The ANSS event ID is ak009c4s2sta and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak009c4s2sta/executive.
2009/09/21 10:41:26 60.893 -147.116 20.6 4.4 Alaska
USGS/SLU Moment Tensor Solution ENS 2009/09/21 10:41:26:0 60.89 -147.12 20.6 4.4 Alaska Stations used: AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CHUM AK.DDM AK.DIV AK.EYAK AK.KLU AK.MCK AK.PPLA AK.RND AK.SCM AK.SSN AT.PMR IU.COLA Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.55e+22 dyne-cm Mw = 4.30 Z = 27 km Plane Strike Dip Rake NP1 195 70 -70 NP2 328 28 -133 Principal Axes: Axis Value Plunge Azimuth T 3.55e+22 22 270 N 0.00e+00 19 8 P -3.55e+22 60 134 Moment Tensor: (dyne-cm) Component Value Mxx -4.27e+21 Mxy 4.52e+21 Mxz 1.06e+22 Myy 2.57e+22 Myz -2.36e+22 Mzz -2.14e+22 ------------## ############--######## ###############----######### ###############--------####### ################-----------####### ################-------------####### ################----------------###### #################-----------------###### ################-------------------##### ################--------------------###### ### ##########---------------------##### ### T ##########---------------------##### ### #########---------- ---------##### ##############---------- P ---------#### ##############---------- ---------#### ############-----------------------### ###########----------------------### ##########----------------------## ########---------------------# ########-------------------# #####----------------- #------------- Global CMT Convention Moment Tensor: R T P -2.14e+22 1.06e+22 2.36e+22 1.06e+22 -4.27e+21 -4.52e+21 2.36e+22 -4.52e+21 2.57e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090921104126/index.html |
STK = 195 DIP = 70 RAKE = -70 MW = 4.30 HS = 27.0
The NDK file is 20090921104126.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 2009/09/21 10:41:26:0 60.89 -147.12 20.6 4.4 Alaska Stations used: AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CHUM AK.DDM AK.DIV AK.EYAK AK.KLU AK.MCK AK.PPLA AK.RND AK.SCM AK.SSN AT.PMR IU.COLA Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.55e+22 dyne-cm Mw = 4.30 Z = 27 km Plane Strike Dip Rake NP1 195 70 -70 NP2 328 28 -133 Principal Axes: Axis Value Plunge Azimuth T 3.55e+22 22 270 N 0.00e+00 19 8 P -3.55e+22 60 134 Moment Tensor: (dyne-cm) Component Value Mxx -4.27e+21 Mxy 4.52e+21 Mxz 1.06e+22 Myy 2.57e+22 Myz -2.36e+22 Mzz -2.14e+22 ------------## ############--######## ###############----######### ###############--------####### ################-----------####### ################-------------####### ################----------------###### #################-----------------###### ################-------------------##### ################--------------------###### ### ##########---------------------##### ### T ##########---------------------##### ### #########---------- ---------##### ##############---------- P ---------#### ##############---------- ---------#### ############-----------------------### ###########----------------------### ##########----------------------## ########---------------------# ########-------------------# #####----------------- #------------- Global CMT Convention Moment Tensor: R T P -2.14e+22 1.06e+22 2.36e+22 1.06e+22 -4.27e+21 -4.52e+21 2.36e+22 -4.52e+21 2.57e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090921104126/index.html |
Moment tensor inversion summary for event 2009/09/21 10:41 Date: 2009/09/21 Time: 10:41 (UTC) Region: Prince William Sound Region of Alaska Mw=4.4 Location: Lat. 60.9199; Lon. -147.1196; Depth 20 km (Best-fitting depth from moment tensor inversion) Solution quality: good; Number of stations = 7 Best Double Couple: strike dip rake Plane 1: 190.5 76.5 -84.1 Plane 2: 346.5 14.7 -113.2 Moment Tensor Parameters: Mo = 5.08063e+22 dyn-cm Mxx = 0.20; Mxy = 0.14; Mxz = 0.96 Myy = 2.17; Myz = -4.26; Mzz = -2.38 Principal Axes: value azimuth plunge T: 4.76 275.58 31.26 N: 0.32 9.08 5.75 P: -5.08 108.39 58.10 |
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.
![]() |
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.
![]() |
|
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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 25 55 -60 3.90 0.3022 WVFGRD96 1.0 20 55 -70 3.96 0.3230 WVFGRD96 2.0 5 45 -85 4.06 0.3468 WVFGRD96 3.0 185 40 -90 4.09 0.2992 WVFGRD96 4.0 40 75 40 4.02 0.3079 WVFGRD96 5.0 40 75 40 4.02 0.3249 WVFGRD96 6.0 40 75 40 4.02 0.3401 WVFGRD96 7.0 40 75 40 4.03 0.3579 WVFGRD96 8.0 25 85 45 4.02 0.3743 WVFGRD96 9.0 200 90 -45 4.03 0.3884 WVFGRD96 10.0 25 85 45 4.07 0.4025 WVFGRD96 11.0 20 90 50 4.07 0.4173 WVFGRD96 12.0 20 90 50 4.08 0.4314 WVFGRD96 13.0 20 90 50 4.10 0.4447 WVFGRD96 14.0 200 90 -50 4.11 0.4573 WVFGRD96 15.0 20 90 50 4.12 0.4688 WVFGRD96 16.0 205 75 -55 4.14 0.4885 WVFGRD96 17.0 200 75 -60 4.15 0.5053 WVFGRD96 18.0 200 70 -60 4.17 0.5228 WVFGRD96 19.0 200 70 -60 4.18 0.5402 WVFGRD96 20.0 200 70 -60 4.22 0.5563 WVFGRD96 21.0 200 70 -65 4.23 0.5727 WVFGRD96 22.0 200 70 -65 4.25 0.5872 WVFGRD96 23.0 200 70 -65 4.26 0.5993 WVFGRD96 24.0 195 70 -70 4.27 0.6095 WVFGRD96 25.0 195 70 -70 4.28 0.6178 WVFGRD96 26.0 195 70 -70 4.29 0.6229 WVFGRD96 27.0 195 70 -70 4.30 0.6245 WVFGRD96 28.0 195 70 -70 4.31 0.6234 WVFGRD96 29.0 190 70 -75 4.32 0.6193 WVFGRD96 30.0 195 75 -75 4.33 0.6151 WVFGRD96 31.0 195 75 -75 4.34 0.6089 WVFGRD96 32.0 195 75 -75 4.34 0.6004 WVFGRD96 33.0 195 75 -75 4.35 0.5902 WVFGRD96 34.0 195 75 -75 4.35 0.5793 WVFGRD96 35.0 195 75 -75 4.36 0.5676 WVFGRD96 36.0 195 75 -75 4.36 0.5550 WVFGRD96 37.0 190 75 -75 4.36 0.5430 WVFGRD96 38.0 190 75 -75 4.36 0.5309 WVFGRD96 39.0 190 75 -75 4.36 0.5187
The best solution is
WVFGRD96 27.0 195 70 -70 4.30 0.6245
The mechanism corresponding 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 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 0.10 n 3
![]() |
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. |
![]() |
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