The ANSS event ID is ak020cwv4p5l and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020cwv4p5l/executive.
2020/10/07 08:59:30 61.643 -151.063 68.8 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/10/07 08:59:30:0 61.64 -151.06 68.8 4.1 Alaska
Stations used:
AK.CAST AK.CUT AK.DHY AK.GHO AK.KNK AK.L19K AK.L20K AK.M20K
AK.N19K AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF
AT.PMR AV.RED AV.SPU AV.STLK TA.M22K TA.O22K
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 = 2.11e+22 dyne-cm
Mw = 4.15
Z = 76 km
Plane Strike Dip Rake
NP1 190 80 -70
NP2 306 22 -153
Principal Axes:
Axis Value Plunge Azimuth
T 2.11e+22 32 263
N 0.00e+00 20 6
P -2.11e+22 51 123
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.23e+21
Mxy 5.53e+21
Mxz 4.48e+21
Myy 9.02e+21
Myz -1.82e+22
Mzz -6.79e+21
----------####
-------------#########
---############----#########
################--------######
#################-----------######
##################-------------#####
##################----------------####
###################-----------------####
##################-------------------###
###################-------------------####
##### ###########--------------------###
##### T ##########---------------------###
##### ##########---------- ---------##
#################---------- P ---------#
################----------- --------##
###############----------------------#
##############----------------------
#############---------------------
###########-------------------
##########------------------
#######---------------
###-----------
Global CMT Convention Moment Tensor:
R T P
-6.79e+21 4.48e+21 1.82e+22
4.48e+21 -2.23e+21 -5.53e+21
1.82e+22 -5.53e+21 9.02e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201007085930/index.html
|
STK = 190
DIP = 80
RAKE = -70
MW = 4.15
HS = 76.0
The NDK file is 20201007085930.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/3.3 -40 o DIST/3.3 +50 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 2.0 155 45 65 3.26 0.1210
WVFGRD96 4.0 295 55 -30 3.33 0.1353
WVFGRD96 6.0 300 60 -25 3.38 0.1621
WVFGRD96 8.0 300 55 -25 3.47 0.1785
WVFGRD96 10.0 120 60 -25 3.51 0.1883
WVFGRD96 12.0 120 65 -25 3.55 0.1945
WVFGRD96 14.0 120 65 -25 3.59 0.1951
WVFGRD96 16.0 225 65 40 3.61 0.1941
WVFGRD96 18.0 215 70 40 3.63 0.1996
WVFGRD96 20.0 210 80 35 3.66 0.2055
WVFGRD96 22.0 210 80 40 3.69 0.2114
WVFGRD96 24.0 210 80 40 3.71 0.2179
WVFGRD96 26.0 210 80 40 3.73 0.2234
WVFGRD96 28.0 210 80 40 3.75 0.2271
WVFGRD96 30.0 210 80 40 3.76 0.2266
WVFGRD96 32.0 210 75 -25 3.78 0.2284
WVFGRD96 34.0 205 70 -30 3.79 0.2382
WVFGRD96 36.0 205 70 -30 3.81 0.2453
WVFGRD96 38.0 205 70 -30 3.84 0.2537
WVFGRD96 40.0 205 80 -45 3.94 0.2663
WVFGRD96 42.0 30 90 50 3.98 0.2738
WVFGRD96 44.0 205 80 -50 3.99 0.2908
WVFGRD96 46.0 205 80 -50 4.01 0.3057
WVFGRD96 48.0 205 80 -50 4.03 0.3230
WVFGRD96 50.0 200 75 -55 4.04 0.3391
WVFGRD96 52.0 190 75 -65 4.06 0.3548
WVFGRD96 54.0 190 75 -65 4.07 0.3698
WVFGRD96 56.0 190 75 -65 4.09 0.3836
WVFGRD96 58.0 190 75 -65 4.10 0.3949
WVFGRD96 60.0 190 75 -65 4.10 0.4043
WVFGRD96 62.0 190 75 -65 4.11 0.4111
WVFGRD96 64.0 190 75 -70 4.12 0.4175
WVFGRD96 66.0 190 75 -70 4.13 0.4210
WVFGRD96 68.0 190 75 -70 4.13 0.4237
WVFGRD96 70.0 190 80 -70 4.14 0.4265
WVFGRD96 72.0 190 80 -70 4.14 0.4290
WVFGRD96 74.0 190 80 -70 4.15 0.4294
WVFGRD96 76.0 190 80 -70 4.15 0.4297
WVFGRD96 78.0 190 80 -70 4.16 0.4286
WVFGRD96 80.0 190 80 -70 4.16 0.4267
WVFGRD96 82.0 190 80 -70 4.16 0.4248
WVFGRD96 84.0 190 80 -70 4.17 0.4209
WVFGRD96 86.0 190 80 -70 4.17 0.4171
WVFGRD96 88.0 190 80 -70 4.17 0.4119
WVFGRD96 90.0 185 80 -75 4.18 0.4069
WVFGRD96 92.0 185 80 -70 4.18 0.4010
WVFGRD96 94.0 190 85 -70 4.18 0.3965
WVFGRD96 96.0 190 85 -70 4.18 0.3914
WVFGRD96 98.0 185 75 -70 4.17 0.3885
The best solution is
WVFGRD96 76.0 190 80 -70 4.15 0.4297
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/3.3 -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:
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 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