Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/08/18 04:44:36:0 58.99 -136.39 3.8 4.0 Alaska Stations used: AK.BARN AK.BCP AK.BESE AK.CTG AK.GRNC AK.JIS AK.LOGN AK.TABL AT.SIT AT.SKAG AT.YKU2 CN.HYT CN.WHY TA.P29M TA.P33M Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.12 n 3 Best Fitting Double Couple Mo = 1.35e+22 dyne-cm Mw = 4.02 Z = 21 km Plane Strike Dip Rake NP1 185 70 80 NP2 32 22 116 Principal Axes: Axis Value Plunge Azimuth T 1.35e+22 64 79 N 0.00e+00 9 188 P -1.35e+22 24 283 Moment Tensor: (dyne-cm) Component Value Mxx -4.47e+20 Mxy 2.91e+21 Mxz -8.88e+19 Myy -8.09e+21 Myz 1.02e+22 Mzz 8.54e+21 --------###### -----------########### -------------##############- -------------################- --------------##################-- ---------------###################-- ---------------#####################-- ----------------#####################--- --- ---------######################--- ---- P ---------########## #########---- ---- ---------########## T #########---- ----------------########## #########---- ----------------#####################----- ---------------#####################---- ---------------####################----- --------------###################----- -------------##################----- ------------################------ -----------#############------ ----------###########------- --------######-------- #####--------- Global CMT Convention Moment Tensor: R T P 8.54e+21 -8.88e+19 -1.02e+22 -8.88e+19 -4.47e+20 -2.91e+21 -1.02e+22 -2.91e+21 -8.09e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170818044436/index.html

Preferred Solution

      STK = 185
      DIP = 70
     RAKE = 80
       MW = 4.02
       HS = 21.0

The NDK file is 20170818044436.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2017/08/18 04:44:36:0 58.99 -136.39 3.8 4.0 Alaska Stations used: AK.BARN AK.BCP AK.BESE AK.CTG AK.GRNC AK.JIS AK.LOGN AK.TABL AT.SIT AT.SKAG AT.YKU2 CN.HYT CN.WHY TA.P29M TA.P33M Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.12 n 3 Best Fitting Double Couple Mo = 1.35e+22 dyne-cm Mw = 4.02 Z = 21 km Plane Strike Dip Rake NP1 185 70 80 NP2 32 22 116 Principal Axes: Axis Value Plunge Azimuth T 1.35e+22 64 79 N 0.00e+00 9 188 P -1.35e+22 24 283 Moment Tensor: (dyne-cm) Component Value Mxx -4.47e+20 Mxy 2.91e+21 Mxz -8.88e+19 Myy -8.09e+21 Myz 1.02e+22 Mzz 8.54e+21 --------###### -----------########### -------------##############- -------------################- --------------##################-- ---------------###################-- ---------------#####################-- ----------------#####################--- --- ---------######################--- ---- P ---------########## #########---- ---- ---------########## T #########---- ----------------########## #########---- ----------------#####################----- ---------------#####################---- ---------------####################----- --------------###################----- -------------##################----- ------------################------ -----------#############------ ----------###########------- --------######-------- #####--------- Global CMT Convention Moment Tensor: R T P 8.54e+21 -8.88e+19 -1.02e+22 -8.88e+19 -4.47e+20 -2.91e+21 -1.02e+22 -2.91e+21 -8.09e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170818044436/index.html

Magnitudes

ML Magnitude

(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.

(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.

Context

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for 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 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 +40
rtr
taper w 0.1
hp c 0.04 n 3 
lp c 0.12 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0     0    50    95   3.73 0.4043
WVFGRD96    2.0     5    50    95   3.85 0.3980
WVFGRD96    3.0   175    55    35   3.82 0.3176
WVFGRD96    4.0   260    45    15   3.75 0.2994
WVFGRD96    5.0   290    35    15   3.72 0.3325
WVFGRD96    6.0   295    30    20   3.72 0.3696
WVFGRD96    7.0   300    30    30   3.74 0.4042
WVFGRD96    8.0   310    25    40   3.74 0.4359
WVFGRD96    9.0   310    25    40   3.76 0.4636
WVFGRD96   10.0   310    25    40   3.81 0.4887
WVFGRD96   11.0   310    30    35   3.83 0.5132
WVFGRD96   12.0   320    25    45   3.84 0.5360
WVFGRD96   13.0   320    25    45   3.86 0.5560
WVFGRD96   14.0   320    25    50   3.88 0.5730
WVFGRD96   15.0   340    20    65   3.89 0.5893
WVFGRD96   16.0   340    20    65   3.91 0.6029
WVFGRD96   17.0   185    70   100   3.93 0.6129
WVFGRD96   18.0   185    70    85   3.95 0.6224
WVFGRD96   19.0   185    70    80   3.97 0.6288
WVFGRD96   20.0   185    70    80   4.01 0.6333
WVFGRD96   21.0   185    70    80   4.02 0.6336
WVFGRD96   22.0   185    70    80   4.04 0.6307
WVFGRD96   23.0   185    70    75   4.05 0.6240
WVFGRD96   24.0   185    70    75   4.06 0.6140
WVFGRD96   25.0   180    75    65   4.09 0.6002
WVFGRD96   26.0   180    75    70   4.09 0.5841
WVFGRD96   27.0   180    75    65   4.10 0.5646
WVFGRD96   28.0   180    75    65   4.11 0.5434
WVFGRD96   29.0   180    75    65   4.11 0.5201

The best solution is

WVFGRD96   21.0   185    70    80   4.02 0.6336

The mechanism correspond 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 and because the velocity model used in the predictions may not be perfect. 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 +40
rtr
taper w 0.1
hp c 0.04 n 3 
lp c 0.12 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.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The CUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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 

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files: