The ANSS event ID is ak023972vvz7 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023972vvz7/executive.
2023/07/19 19:15:12 62.225 -147.865 31.8 3.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/07/19 19:15:12:0 62.22 -147.87 31.8 3.4 Alaska
Stations used:
AK.DHY AK.FID AK.GHO AK.GLI AK.HARP AK.KNK AK.L22K AK.PAX
AK.SAW AK.SCM AK.SKN AT.PMR
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 = 2.32e+21 dyne-cm
Mw = 3.51
Z = 40 km
Plane Strike Dip Rake
NP1 35 60 -90
NP2 215 30 -90
Principal Axes:
Axis Value Plunge Azimuth
T 2.32e+21 15 125
N 0.00e+00 -0 35
P -2.32e+21 75 305
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.60e+20
Mxy -9.43e+20
Mxz -6.65e+20
Myy 1.35e+21
Myz 9.49e+20
Mzz -2.01e+21
##############
############---------#
##########----------------##
########-------------------###
########---------------------#####
#######-----------------------######
#######------------------------#######
#######------------------------#########
######--------- -------------#########
######---------- P ------------###########
######---------- -----------############
#####------------------------#############
#####-----------------------##############
####----------------------##############
####---------------------###############
###-------------------########## ###
###----------------############ T ##
##--------------############## #
#-----------##################
#------#####################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-2.01e+21 -6.65e+20 -9.49e+20
-6.65e+20 6.60e+20 9.43e+20
-9.49e+20 9.43e+20 1.35e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230719191512/index.html
|
STK = 215
DIP = 30
RAKE = -90
MW = 3.51
HS = 40.0
The NDK file is 20230719191512.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 1.0 25 45 75 2.66 0.1593
WVFGRD96 2.0 30 45 85 2.83 0.2250
WVFGRD96 3.0 30 40 85 2.87 0.2181
WVFGRD96 4.0 190 25 60 2.88 0.2488
WVFGRD96 5.0 5 25 30 2.87 0.2794
WVFGRD96 6.0 0 25 25 2.88 0.3063
WVFGRD96 7.0 55 85 -75 2.91 0.3330
WVFGRD96 8.0 60 80 -75 3.00 0.3524
WVFGRD96 9.0 50 75 -80 3.05 0.3738
WVFGRD96 10.0 45 75 -85 3.08 0.3883
WVFGRD96 11.0 210 20 -105 3.10 0.3957
WVFGRD96 12.0 210 20 -100 3.13 0.3963
WVFGRD96 13.0 40 75 -90 3.15 0.3924
WVFGRD96 14.0 40 75 -90 3.16 0.3841
WVFGRD96 15.0 40 75 -90 3.17 0.3731
WVFGRD96 16.0 200 15 -110 3.18 0.3614
WVFGRD96 17.0 200 10 -110 3.18 0.3519
WVFGRD96 18.0 230 90 70 3.14 0.3453
WVFGRD96 19.0 50 90 -65 3.15 0.3450
WVFGRD96 20.0 50 90 -65 3.16 0.3479
WVFGRD96 21.0 55 90 -65 3.17 0.3512
WVFGRD96 22.0 55 90 -65 3.19 0.3573
WVFGRD96 23.0 55 90 -60 3.19 0.3633
WVFGRD96 24.0 200 20 -95 3.30 0.3682
WVFGRD96 25.0 200 20 -95 3.31 0.3732
WVFGRD96 26.0 215 20 -75 3.34 0.3756
WVFGRD96 27.0 65 75 -50 3.24 0.3794
WVFGRD96 28.0 75 80 -40 3.24 0.3835
WVFGRD96 29.0 75 80 -40 3.25 0.3867
WVFGRD96 30.0 75 80 -40 3.25 0.3934
WVFGRD96 31.0 75 80 -40 3.27 0.4059
WVFGRD96 32.0 75 80 -40 3.28 0.4156
WVFGRD96 33.0 40 60 -80 3.36 0.4382
WVFGRD96 34.0 40 60 -80 3.37 0.4611
WVFGRD96 35.0 35 60 -90 3.39 0.4824
WVFGRD96 36.0 35 60 -90 3.39 0.4995
WVFGRD96 37.0 210 35 -95 3.40 0.5120
WVFGRD96 38.0 215 30 -90 3.40 0.5189
WVFGRD96 39.0 35 55 -90 3.41 0.5245
WVFGRD96 40.0 215 30 -90 3.51 0.5569
WVFGRD96 41.0 215 30 -90 3.52 0.5560
WVFGRD96 42.0 215 30 -90 3.52 0.5538
WVFGRD96 43.0 215 30 -90 3.53 0.5498
WVFGRD96 44.0 215 30 -90 3.53 0.5473
WVFGRD96 45.0 220 30 -85 3.54 0.5430
WVFGRD96 46.0 220 30 -85 3.54 0.5422
WVFGRD96 47.0 220 30 -85 3.54 0.5378
WVFGRD96 48.0 220 30 -85 3.55 0.5371
WVFGRD96 49.0 220 30 -85 3.55 0.5328
WVFGRD96 50.0 220 30 -85 3.55 0.5316
WVFGRD96 51.0 225 35 -75 3.56 0.5293
WVFGRD96 52.0 225 35 -75 3.56 0.5279
WVFGRD96 53.0 225 35 -75 3.56 0.5286
WVFGRD96 54.0 225 35 -75 3.56 0.5261
WVFGRD96 55.0 225 35 -75 3.56 0.5266
WVFGRD96 56.0 225 35 -75 3.56 0.5250
WVFGRD96 57.0 225 35 -75 3.57 0.5255
WVFGRD96 58.0 230 35 -70 3.57 0.5232
WVFGRD96 59.0 230 35 -70 3.57 0.5245
The best solution is
WVFGRD96 40.0 215 30 -90 3.51 0.5569
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