The ANSS event ID is ak018afourf2 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018afourf2/executive.
2018/08/15 16:56:20 62.125 -149.705 49.7 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/08/15 16:56:20:0 62.12 -149.71 49.7 4.2 Alaska
Stations used:
AK.CUT AK.DHY AK.DIV AK.GHO AK.GLI AK.KLU AK.KNK AK.MCK
AK.PAX AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SWD
AT.PMR AV.SPU TA.L19K TA.M19K TA.M20K TA.M22K TA.M24K
TA.O22K
Filtering commands used:
cut o DIST/3.4 -30 o DIST/3.4 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.43e+22 dyne-cm
Mw = 4.19
Z = 58 km
Plane Strike Dip Rake
NP1 225 75 -75
NP2 359 21 -134
Principal Axes:
Axis Value Plunge Azimuth
T 2.43e+22 28 303
N 0.00e+00 14 41
P -2.43e+22 57 155
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.07e+20
Mxy -5.86e+21
Mxz 1.55e+22
Myy 1.19e+22
Myz -1.32e+22
Mzz -1.17e+22
###########---
##################----
########################----
##############################
##########################----####
#### #################--------####
##### T ###############-----------####
###### ############---------------####
###################-----------------####
##################-------------------#####
#################---------------------####
###############-----------------------####
#############------------------------#####
###########-------------------------####
##########------------ -----------####
#######-------------- P ----------####
#####--------------- ---------####
###---------------------------####
#--------------------------###
------------------------####
-------------------###
-----------###
Global CMT Convention Moment Tensor:
R T P
-1.17e+22 1.55e+22 1.32e+22
1.55e+22 -2.07e+20 5.86e+21
1.32e+22 5.86e+21 1.19e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180815165620/index.html
|
STK = 225
DIP = 75
RAKE = -75
MW = 4.19
HS = 58.0
The NDK file is 20180815165620.ndk The waveform inversion is preferred.
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 ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML 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.
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.
 |
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.
|
|
|
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/3.4 -30 o DIST/3.4 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 240 50 -85 3.37 0.2030
WVFGRD96 4.0 285 35 10 3.42 0.1973
WVFGRD96 6.0 290 40 20 3.46 0.2404
WVFGRD96 8.0 305 40 30 3.55 0.2646
WVFGRD96 10.0 315 40 45 3.59 0.2838
WVFGRD96 12.0 320 45 50 3.63 0.2940
WVFGRD96 14.0 120 55 20 3.65 0.3002
WVFGRD96 16.0 125 50 25 3.67 0.3092
WVFGRD96 18.0 125 50 25 3.70 0.3163
WVFGRD96 20.0 125 50 25 3.73 0.3196
WVFGRD96 22.0 130 45 30 3.75 0.3166
WVFGRD96 24.0 125 45 25 3.77 0.3091
WVFGRD96 26.0 105 55 10 3.79 0.3085
WVFGRD96 28.0 100 45 -15 3.79 0.3171
WVFGRD96 30.0 95 40 -20 3.81 0.3355
WVFGRD96 32.0 90 35 -30 3.83 0.3589
WVFGRD96 34.0 90 30 -30 3.85 0.3803
WVFGRD96 36.0 215 70 -85 3.88 0.4082
WVFGRD96 38.0 220 70 -80 3.90 0.4398
WVFGRD96 40.0 225 75 -80 4.04 0.4698
WVFGRD96 42.0 225 70 -80 4.06 0.4934
WVFGRD96 44.0 225 70 -80 4.08 0.5127
WVFGRD96 46.0 225 70 -80 4.10 0.5283
WVFGRD96 48.0 225 70 -80 4.12 0.5407
WVFGRD96 50.0 225 70 -75 4.14 0.5511
WVFGRD96 52.0 225 70 -75 4.15 0.5585
WVFGRD96 54.0 225 75 -75 4.16 0.5636
WVFGRD96 56.0 225 75 -75 4.18 0.5683
WVFGRD96 58.0 225 75 -75 4.19 0.5708
WVFGRD96 60.0 225 75 -75 4.20 0.5697
WVFGRD96 62.0 225 75 -75 4.21 0.5671
WVFGRD96 64.0 225 80 -75 4.21 0.5643
WVFGRD96 66.0 225 80 -75 4.22 0.5633
WVFGRD96 68.0 225 80 -75 4.22 0.5636
WVFGRD96 70.0 225 80 -75 4.23 0.5612
WVFGRD96 72.0 225 80 -75 4.23 0.5554
WVFGRD96 74.0 225 85 -75 4.23 0.5546
WVFGRD96 76.0 225 85 -75 4.24 0.5533
WVFGRD96 78.0 225 85 -75 4.24 0.5500
WVFGRD96 80.0 45 90 75 4.23 0.5309
WVFGRD96 82.0 225 85 -75 4.25 0.5393
WVFGRD96 84.0 45 90 75 4.24 0.5255
WVFGRD96 86.0 45 90 75 4.25 0.5207
WVFGRD96 88.0 40 90 75 4.25 0.5147
WVFGRD96 90.0 220 85 -75 4.25 0.5111
WVFGRD96 92.0 220 85 -75 4.25 0.5029
WVFGRD96 94.0 45 90 70 4.26 0.4924
WVFGRD96 96.0 45 90 70 4.26 0.4847
WVFGRD96 98.0 225 90 -70 4.26 0.4765
The best solution is
WVFGRD96 58.0 225 75 -75 4.19 0.5708
The mechanism corresponding to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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/3.4 -30 o DIST/3.4 +60 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:
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.
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