The ANSS event ID is ak008epgg4c9 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak008epgg4c9/executive.
2008/11/15 14:23:34 66.393 -157.572 8.1 4.7 Alaska
USGS/SLU Moment Tensor Solution ENS 2008/11/15 14:23:34:0 66.39 -157.57 8.1 4.7 Alaska Stations used: AK.BPAW AK.COLD AK.MCK AK.PPLA AK.TNA AK.TRF AT.SVW2 IU.COLA Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 1.04e+23 dyne-cm Mw = 4.61 Z = 17 km Plane Strike Dip Rake NP1 350 85 -20 NP2 82 70 -175 Principal Axes: Axis Value Plunge Azimuth T 1.04e+23 10 37 N 0.00e+00 69 157 P -1.04e+23 18 304 Moment Tensor: (dyne-cm) Component Value Mxx 3.33e+22 Mxy 9.21e+22 Mxz -2.29e+21 Myy -2.72e+22 Myz 3.58e+22 Mzz -6.15e+21 ----########## --------############ ------------########### T ## -------------########### ### - ------------################## -- P ------------################### --- -------------################### --------------------#################### --------------------#################### ----------------------###################- ----------------------#################--- ----------------------##############------ -----------------------########----------- ####------------------#----------------- ######################------------------ ######################---------------- #####################--------------- ####################-------------- ##################------------ #################----------- ###############------- ###########--- Global CMT Convention Moment Tensor: R T P -6.15e+21 -2.29e+21 -3.58e+22 -2.29e+21 3.33e+22 -9.21e+22 -3.58e+22 -9.21e+22 -2.72e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081115142334/index.html |
STK = 350 DIP = 85 RAKE = -20 MW = 4.61 HS = 17.0
The NDK file is 20081115142334.ndk The waveform inversion is preferred.
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
USGS/SLU Moment Tensor Solution ENS 2008/11/15 14:23:34:0 66.39 -157.57 8.1 4.7 Alaska Stations used: AK.BPAW AK.COLD AK.MCK AK.PPLA AK.TNA AK.TRF AT.SVW2 IU.COLA Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 1.04e+23 dyne-cm Mw = 4.61 Z = 17 km Plane Strike Dip Rake NP1 350 85 -20 NP2 82 70 -175 Principal Axes: Axis Value Plunge Azimuth T 1.04e+23 10 37 N 0.00e+00 69 157 P -1.04e+23 18 304 Moment Tensor: (dyne-cm) Component Value Mxx 3.33e+22 Mxy 9.21e+22 Mxz -2.29e+21 Myy -2.72e+22 Myz 3.58e+22 Mzz -6.15e+21 ----########## --------############ ------------########### T ## -------------########### ### - ------------################## -- P ------------################### --- -------------################### --------------------#################### --------------------#################### ----------------------###################- ----------------------#################--- ----------------------##############------ -----------------------########----------- ####------------------#----------------- ######################------------------ ######################---------------- #####################--------------- ####################-------------- ##################------------ #################----------- ###############------- ###########--- Global CMT Convention Moment Tensor: R T P -6.15e+21 -2.29e+21 -3.58e+22 -2.29e+21 3.33e+22 -9.21e+22 -3.58e+22 -9.21e+22 -2.72e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081115142334/index.html |
Moment tensor inversion summary for event 2008/11/15 14:23 This is a fully automatic solution. It has not yet been reviewed by a seismologist.2008/11/15 14:23 Date 2008/11/15 Region: North-central Region of Alaska Mw=4.7 Centroid Location: Time 14:23; Lat. 66.29N; Lon. 203.09W; Depth 20 km Best Double Couple: Plane 1: strike = 155; dip = 58; rake = -42 Plane 2: strike = 270; dip = 56; rake = -140 Moment Tensor: Mo = 1.23456e+23 dyn-cm Mxx = 835.143; Mxy = 967.439; Mxz = 107.083 Myy = -52.712; Myz = -262.199; Mzz = -782.432 Principal Axes: T: value = 84.000; azimuth = 213; plunge = 1 N: value = 80.000; azimuth = 121; plunge = 51 P: value = 78.000; azimuth = 304; plunge = 39 |
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Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 165 65 -25 4.29 0.4312 WVFGRD96 1.0 165 70 -30 4.32 0.4472 WVFGRD96 2.0 165 70 -30 4.39 0.5197 WVFGRD96 3.0 165 70 -30 4.42 0.5430 WVFGRD96 4.0 165 75 -30 4.43 0.5624 WVFGRD96 5.0 170 85 -25 4.44 0.5851 WVFGRD96 6.0 170 85 -25 4.46 0.6080 WVFGRD96 7.0 170 85 -20 4.48 0.6307 WVFGRD96 8.0 170 85 -25 4.51 0.6524 WVFGRD96 9.0 170 85 -25 4.53 0.6688 WVFGRD96 10.0 170 85 -20 4.54 0.6818 WVFGRD96 11.0 350 90 20 4.55 0.6892 WVFGRD96 12.0 170 85 -20 4.56 0.6949 WVFGRD96 13.0 350 90 20 4.57 0.6934 WVFGRD96 14.0 350 80 -20 4.58 0.6932 WVFGRD96 15.0 350 80 -20 4.59 0.6991 WVFGRD96 16.0 350 80 -20 4.60 0.7026 WVFGRD96 17.0 350 85 -20 4.61 0.7045 WVFGRD96 18.0 170 90 20 4.61 0.7026 WVFGRD96 19.0 170 90 15 4.63 0.7013 WVFGRD96 20.0 170 90 15 4.64 0.7001 WVFGRD96 21.0 350 85 -15 4.64 0.6968 WVFGRD96 22.0 170 90 15 4.65 0.6916 WVFGRD96 23.0 170 90 15 4.66 0.6850 WVFGRD96 24.0 350 90 -15 4.66 0.6770 WVFGRD96 25.0 350 90 -15 4.67 0.6677 WVFGRD96 26.0 170 85 15 4.68 0.6586 WVFGRD96 27.0 170 85 10 4.69 0.6481 WVFGRD96 28.0 170 85 10 4.69 0.6372 WVFGRD96 29.0 170 85 10 4.70 0.6255
The best solution is
WVFGRD96 17.0 350 85 -20 4.61 0.7045
The mechanism corresponding 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 0.06 n 3
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Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
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