Location
SLU Location
To check the ANSS location or to compare the observed P-wave first motions to the moment tensor solution, P- and S-wave first arrival times were manually read together with the P-wave first motions. The subsequent output of the program elocate is given in the file elocate.txt. The first motion plot is shown below.
Location ANSS
The ANSS event ID is ak024fhjxlyu and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak024fhjxlyu/executive.
2024/12/02 13:14:08 61.469 -146.616 7.5 3.7 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2024/12/02 13:14:08:0 61.47 -146.62 7.5 3.7 Alaska
Stations used:
AK.BAE AK.BMR AK.CAST AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO
AK.HIN AK.KLU AK.KNK AK.L19K AK.L26K AK.M20K AK.PWL AK.RC01
AK.RIDG AK.SCM AK.WAT6 AT.PMR AV.WAZA CN.BVCY IU.COLA
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 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 5.89e+21 dyne-cm
Mw = 3.78
Z = 41 km
Plane Strike Dip Rake
NP1 191 57 -103
NP2 35 35 -70
Principal Axes:
Axis Value Plunge Azimuth
T 5.89e+21 11 291
N 0.00e+00 11 198
P -5.89e+21 74 65
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.25e+20
Mxy -2.05e+21
Mxz -2.66e+20
Myy 4.57e+21
Myz -2.50e+21
Mzz -5.20e+21
#########-----
###########-----------
############---------------#
############-----------------#
#############------------------###
#############--------------------###
##########---------------------####
# T #########----------------------#####
# #########----------- --------#####
#############------------ P --------######
#############------------ --------######
#############----------------------#######
############-----------------------#######
###########----------------------#######
###########---------------------########
##########--------------------########
##########-----------------#########
#########---------------##########
#######-------------##########
#######---------############
--###-################
--############
Global CMT Convention Moment Tensor:
R T P
-5.20e+21 -2.66e+20 2.50e+21
-2.66e+20 6.25e+20 2.05e+21
2.50e+21 2.05e+21 4.57e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20241202131408/index.html
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 35
DIP = 35
RAKE = -70
MW = 3.78
HS = 41.0
The NDK file is 20241202131408.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
| SLU |
SLUFM |
USGS/SLU Moment Tensor Solution
ENS 2024/12/02 13:14:08:0 61.47 -146.62 7.5 3.7 Alaska
Stations used:
AK.BAE AK.BMR AK.CAST AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO
AK.HIN AK.KLU AK.KNK AK.L19K AK.L26K AK.M20K AK.PWL AK.RC01
AK.RIDG AK.SCM AK.WAT6 AT.PMR AV.WAZA CN.BVCY IU.COLA
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 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 5.89e+21 dyne-cm
Mw = 3.78
Z = 41 km
Plane Strike Dip Rake
NP1 191 57 -103
NP2 35 35 -70
Principal Axes:
Axis Value Plunge Azimuth
T 5.89e+21 11 291
N 0.00e+00 11 198
P -5.89e+21 74 65
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.25e+20
Mxy -2.05e+21
Mxz -2.66e+20
Myy 4.57e+21
Myz -2.50e+21
Mzz -5.20e+21
#########-----
###########-----------
############---------------#
############-----------------#
#############------------------###
#############--------------------###
##########---------------------####
# T #########----------------------#####
# #########----------- --------#####
#############------------ P --------######
#############------------ --------######
#############----------------------#######
############-----------------------#######
###########----------------------#######
###########---------------------########
##########--------------------########
##########-----------------#########
#########---------------##########
#######-------------##########
#######---------############
--###-################
--############
Global CMT Convention Moment Tensor:
R T P
-5.20e+21 -2.66e+20 2.50e+21
-2.66e+20 6.25e+20 2.05e+21
2.50e+21 2.05e+21 4.57e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20241202131408/index.html
|
First motions and takeoff angles from an elocate run.
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Magnitudes
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.
ML Magnitude
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.
Context
The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.
Waveform Inversion using wvfgrd96
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.
