Location

Location ANSS

The ANSS event ID is ak019ft9h8bj and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019ft9h8bj/executive.

2019/12/10 21:17:45 66.305 -157.213 10.1 3.3 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/12/10 21:17:45:0 66.31 -157.21 10.1 3.3 Alaska Stations used: AK.J19K AK.J20K AK.MLY AK.RDOG TA.C18K TA.C19K TA.E24K TA.F19K TA.F21K TA.G16K TA.G18K TA.G19K TA.G21K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.70e+21 dyne-cm Mw = 3.42 Z = 9 km Plane Strike Dip Rake NP1 335 65 -60 NP2 101 38 -137 Principal Axes: Axis Value Plunge Azimuth T 1.70e+21 15 44 N 0.00e+00 27 141 P -1.70e+21 59 288 Moment Tensor: (dyne-cm) Component Value Mxx 7.91e+20 Mxy 9.26e+20 Mxz 7.43e+19 Myy 3.36e+20 Myz 1.01e+21 Mzz -1.13e+21 ############## -----################# ----------############## # -------------############ T ## -----------------########## #### -------------------################# ----------------------################ ------------------------################ ----------- -----------############### #----------- P ------------############### #----------- -------------############## ##--------------------------############## ###--------------------------############# ####------------------------############ #####------------------------#########-- #######---------------------########-- #########------------------#####---- ############--------------#------- #########################----- #######################----- ####################-- ############## Global CMT Convention Moment Tensor: R T P -1.13e+21 7.43e+19 -1.01e+21 7.43e+19 7.91e+20 -9.26e+20 -1.01e+21 -9.26e+20 3.36e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191210211745/index.html

Preferred Solution

      STK = 335
      DIP = 65
     RAKE = -60
       MW = 3.42
       HS = 9.0

The NDK file is 20191210211745.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.

mLg Magnitude

Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.

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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 
br c 0.12 0.25 n 4 p 2

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   355    90     0   3.00 0.3208
WVFGRD96    2.0    -5    90     0   3.13 0.4505
WVFGRD96    3.0   355    85   -10   3.18 0.4751
WVFGRD96    4.0     0    80    45   3.29 0.4896
WVFGRD96    5.0     5    75    50   3.34 0.5191
WVFGRD96    6.0   355    90   -55   3.36 0.5420
WVFGRD96    7.0   335    65   -60   3.38 0.5613
WVFGRD96    8.0   335    65   -60   3.42 0.5694
WVFGRD96    9.0   335    65   -60   3.42 0.5697
WVFGRD96   10.0   340    65   -50   3.40 0.5659
WVFGRD96   11.0   345    70   -45   3.39 0.5620
WVFGRD96   12.0   345    70   -40   3.39 0.5579
WVFGRD96   13.0   345    70   -40   3.39 0.5545
WVFGRD96   14.0   345    70   -40   3.40 0.5504
WVFGRD96   15.0   350    75   -35   3.41 0.5465
WVFGRD96   16.0   350    70   -30   3.42 0.5435
WVFGRD96   17.0   350    70   -30   3.43 0.5400
WVFGRD96   18.0   350    70   -30   3.44 0.5358
WVFGRD96   19.0   350    75   -30   3.44 0.5308
WVFGRD96   20.0   350    75   -30   3.45 0.5264
WVFGRD96   21.0   350    75   -30   3.46 0.5211
WVFGRD96   22.0   350    75   -30   3.47 0.5155
WVFGRD96   23.0   350    75   -30   3.47 0.5099
WVFGRD96   24.0   350    75   -30   3.48 0.5039
WVFGRD96   25.0   350    75   -30   3.49 0.4981
WVFGRD96   26.0   350    75   -30   3.50 0.4926
WVFGRD96   27.0   345    75   -35   3.49 0.4884
WVFGRD96   28.0   345    75   -35   3.50 0.4848
WVFGRD96   29.0   345    75   -35   3.51 0.4811

The best solution is

WVFGRD96    9.0   335    65   -60   3.42 0.5697

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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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