Location
Location ANSS
The ANSS event ID is ak0156chdx4c and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0156chdx4c/executive.
2015/05/18 15:49:10 61.940 -150.450 21.5 4.3 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2015/05/18 15:49:10:0 61.94 -150.45 21.5 4.3 Alaska
Stations used:
AK.BARN AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CAPN AK.CCB
AK.COLD AK.CTG AK.DOT AK.EYAK AK.FID AK.FIRE AK.FYU AK.GHO
AK.GLB AK.GLI AK.GRNC AK.HIN AK.HOM AK.KLU AK.MCK AK.MDM
AK.MESA AK.MLY AK.NEA2 AK.PAX AK.PIN AK.PPD AK.PPLA AK.PWL
AK.RC01 AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN AK.SWD
AK.TABL AK.TRF AK.WAT3 AK.WAT4 AK.WAT5 AK.WRH AK.YAH
AT.MENT AT.PMR AT.SVW2 IM.IL31 IU.COLA TA.N25K US.EGAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 1.84e+22 dyne-cm
Mw = 4.11
Z = 28 km
Plane Strike Dip Rake
NP1 340 55 80
NP2 177 36 104
Principal Axes:
Axis Value Plunge Azimuth
T 1.84e+22 77 216
N 0.00e+00 8 346
P -1.84e+22 9 77
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.10e+20
Mxy -3.47e+21
Mxz -3.84e+21
Myy -1.67e+22
Myz -5.20e+21
Mzz 1.70e+22
###-----------
-----###--------------
------########--------------
------###########-------------
-------#############--------------
-------################-------------
-------##################-------------
-------####################----------
-------#####################--------- P
--------#####################--------- -
--------######################------------
--------########## #########------------
--------########## T ##########-----------
-------########## ##########----------
--------######################----------
-------######################---------
-------#####################--------
-------####################-------
------###################-----
-------################-----
------#############---
-----#########
Global CMT Convention Moment Tensor:
R T P
1.70e+22 -3.84e+21 5.20e+21
-3.84e+21 -3.10e+20 3.47e+21
5.20e+21 3.47e+21 -1.67e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150518154910/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 = 340
DIP = 55
RAKE = 80
MW = 4.11
HS = 28.0
The NDK file is 20150518154910.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 2015/05/18 15:49:10:0 61.94 -150.45 21.5 4.3 Alaska
Stations used:
AK.BARN AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CAPN AK.CCB
AK.COLD AK.CTG AK.DOT AK.EYAK AK.FID AK.FIRE AK.FYU AK.GHO
AK.GLB AK.GLI AK.GRNC AK.HIN AK.HOM AK.KLU AK.MCK AK.MDM
AK.MESA AK.MLY AK.NEA2 AK.PAX AK.PIN AK.PPD AK.PPLA AK.PWL
AK.RC01 AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN AK.SWD
AK.TABL AK.TRF AK.WAT3 AK.WAT4 AK.WAT5 AK.WRH AK.YAH
AT.MENT AT.PMR AT.SVW2 IM.IL31 IU.COLA TA.N25K US.EGAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 1.84e+22 dyne-cm
Mw = 4.11
Z = 28 km
Plane Strike Dip Rake
NP1 340 55 80
NP2 177 36 104
Principal Axes:
Axis Value Plunge Azimuth
T 1.84e+22 77 216
N 0.00e+00 8 346
P -1.84e+22 9 77
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.10e+20
Mxy -3.47e+21
Mxz -3.84e+21
Myy -1.67e+22
Myz -5.20e+21
Mzz 1.70e+22
###-----------
-----###--------------
------########--------------
------###########-------------
-------#############--------------
-------################-------------
-------##################-------------