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Location of broadband stations used for waveform inversion
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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 0.08 n 3
br c 0.12 0.25 n 4 p 2
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 20 45 90 3.08 0.2972
WVFGRD96 2.0 295 45 90 3.28 0.4307
WVFGRD96 3.0 35 50 -75 3.27 0.4253
WVFGRD96 4.0 45 60 -60 3.30 0.4419
WVFGRD96 5.0 40 60 -70 3.35 0.4495
WVFGRD96 6.0 245 40 -10 3.30 0.4627
WVFGRD96 7.0 250 70 50 3.34 0.4792
WVFGRD96 8.0 240 35 -20 3.37 0.4845
WVFGRD96 9.0 250 70 50 3.39 0.4951
WVFGRD96 10.0 255 60 45 3.39 0.5085
WVFGRD96 11.0 255 60 45 3.40 0.5196
WVFGRD96 12.0 255 60 40 3.40 0.5283
WVFGRD96 13.0 230 40 -40 3.42 0.5399
WVFGRD96 14.0 230 40 -45 3.43 0.5506
WVFGRD96 15.0 230 45 -40 3.44 0.5612
WVFGRD96 16.0 230 45 -40 3.45 0.5696
WVFGRD96 17.0 230 50 -40 3.46 0.5777
WVFGRD96 18.0 230 50 -40 3.47 0.5851
WVFGRD96 19.0 230 50 -40 3.48 0.5906
WVFGRD96 20.0 230 50 -40 3.49 0.5941
WVFGRD96 21.0 230 50 -40 3.50 0.5957
WVFGRD96 22.0 230 50 -40 3.51 0.5975
WVFGRD96 23.0 230 55 -40 3.51 0.5985
WVFGRD96 24.0 230 55 -40 3.52 0.5995
WVFGRD96 25.0 230 60 -45 3.53 0.6001
WVFGRD96 26.0 230 60 -45 3.53 0.6019
WVFGRD96 27.0 230 60 -45 3.54 0.6029
WVFGRD96 28.0 230 60 -45 3.55 0.6029
WVFGRD96 29.0 225 55 -50 3.56 0.6020
WVFGRD96 30.0 225 60 -50 3.57 0.6004
WVFGRD96 31.0 225 55 -50 3.58 0.5984
WVFGRD96 32.0 45 45 -60 3.60 0.5982
WVFGRD96 33.0 50 45 -55 3.61 0.6044
WVFGRD96 34.0 45 45 -60 3.63 0.6100
WVFGRD96 35.0 40 40 -65 3.64 0.6155
WVFGRD96 36.0 40 40 -65 3.65 0.6204
WVFGRD96 37.0 40 40 -65 3.66 0.6239
WVFGRD96 38.0 40 40 -65 3.68 0.6263
WVFGRD96 39.0 40 40 -65 3.69 0.6274
WVFGRD96 40.0 35 35 -70 3.77 0.6302
WVFGRD96 41.0 35 35 -70 3.78 0.6310
WVFGRD96 42.0 35 35 -70 3.78 0.6290
WVFGRD96 43.0 35 35 -70 3.79 0.6257
WVFGRD96 44.0 35 35 -70 3.80 0.6207
WVFGRD96 45.0 35 35 -70 3.80 0.6144
WVFGRD96 46.0 35 35 -70 3.81 0.6076
WVFGRD96 47.0 35 35 -70 3.81 0.5994
WVFGRD96 48.0 35 35 -70 3.81 0.5911
WVFGRD96 49.0 35 35 -70 3.82 0.5814
WVFGRD96 50.0 35 35 -70 3.82 0.5724
WVFGRD96 51.0 40 35 -65 3.82 0.5618
WVFGRD96 52.0 45 40 -55 3.80 0.5533
WVFGRD96 53.0 50 45 -50 3.80 0.5481
WVFGRD96 54.0 50 45 -50 3.80 0.5437
WVFGRD96 55.0 50 45 -50 3.80 0.5389
WVFGRD96 56.0 50 45 -50 3.80 0.5338
WVFGRD96 57.0 55 50 -45 3.79 0.5292
WVFGRD96 58.0 55 50 -45 3.79 0.5254
WVFGRD96 59.0 55 50 -45 3.79 0.5212
The best solution is
WVFGRD96 41.0 35 35 -70 3.78 0.6310
The mechanism corresponding to the best fit is
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Figure 1. Waveform inversion focal mechanism
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The best fit as a function of depth is given in the following figure:
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Figure 2. Depth sensitivity for waveform mechanism
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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 0.08 n 3
br c 0.12 0.25 n 4 p 2
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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.
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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.
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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)
Assuming only a mislocation, the time shifts are fit to a functional form:
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.
Velocity Model
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
Last Changed Wed Dec 4 08:41:05 CST 2024
