Location

Location ANSS

The ANSS event ID is ak023azg5evm and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023azg5evm/executive.

2023/08/27 11:27:00 61.458 -150.692 41.8 3.7 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2023/08/27 11:27:00:0 61.46 -150.69 41.8 3.7 Alaska Stations used: AK.CUT AK.DIV AK.FID AK.FIRE AK.GHO AK.KNK AK.L20K AK.L22K AK.M20K AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SWD AK.WAT6 AT.PMR AV.RED AV.SPCP AV.STLK II.KDAK Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 8.04e+21 dyne-cm Mw = 3.87 Z = 62 km Plane Strike Dip Rake NP1 10 75 65 NP2 251 29 148 Principal Axes: Axis Value Plunge Azimuth T 8.04e+21 53 250 N 0.00e+00 24 17 P -8.04e+21 26 119 Moment Tensor: (dyne-cm) Component Value Mxx -1.23e+21 Mxy 3.71e+21 Mxz 2.30e+20 Myy -2.41e+21 Myz -6.36e+21 Mzz 3.64e+21 -----------### ---------------####### -------------------######### ---------###########------#### --------##############---------### ------#################------------# -----###################-------------# ----#####################--------------- ---######################--------------- ---######################----------------- --#######################----------------- --######### ###########----------------- --######### T ##########------------------ ########## ##########----------------- #######################--------- ----- #####################---------- P ---- ###################----------- --- ##################---------------- ###############--------------- #############--------------- #########------------- ####---------- Global CMT Convention Moment Tensor: R T P 3.64e+21 2.30e+20 6.36e+21 2.30e+20 -1.23e+21 -3.71e+21 6.36e+21 -3.71e+21 -2.41e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230827112700/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is

      STK = 10
      DIP = 75
     RAKE = 65
       MW = 3.87
       HS = 62.0

The NDK file is 20230827112700.ndk The waveform inversion is preferred.

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.

ML Magnitude

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.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

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:

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3

The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0   195    50   -95   3.17 0.2205
WVFGRD96    4.0   220    75   -50   3.24 0.2004
WVFGRD96    6.0   240    65    35   3.26 0.2300
WVFGRD96    8.0   240    65    40   3.35 0.2452
WVFGRD96   10.0   255    55    45   3.38 0.2553
WVFGRD96   12.0   260    55    50   3.41 0.2587
WVFGRD96   14.0   175    60    45   3.41 0.2711
WVFGRD96   16.0   170    60    40   3.44 0.2842
WVFGRD96   18.0   170    65    40   3.45 0.2941
WVFGRD96   20.0   170    65    40   3.47 0.3051
WVFGRD96   22.0   170    65    40   3.50 0.3143
WVFGRD96   24.0   170    65    40   3.51 0.3205
WVFGRD96   26.0   170    70    40   3.52 0.3237
WVFGRD96   28.0   170    45    30   3.59 0.3333
WVFGRD96   30.0   175    45    30   3.61 0.3428
WVFGRD96   32.0   175    45    35   3.62 0.3507
WVFGRD96   34.0   175    45    35   3.63 0.3540
WVFGRD96   36.0   180    40    35   3.66 0.3546
WVFGRD96   38.0    10    75    50   3.65 0.3631
WVFGRD96   40.0    15    75    65   3.77 0.3784
WVFGRD96   42.0    15    75    65   3.79 0.3988
WVFGRD96   44.0    15    75    65   3.80 0.4142
WVFGRD96   46.0    15    75    65   3.81 0.4263
WVFGRD96   48.0    15    75    65   3.83 0.4418
WVFGRD96   50.0    15    75    65   3.84 0.4550
WVFGRD96   52.0    15    75    65   3.85 0.4642
WVFGRD96   54.0    10    75    65   3.85 0.4723
WVFGRD96   56.0    15    70    65   3.87 0.4775
WVFGRD96   58.0    10    75    65   3.86 0.4810
WVFGRD96   60.0    10    75    65   3.87 0.4825
WVFGRD96   62.0    10    75    65   3.87 0.4826
WVFGRD96   64.0    10    75    70   3.88 0.4809
WVFGRD96   66.0    10    75    70   3.88 0.4783
WVFGRD96   68.0    10    75    70   3.89 0.4743
WVFGRD96   70.0    10    75    70   3.89 0.4692
WVFGRD96   72.0    10    75    70   3.89 0.4636
WVFGRD96   74.0    10    75    70   3.90 0.4578
WVFGRD96   76.0    10    75    70   3.90 0.4509
WVFGRD96   78.0    10    75    70   3.90 0.4446
WVFGRD96   80.0    10    75    70   3.90 0.4380
WVFGRD96   82.0    10    75    70   3.90 0.4320
WVFGRD96   84.0    15    70    70   3.91 0.4255
WVFGRD96   86.0    10    70    70   3.90 0.4191
WVFGRD96   88.0    10    70    70   3.91 0.4133
WVFGRD96   90.0    10    70    70   3.91 0.4072
WVFGRD96   92.0    10    70    70   3.91 0.4009
WVFGRD96   94.0    10    70    70   3.91 0.3945
WVFGRD96   96.0   175    10    70   3.95 0.3910
WVFGRD96   98.0   175    10    70   3.95 0.3869

The best solution is

WVFGRD96   62.0    10    75    65   3.87 0.4826

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

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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:

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)

Assuming only a mislocation, the time shifts are fit to a functional form:

 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

Last Changed Tue Apr 23 03:12:10 AM CDT 2024