Location
Location ANSS
The ANSS event ID is ak0159a4e6u2 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0159a4e6u2/executive.
2015/07/21 03:08:32 62.340 -149.701 52.5 4.4 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2015/07/21 03:08:32:0 62.34 -149.70 52.5 4.4 Alaska
Stations used:
AK.BWN AK.CCB AK.FID AK.GLI AK.HDA AK.KLU AK.KNK AK.KTH
AK.MCK AK.MDM AK.NEA2 AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW
AK.SCM AK.SKN AK.SSN AK.TRF AK.WRH IM.IL31 TA.M24K TA.N25K
TA.POKR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 2.60e+22 dyne-cm
Mw = 4.21
Z = 58 km
Plane Strike Dip Rake
NP1 317 69 -131
NP2 205 45 -30
Principal Axes:
Axis Value Plunge Azimuth
T 2.60e+22 14 76
N 0.00e+00 38 334
P -2.60e+22 49 183
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.88e+21
Mxy 5.26e+21
Mxz 1.44e+22
Myy 2.29e+22
Myz 6.73e+21
Mzz -1.30e+22
--------------
--------------########
-------------###############
#####-------##################
###########-######################
###########----#####################
###########-------####################
###########----------############### #
##########-------------############# T #
##########----------------########### ##
#########------------------###############
#########--------------------#############
#########---------------------############
#######-----------------------##########
#######------------------------#########
######----------- -----------#######
######---------- P ------------#####
#####---------- ------------####
###--------------------------#
###-------------------------
#---------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-1.30e+22 1.44e+22 -6.73e+21
1.44e+22 -9.88e+21 -5.26e+21
-6.73e+21 -5.26e+21 2.29e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150721030832/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 = 205
DIP = 45
RAKE = -30
MW = 4.21
HS = 58.0
The NDK file is 20150721030832.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 2015/07/21 03:08:32:0 62.34 -149.70 52.5 4.4 Alaska
Stations used:
AK.BWN AK.CCB AK.FID AK.GLI AK.HDA AK.KLU AK.KNK AK.KTH
AK.MCK AK.MDM AK.NEA2 AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW
AK.SCM AK.SKN AK.SSN AK.TRF AK.WRH IM.IL31 TA.M24K TA.N25K
TA.POKR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 2.60e+22 dyne-cm
Mw = 4.21
Z = 58 km
Plane Strike Dip Rake
NP1 317 69 -131
NP2 205 45 -30
Principal Axes:
Axis Value Plunge Azimuth
T 2.60e+22 14 76
N 0.00e+00 38 334
P -2.60e+22 49 183
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.88e+21
Mxy 5.26e+21
Mxz 1.44e+22
Myy 2.29e+22
Myz 6.73e+21
Mzz -1.30e+22
--------------
--------------########
-------------###############
#####-------##################
###########-######################
###########----#####################
###########-------####################
###########----------############### #
##########-------------############# T #
##########----------------########### ##
#########------------------###############
#########--------------------#############
#########---------------------############
#######-----------------------##########
#######------------------------#########
######----------- -----------#######
######---------- P ------------#####
#####---------- ------------####
###--------------------------#
###-------------------------
#---------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-1.30e+22 1.44e+22 -6.73e+21
1.44e+22 -9.88e+21 -5.26e+21
-6.73e+21 -5.26e+21 2.29e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150721030832/index.html
|
Regional Moment Tensor (Mwr)
Moment 2.378e+15 N-m
Magnitude 4.18
Depth 57.0 km
Percent DC 87%
Half Duration –
Catalog AK (ak11651501)
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 208 53 -30
NP2 317 66 -139
Principal Axes
Axis Value Plunge Azimuth
T 2.295 8 79
N 0.158 43 341
P -2.453 45 178
|
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 0.02 n 3
lp c 0.05 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 160 45 -90 3.54 0.1797
WVFGRD96 4.0 260 45 85 3.62 0.1911
WVFGRD96 6.0 230 65 20 3.60 0.1911
WVFGRD96 8.0 230 50 30 3.67 0.2065
WVFGRD96 10.0 215 50 -10 3.67 0.2208
WVFGRD96 12.0 215 50 -10 3.70 0.2422
WVFGRD96 14.0 210 55 -20 3.72 0.2694
WVFGRD96 16.0 210 50 -20 3.75 0.2973
WVFGRD96 18.0 210 50 -20 3.77 0.3237
WVFGRD96 20.0 210 50 -20 3.79 0.3471
WVFGRD96 22.0 210 50 -20 3.82 0.3685
WVFGRD96 24.0 210 50 -20 3.84 0.3885
WVFGRD96 26.0 210 50 -20 3.86 0.4068
WVFGRD96 28.0 210 50 -20 3.88 0.4232
WVFGRD96 30.0 210 50 -20 3.90 0.4379
WVFGRD96 32.0 210 50 -20 3.91 0.4507
WVFGRD96 34.0 210 50 -20 3.93 0.4615
WVFGRD96 36.0 210 50 -20 3.95 0.4705
WVFGRD96 38.0 210 55 -20 3.97 0.4776
WVFGRD96 40.0 205 40 -25 4.08 0.4891
WVFGRD96 42.0 205 40 -30 4.10 0.5019
WVFGRD96 44.0 205 45 -30 4.11 0.5129
WVFGRD96 46.0 205 45 -30 4.13 0.5238
WVFGRD96 48.0 205 45 -30 4.14 0.5325
WVFGRD96 50.0 205 45 -30 4.16 0.5398
WVFGRD96 52.0 205 45 -30 4.17 0.5451
WVFGRD96 54.0 205 45 -30 4.18 0.5492
WVFGRD96 56.0 205 45 -30 4.20 0.5519
WVFGRD96 58.0 205 45 -30 4.21 0.5531
WVFGRD96 60.0 205 45 -30 4.22 0.5529
WVFGRD96 62.0 205 45 -30 4.23 0.5516
WVFGRD96 64.0 205 45 -30 4.24 0.5488
WVFGRD96 66.0 205 45 -30 4.25 0.5444
WVFGRD96 68.0 210 50 -25 4.26 0.5392
WVFGRD96 70.0 210 50 -25 4.27 0.5339
WVFGRD96 72.0 210 50 -25 4.28 0.5276
WVFGRD96 74.0 210 50 -25 4.28 0.5198
WVFGRD96 76.0 210 50 -25 4.29 0.5111
WVFGRD96 78.0 215 50 -20 4.30 0.5025
WVFGRD96 80.0 215 50 -20 4.31 0.4939
WVFGRD96 82.0 215 50 -15 4.31 0.4862
WVFGRD96 84.0 215 50 -15 4.32 0.4787
WVFGRD96 86.0 215 50 -15 4.33 0.4706
WVFGRD96 88.0 215 50 -15 4.33 0.4617
WVFGRD96 90.0 215 50 -15 4.34 0.4520
WVFGRD96 92.0 215 50 -15 4.34 0.4420
WVFGRD96 94.0 215 50 -15 4.35 0.4313
WVFGRD96 96.0 215 50 -15 4.35 0.4204
WVFGRD96 98.0 215 50 -10 4.35 0.4095
The best solution is
WVFGRD96 58.0 205 45 -30 4.21 0.5531
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 0.02 n 3
lp c 0.05 n 3
|
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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 CUS.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
CUS Model with Q from simple gamma values
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
1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00
9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00
10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00
20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00
0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00
Last Changed Fri Apr 26 08:10:17 PM CDT 2024
