Location
Location ANSS
The ANSS event ID is ak0159nc9dk8 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0159nc9dk8/executive.
2015/07/29 02:35:58 59.894 -153.196 119.3 6.4 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2015/07/29 02:35:58:0 59.89 -153.20 119.3 6.4 Alaska
Stations used:
AK.BPAW AK.BRLK AK.BWN AK.CUT AK.EYAK AK.FID AK.GLI AK.HIN
AK.KLU AK.KNK AK.KTH AK.MCK AK.PPLA AK.PWL AK.RAG AK.RND
AK.SAW AK.SCM AK.SII AK.SKN AK.SSN AK.SWD AK.TRF AT.MID
AT.OHAK AT.PMR AT.SVW2 II.KDAK TA.N25K TA.Q23K
Filtering commands used:
cut o DIST/3.4 -50 o DIST/3.4 +100
rtr
taper w 0.1
hp c 0.01 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 3.31e+25 dyne-cm
Mw = 6.28
Z = 114 km
Plane Strike Dip Rake
NP1 321 60 145
NP2 70 60 35
Principal Axes:
Axis Value Plunge Azimuth
T 3.31e+25 45 285
N 0.00e+00 45 106
P -3.31e+25 0 15
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.96e+25
Mxy -1.27e+25
Mxz 4.29e+24
Myy 1.32e+25
Myz -1.60e+25
Mzz 1.64e+25
----------- P
--------------- ----
####------------------------
##########--------------------
###############-------------------
##################------------------
######################----------------
########################---------------#
######## ###############------------##
######### T ################----------####
######### ##################------######
###############################----#######
##########################################
############################---#########
########################--------########
--################--------------######
-------------------------------#####
------------------------------####
----------------------------##
---------------------------#
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
1.64e+25 4.29e+24 1.60e+25
4.29e+24 -2.96e+25 1.27e+25
1.60e+25 1.27e+25 1.32e+25
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150729023558/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 = 70
DIP = 60
RAKE = 35
MW = 6.28
HS = 114.0
The NDK file is 20150729023558.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 |
USGSW |
USGS/SLU Moment Tensor Solution
ENS 2015/07/29 02:35:58:0 59.89 -153.20 119.3 6.4 Alaska
Stations used:
AK.BPAW AK.BRLK AK.BWN AK.CUT AK.EYAK AK.FID AK.GLI AK.HIN
AK.KLU AK.KNK AK.KTH AK.MCK AK.PPLA AK.PWL AK.RAG AK.RND
AK.SAW AK.SCM AK.SII AK.SKN AK.SSN AK.SWD AK.TRF AT.MID
AT.OHAK AT.PMR AT.SVW2 II.KDAK TA.N25K TA.Q23K
Filtering commands used:
cut o DIST/3.4 -50 o DIST/3.4 +100
rtr
taper w 0.1
hp c 0.01 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 3.31e+25 dyne-cm
Mw = 6.28
Z = 114 km
Plane Strike Dip Rake
NP1 321 60 145
NP2 70 60 35
Principal Axes:
Axis Value Plunge Azimuth
T 3.31e+25 45 285
N 0.00e+00 45 106
P -3.31e+25 0 15
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.96e+25
Mxy -1.27e+25
Mxz 4.29e+24
Myy 1.32e+25
Myz -1.60e+25
Mzz 1.64e+25
----------- P
--------------- ----
####------------------------
##########--------------------
###############-------------------
##################------------------
######################----------------
########################---------------#
######## ###############------------##
######### T ################----------####
######### ##################------######
###############################----#######
##########################################
############################---#########
########################--------########
