Location

Location ANSS

The ANSS event ID is ak016tu367t and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak016tu367t/executive.

2016/01/18 04:05:55 62.103 -150.640 10.1 4.5 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2016/01/18 04:05:55:0 62.10 -150.64 10.1 4.5 Alaska Stations used: AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CAST AK.CCB AK.CNP AK.CUT AK.DHY AK.DIV AK.DOT AK.EYAK AK.FID AK.FIRE AK.GHO AK.GLB AK.GLI AK.HDA AK.HOM AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.PWL AK.RAG AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD AK.TRF AK.WRH AT.MENT AT.MID AT.PMR AT.SVW2 AT.TTA IM.IL31 IU.COLA TA.H21K TA.H24K TA.I21K TA.I23K TA.J20K TA.J26L TA.K20K TA.L19K TA.L26K TA.L27K TA.M22K TA.M24K TA.N19K TA.N25K TA.O18K TA.O19K TA.P18K TA.POKR TA.Q23K TA.TCOL Filtering commands used: cut o DIST/3.3 -20 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 6.61e+22 dyne-cm Mw = 4.48 Z = 16 km Plane Strike Dip Rake NP1 160 70 30 NP2 59 62 157 Principal Axes: Axis Value Plunge Azimuth T 6.61e+22 35 22 N 0.00e+00 54 191 P -6.61e+22 5 288 Moment Tensor: (dyne-cm) Component Value Mxx 3.21e+22 Mxy 3.44e+22 Mxz 2.70e+22 Myy -5.33e+22 Myz 1.71e+22 Mzz 2.12e+22 ############## ---################### ------###################### -------########### ######### ---------########### T ########### ----------########### ############ ----------########################-- P ----------#######################---- ----------######################----- ---------------####################------- ---------------##################--------- ----------------###############----------- -----------------############------------- ----------------##########-------------- -----------------######----------------- -----------------#-------------------- -------------####------------------- #################----------------- #################------------- #################----------- ################------ ############## Global CMT Convention Moment Tensor: R T P 2.12e+22 2.70e+22 -1.71e+22 2.70e+22 3.21e+22 -3.44e+22 -1.71e+22 -3.44e+22 -5.33e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160118040555/index.html

Preferred Solution

      STK = 160
      DIP = 70
     RAKE = 30
       MW = 4.48
       HS = 16.0

The NDK file is 20160118040555.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

USGSMT

USGS/SLU Moment Tensor Solution ENS 2016/01/18 04:05:55:0 62.10 -150.64 10.1 4.5 Alaska Stations used: AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CAST AK.CCB AK.CNP AK.CUT AK.DHY AK.DIV AK.DOT AK.EYAK AK.FID AK.FIRE AK.GHO AK.GLB AK.GLI AK.HDA AK.HOM AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.PWL AK.RAG AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD AK.TRF AK.WRH AT.MENT AT.MID AT.PMR AT.SVW2 AT.TTA IM.IL31 IU.COLA TA.H21K TA.H24K TA.I21K TA.I23K TA.J20K TA.J26L TA.K20K TA.L19K TA.L26K TA.L27K TA.M22K TA.M24K TA.N19K TA.N25K TA.O18K TA.O19K TA.P18K TA.POKR TA.Q23K TA.TCOL Filtering commands used: cut o DIST/3.3 -20 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 6.61e+22 dyne-cm Mw = 4.48 Z = 16 km Plane Strike Dip Rake NP1 160 70 30 NP2 59 62 157 Principal Axes: Axis Value Plunge Azimuth T 6.61e+22 35 22 N 0.00e+00 54 191 P -6.61e+22 5 288 Moment Tensor: (dyne-cm) Component Value Mxx 3.21e+22 Mxy 3.44e+22 Mxz 2.70e+22 Myy -5.33e+22 Myz 1.71e+22 Mzz 2.12e+22 ############## ---################### ------###################### -------########### ######### ---------########### T ########### ----------########### ############ ----------########################-- P ----------#######################---- ----------######################----- ---------------####################------- ---------------##################--------- ----------------###############----------- -----------------############------------- ----------------##########-------------- -----------------######----------------- -----------------#-------------------- -------------####------------------- #################----------------- #################------------- #################----------- ################------ ############## Global CMT Convention Moment Tensor: R T P 2.12e+22 2.70e+22 -1.71e+22 2.70e+22 3.21e+22 -3.44e+22 -1.71e+22 -3.44e+22 -5.33e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160118040555/index.html

Data Source US2 Regional Moment Tensor (Mwr) Moment 7.075e+15 N-m Magnitude 4.50 Depth 14.0 km Percent DC 91% Half Duration – Catalog US (us10004fac) Data Source US2 Contributor US2 Nodal Planes Plane Strike Dip Rake NP1 162 65 35 NP2 56 59 151 Principal Axes Axis Value Plunge Azimuth T 6.908 41 21 N 0.323 48 193 P -7.231 4 287

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 -20 o DIST/3.3 +70
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   160    80   -10   4.05 0.2589
WVFGRD96    2.0   150    65   -30   4.20 0.3192
WVFGRD96    3.0   325    55   -40   4.27 0.3339
WVFGRD96    4.0   325    55   -40   4.30 0.3416
WVFGRD96    5.0   330    65   -35   4.30 0.3457
WVFGRD96    6.0   335    80   -40   4.32 0.3624
WVFGRD96    7.0   335    85   -40   4.33 0.3856
WVFGRD96    8.0   335    85   -45   4.39 0.4101
WVFGRD96    9.0   335    85   -45   4.40 0.4305
WVFGRD96   10.0   165    65    40   4.43 0.4566
WVFGRD96   11.0   165    65    40   4.44 0.4847
WVFGRD96   12.0   165    65    40   4.45 0.5043
WVFGRD96   13.0   165    65    35   4.46 0.5177
WVFGRD96   14.0   165    65    35   4.47 0.5256
WVFGRD96   15.0   160    70    30   4.48 0.5299
WVFGRD96   16.0   160    70    30   4.48 0.5315
WVFGRD96   17.0   160    70    30   4.49 0.5300
WVFGRD96   18.0   160    70    30   4.50 0.5262
WVFGRD96   19.0   160    70    30   4.50 0.5205
WVFGRD96   20.0   160    70    30   4.51 0.5131
WVFGRD96   21.0   160    70    30   4.52 0.5045
WVFGRD96   22.0   160    70    25   4.52 0.4950
WVFGRD96   23.0   160    70    25   4.53 0.4849
WVFGRD96   24.0   160    70    25   4.53 0.4739
WVFGRD96   25.0   160    70    25   4.53 0.4627
WVFGRD96   26.0   160    70    25   4.54 0.4514
WVFGRD96   27.0   160    70    25   4.54 0.4397
WVFGRD96   28.0   160    70    25   4.55 0.4281
WVFGRD96   29.0   160    70    25   4.55 0.4166

The best solution is

WVFGRD96   16.0   160    70    30   4.48 0.5315

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 -20 o DIST/3.3 +70
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