Location

Location ANSS

The ANSS event ID is ak02098f8b3x and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02098f8b3x/executive.

2020/07/19 00:35:28 62.150 -154.930 18.3 4.2 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2020/07/19 00:35:28:0 62.15 -154.93 18.3 4.2 Alaska Stations used: AK.BWN AK.CAST AK.CNP AK.CUT AK.DHY AK.GHO AK.GLI AK.H21K AK.I21K AK.I23K AK.J17K AK.J19K AK.J20K AK.K20K AK.KNK AK.KTH AK.L18K AK.L22K AK.M20K AK.MCK AK.MLY AK.N18K AK.N19K AK.NEA2 AK.O18K AK.O19K AK.P16K AK.P17K AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AK.WRH AT.PMR AV.ACH AV.ILSW AV.SPU AV.STLK TA.H17K TA.H18K TA.H19K TA.I17K TA.I20K TA.J16K TA.J18K TA.K15K TA.K17K TA.L16K TA.M14K TA.M16K TA.M17K TA.N14K TA.N15K TA.N17K TA.O15K TA.O16K 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 0.10 n 3 Best Fitting Double Couple Mo = 8.04e+21 dyne-cm Mw = 3.87 Z = 18 km Plane Strike Dip Rake NP1 140 85 30 NP2 47 60 174 Principal Axes: Axis Value Plunge Azimuth T 8.04e+21 24 8 N 0.00e+00 60 149 P -8.04e+21 17 270 Moment Tensor: (dyne-cm) Component Value Mxx 6.54e+21 Mxy 8.60e+20 Mxz 3.01e+21 Myy -7.24e+21 Myz 2.64e+21 Mzz 6.98e+20 ############## ############ ####### -############## T ########## ---############# ########### ------##########################-- --------#########################--- -----------######################----- -------------#####################------ ---------------##################------- -----------------################--------- -- --------------#############---------- -- P ---------------##########------------ -- -----------------#######------------- ----------------------####-------------- ---------------------------------------- ---------------------####------------- -----------------########----------- -------------#############-------- -----#####################---- ###########################- ###################### ############## Global CMT Convention Moment Tensor: R T P 6.98e+20 3.01e+21 -2.64e+21 3.01e+21 6.54e+21 -8.60e+20 -2.64e+21 -8.60e+20 -7.24e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200719003528/index.html

Preferred Solution

      STK = 140
      DIP = 85
     RAKE = 30
       MW = 3.87
       HS = 18.0

The NDK file is 20200719003528.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

USGSMWR

USGS/SLU Moment Tensor Solution ENS 2020/07/19 00:35:28:0 62.15 -154.93 18.3 4.2 Alaska Stations used: AK.BWN AK.CAST AK.CNP AK.CUT AK.DHY AK.GHO AK.GLI AK.H21K AK.I21K AK.I23K AK.J17K AK.J19K AK.J20K AK.K20K AK.KNK AK.KTH AK.L18K AK.L22K AK.M20K AK.MCK AK.MLY AK.N18K AK.N19K AK.NEA2 AK.O18K AK.O19K AK.P16K AK.P17K AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AK.WRH AT.PMR AV.ACH AV.ILSW AV.SPU AV.STLK TA.H17K TA.H18K TA.H19K TA.I17K TA.I20K TA.J16K TA.J18K TA.K15K TA.K17K TA.L16K TA.M14K TA.M16K TA.M17K TA.N14K TA.N15K TA.N17K TA.O15K TA.O16K 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 0.10 n 3 Best Fitting Double Couple Mo = 8.04e+21 dyne-cm Mw = 3.87 Z = 18 km Plane Strike Dip Rake NP1 140 85 30 NP2 47 60 174 Principal Axes: Axis Value Plunge Azimuth T 8.04e+21 24 8 N 0.00e+00 60 149 P -8.04e+21 17 270 Moment Tensor: (dyne-cm) Component Value Mxx 6.54e+21 Mxy 8.60e+20 Mxz 3.01e+21 Myy -7.24e+21 Myz 2.64e+21 Mzz 6.98e+20 ############## ############ ####### -############## T ########## ---############# ########### ------##########################-- --------#########################--- -----------######################----- -------------#####################------ ---------------##################------- -----------------################--------- -- --------------#############---------- -- P ---------------##########------------ -- -----------------#######------------- ----------------------####-------------- ---------------------------------------- ---------------------####------------- -----------------########----------- -------------#############-------- -----#####################---- ###########################- ###################### ############## Global CMT Convention Moment Tensor: R T P 6.98e+20 3.01e+21 -2.64e+21 3.01e+21 6.54e+21 -8.60e+20 -2.64e+21 -8.60e+20 -7.24e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200719003528/index.html

Regional Moment Tensor (Mwr) Moment 8.742e+14 N-m Magnitude 3.89 Mwr Depth 17.0 km Percent DC 78% Half Duration - Catalog US Data Source US 2 Contributor US 2 Nodal Planes Plane Strike Dip Rake NP1 319 83 -29 NP2 52 61 -172 Principal Axes Axis Value Plunge Azimuth T 9.201e+14 N-m 15 9 N -1.004e+14 N-m 60 127 P -8.197e+14 N-m 25 272

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 0.10 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   320    80    10   3.28 0.2490
WVFGRD96    2.0   320    80    15   3.41 0.3144
WVFGRD96    3.0   320    80    20   3.47 0.3274
WVFGRD96    4.0   140    90   -30   3.52 0.3367
WVFGRD96    5.0   320    90    35   3.55 0.3565
WVFGRD96    6.0   135    85   -35   3.57 0.3833
WVFGRD96    7.0   140    90    35   3.60 0.4099
WVFGRD96    8.0   140    90    40   3.66 0.4477
WVFGRD96    9.0   315    85   -40   3.69 0.4869
WVFGRD96   10.0   315    85   -40   3.71 0.5200
WVFGRD96   11.0   140    90    35   3.74 0.5510
WVFGRD96   12.0   320    90   -35   3.76 0.5773
WVFGRD96   13.0   140    85    35   3.78 0.6001
WVFGRD96   14.0   140    85    30   3.81 0.6196
WVFGRD96   15.0   320    90   -30   3.83 0.6309
WVFGRD96   16.0   140    85    30   3.84 0.6423
WVFGRD96   17.0   320    90   -30   3.86 0.6438
WVFGRD96   18.0   140    85    30   3.87 0.6466
WVFGRD96   19.0   140    85    30   3.89 0.6415
WVFGRD96   20.0   140    85    30   3.90 0.6321
WVFGRD96   21.0   140    85    30   3.91 0.6192
WVFGRD96   22.0   320    90   -30   3.92 0.6000
WVFGRD96   23.0   140    85    30   3.92 0.5846
WVFGRD96   24.0   140    85    30   3.93 0.5638
WVFGRD96   25.0   140    85    30   3.93 0.5416
WVFGRD96   26.0   140    85    30   3.94 0.5187
WVFGRD96   27.0   320    90   -35   3.94 0.4933
WVFGRD96   28.0   140    85    35   3.94 0.4726
WVFGRD96   29.0   140    85    35   3.95 0.4499

The best solution is

WVFGRD96   18.0   140    85    30   3.87 0.6466

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 0.10 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:

• 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