Location

Location ANSS

The ANSS event ID is ak0192zumrhu and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0192zumrhu/executive.

2019/03/06 21:33:14 66.311 -157.219 9.1 5.2 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/03/06 21:33:14:0 66.31 -157.22 9.1 5.2 Alaska Stations used: AK.ANM AK.BPAW AK.CAST AK.CCB AK.CHUM AK.COLD AK.FA01 AK.FA02 AK.FA05 AK.FA06 AK.GCSA AK.HDA AK.KTH AK.NEA2 AK.PPD AK.PPLA AK.RDOG AK.RIDG AK.SAW AK.SKN AK.TNA AK.TRF AK.WRH AT.TTA AV.SPBG AV.SPBL AV.SPCG AV.SPCR AV.SPU AV.STLK IU.COLA TA.B18K TA.B21K TA.C17K TA.D17K TA.D22K TA.D23K TA.E22K TA.E24K TA.E25K TA.F18K TA.F22K TA.F26K TA.G26K TA.H16K TA.H19K TA.H20K TA.H24K TA.I21K TA.I23K TA.K15K TA.K17K TA.L14K TA.L16K TA.L18K TA.M17K TA.N18K TA.N20K TA.POKR TA.TOLK XV.F1TN XV.F2TN XV.F6TP XV.F7TV XV.F8KN XV.FAPT XV.FNN1 XV.FPAP XV.FTGH 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 = 7.16e+23 dyne-cm Mw = 5.17 Z = 11 km Plane Strike Dip Rake NP1 355 85 10 NP2 264 80 175 Principal Axes: Axis Value Plunge Azimuth T 7.16e+23 11 220 N 0.00e+00 79 21 P -7.16e+23 3 129 Moment Tensor: (dyne-cm) Component Value Mxx 1.22e+23 Mxy 6.90e+23 Mxz -7.19e+22 Myy -1.43e+23 Myz -1.17e+23 Mzz 2.16e+22 ------######## ----------############ -------------############### ---------------############### -----------------################# ------------------################## -------------------################### ---------------------################### ---------------------################### ----------------------#---------------#### ----------############-------------------- ---###################-------------------- #######################------------------- ######################------------------ ######################------------------ #####################----------------- ####################------------ - ### #############------------ P # T #############------------ #############------------ #############--------- #########----- Global CMT Convention Moment Tensor: R T P 2.16e+22 -7.19e+22 1.17e+23 -7.19e+22 1.22e+23 -6.90e+23 1.17e+23 -6.90e+23 -1.43e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190306213314/index.html

Preferred Solution

      STK = 355
      DIP = 85
     RAKE = 10
       MW = 5.17
       HS = 11.0

The NDK file is 20190306213314.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 2019/03/06 21:33:14:0 66.31 -157.22 9.1 5.2 Alaska Stations used: AK.ANM AK.BPAW AK.CAST AK.CCB AK.CHUM AK.COLD AK.FA01 AK.FA02 AK.FA05 AK.FA06 AK.GCSA AK.HDA AK.KTH AK.NEA2 AK.PPD AK.PPLA AK.RDOG AK.RIDG AK.SAW AK.SKN AK.TNA AK.TRF AK.WRH AT.TTA AV.SPBG AV.SPBL AV.SPCG AV.SPCR AV.SPU AV.STLK IU.COLA TA.B18K TA.B21K TA.C17K TA.D17K TA.D22K TA.D23K TA.E22K TA.E24K TA.E25K TA.F18K TA.F22K TA.F26K TA.G26K TA.H16K TA.H19K TA.H20K TA.H24K TA.I21K TA.I23K TA.K15K TA.K17K TA.L14K TA.L16K TA.L18K TA.M17K TA.N18K TA.N20K TA.POKR TA.TOLK XV.F1TN XV.F2TN XV.F6TP XV.F7TV XV.F8KN XV.FAPT XV.FNN1 XV.FPAP XV.FTGH 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 = 7.16e+23 dyne-cm Mw = 5.17 Z = 11 km Plane Strike Dip Rake NP1 355 85 10 NP2 264 80 175 Principal Axes: Axis Value Plunge Azimuth T 7.16e+23 11 220 N 0.00e+00 79 21 P -7.16e+23 3 129 Moment Tensor: (dyne-cm) Component Value Mxx 1.22e+23 Mxy 6.90e+23 Mxz -7.19e+22 Myy -1.43e+23 Myz -1.17e+23 Mzz 2.16e+22 ------######## ----------############ -------------############### ---------------############### -----------------################# ------------------################## -------------------################### ---------------------################### ---------------------################### ----------------------#---------------#### ----------############-------------------- ---###################-------------------- #######################------------------- ######################------------------ ######################------------------ #####################----------------- ####################------------ - ### #############------------ P # T #############------------ #############------------ #############--------- #########----- Global CMT Convention Moment Tensor: R T P 2.16e+22 -7.19e+22 1.17e+23 -7.19e+22 1.22e+23 -6.90e+23 1.17e+23 -6.90e+23 -1.43e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190306213314/index.html

Moment Tensor (Mww) Moment 8.915e+16 N-m Magnitude 5.23 Mww Depth 11.5 km Percent DC 51% Half Duration 1.06 s Catalog US Data Source US 2 Contributor US 2 Nodal Planes Plane Strike Dip Rake NP1 262 87 166 NP2 353 76 3 Principal Axes Axis Value Plunge Azimuth T 9.873e+16 N-m 12 217 N -2.413e+16 N-m 76 69 P -7.460e+16 N-m 7 308

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   170    75   -20   4.72 0.3008
WVFGRD96    2.0   170    70   -25   4.88 0.4183
WVFGRD96    3.0   175    85     5   4.90 0.4758
WVFGRD96    4.0   175    85     5   4.95 0.5157
WVFGRD96    5.0   175    85   -10   4.99 0.5473
WVFGRD96    6.0   175    85   -10   5.03 0.5739
WVFGRD96    7.0   175    90   -10   5.06 0.5998
WVFGRD96    8.0   175    90   -10   5.11 0.6251
WVFGRD96    9.0   355    85    10   5.13 0.6382
WVFGRD96   10.0   355    85    10   5.15 0.6441
WVFGRD96   11.0   355    85    10   5.17 0.6452
WVFGRD96   12.0   175    90    -5   5.19 0.6406
WVFGRD96   13.0   175    90    -5   5.21 0.6346
WVFGRD96   14.0   175    90    -5   5.22 0.6257
WVFGRD96   15.0   175    90    -5   5.23 0.6142
WVFGRD96   16.0   175    90    -5   5.24 0.6002
WVFGRD96   17.0   175    90    -5   5.25 0.5854
WVFGRD96   18.0   355    85     5   5.26 0.5706
WVFGRD96   19.0   355    85     5   5.27 0.5529
WVFGRD96   20.0   175    90    -5   5.27 0.5322
WVFGRD96   21.0   355    85     5   5.28 0.5149
WVFGRD96   22.0   355    85     5   5.28 0.4961
WVFGRD96   23.0   175    90    -5   5.29 0.4762
WVFGRD96   24.0   175    90    -5   5.29 0.4583
WVFGRD96   25.0   175    90    -5   5.29 0.4397
WVFGRD96   26.0   175    90   -10   5.29 0.4239
WVFGRD96   27.0   175    90   -10   5.29 0.4079
WVFGRD96   28.0   175    90   -10   5.29 0.3912
WVFGRD96   29.0   265    90    -5   5.30 0.3833

The best solution is

WVFGRD96   11.0   355    85    10   5.17 0.6452

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