Location

Location ANSS

The ANSS event ID is ak023dtut7mp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023dtut7mp/executive.

2023/10/28 03:44:10 59.003 -136.550 5.9 5.3 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2023/10/28 03:44:10:0 59.00 -136.55 5.9 5.3 Alaska Stations used: AK.BAGL AK.BAL AK.BARK AK.BARN AK.BERG AK.BESE AK.BMR AK.CHX AK.CRQ AK.DOT AK.ISLE AK.JIS AK.KHIT AK.KIAG AK.LOGN AK.M27K AK.MCAR AK.MESA AK.PAX AK.PIN AK.PNL AK.PS12 AK.PTPK AK.R32K AK.RKAV AK.S31K AK.S32K AK.SAMH AK.TGL AK.U33K AK.V35K AK.VRDI AK.WAX AT.CRAG AT.MENT AT.SIT AT.SKAG AV.EDCR AV.EDES AV.EDSO AV.N25K CN.ATLI CN.BRWY CN.DAWY CN.HYT CN.PLBC CN.RUBB CN.WHY CN.YUK3 CN.YUK4 CN.YUK6 CN.YUK7 CN.YUK8 EO.KLRS NY.MAYO NY.MMPY NY.WTLY US.WRAK 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 0.06 n 3 Best Fitting Double Couple Mo = 7.16e+23 dyne-cm Mw = 5.17 Z = 4 km Plane Strike Dip Rake NP1 2 56 113 NP2 145 40 60 Principal Axes: Axis Value Plunge Azimuth T 7.16e+23 69 323 N 0.00e+00 19 169 P -7.16e+23 9 76 Moment Tensor: (dyne-cm) Component Value Mxx 1.53e+22 Mxy -2.08e+23 Mxz 1.63e+23 Myy -6.26e+23 Myz -2.46e+23 Mzz 6.11e+23 #########----- ###############------- -##################--------- --###################--------- ---#####################---------- ----#####################----------- ----#######################----------- -----########## ##########--------- -----########## T ##########--------- P -------######### ###########-------- - -------#######################------------ --------######################------------ --------#####################------------- --------####################------------ ---------###################------------ ----------################------------ ----------###############----------- -----------############----------- ------------########---------- --------------####---------- --------------######## --------###### Global CMT Convention Moment Tensor: R T P 6.11e+23 1.63e+23 2.46e+23 1.63e+23 1.53e+22 2.08e+23 2.46e+23 2.08e+23 -6.26e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231028034410/index.html

Preferred Solution

      STK = 145
      DIP = 40
     RAKE = 60
       MW = 5.17
       HS = 4.0

The NDK file is 20231028034410.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

USGSW

USGS/SLU Moment Tensor Solution ENS 2023/10/28 03:44:10:0 59.00 -136.55 5.9 5.3 Alaska Stations used: AK.BAGL AK.BAL AK.BARK AK.BARN AK.BERG AK.BESE AK.BMR AK.CHX AK.CRQ AK.DOT AK.ISLE AK.JIS AK.KHIT AK.KIAG AK.LOGN AK.M27K AK.MCAR AK.MESA AK.PAX AK.PIN AK.PNL AK.PS12 AK.PTPK AK.R32K AK.RKAV AK.S31K AK.S32K AK.SAMH AK.TGL AK.U33K AK.V35K AK.VRDI AK.WAX AT.CRAG AT.MENT AT.SIT AT.SKAG AV.EDCR AV.EDES AV.EDSO AV.N25K CN.ATLI CN.BRWY CN.DAWY CN.HYT CN.PLBC CN.RUBB CN.WHY CN.YUK3 CN.YUK4 CN.YUK6 CN.YUK7 CN.YUK8 EO.KLRS NY.MAYO NY.MMPY NY.WTLY US.WRAK 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 0.06 n 3 Best Fitting Double Couple Mo = 7.16e+23 dyne-cm Mw = 5.17 Z = 4 km Plane Strike Dip Rake NP1 2 56 113 NP2 145 40 60 Principal Axes: Axis Value Plunge Azimuth T 7.16e+23 69 323 N 0.00e+00 19 169 P -7.16e+23 9 76 Moment Tensor: (dyne-cm) Component Value Mxx 1.53e+22 Mxy -2.08e+23 Mxz 1.63e+23 Myy -6.26e+23 Myz -2.46e+23 Mzz 6.11e+23 #########----- ###############------- -##################--------- --###################--------- ---#####################---------- ----#####################----------- ----#######################----------- -----########## ##########--------- -----########## T ##########--------- P -------######### ###########-------- - -------#######################------------ --------######################------------ --------#####################------------- --------####################------------ ---------###################------------ ----------################------------ ----------###############----------- -----------############----------- ------------########---------- --------------####---------- --------------######## --------###### Global CMT Convention Moment Tensor: R T P 6.11e+23 1.63e+23 2.46e+23 1.63e+23 1.53e+22 2.08e+23 2.46e+23 2.08e+23 -6.26e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231028034410/index.html

W-phase Moment Tensor (Mww) Moment 1.135e+17 N-m Magnitude 5.30 Mww Depth 11.5 km Percent DC 70% Half Duration 1.17 s Catalog US Data Source US 3 Contributor US 3 Nodal Planes Plane Strike Dip Rake NP1 337 59 85 NP2 166 31 98 Principal Axes Axis Value Plunge Azimuth T 1.032e+17 N-m 75 234 N 0.184e+17 N-m 4 340 P -1.216e+17 N-m 14 71

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

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 0.06 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   135    30    30   5.17 0.5424
WVFGRD96    2.0   135    40    40   5.13 0.5795
WVFGRD96    3.0   140    40    55   5.16 0.5985
WVFGRD96    4.0   145    40    60   5.17 0.5986
WVFGRD96    5.0   150    40    70   5.18 0.5770
WVFGRD96    6.0   150    40    70   5.17 0.5386
WVFGRD96    7.0   120    60    10   5.06 0.5118
WVFGRD96    8.0   290    60   -25   5.08 0.5167
WVFGRD96    9.0   290    60   -30   5.09 0.5263
WVFGRD96   10.0   290    60   -30   5.11 0.5322
WVFGRD96   11.0   290    60   -30   5.11 0.5378
WVFGRD96   12.0   290    60   -25   5.11 0.5399
WVFGRD96   13.0   290    60   -25   5.11 0.5391
WVFGRD96   14.0   290    65   -25   5.12 0.5380
WVFGRD96   15.0   290    65   -25   5.13 0.5361
WVFGRD96   16.0   290    65   -25   5.13 0.5327
WVFGRD96   17.0   290    65   -20   5.13 0.5283
WVFGRD96   18.0   290    65   -20   5.14 0.5234
WVFGRD96   19.0   290    65   -20   5.14 0.5179
WVFGRD96   20.0   290    65   -20   5.16 0.5109
WVFGRD96   21.0   290    65   -20   5.16 0.5035
WVFGRD96   22.0   290    65   -20   5.16 0.4957
WVFGRD96   23.0   290    65   -20   5.17 0.4874
WVFGRD96   24.0   290    65   -20   5.17 0.4787
WVFGRD96   25.0   290    65   -20   5.17 0.4700
WVFGRD96   26.0   290    65   -20   5.18 0.4612
WVFGRD96   27.0   110    65   -10   5.20 0.4497
WVFGRD96   28.0   110    70   -10   5.20 0.4429
WVFGRD96   29.0   110    70   -10   5.21 0.4368

The best solution is

WVFGRD96    4.0   145    40    60   5.17 0.5986

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 0.06 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)

 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