Location
Location ANSS
The ANSS event ID is ak016690dapx and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak016690dapx/executive.
2016/05/15 05:51:00 63.077 -150.946 131.5 5.3 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2016/05/15 05:51:00:0 63.08 -150.95 131.5 5.3 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.FID
AK.FIRE AK.GHO AK.GLI AK.HDA AK.KLU AK.KNK AK.KTH AK.MCK
AK.MDM AK.NEA2 AK.PAX AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM
AK.TRF AK.WRH AT.PMR AT.SVW2 AT.TTA IM.IL31 IU.COLA TA.H21K
TA.H23K TA.H24K TA.I21K TA.I23K TA.J20K TA.K20K TA.L19K
TA.M19K TA.M22K TA.N19K TA.O22K TA.POKR TA.TCOL
Filtering commands used:
cut o DIST/3.3 -50 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 9.12e+23 dyne-cm
Mw = 5.24
Z = 126 km
Plane Strike Dip Rake
NP1 50 60 65
NP2 273 38 126
Principal Axes:
Axis Value Plunge Azimuth
T 9.12e+23 65 274
N 0.00e+00 21 63
P -9.12e+23 12 158
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.49e+23
Mxy 2.95e+23
Mxz 1.93e+23
Myy 3.30e+22
Myz -4.13e+23
Mzz 7.16e+23
--------------
----------------------
----------------------------
------------------------------
----------#############----------#
------#######################-----##
----#############################-####
---###############################--####
-################################-----##
-############ #################-------##
############# T ################---------#
############# ###############-----------
#############################-------------
##########################--------------
########################----------------
#####################-----------------
#################-------------------
###########-----------------------
------------------------------
------------------- ------
---------------- P ---
------------
Global CMT Convention Moment Tensor:
R T P
7.16e+23 1.93e+23 4.13e+23
1.93e+23 -7.49e+23 -2.95e+23
4.13e+23 -2.95e+23 3.30e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160515055100/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 = 60
RAKE = 65
MW = 5.24
HS = 126.0
The NDK file is 20160515055100.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 2016/05/15 05:51:00:0 63.08 -150.95 131.5 5.3 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.FID
AK.FIRE AK.GHO AK.GLI AK.HDA AK.KLU AK.KNK AK.KTH AK.MCK
AK.MDM AK.NEA2 AK.PAX AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM
AK.TRF AK.WRH AT.PMR AT.SVW2 AT.TTA IM.IL31 IU.COLA TA.H21K
TA.H23K TA.H24K TA.I21K TA.I23K TA.J20K TA.K20K TA.L19K
TA.M19K TA.M22K TA.N19K TA.O22K TA.POKR TA.TCOL
Filtering commands used:
cut o DIST/3.3 -50 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 9.12e+23 dyne-cm
Mw = 5.24
Z = 126 km
Plane Strike Dip Rake
NP1 50 60 65
NP2 273 38 126
Principal Axes:
Axis Value Plunge Azimuth
T 9.12e+23 65 274
N 0.00e+00 21 63
P -9.12e+23 12 158
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.49e+23
Mxy 2.95e+23
Mxz 1.93e+23
Myy 3.30e+22
Myz -4.13e+23
Mzz 7.16e+23
--------------
----------------------
----------------------------
------------------------------
----------#############----------#
------#######################-----##
----#############################-####
---###############################--####
-################################-----##
-############ #################-------##
############# T ################---------#
############# ###############-----------
#############################-------------
##########################--------------
########################----------------
#####################-----------------
#################-------------------
###########-----------------------
------------------------------
------------------- ------
---------------- P ---
------------
Global CMT Convention Moment Tensor:
R T P
7.16e+23 1.93e+23 4.13e+23
1.93e+23 -7.49e+23 -2.95e+23
4.13e+23 -2.95e+23 3.30e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160515055100/index.html
|
Body-wave Moment Tensor (Mwb)
Moment 1.053e+17 N-m
Magnitude 5.3 Mwb
Depth 130.0 km
Percent DC 87 %
Half Duration –
Catalog US
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 266 32 123
NP2 48 64 71
Principal Axes
Axis Value Plunge Azimuth
T 1.087e+17 N-m 66 284
N -0.072e+17 N-m 17 57
P -1.015e+17 N-m 17 152
|
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 -50 o DIST/3.3 +60
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 70.0 55 50 65 5.11 0.3255
WVFGRD96 72.0 50 55 60 5.13 0.3645
WVFGRD96 74.0 50 55 60 5.14 0.4045
WVFGRD96 76.0 50 55 60 5.15 0.4459
WVFGRD96 78.0 50 55 60 5.17 0.4790
WVFGRD96 80.0 50 55 60 5.17 0.4992
WVFGRD96 82.0 50 55 60 5.18 0.5133
WVFGRD96 84.0 50 55 60 5.18 0.5258
WVFGRD96 86.0 50 55 60 5.18 0.5361
WVFGRD96 88.0 50 55 60 5.19 0.5452
WVFGRD96 90.0 50 60 65 5.19 0.5541
WVFGRD96 92.0 50 60 65 5.20 0.5611
WVFGRD96 94.0 50 60 65 5.20 0.5676
WVFGRD96 96.0 50 60 65 5.20 0.5734
WVFGRD96 98.0 50 60 65 5.21 0.5794
WVFGRD96 100.0 50 60 65 5.21 0.5841
WVFGRD96 102.0 50 60 65 5.21 0.5885
WVFGRD96 104.0 50 60 65 5.21 0.5920
WVFGRD96 106.0 50 60 65 5.22 0.5949
WVFGRD96 108.0 50 60 65 5.22 0.5991
WVFGRD96 110.0 50 60 65 5.22 0.6010
WVFGRD96 112.0 50 60 65 5.22 0.6038
WVFGRD96 114.0 50 60 65 5.23 0.6062
WVFGRD96 116.0 50 60 65 5.23 0.6078
WVFGRD96 118.0 50 60 65 5.23 0.6097
WVFGRD96 120.0 50 60 65 5.23 0.6111
WVFGRD96 122.0 50 60 65 5.23 0.6120
WVFGRD96 124.0 50 60 65 5.24 0.6122
WVFGRD96 126.0 50 60 65 5.24 0.6129
WVFGRD96 128.0 50 60 70 5.24 0.6121
WVFGRD96 130.0 50 60 70 5.24 0.6128
WVFGRD96 132.0 50 60 70 5.25 0.6113
WVFGRD96 134.0 50 60 70 5.25 0.6104
WVFGRD96 136.0 50 60 70 5.25 0.6090
WVFGRD96 138.0 50 60 70 5.25 0.6071
WVFGRD96 140.0 50 60 70 5.25 0.6059
WVFGRD96 142.0 50 60 70 5.25 0.6026
WVFGRD96 144.0 50 60 70 5.26 0.6011
WVFGRD96 146.0 50 55 65 5.26 0.5982
WVFGRD96 148.0 50 55 65 5.26 0.5956
The best solution is
WVFGRD96 126.0 50 60 65 5.24 0.6129
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 -50 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 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 04:47:26 PM CDT 2024
