The ANSS event ID is ak0206e332ic and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0206e332ic/executive.
2020/05/18 12:40:26 58.969 -154.292 116.5 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/05/18 12:40:26:0 58.97 -154.29 116.5 3.9 Alaska
Stations used:
AK.CNP AK.HOM AK.N18K AK.N19K AK.O18K AK.P16K AK.P17K
AK.Q19K AV.ILSW II.KDAK TA.N17K TA.O16K
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 = 1.35e+22 dyne-cm
Mw = 4.02
Z = 118 km
Plane Strike Dip Rake
NP1 230 85 -45
NP2 325 45 -173
Principal Axes:
Axis Value Plunge Azimuth
T 1.35e+22 26 286
N 0.00e+00 45 45
P -1.35e+22 34 177
Moment Tensor: (dyne-cm)
Component Value
Mxx -8.39e+21
Mxy -2.47e+21
Mxz 7.73e+21
Myy 1.00e+22
Myz -5.40e+21
Mzz -1.66e+21
--------------
----------------------
############----------------
################--------------
#####################----------###
########################-----#######
######################################
### ####################---###########
### T ##################------##########
#### ###############----------##########
####################-------------#########
##################----------------########
################------------------########
#############---------------------######
###########-----------------------######
########-------------------------#####
#####---------------------------####
##-------------- ------------###
-------------- P -----------##
------------- -----------#
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-1.66e+21 7.73e+21 5.40e+21
7.73e+21 -8.39e+21 2.47e+21
5.40e+21 2.47e+21 1.00e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200518124026/index.html
|
STK = 230
DIP = 85
RAKE = -45
MW = 4.02
HS = 118.0
The NDK file is 20200518124026.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 2.0 315 75 -5 3.05 0.2007
WVFGRD96 4.0 135 60 -10 3.18 0.2409
WVFGRD96 6.0 135 65 -15 3.25 0.2777
WVFGRD96 8.0 130 65 -35 3.36 0.3114
WVFGRD96 10.0 130 65 -35 3.41 0.3288
WVFGRD96 12.0 130 70 -35 3.46 0.3355
WVFGRD96 14.0 130 70 -30 3.49 0.3334
WVFGRD96 16.0 135 70 -30 3.51 0.3213
WVFGRD96 18.0 135 70 -30 3.54 0.2989
WVFGRD96 20.0 135 70 -30 3.55 0.2654
WVFGRD96 22.0 230 80 35 3.57 0.2800
WVFGRD96 24.0 230 85 30 3.60 0.3034
WVFGRD96 26.0 230 85 30 3.63 0.3217
WVFGRD96 28.0 230 80 25 3.65 0.3330
WVFGRD96 30.0 230 80 25 3.66 0.3361
WVFGRD96 32.0 230 85 25 3.67 0.3367
WVFGRD96 34.0 230 85 25 3.69 0.3335
WVFGRD96 36.0 225 85 15 3.71 0.3374
WVFGRD96 38.0 230 70 30 3.76 0.3441
WVFGRD96 40.0 235 65 40 3.85 0.3604
WVFGRD96 42.0 235 65 40 3.88 0.3637
WVFGRD96 44.0 230 70 30 3.88 0.3677
WVFGRD96 46.0 230 70 30 3.89 0.3730
WVFGRD96 48.0 230 70 30 3.91 0.3773
WVFGRD96 50.0 230 70 30 3.92 0.3814
WVFGRD96 52.0 230 70 30 3.93 0.3846
WVFGRD96 54.0 230 70 30 3.94 0.3875
WVFGRD96 56.0 230 70 30 3.95 0.3899
WVFGRD96 58.0 225 70 25 3.95 0.3933
WVFGRD96 60.0 225 75 25 3.95 0.4002
WVFGRD96 62.0 50 65 25 3.95 0.4042
WVFGRD96 64.0 225 85 -20 3.92 0.4324
WVFGRD96 66.0 225 85 -25 3.94 0.4910
WVFGRD96 68.0 50 90 30 3.95 0.5452
WVFGRD96 70.0 225 85 -35 3.96 0.5950
WVFGRD96 72.0 50 90 40 3.98 0.6114
WVFGRD96 74.0 230 90 -40 3.98 0.6200
WVFGRD96 76.0 50 90 40 3.98 0.6245
WVFGRD96 78.0 50 90 40 3.98 0.6306
WVFGRD96 80.0 230 90 -40 3.98 0.6343
WVFGRD96 82.0 230 90 -40 3.99 0.6382
WVFGRD96 84.0 50 90 40 3.99 0.6421
WVFGRD96 86.0 230 90 -40 3.99 0.6444
WVFGRD96 88.0 230 90 -40 3.99 0.6464
WVFGRD96 90.0 230 90 -40 3.99 0.6468
WVFGRD96 92.0 230 85 -40 3.99 0.6505
WVFGRD96 94.0 50 90 40 4.00 0.6506
WVFGRD96 96.0 50 90 45 4.00 0.6533
WVFGRD96 98.0 230 85 -40 3.99 0.6564
WVFGRD96 100.0 55 90 45 4.01 0.6558
WVFGRD96 102.0 230 85 -45 4.00 0.6603
WVFGRD96 104.0 55 90 45 4.01 0.6577
WVFGRD96 106.0 230 85 -45 4.01 0.6627
WVFGRD96 108.0 230 85 -45 4.01 0.6627
WVFGRD96 110.0 230 85 -45 4.01 0.6635
WVFGRD96 112.0 230 85 -45 4.01 0.6639
WVFGRD96 114.0 230 85 -45 4.01 0.6639
WVFGRD96 116.0 230 85 -45 4.02 0.6659
WVFGRD96 118.0 230 85 -45 4.02 0.6660
WVFGRD96 120.0 230 85 -45 4.02 0.6649
WVFGRD96 122.0 230 85 -45 4.02 0.6648
WVFGRD96 124.0 230 85 -45 4.03 0.6630
WVFGRD96 126.0 230 85 -45 4.03 0.6631
WVFGRD96 128.0 230 85 -45 4.03 0.6637
WVFGRD96 130.0 230 85 -45 4.03 0.6626
WVFGRD96 132.0 230 85 -45 4.03 0.6619
WVFGRD96 134.0 230 80 -45 4.03 0.6615
WVFGRD96 136.0 230 80 -45 4.04 0.6604
WVFGRD96 138.0 230 80 -45 4.04 0.6600
WVFGRD96 140.0 230 80 -45 4.04 0.6592
WVFGRD96 142.0 230 80 -45 4.04 0.6573
WVFGRD96 144.0 230 80 -45 4.04 0.6556
WVFGRD96 146.0 230 80 -45 4.05 0.6537
WVFGRD96 148.0 230 80 -45 4.05 0.6527
The best solution is
WVFGRD96 118.0 230 85 -45 4.02 0.6660
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