-------####################----------
-------#####################--------- P
--------#####################--------- -
--------######################------------
--------########## #########------------
--------########## T ##########-----------
-------########## ##########----------
--------######################----------
-------######################---------
-------#####################--------
-------####################-------
------###################-----
-------################-----
------#############---
-----#########
Global CMT Convention Moment Tensor:
R T P
1.70e+22 -3.84e+21 5.20e+21
-3.84e+21 -3.10e+20 3.47e+21
5.20e+21 3.47e+21 -1.67e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150518154910/index.html
|
Regional Moment Tensor (Mwr)
Moment 1.957e+15 N-m
Magnitude 4.13
Depth 28.0 km
Percent DC 82%
Half Duration –
Catalog AK (ak11600239)
Data Source US2
Contributor US2
Nodal Planes
Plane Strike Dip Rake
NP1 338° 66° 73°
NP2 195° 29° 123°
Principal Axes
Axis Value Plunge Azimuth
T 2.040 65° 219°
N -0.179 15° 345°
P -1.861 19° 81°
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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 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.07 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 345 45 -90 3.74 0.3509
WVFGRD96 4.0 55 60 -40 3.76 0.2238
WVFGRD96 6.0 205 15 -40 3.77 0.2633
WVFGRD96 8.0 195 15 -50 3.86 0.3182
WVFGRD96 10.0 190 15 -55 3.88 0.3704
WVFGRD96 12.0 175 10 -70 3.89 0.4101
WVFGRD96 14.0 320 20 75 3.93 0.4522
WVFGRD96 16.0 170 30 105 3.97 0.5142
WVFGRD96 18.0 335 60 80 4.00 0.5732
WVFGRD96 20.0 335 60 80 4.03 0.6205
WVFGRD96 22.0 340 60 80 4.06 0.6543
WVFGRD96 24.0 340 60 80 4.08 0.6807
WVFGRD96 26.0 340 55 80 4.09 0.6972
WVFGRD96 28.0 340 55 80 4.11 0.7049
WVFGRD96 30.0 335 55 75 4.13 0.7019
WVFGRD96 32.0 335 55 75 4.14 0.6874
WVFGRD96 34.0 335 55 75 4.15 0.6599
WVFGRD96 36.0 335 55 70 4.17 0.6243
WVFGRD96 38.0 330 55 65 4.19 0.5868
WVFGRD96 40.0 335 65 70 4.29 0.5091
WVFGRD96 42.0 175 50 95 4.28 0.5022
WVFGRD96 44.0 350 40 85 4.30 0.4875
WVFGRD96 46.0 345 40 80 4.31 0.4681
WVFGRD96 48.0 345 40 80 4.32 0.4451
WVFGRD96 50.0 340 40 75 4.32 0.4204
WVFGRD96 52.0 340 40 75 4.33 0.3947
WVFGRD96 54.0 335 40 70 4.33 0.3709
WVFGRD96 56.0 330 40 65 4.34 0.3492
WVFGRD96 58.0 355 25 90 4.32 0.3337
WVFGRD96 60.0 365 25 100 4.32 0.3233
WVFGRD96 62.0 355 20 95 4.33 0.3121
WVFGRD96 64.0 170 70 90 4.33 0.3020
WVFGRD96 66.0 175 70 90 4.34 0.2923
WVFGRD96 68.0 350 20 85 4.34 0.2826
WVFGRD96 70.0 155 30 50 4.29 0.2848
WVFGRD96 72.0 150 35 45 4.30 0.2895
WVFGRD96 74.0 150 35 50 4.30 0.2945
WVFGRD96 76.0 150 35 50 4.31 0.2989
WVFGRD96 78.0 150 35 50 4.31 0.3025
WVFGRD96 80.0 155 35 55 4.32 0.3052
WVFGRD96 82.0 155 35 55 4.32 0.3075
WVFGRD96 84.0 155 35 55 4.33 0.3093
WVFGRD96 86.0 155 35 55 4.33 0.3104
WVFGRD96 88.0 155 35 55 4.34 0.3108
WVFGRD96 90.0 145 35 60 4.34 0.3113
WVFGRD96 92.0 150 35 65 4.34 0.3116
WVFGRD96 94.0 150 35 65 4.34 0.3131
WVFGRD96 96.0 155 35 70 4.35 0.3142
WVFGRD96 98.0 155 35 70 4.35 0.3156
The best solution is
WVFGRD96 28.0 340 55 80 4.11 0.7049
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 0.07 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 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 Fri Apr 26 04:51:57 PM CDT 2024
