2007/11/03 19:55:05 66.05 -142.17 10.0 4.7 Alaska
USGS Felt map for this earthquake
SLU Moment Tensor Solution 2007/11/03 19:55:05 66.05 -142.17 10.0 4.7 Alaska Best Fitting Double Couple Mo = 8.91e+21 dyne-cm Mw = 3.90 Z = 13 km Plane Strike Dip Rake NP1 330 90 -160 NP2 240 70 0 Principal Axes: Axis Value Plunge Azimuth T 8.91e+21 14 103 N 0.00e+00 70 330 P -8.91e+21 14 197 Moment Tensor: (dyne-cm) Component Value Mxx -7.25e+21 Mxy -4.19e+21 Mxz 1.52e+21 Myy 7.25e+21 Myz 2.64e+21 Mzz 0.00e+00 -------------- #--------------------- #####----------------------- #######----------------------- ##########------------------------ ############-------------------##### ##############-----------############# ################-----################### #################-###################### ################---####################### #############-------###################### ###########-----------#################### #########--------------############## ## ######-----------------############# T # ####--------------------############ # ##----------------------############## ------------------------############ ------------------------########## -----------------------####### ------- -------------##### ---- P --------------# ----------- Harvard Convention Moment Tensor: R T F 0.00e+00 1.52e+21 -2.64e+21 1.52e+21 -7.25e+21 4.19e+21 -2.64e+21 4.19e+21 7.25e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20071103195505/index.html |
STK = 240 DIP = 70 RAKE = 0 MW = 3.90 HS = 13
The mechanism is strike-slip but depth control is poor. The moment magnitude could be smaller is the earthquake is shallower.
The following compares this source inversion to others
SLU Moment Tensor Solution 2007/11/03 19:55:05 66.05 -142.17 10.0 4.7 Alaska Best Fitting Double Couple Mo = 8.91e+21 dyne-cm Mw = 3.90 Z = 13 km Plane Strike Dip Rake NP1 330 90 -160 NP2 240 70 0 Principal Axes: Axis Value Plunge Azimuth T 8.91e+21 14 103 N 0.00e+00 70 330 P -8.91e+21 14 197 Moment Tensor: (dyne-cm) Component Value Mxx -7.25e+21 Mxy -4.19e+21 Mxz 1.52e+21 Myy 7.25e+21 Myz 2.64e+21 Mzz 0.00e+00 -------------- #--------------------- #####----------------------- #######----------------------- ##########------------------------ ############-------------------##### ##############-----------############# ################-----################### #################-###################### ################---####################### #############-------###################### ###########-----------#################### #########--------------############## ## ######-----------------############# T # ####--------------------############ # ##----------------------############## ------------------------############ ------------------------########## -----------------------####### ------- -------------##### ---- P --------------# ----------- Harvard Convention Moment Tensor: R T F 0.00e+00 1.52e+21 -2.64e+21 1.52e+21 -7.25e+21 4.19e+21 -2.64e+21 4.19e+21 7.25e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20071103195505/index.html |
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.
|
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 0.01 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 235 55 -15 3.81 0.4411 WVFGRD96 1.0 235 55 -15 3.82 0.4523 WVFGRD96 2.0 240 75 -5 3.78 0.4665 WVFGRD96 3.0 240 75 0 3.79 0.4756 WVFGRD96 4.0 240 60 5 3.83 0.4815 WVFGRD96 5.0 240 60 10 3.84 0.4875 WVFGRD96 6.0 240 60 10 3.85 0.4912 WVFGRD96 7.0 240 60 10 3.86 0.4932 WVFGRD96 8.0 235 65 5 3.86 0.4952 WVFGRD96 9.0 240 65 5 3.86 0.4984 WVFGRD96 10.0 240 65 5 3.88 0.5015 WVFGRD96 11.0 240 65 0 3.88 0.5037 WVFGRD96 12.0 240 70 0 3.89 0.5052 WVFGRD96 13.0 240 70 0 3.90 0.5056 WVFGRD96 14.0 240 70 0 3.91 0.5044 WVFGRD96 15.0 240 80 5 3.91 0.5043 WVFGRD96 16.0 240 80 5 3.92 0.5025 WVFGRD96 17.0 240 75 5 3.93 0.5000 WVFGRD96 18.0 240 75 5 3.94 0.4983 WVFGRD96 19.0 240 75 5 3.95 0.4930 WVFGRD96 20.0 235 70 5 3.98 0.4890 WVFGRD96 21.0 240 70 5 3.98 0.4840 WVFGRD96 22.0 235 75 0 3.99 0.4782 WVFGRD96 23.0 235 70 0 4.00 0.4726 WVFGRD96 24.0 235 70 0 4.01 0.4662 WVFGRD96 25.0 235 70 0 4.01 0.4591 WVFGRD96 26.0 235 65 0 4.03 0.4525 WVFGRD96 27.0 235 65 0 4.03 0.4456 WVFGRD96 28.0 235 70 0 4.04 0.4387 WVFGRD96 29.0 235 65 0 4.05 0.4327
The best solution is
WVFGRD96 13.0 240 70 0 3.90 0.5056
The mechanism correspond to the best fit is
|
The best fit as a function of depth is given in the following figure:
|
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 0.01 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
|
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.
|
The surface-wave determined focal mechanism is shown here.
NODAL PLANES STK= 314.99 DIP= 80.00 RAKE= -165.00 OR STK= 222.32 DIP= 75.23 RAKE= -10.35 DEPTH = 11.0 km Mw = 3.94 Best Fit 0.8071 - P-T axis plot gives solutions with FIT greater than FIT90
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion EGAK 161 149 -12345 DOT 199 282 -12345 PAX 206 378 -12345 COLD 294 379 -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.
|
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. |
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) EGAK 161 149 DAWY 148 257 DOT 199 282 PAX 206 378 COLD 294 379 MCK 234 411 INK 52 450 BPAW 246 468 TRF 237 482 KTH 240 500 SAW 215 561 PPLA 238 595 PNL 168 725 DLBC 138 1054 WRAK 149 1193
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 0.01 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
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 Nov 6 15:02:05 CST 2007