--################--------------######
-------------------------------#####
------------------------------####
----------------------------##
---------------------------#
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
1.64e+25 4.29e+24 1.60e+25
4.29e+24 -2.96e+25 1.27e+25
1.60e+25 1.27e+25 1.32e+25
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150729023558/index.html
|
W-phase Moment Tensor (Mww)
Moment 3.391e+18 N-m
Magnitude 6.29
Depth 120.5 km
Percent DC 83%
Half Duration –
Catalog US (us2000314u)
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 323 63 147
NP2 69 61 31
Principal Axes
Axis Value Plunge Azimuth
T 3.527 42 285
N -0.292 48 108
P -3.236 1 16
|
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/3.4 -50 o DIST/3.4 +100
rtr
taper w 0.1
hp c 0.01 n 3
lp c 0.05 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 150 85 -20 5.36 0.1242
WVFGRD96 4.0 330 90 10 5.43 0.1464
WVFGRD96 6.0 150 90 -10 5.48 0.1557
WVFGRD96 8.0 150 85 -15 5.53 0.1611
WVFGRD96 10.0 240 90 -25 5.57 0.1627
WVFGRD96 12.0 65 80 25 5.59 0.1720
WVFGRD96 14.0 65 80 25 5.61 0.1801
WVFGRD96 16.0 65 75 25 5.63 0.1879
WVFGRD96 18.0 65 75 20 5.65 0.1964
WVFGRD96 20.0 65 75 20 5.66 0.2053
WVFGRD96 22.0 65 75 20 5.68 0.2131
WVFGRD96 24.0 65 70 20 5.70 0.2206
WVFGRD96 26.0 65 70 20 5.71 0.2271
WVFGRD96 28.0 65 70 20 5.73 0.2327
WVFGRD96 30.0 65 75 20 5.76 0.2385
WVFGRD96 32.0 65 75 20 5.78 0.2440
WVFGRD96 34.0 65 75 20 5.80 0.2494
WVFGRD96 36.0 65 75 20 5.82 0.2551
WVFGRD96 38.0 65 75 20 5.85 0.2613
WVFGRD96 40.0 65 75 25 5.93 0.2668
WVFGRD96 42.0 65 70 20 5.94 0.2729
WVFGRD96 44.0 65 70 15 5.96 0.2792
WVFGRD96 46.0 65 70 15 5.97 0.2853
WVFGRD96 48.0 65 70 15 5.99 0.2910
WVFGRD96 50.0 65 70 15 6.00 0.2965
WVFGRD96 52.0 65 75 20 6.02 0.3032
WVFGRD96 54.0 65 75 20 6.04 0.3112
WVFGRD96 56.0 65 70 20 6.05 0.3192
WVFGRD96 58.0 65 70 20 6.06 0.3291
WVFGRD96 60.0 65 65 15 6.07 0.3424
WVFGRD96 62.0 65 65 15 6.09 0.3582
WVFGRD96 64.0 65 65 15 6.10 0.3767
WVFGRD96 66.0 65 65 15 6.12 0.3956
WVFGRD96 68.0 65 65 15 6.13 0.4143
WVFGRD96 70.0 65 65 20 6.15 0.4336
WVFGRD96 72.0 65 65 20 6.16 0.4533
WVFGRD96 74.0 65 65 20 6.17 0.4723
WVFGRD96 76.0 65 65 20 6.19 0.4906
WVFGRD96 78.0 65 65 20 6.20 0.5077
WVFGRD96 80.0 65 65 20 6.21 0.5246
WVFGRD96 82.0 65 65 25 6.21 0.5407
WVFGRD96 84.0 65 65 25 6.22 0.5560
WVFGRD96 86.0 65 65 25 6.23 0.5698
WVFGRD96 88.0 65 65 25 6.24 0.5824
WVFGRD96 90.0 70 60 30 6.24 0.5946
WVFGRD96 92.0 70 60 30 6.24 0.6058
WVFGRD96 94.0 70 60 30 6.25 0.6159
WVFGRD96 96.0 70 60 30 6.26 0.6245
WVFGRD96 98.0 70 60 30 6.26 0.6319
WVFGRD96 100.0 70 60 30 6.27 0.6383
WVFGRD96 102.0 70 60 30 6.27 0.6433
WVFGRD96 104.0 70 60 30 6.27 0.6475
WVFGRD96 106.0 70 60 35 6.27 0.6511
WVFGRD96 108.0 70 60 35 6.28 0.6539
WVFGRD96 110.0 70 60 35 6.28 0.6557
WVFGRD96 112.0 70 60 35 6.28 0.6566
WVFGRD96 114.0 70 60 35 6.28 0.6567
WVFGRD96 116.0 70 60 35 6.29 0.6560
WVFGRD96 118.0 70 60 35 6.29 0.6546
WVFGRD96 120.0 70 60 35 6.29 0.6527
WVFGRD96 122.0 70 60 40 6.29 0.6503
WVFGRD96 124.0 70 60 40 6.29 0.6477
WVFGRD96 126.0 70 60 40 6.29 0.6449
WVFGRD96 128.0 70 60 40 6.29 0.6411
The best solution is
WVFGRD96 114.0 70 60 35 6.28 0.6567
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/3.4 -50 o DIST/3.4 +100
rtr
taper w 0.1
hp c 0.01 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 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 09:21:18 PM CDT 2024
