####---------------------#############
###---------------------------######
##--------------------------------
------------------------------
----------------------------
----------- --------
------- P ----
Harvard Convention
Moment Tensor:
R T F
3.54e+21 2.37e+21 -4.19e+21
2.37e+21 -8.39e+21 -1.77e+21
-4.19e+21 -1.77e+21 4.85e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080805194810/index.html
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STK = 225
DIP = 55
RAKE = 25
MW = 3.90
HS = 10.0
The waveform inversion is preferred.
The following compares this source inversion to others
SLU Moment Tensor Solution
2008/08/05 19:48:10 61.4460 -141.5240 5.0 3.60 Alaska
Best Fitting Double Couple
Mo = 8.91e+21 dyne-cm
Mw = 3.90
Z = 10 km
Plane Strike Dip Rake
NP1 120 70 142
NP2 225 55 25
Principal Axes:
Axis Value Plunge Azimuth
T 8.91e+21 41 77
N 0.00e+00 48 276
P -8.91e+21 9 175
Moment Tensor: (dyne-cm)
Component Value
Mxx -8.39e+21
Mxy 1.77e+21
Mxz 2.37e+21
Myy 4.85e+21
Myz 4.19e+21
Mzz 3.54e+21
--------------
----------------------
----------------------------
---------------------#########
------------------################
----------------####################
##-------------#######################
####----------##########################
######------################### ######
#########---#################### T #######
##########-##################### #######
#########----#############################
########--------##########################
#######-----------######################
######---------------###################
####---------------------#############
###---------------------------######
##--------------------------------
------------------------------
----------------------------
----------- --------
------- P ----
Harvard Convention
Moment Tensor:
R T F
3.54e+21 2.37e+21 -4.19e+21
2.37e+21 -8.39e+21 -1.77e+21
-4.19e+21 -1.77e+21 4.85e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080805194810/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 40 85 0 3.69 0.5135
WVFGRD96 1.0 40 70 0 3.74 0.5297
WVFGRD96 2.0 40 70 -10 3.79 0.5327
WVFGRD96 3.0 210 60 -30 3.84 0.5199
WVFGRD96 4.0 225 45 20 3.84 0.5197
WVFGRD96 5.0 225 45 20 3.84 0.5464
WVFGRD96 6.0 225 55 25 3.86 0.5724
WVFGRD96 7.0 225 55 25 3.86 0.5955
WVFGRD96 8.0 225 55 25 3.87 0.6087
WVFGRD96 9.0 220 60 20 3.88 0.6165
WVFGRD96 10.0 225 55 25 3.90 0.6184
WVFGRD96 11.0 225 60 20 3.92 0.6177
WVFGRD96 12.0 225 60 20 3.92 0.6133
WVFGRD96 13.0 220 60 20 3.93 0.6046
WVFGRD96 14.0 220 60 20 3.93 0.5921
WVFGRD96 15.0 220 65 20 3.95 0.5781
WVFGRD96 16.0 220 65 20 3.95 0.5615
WVFGRD96 17.0 220 65 20 3.96 0.5427
WVFGRD96 18.0 220 65 20 3.96 0.5217
WVFGRD96 19.0 215 55 -20 3.96 0.4981
WVFGRD96 20.0 210 45 -30 3.97 0.4819
WVFGRD96 21.0 210 45 -30 3.97 0.4633
WVFGRD96 22.0 210 45 -30 3.97 0.4434
WVFGRD96 23.0 210 40 -30 3.97 0.4238
WVFGRD96 24.0 210 40 -30 3.98 0.4042
WVFGRD96 25.0 30 55 -35 3.99 0.3890
WVFGRD96 26.0 30 55 -35 3.99 0.3727
WVFGRD96 27.0 30 50 -35 3.99 0.3562
WVFGRD96 28.0 30 50 -40 3.99 0.3403
WVFGRD96 29.0 30 50 -40 3.99 0.3242
The best solution is
WVFGRD96 10.0 225 55 25 3.90 0.6184
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=Thu Aug 7 11:42:47 CDT 2008