Location

Location ANSS

The ANSS event ID is ak018694j0sh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018694j0sh/executive.

2018/05/16 12:49:55 66.349 -157.520 7.2 3.5 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/05/16 12:49:55:0 66.35 -157.52 7.2 3.5 Alaska Stations used: AK.BPAW AK.CAST AK.KTH AK.MLY AK.NEA2 AK.RDOG TA.C18K TA.C19K TA.D19K TA.D22K TA.E18K TA.E19K TA.F15K TA.F17K TA.F21K TA.F24K TA.G16K TA.G18K TA.G21K TA.G23K TA.H18K TA.H19K TA.H21K TA.H23K TA.I20K TA.I23K TA.J16K TA.J17K TA.J18K TA.J19K TA.J20K TA.K17K TA.K20K TA.TOLK 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 0.07 n 3 Best Fitting Double Couple Mo = 6.76e+21 dyne-cm Mw = 3.82 Z = 3 km Plane Strike Dip Rake NP1 258 69 -148 NP2 155 60 -25 Principal Axes: Axis Value Plunge Azimuth T 6.76e+21 5 25 N 0.00e+00 52 288 P -6.76e+21 38 119 Moment Tensor: (dyne-cm) Component Value Mxx 4.51e+21 Mxy 4.36e+21 Mxz 2.17e+21 Myy -2.03e+21 Myz -2.59e+21 Mzz -2.47e+21 ############## --################ T # ----################# #### -----######################### -------########################### --------############################ ---------############################# ----------############################## ----------######------------------------ -----------#------------------------------ -------#####------------------------------ ----#########----------------------------- --############---------------------------- ##############---------------- ------- ###############--------------- P ------- ###############-------------- ------ ###############--------------------- ################------------------ ################-------------- #################----------- #################----- ############## Global CMT Convention Moment Tensor: R T P -2.47e+21 2.17e+21 2.59e+21 2.17e+21 4.51e+21 -4.36e+21 2.59e+21 -4.36e+21 -2.03e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180516124955/index.html

Preferred Solution

      STK = 155
      DIP = 60
     RAKE = -25
       MW = 3.82
       HS = 3.0

The NDK file is 20180516124955.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	SLUFM
USGS/SLU Moment Tensor Solution ENS 2018/05/16 12:49:55:0 66.35 -157.52 7.2 3.5 Alaska Stations used: AK.BPAW AK.CAST AK.KTH AK.MLY AK.NEA2 AK.RDOG TA.C18K TA.C19K TA.D19K TA.D22K TA.E18K TA.E19K TA.F15K TA.F17K TA.F21K TA.F24K TA.G16K TA.G18K TA.G21K TA.G23K TA.H18K TA.H19K TA.H21K TA.H23K TA.I20K TA.I23K TA.J16K TA.J17K TA.J18K TA.J19K TA.J20K TA.K17K TA.K20K TA.TOLK 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 0.07 n 3 Best Fitting Double Couple Mo = 6.76e+21 dyne-cm Mw = 3.82 Z = 3 km Plane Strike Dip Rake NP1 258 69 -148 NP2 155 60 -25 Principal Axes: Axis Value Plunge Azimuth T 6.76e+21 5 25 N 0.00e+00 52 288 P -6.76e+21 38 119 Moment Tensor: (dyne-cm) Component Value Mxx 4.51e+21 Mxy 4.36e+21 Mxz 2.17e+21 Myy -2.03e+21 Myz -2.59e+21 Mzz -2.47e+21 ############## --################ T # ----################# #### -----######################### -------########################### --------############################ ---------############################# ----------############################## ----------######------------------------ -----------#------------------------------ -------#####------------------------------ ----#########----------------------------- --############---------------------------- ##############---------------- ------- ###############--------------- P ------- ###############-------------- ------ ###############--------------------- ################------------------ ################-------------- #################----------- #################----- ############## Global CMT Convention Moment Tensor: R T P -2.47e+21 2.17e+21 2.59e+21 2.17e+21 4.51e+21 -4.36e+21 2.59e+21 -4.36e+21 -2.03e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180516124955/index.html	egional Moment Tensor (Mwr) Moment 7.111e+14 N-m Magnitude 3.8 Mwr Depth 3.0 km Percent DC 94 % Half Duration – Catalog US Data Source US2 Contributor US2 Nodal Planes Plane Strike Dip Rake NP1 153 61 -31 NP2 260 63 -148 Principal Axes Axis Value Plunge Azimuth T 7.002e+14 N-m 1 26 N 0.214e+14 N-m 49 295 P -7.216e+14 N-m 41 117	First motions and takeoff angles from an elocate run.

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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   160    55    -5   3.77 0.5572
WVFGRD96    2.0   155    60   -25   3.80 0.5832
WVFGRD96    3.0   155    60   -25   3.82 0.5907
WVFGRD96    4.0   155    65   -25   3.82 0.5781
WVFGRD96    5.0   160    65   -15   3.81 0.5601
WVFGRD96    6.0   160    65   -10   3.81 0.5485
WVFGRD96    7.0   165    65    20   3.81 0.5442
WVFGRD96    8.0   165    65    20   3.82 0.5491
WVFGRD96    9.0   160    70    15   3.82 0.5523
WVFGRD96   10.0   165    65    20   3.84 0.5558
WVFGRD96   11.0   165    65    20   3.85 0.5559
WVFGRD96   12.0   165    65    20   3.85 0.5539
WVFGRD96   13.0   165    65    20   3.86 0.5513
WVFGRD96   14.0   160    70    15   3.86 0.5478
WVFGRD96   15.0   160    70    15   3.87 0.5440
WVFGRD96   16.0   345    75    25   3.89 0.5432
WVFGRD96   17.0   345    75    20   3.89 0.5406
WVFGRD96   18.0   345    75    20   3.90 0.5376
WVFGRD96   19.0   345    75    20   3.91 0.5342
WVFGRD96   20.0   345    75    25   3.92 0.5305
WVFGRD96   21.0   155    75   -25   3.92 0.5231
WVFGRD96   22.0   155    75   -25   3.93 0.5184
WVFGRD96   23.0   155    75   -25   3.93 0.5129
WVFGRD96   24.0   160    80   -20   3.94 0.5068
WVFGRD96   25.0   160    80   -20   3.95 0.5000
WVFGRD96   26.0   160    80   -20   3.96 0.4927
WVFGRD96   27.0   160    80   -20   3.96 0.4851
WVFGRD96   28.0   160    80   -20   3.97 0.4770
WVFGRD96   29.0   160    80   -20   3.97 0.4687

The best solution is

WVFGRD96    3.0   155    60   -25   3.82 0.5907

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 0.07 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 CUS.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
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 0.00  0.00  1.00  1.00