Location

Location ANSS

The ANSS event ID is ak008exoev7b and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak008exoev7b/executive.

2008/11/20 11:34:09 69.107 -144.790 27.8 4.3 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2008/11/20 11:34:09:0 69.11 -144.79 27.8 4.3 Alaska Stations used: AK.BPAW AK.COLD AK.MCK AK.PAX AK.TRF CN.DAWY CN.INK IU.COLA US.EGAK Filtering commands used: rtr taper w 0.05 hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.45e+22 dyne-cm Mw = 4.04 Z = 11 km Plane Strike Dip Rake NP1 117 86 150 NP2 210 60 5 Principal Axes: Axis Value Plunge Azimuth T 1.45e+22 24 70 N 0.00e+00 60 290 P -1.45e+22 17 168 Moment Tensor: (dyne-cm) Component Value Mxx -1.11e+22 Mxy 6.71e+21 Mxz 5.92e+21 Myy 9.98e+21 Myz 4.15e+21 Mzz 1.09e+21 -------------- --------------------## -------------------######### -----------------############# -----------------################# ----------------#################### ##-------------################# ### ######---------################## T #### #########-----################### #### ########################################## #############----######################### ############--------###################### ###########-------------################## ##########-----------------############# #########----------------------######### ########--------------------------#### #######----------------------------- ######---------------------------- ####-------------------------- ###------------- --------- ------------- P ------ --------- -- Global CMT Convention Moment Tensor: R T P 1.09e+21 5.92e+21 -4.15e+21 5.92e+21 -1.11e+22 -6.71e+21 -4.15e+21 -6.71e+21 9.98e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081120113409/index.html

Preferred Solution

      STK = 210
      DIP = 60
     RAKE = 5
       MW = 4.04
       HS = 11.0

The NDK file is 20081120113409.ndk The waveform inversion is preferred.

Moment Tensor Comparison

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.

SLU	AEIC
USGS/SLU Moment Tensor Solution ENS 2008/11/20 11:34:09:0 69.11 -144.79 27.8 4.3 Alaska Stations used: AK.BPAW AK.COLD AK.MCK AK.PAX AK.TRF CN.DAWY CN.INK IU.COLA US.EGAK Filtering commands used: rtr taper w 0.05 hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.45e+22 dyne-cm Mw = 4.04 Z = 11 km Plane Strike Dip Rake NP1 117 86 150 NP2 210 60 5 Principal Axes: Axis Value Plunge Azimuth T 1.45e+22 24 70 N 0.00e+00 60 290 P -1.45e+22 17 168 Moment Tensor: (dyne-cm) Component Value Mxx -1.11e+22 Mxy 6.71e+21 Mxz 5.92e+21 Myy 9.98e+21 Myz 4.15e+21 Mzz 1.09e+21 -------------- --------------------## -------------------######### -----------------############# -----------------################# ----------------#################### ##-------------################# ### ######---------################## T #### #########-----################### #### ########################################## #############----######################### ############--------###################### ###########-------------################## ##########-----------------############# #########----------------------######### ########--------------------------#### #######----------------------------- ######---------------------------- ####-------------------------- ###------------- --------- ------------- P ------ --------- -- Global CMT Convention Moment Tensor: R T P 1.09e+21 5.92e+21 -4.15e+21 5.92e+21 -1.11e+22 -6.71e+21 -4.15e+21 -6.71e+21 9.98e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081120113409/index.html			Moment tensor inversion summary for event 2008/11/20 11:34 Date: 2008/11/20 Time: 11:34 (UTC) Region: Northeast Brooks Range, Alaska Mw=4.3 Location: Lat. 69.1067; Lon. -144.7887; Depth 25 km (Best-fitting depth from moment tensor inversion) Solution quality: poor Number of stations = 3 Best Double Couple: strike dip rake Plane 1: 342.8 88.3 159.8 Plane 2: 73.5 69.8 1.8 Moment Tensor Parameters: Mo = 2.77984e+22 dyn-cm Mxx = -1.41; Mxy = -2.16; Mxz = -0.12 Myy = 1.51; Myz = -0.97; Mzz = -0.10 Principal Axes: value azimuth plunge T: 2.88 296.32 15.39 N: -0.20 158.14 69.73 P: -2.68 29.92 12.87

Magnitudes

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 M_L by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for M_L is adjusted to remove a linear distance trend in residuals to give a regionally defined M_L. The defined M_L 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.

Context

Waveform Inversion using wvfgrd96

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.

Location of broadband stations used for waveform inversion

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:

rtr
taper w 0.05
hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   185    65   -30   3.80 0.5133
WVFGRD96    1.0   185    70   -30   3.82 0.5188
WVFGRD96    2.0   180    65   -40   3.89 0.5449
WVFGRD96    3.0    10    40   -20   3.97 0.5351
WVFGRD96    4.0    25    45   -15   3.98 0.5683
WVFGRD96    5.0   210    45     0   3.98 0.5862
WVFGRD96    6.0   210    50     5   3.97 0.5995
WVFGRD96    7.0   210    55    10   3.97 0.6079
WVFGRD96    8.0   210    50     5   4.02 0.6128
WVFGRD96    9.0   210    55     5   4.02 0.6160
WVFGRD96   10.0   210    55    10   4.03 0.6179
WVFGRD96   11.0   210    60     5   4.04 0.6179
WVFGRD96   12.0   210    60    10   4.05 0.6176
WVFGRD96   13.0   210    60    10   4.05 0.6157
WVFGRD96   14.0   210    65     5   4.07 0.6112
WVFGRD96   15.0   210    65     0   4.09 0.6080
WVFGRD96   16.0   210    65     0   4.09 0.6023
WVFGRD96   17.0   210    65     5   4.10 0.5950
WVFGRD96   18.0   210    65     5   4.11 0.5875
WVFGRD96   19.0    25    65    25   4.11 0.5813
WVFGRD96   20.0    25    65    25   4.12 0.5744
WVFGRD96   21.0    25    65    25   4.13 0.5662
WVFGRD96   22.0    25    65    25   4.14 0.5573
WVFGRD96   23.0    25    65    25   4.15 0.5473
WVFGRD96   24.0    25    65    25   4.15 0.5361
WVFGRD96   25.0    25    65    25   4.16 0.5247
WVFGRD96   26.0    25    70    30   4.18 0.5118
WVFGRD96   27.0    25    70    30   4.18 0.4999
WVFGRD96   28.0    25    70    30   4.19 0.4868
WVFGRD96   29.0    25    70    30   4.19 0.4739

The best solution is

WVFGRD96   11.0   210    60     5   4.04 0.6179

The mechanism corresponding to the best fit is

Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

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

rtr
taper w 0.05
hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2

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:

• The origin time and epicentral distance are incorrect
• The velocity model used for the inversion is incorrect
• The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

 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.

Velocity Model

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