Location
Location ANSS
The ANSS event ID is ak0175bwasnf and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0175bwasnf/executive.
2017/04/26 03:29:09 62.227 -149.345 41.7 4.2 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2017/04/26 03:29:09:0 62.23 -149.35 41.7 4.2 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAPN AK.CAST AK.CUT AK.DIV AK.GHO AK.KLU
AK.KNK AK.KTH AK.MCK AK.NEA2 AK.PAX AK.PPLA AK.PWL AK.RC01
AK.RIDG AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR
TA.M20K TA.M22K TA.M24K TA.N25K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.26e+22 dyne-cm
Mw = 4.17
Z = 58 km
Plane Strike Dip Rake
NP1 50 50 -75
NP2 207 42 -107
Principal Axes:
Axis Value Plunge Azimuth
T 2.26e+22 4 129
N 0.00e+00 11 220
P -2.26e+22 78 21
Moment Tensor: (dyne-cm)
Component Value
Mxx 8.22e+21
Mxy -1.14e+22
Mxz -5.33e+21
Myy 1.33e+22
Myz -4.45e+20
Mzz -2.15e+22
##############
##############--------
#############---------------
###########-------------------
###########----------------------#
###########-----------------------##
##########-------------------------###
##########--------------------------####
#########----------- ------------#####
#########------------ P -----------#######
#########------------ -----------#######
########-------------------------#########
########------------------------##########
#######-----------------------##########
######----------------------############
######-------------------#############
#####----------------########### #
####-------------############## T
-##------####################
---#########################
-#####################
##############
Global CMT Convention Moment Tensor:
R T P
-2.15e+22 -5.33e+21 4.45e+20
-5.33e+21 8.22e+21 1.14e+22
4.45e+20 1.14e+22 1.33e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170426032909/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 = 50
DIP = 50
RAKE = -75
MW = 4.17
HS = 58.0
The NDK file is 20170426032909.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 |
USGSMT |
USGS/SLU Moment Tensor Solution
ENS 2017/04/26 03:29:09:0 62.23 -149.35 41.7 4.2 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAPN AK.CAST AK.CUT AK.DIV AK.GHO AK.KLU
AK.KNK AK.KTH AK.MCK AK.NEA2 AK.PAX AK.PPLA AK.PWL AK.RC01
AK.RIDG AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR
TA.M20K TA.M22K TA.M24K TA.N25K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.26e+22 dyne-cm
Mw = 4.17
Z = 58 km
Plane Strike Dip Rake
NP1 50 50 -75
NP2 207 42 -107
Principal Axes:
Axis Value Plunge Azimuth
T 2.26e+22 4 129
N 0.00e+00 11 220
P -2.26e+22 78 21
Moment Tensor: (dyne-cm)
Component Value
Mxx 8.22e+21
Mxy -1.14e+22
Mxz -5.33e+21
Myy 1.33e+22
Myz -4.45e+20
Mzz -2.15e+22
##############
##############--------
#############---------------
###########-------------------
###########----------------------#
###########-----------------------##
##########-------------------------###
##########--------------------------####
#########----------- ------------#####
#########------------ P -----------#######
#########------------ -----------#######
########-------------------------#########
########------------------------##########
#######-----------------------##########
######----------------------############
######-------------------#############
#####----------------########### #
####-------------############## T
-##------####################
---#########################
-#####################
##############
Global CMT Convention Moment Tensor:
R T P
-2.15e+22 -5.33e+21 4.45e+20
-5.33e+21 8.22e+21 1.14e+22
4.45e+20 1.14e+22 1.33e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170426032909/index.html
|
Regional Moment Tensor (Mwr)
Moment 2.351e+15 N-m
Magnitude 4.2 Mwr
Depth 59.0 km
Percent DC 89 %
Half Duration –
Catalog AK
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 206 48 -110
NP2 54 46 -69
Principal Axes
Axis Value Plunge Azimuth
T 2.283e+15 N-m 1 310
N 0.130e+15 N-m 15 219
P -2.413e+15 N-m 75 44
|
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 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 225 50 95 3.45 0.2112
WVFGRD96 4.0 190 25 40 3.49 0.1979
WVFGRD96 6.0 185 30 35 3.50 0.2275
WVFGRD96 8.0 205 65 80 3.62 0.2519
WVFGRD96 10.0 200 65 70 3.64 0.2647
WVFGRD96 12.0 45 50 -70 3.67 0.2675
WVFGRD96 14.0 50 50 -65 3.69 0.2659
WVFGRD96 16.0 55 50 -60 3.72 0.2624
WVFGRD96 18.0 55 50 -60 3.74 0.2586
WVFGRD96 20.0 60 55 -55 3.76 0.2513
WVFGRD96 22.0 70 50 -40 3.79 0.2450
WVFGRD96 24.0 70 50 -40 3.80 0.2425
WVFGRD96 26.0 265 70 50 3.82 0.2440
WVFGRD96 28.0 265 75 50 3.84 0.2439
WVFGRD96 30.0 70 85 -50 3.86 0.2471
WVFGRD96 32.0 70 80 -50 3.88 0.2583
WVFGRD96 34.0 70 65 -50 3.90 0.2855
WVFGRD96 36.0 70 65 -50 3.92 0.3186
WVFGRD96 38.0 65 60 -55 3.95 0.3513
WVFGRD96 40.0 60 60 -65 4.06 0.3908
WVFGRD96 42.0 60 60 -65 4.09 0.4076
WVFGRD96 44.0 60 60 -65 4.11 0.4250
WVFGRD96 46.0 55 55 -70 4.13 0.4405
WVFGRD96 48.0 55 55 -70 4.14 0.4521
WVFGRD96 50.0 55 55 -70 4.15 0.4590
WVFGRD96 52.0 55 55 -70 4.16 0.4643
WVFGRD96 54.0 55 55 -70 4.16 0.4659
WVFGRD96 56.0 50 50 -75 4.17 0.4682
WVFGRD96 58.0 50 50 -75 4.17 0.4686
WVFGRD96 60.0 50 50 -75 4.18 0.4682
WVFGRD96 62.0 50 50 -75 4.18 0.4668
WVFGRD96 64.0 55 50 -70 4.18 0.4643
WVFGRD96 66.0 55 50 -70 4.18 0.4609
WVFGRD96 68.0 50 45 -75 4.18 0.4568
WVFGRD96 70.0 50 45 -75 4.18 0.4535
WVFGRD96 72.0 50 45 -75 4.18 0.4500
WVFGRD96 74.0 55 45 -70 4.18 0.4458
WVFGRD96 76.0 55 45 -70 4.18 0.4407
WVFGRD96 78.0 55 45 -70 4.18 0.4364
WVFGRD96 80.0 55 45 -70 4.18 0.4325
WVFGRD96 82.0 55 45 -70 4.18 0.4279
WVFGRD96 84.0 55 45 -70 4.18 0.4239
WVFGRD96 86.0 60 45 -65 4.19 0.4200
WVFGRD96 88.0 60 45 -65 4.19 0.4149
WVFGRD96 90.0 60 45 -65 4.19 0.4110
WVFGRD96 92.0 60 45 -65 4.19 0.4073
WVFGRD96 94.0 60 45 -65 4.19 0.4031
WVFGRD96 96.0 60 45 -65 4.19 0.3983
WVFGRD96 98.0 60 45 -65 4.19 0.3940
The best solution is
WVFGRD96 58.0 50 50 -75 4.17 0.4686
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 -30 o DIST/3.3 +70
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.
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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 Sat Apr 27 12:09:40 PM CDT 2024
