Location
2008/01/03 13:53:10 66.317 -142.552 15.0 4.0 Alaska
Arrival Times (from USGS)
Arrival time list
Felt Map
USGS Felt map for this earthquake
USGS Felt reports page for
Focal Mechanism
| SLU | AEIC |
SLU Moment Tensor Solution
2008/01/03 13:53:10 66.317 -142.552 15.0 4.0 Alaska
Best Fitting Double Couple
Mo = 7.24e+21 dyne-cm
Mw = 3.84
Z = 10 km
Plane Strike Dip Rake
NP1 50 80 15
NP2 317 75 170
Principal Axes:
Axis Value Plunge Azimuth
T 7.24e+21 18 274
N 0.00e+00 72 83
P -7.24e+21 3 183
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.16e+21
Mxy -8.81e+20
Mxz 5.69e+20
Myy 6.52e+21
Myz -2.06e+21
Mzz 6.41e+20
--------------
----------------------
----------------------------
####--------------------------
#########-----------------------##
#############-------------------####
################---------------#######
###################------------#########
#####################--------###########
## ###################----##############
## T ####################-################
## ###################---###############
######################------##############
##################-----------###########
################--------------##########
############------------------########
########----------------------######
####--------------------------####
-----------------------------#
----------------------------
-------- -----------
---- P -------
Harvard Convention
Moment Tensor:
R T F
6.41e+20 5.69e+20 2.06e+21
5.69e+20 -7.16e+21 8.81e+20
2.06e+21 8.81e+20 6.52e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080103135310/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 = 50
DIP = 80
RAKE = 15
MW = 3.84
HS = 10
The waveform inversion is preferred. The surface-wave solution is compatible.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
SLU Moment Tensor Solution
2008/01/03 13:53:10 66.317 -142.552 15.0 4.0 Alaska
Best Fitting Double Couple
Mo = 7.24e+21 dyne-cm
Mw = 3.84
Z = 10 km
Plane Strike Dip Rake
NP1 50 80 15
NP2 317 75 170
Principal Axes:
Axis Value Plunge Azimuth
T 7.24e+21 18 274
N 0.00e+00 72 83
P -7.24e+21 3 183
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.16e+21
Mxy -8.81e+20
Mxz 5.69e+20
Myy 6.52e+21
Myz -2.06e+21
Mzz 6.41e+20
--------------
----------------------
----------------------------
####--------------------------
#########-----------------------##
#############-------------------####
################---------------#######
###################------------#########
#####################--------###########
## ###################----##############
## T ####################-################
## ###################---###############
######################------##############
##################-----------###########
################--------------##########
############------------------########
########----------------------######
####--------------------------####
-----------------------------#
----------------------------
-------- -----------
---- P -------
Harvard Convention
Moment Tensor:
R T F
6.41e+20 5.69e+20 2.06e+21
5.69e+20 -7.16e+21 8.81e+20
2.06e+21 8.81e+20 6.52e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080103135310/index.html
|
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 240 65 15 3.71 0.5151
WVFGRD96 1.0 230 80 0 3.71 0.5333
WVFGRD96 2.0 230 75 5 3.75 0.5592
WVFGRD96 3.0 225 85 0 3.77 0.5756
WVFGRD96 4.0 225 80 0 3.79 0.5819
WVFGRD96 5.0 50 80 20 3.79 0.5813
WVFGRD96 6.0 50 80 20 3.81 0.5901
WVFGRD96 7.0 50 80 20 3.81 0.5957
WVFGRD96 8.0 50 80 20 3.82 0.5986
WVFGRD96 9.0 50 80 15 3.83 0.6014
WVFGRD96 10.0 50 80 15 3.84 0.6024
WVFGRD96 11.0 230 80 -15 3.84 0.6009
WVFGRD96 12.0 230 80 -10 3.85 0.6008
WVFGRD96 13.0 230 80 -10 3.86 0.5994
WVFGRD96 14.0 230 80 -10 3.87 0.5968
WVFGRD96 15.0 230 80 -10 3.87 0.5930
WVFGRD96 16.0 230 80 -10 3.88 0.5880
WVFGRD96 17.0 230 80 -10 3.89 0.5817
WVFGRD96 18.0 230 80 -10 3.90 0.5744
WVFGRD96 19.0 140 80 -10 3.91 0.5669
WVFGRD96 20.0 140 80 -15 3.92 0.5636
WVFGRD96 21.0 140 80 -15 3.93 0.5628
WVFGRD96 22.0 140 80 -10 3.94 0.5612
WVFGRD96 23.0 140 80 -15 3.95 0.5610
WVFGRD96 24.0 140 80 -15 3.96 0.5600
WVFGRD96 25.0 140 80 -15 3.97 0.5597
WVFGRD96 26.0 140 80 -15 3.98 0.5582
WVFGRD96 27.0 140 75 -15 3.99 0.5574
WVFGRD96 28.0 140 75 -15 4.00 0.5557
WVFGRD96 29.0 140 75 -15 4.01 0.5539
The best solution is
WVFGRD96 10.0 50 80 15 3.84 0.6024
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
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Figure 3. Waveform comparison for depth of 8 km
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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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Surface-Wave Focal Mechanism
The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 125.00
DIP= 90.00
RAKE= -160.00
OR
STK= 35.00
DIP= 70.00
RAKE= 0.00
DEPTH = 7.0 km
Mw = 3.97
Best Fit 0.8574 - P-T axis plot gives solutions with FIT greater than FIT90
First motion data
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion
EGAK 159 183 -12345
COLA 239 292 -12345
COLD 290 351 -12345
MCK 229 416 -12345
TRF 232 486 -12345
KTH 236 501 -12345
WHY 145 736 -12345
Surface-wave analysis
Surface wave analysis was performed using codes from
Computer Programs in Seismology, specifically the
multiple filter analysis program do_mft and the surface-wave
radiation pattern search program srfgrd96.
Data preparation
Digital data were collected, instrument response removed and traces converted
to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.
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Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection
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Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the
possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here.
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns.
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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled.
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Love-wave radiation patterns
Rayleigh-wave radiation patterns
Broadband station distribution
The distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km)
EGAK 159 183
COLA 239 292
COLD 290 351
MCK 229 416
BPAW 242 465
TRF 232 486
KTH 236 501
EYAK 195 662
WHY 145 736
TNA 278 1150
WRAK 149 1227
FNBB 120 1291
RES 41 1932
SHB 141 2147
EDM 118 2159
Waveform comparison for this mechanism
Since the analysis of the surface-wave radiation patterns uses only spectral
amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were
emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute
forward synthetics and compare the observed and predicted waveforms.
The fits to the waveforms with the given mechanism are show below:
This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the
observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:
hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
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.
Appendix A
Spectra fit plots to each station
Velocity Model
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
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=Thu Jan 3 11:03:25 CST 2008
Last Changed 2008/01/03
