The ANSS event ID is ak008cd72w8v and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak008cd72w8v/executive.
2008/09/25 19:34:07 63.845 -148.744 10.7 3 Alaska
USGS/SLU Moment Tensor Solution ENS 2008/09/25 19:34:07:0 63.85 -148.74 10.7 3.0 Alaska Stations used: AK.BPAW AK.MCK AK.PAX AK.SWD AK.TRF AT.PMR IU.COLA US.EGAK Filtering commands used: hp c 0.02 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 1.06e+22 dyne-cm Mw = 3.95 Z = 15 km Plane Strike Dip Rake NP1 70 65 85 NP2 262 25 101 Principal Axes: Axis Value Plunge Azimuth T 1.06e+22 70 330 N 0.00e+00 5 72 P -1.06e+22 20 164 Moment Tensor: (dyne-cm) Component Value Mxx -7.68e+21 Mxy 1.96e+21 Mxz 6.24e+21 Myy -4.08e+20 Myz -2.69e+21 Mzz 8.08e+21 -------------- ---------------------- --------------##------------ -------##################----- ------########################---- ----##############################-- ---#################################-- ---############# ##################### --############## T ###################-- --############### #################----- -##################################------- -################################--------- ##############################------------ #########################--------------- #####################------------------- ----#######--------------------------- ------------------------------------ ---------------------------------- ------------------ --------- ----------------- P -------- -------------- ----- -------------- Global CMT Convention Moment Tensor: R T P 8.08e+21 6.24e+21 2.69e+21 6.24e+21 -7.68e+21 -1.96e+21 2.69e+21 -1.96e+21 -4.08e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080925193407/index.html |
STK = 70 DIP = 65 RAKE = 85 MW = 3.95 HS = 15.0
The NDK file is 20080925193407.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 2008/09/25 19:34:07:0 63.85 -148.74 10.7 3.0 Alaska Stations used: AK.BPAW AK.MCK AK.PAX AK.SWD AK.TRF AT.PMR IU.COLA US.EGAK Filtering commands used: hp c 0.02 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 1.06e+22 dyne-cm Mw = 3.95 Z = 15 km Plane Strike Dip Rake NP1 70 65 85 NP2 262 25 101 Principal Axes: Axis Value Plunge Azimuth T 1.06e+22 70 330 N 0.00e+00 5 72 P -1.06e+22 20 164 Moment Tensor: (dyne-cm) Component Value Mxx -7.68e+21 Mxy 1.96e+21 Mxz 6.24e+21 Myy -4.08e+20 Myz -2.69e+21 Mzz 8.08e+21 -------------- ---------------------- --------------##------------ -------##################----- ------########################---- ----##############################-- ---#################################-- ---############# ##################### --############## T ###################-- --############### #################----- -##################################------- -################################--------- ##############################------------ #########################--------------- #####################------------------- ----#######--------------------------- ------------------------------------ ---------------------------------- ------------------ --------- ----------------- P -------- -------------- ----- -------------- Global CMT Convention Moment Tensor: R T P 8.08e+21 6.24e+21 2.69e+21 6.24e+21 -7.68e+21 -1.96e+21 2.69e+21 -1.96e+21 -4.08e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080925193407/index.html |
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Moment tensor inversion summary for event 2008/09/25 19:34 Date: 2008/09/25 Time: 19:34 (UTC) Region: Central Region of Alaska Mw=4.0 Location: Lat. 63.8273; Lon. -148.7596; Depth 15 km (Best-fitting depth from moment tensor inversion) Solution quality: good; Number of stations = 3 Best Double Couple: strike dip rake Plane 1: 69.3 63.6 91.1 Plane 2: 246.9 26.4 87.9 Moment Tensor Parameters: Mo = 1.03529e+22 dyn-cm Mxx = -0.69; Mxy = 0.25; Mxz = 0.59 Myy = -0.17; Myz = -0.21; Mzz = 0.86 Principal Axes: value azimuth plunge T: 1.07 341.63 71.38 N: -0.07 248.83 0.94 P: -1.00 158.51 18.59 |
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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 0.05 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 255 65 -10 3.82 0.4392 WVFGRD96 1.0 255 55 -5 3.88 0.4413 WVFGRD96 2.0 250 45 -5 3.95 0.4198 WVFGRD96 3.0 235 35 -5 3.96 0.4074 WVFGRD96 4.0 215 20 -20 3.93 0.4488 WVFGRD96 5.0 210 20 -20 3.90 0.4902 WVFGRD96 6.0 185 20 15 3.95 0.5285 WVFGRD96 7.0 195 20 25 3.92 0.5589 WVFGRD96 8.0 205 20 40 3.92 0.5801 WVFGRD96 9.0 230 20 65 3.91 0.6020 WVFGRD96 10.0 230 20 70 3.95 0.6155 WVFGRD96 11.0 240 20 80 3.95 0.6271 WVFGRD96 12.0 250 25 90 3.95 0.6358 WVFGRD96 13.0 70 65 90 3.95 0.6441 WVFGRD96 14.0 70 65 85 3.96 0.6448 WVFGRD96 15.0 70 65 85 3.95 0.6475 WVFGRD96 16.0 255 25 95 3.94 0.6459 WVFGRD96 17.0 70 65 85 3.95 0.6443 WVFGRD96 18.0 70 65 90 3.94 0.6425 WVFGRD96 19.0 260 25 100 3.95 0.6375 WVFGRD96 20.0 70 65 90 3.97 0.6342 WVFGRD96 21.0 255 25 95 3.97 0.6301 WVFGRD96 22.0 70 65 90 3.97 0.6264 WVFGRD96 23.0 70 65 95 3.97 0.6205 WVFGRD96 24.0 245 25 85 3.97 0.6150 WVFGRD96 25.0 235 25 75 3.97 0.6088 WVFGRD96 26.0 240 25 80 3.98 0.6031 WVFGRD96 27.0 235 30 80 3.98 0.5973 WVFGRD96 28.0 225 30 70 3.98 0.5913 WVFGRD96 29.0 235 30 80 3.99 0.5849 WVFGRD96 30.0 225 30 70 3.99 0.5786 WVFGRD96 31.0 235 30 75 3.99 0.5721 WVFGRD96 32.0 235 30 75 4.00 0.5656 WVFGRD96 33.0 235 30 75 4.00 0.5594 WVFGRD96 34.0 235 30 75 4.01 0.5525 WVFGRD96 35.0 235 30 75 4.02 0.5460 WVFGRD96 36.0 235 30 75 4.02 0.5384 WVFGRD96 37.0 230 35 75 4.03 0.5313 WVFGRD96 38.0 240 35 80 4.04 0.5232 WVFGRD96 39.0 230 35 75 4.05 0.5147 WVFGRD96 40.0 240 30 80 4.17 0.5070 WVFGRD96 41.0 240 30 80 4.17 0.4996 WVFGRD96 42.0 240 30 80 4.18 0.4926 WVFGRD96 43.0 240 30 80 4.18 0.4847 WVFGRD96 44.0 240 30 80 4.18 0.4771 WVFGRD96 45.0 240 30 80 4.19 0.4693 WVFGRD96 46.0 245 30 85 4.20 0.4613 WVFGRD96 47.0 245 30 85 4.20 0.4538 WVFGRD96 48.0 245 30 85 4.20 0.4462 WVFGRD96 49.0 245 35 85 4.21 0.4385 WVFGRD96 50.0 245 35 85 4.21 0.4313
The best solution is
WVFGRD96 15.0 70 65 85 3.95 0.6475
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 0.05 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 CUS.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 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