2008/03/11 19:42:25 69.824 -146.655 10.0 4.0 Alaska
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2008/03/11 19:42:25 69.824 -146.655 10.0 4.0 Alaska
Best Fitting Double Couple
Mo = 3.09e+22 dyne-cm
Mw = 4.26
Z = 18 km
Plane Strike Dip Rake
NP1 269 76 154
NP2 5 65 15
Principal Axes:
Axis Value Plunge Azimuth
T 3.09e+22 28 225
N 0.00e+00 61 63
P -3.09e+22 8 319
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.74e+21
Mxy 2.72e+22
Mxz -1.21e+22
Myy -1.38e+21
Myz -6.22e+21
Mzz 6.13e+21
----------####
---------------#######
-----------------#########
P -----------------##########
-- ------------------###########
------------------------############
-------------------------#############
--------------------------##############
--------------------------##############
------------################-----#########
----#######################--------------#
###########################---------------
###########################---------------
##########################--------------
#########################---------------
####### ##############--------------
###### T #############--------------
##### #############-------------
##################------------
################------------
############----------
######--------
Harvard Convention
Moment Tensor:
R T F
6.13e+21 -1.21e+22 6.22e+21
-1.21e+22 -4.74e+21 -2.72e+22
6.22e+21 -2.72e+22 -1.38e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080311194225/index.html
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STK = 5
DIP = 65
RAKE = 15
MW = 4.26
HS = 18.0
The waveform inversion is preferred. the surface-wave datga were important since they showed that the data required a different dispersion model than the assumed CUS model. The WUS model provides a better fit to the regional dispersion, and thus also provides a somewhat better fit to the waveforms. The two techniques provide similar answers, but in this case the waveforms inversion provides a better depth constraint.
The following compares this source inversion to others
SLU Moment Tensor Solution
2008/03/11 19:42:25 69.824 -146.655 10.0 4.0 Alaska
Best Fitting Double Couple
Mo = 3.09e+22 dyne-cm
Mw = 4.26
Z = 18 km
Plane Strike Dip Rake
NP1 269 76 154
NP2 5 65 15
Principal Axes:
Axis Value Plunge Azimuth
T 3.09e+22 28 225
N 0.00e+00 61 63
P -3.09e+22 8 319
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.74e+21
Mxy 2.72e+22
Mxz -1.21e+22
Myy -1.38e+21
Myz -6.22e+21
Mzz 6.13e+21
----------####
---------------#######
-----------------#########
P -----------------##########
-- ------------------###########
------------------------############
-------------------------#############
--------------------------##############
--------------------------##############
------------################-----#########
----#######################--------------#
###########################---------------
###########################---------------
##########################--------------
#########################---------------
####### ##############--------------
###### T #############--------------
##### #############-------------
##################------------
################------------
############----------
######--------
Harvard Convention
Moment Tensor:
R T F
6.13e+21 -1.21e+22 6.22e+21
-1.21e+22 -4.74e+21 -2.72e+22
6.22e+21 -2.72e+22 -1.38e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080311194225/index.html
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AEIC
http://www.aeic.alaska.edu/moment_tensors/current_year/20080311194229/20080311194229_info.html
Moment tensor inversion summary for event 2008/03/11 19:42
Date: 2008/03/11
Time: 19:42 (UTC)
Region: North Slope Region of Alaska
Mw=4.4
Location:
Lat. 69.5812; Lon. -146.8216; Depth 5 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 1
Best Double Couple:
strike dip rake
Plane 1: 90.5 86.1 -143.7
Plane 2: 357.7 53.8 -4.8
Moment Tensor Parameters:
Mo = 4.01584e+22 dyn-cm
Mxx = 0.18; Mxy = 3.22; Mxz = -2.37
Myy = 0.06; Myz = 0.31; Mzz = -0.25
Principal Axes:
value azimuth plunge
T: 3.94 218.36 21.69
N: 0.16 95.80 53.53
P: -4.09 320.44 27.74
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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 0.05 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 180 90 15 3.96 0.4608
WVFGRD96 1.0 180 90 5 3.97 0.4851
WVFGRD96 2.0 0 85 -5 4.03 0.5432
WVFGRD96 3.0 175 75 -20 4.08 0.5737
WVFGRD96 4.0 355 75 -20 4.11 0.5978
WVFGRD96 5.0 355 80 -15 4.14 0.6047
WVFGRD96 6.0 355 75 -15 4.15 0.6062
WVFGRD96 7.0 175 75 -5 4.17 0.6014
WVFGRD96 8.0 355 65 -10 4.20 0.6127
WVFGRD96 9.0 -5 60 -10 4.21 0.6102
WVFGRD96 10.0 0 60 0 4.20 0.6162
WVFGRD96 11.0 5 65 20 4.21 0.6240
WVFGRD96 12.0 5 65 20 4.22 0.6333
WVFGRD96 13.0 5 65 20 4.23 0.6414
WVFGRD96 14.0 5 65 20 4.23 0.6483
WVFGRD96 15.0 5 65 15 4.23 0.6535
WVFGRD96 16.0 5 65 15 4.24 0.6576
WVFGRD96 17.0 5 65 15 4.25 0.6602
WVFGRD96 18.0 5 65 15 4.26 0.6613
WVFGRD96 19.0 5 65 15 4.26 0.6610
WVFGRD96 20.0 5 70 15 4.27 0.6605
WVFGRD96 21.0 5 65 15 4.28 0.6586
WVFGRD96 22.0 5 70 15 4.29 0.6562
WVFGRD96 23.0 5 70 15 4.30 0.6529
WVFGRD96 24.0 5 70 15 4.30 0.6491
WVFGRD96 25.0 5 70 15 4.31 0.6444
WVFGRD96 26.0 5 70 15 4.32 0.6388
WVFGRD96 27.0 5 70 15 4.32 0.6327
WVFGRD96 28.0 5 70 10 4.33 0.6266
WVFGRD96 29.0 5 70 10 4.33 0.6201
The best solution is
WVFGRD96 18.0 5 65 15 4.26 0.6613
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 0.05 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. |
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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The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 94.99
DIP= 75.00
RAKE= -155.00
OR
STK= 358.10
DIP= 65.90
RAKE= -16.48
DEPTH = 18.0 km
Mw = 4.32
Best Fit 0.8472 - P-T axis plot gives solutions with FIT greater than FIT90
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion COLD 208 324 -12345 INK 102 549 -12345 EGAK 155 610 -12345 BPAW 199 665 -12345 MCK 189 687 -12345 DAWY 150 715 -12345 KTH 197 724 -12345 TRF 194 728 -12345 DOT 171 776 -12345 PPLA 200 783 -12345
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.
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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| 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. |
The distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km) COLD 208 324 INK 102 549 EGAK 155 610 BPAW 198 665 MCK 190 687 CHUM 203 707 DAWY 150 715 KTH 197 724 TRF 194 728 PAX 176 766 PPLA 200 810 SAW 186 897 RC01 190 984 WHY 146 1156 PNL 160 1182 PLBC 152 1252 ROMN 102 1371 CTLN 100 1439 DLBC 140 1495 JERN 89 1512 FNBB 126 1649 RES 49 1776 RUBB 146 1914 BMBC 129 1949 DIB 151 1988 MOBC 150 2004 BBB 144 2186 PHC 145 2356 ILON 60 2405 NLWA 142 2789 NEW 131 2877 HAWA 136 3003 MSO 128 3118 DGMT 114 3275 HLID 131 3440 RSSD 118 3713 EYMN 101 3792
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:
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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 0.05 n 3
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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 WUS.REG 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
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Tue Mar 11 19:34:01 CDT 2008