Location
Location ANSS
The ANSS event ID is ak0198ey5yyy and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0198ey5yyy/executive.
2019/07/02 18:17:07 66.270 -157.181 7.9 4.5 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2019/07/02 18:17:07:0 66.27 -157.18 7.9 4.5 Alaska
Stations used:
AK.ANM AK.BPAW AK.BWN AK.CAST AK.COLD AK.KTH AK.MLY AK.NEA2
AK.RDOG TA.B20K TA.B21K TA.C18K TA.C19K TA.D20K TA.D22K
TA.E18K TA.E19K TA.E21K TA.E22K TA.E23K TA.E24K TA.F15K
TA.F17K TA.F19K TA.F20K TA.F21K TA.F24K TA.G16K TA.G18K
TA.G19K TA.G21K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K
TA.H21K TA.H23K TA.H24K TA.I20K TA.I23K TA.J16K TA.J17K
TA.J18K TA.J19K TA.J20K TA.K17K TA.K20K TA.TOLK
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.19e+23 dyne-cm
Mw = 4.65
Z = 10 km
Plane Strike Dip Rake
NP1 170 90 10
NP2 80 80 180
Principal Axes:
Axis Value Plunge Azimuth
T 1.19e+23 7 35
N 0.00e+00 80 170
P -1.19e+23 7 305
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.00e+22
Mxy 1.10e+23
Mxz 3.58e+21
Myy -4.00e+22
Myz 2.03e+22
Mzz -1.80e+15
----##########
--------#############
-----------############# T #
-------------############ ##
-------------###################
P -------------####################
- --------------####################
-------------------#####################
-------------------#####################
---------------------###################--
---------------------##############-------
---------------------########-------------
--------------------#---------------------
#####################-------------------
#####################-------------------
####################------------------
####################----------------
###################---------------
#################-------------
#################-----------
##############--------
##########----
Global CMT Convention Moment Tensor:
R T P
-1.80e+15 3.58e+21 -2.03e+22
3.58e+21 4.00e+22 -1.10e+23
-2.03e+22 -1.10e+23 -4.00e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190702181707/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 = 170
DIP = 90
RAKE = 10
MW = 4.65
HS = 10.0
The NDK file is 20190702181707.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 |
USGSMWR |
USGS/SLU Moment Tensor Solution
ENS 2019/07/02 18:17:07:0 66.27 -157.18 7.9 4.5 Alaska
Stations used:
AK.ANM AK.BPAW AK.BWN AK.CAST AK.COLD AK.KTH AK.MLY AK.NEA2
AK.RDOG TA.B20K TA.B21K TA.C18K TA.C19K TA.D20K TA.D22K
TA.E18K TA.E19K TA.E21K TA.E22K TA.E23K TA.E24K TA.F15K
TA.F17K TA.F19K TA.F20K TA.F21K TA.F24K TA.G16K TA.G18K
TA.G19K TA.G21K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K
TA.H21K TA.H23K TA.H24K TA.I20K TA.I23K TA.J16K TA.J17K
TA.J18K TA.J19K TA.J20K TA.K17K TA.K20K TA.TOLK
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.19e+23 dyne-cm
Mw = 4.65
Z = 10 km
Plane Strike Dip Rake
NP1 170 90 10
NP2 80 80 180
Principal Axes:
Axis Value Plunge Azimuth
T 1.19e+23 7 35
N 0.00e+00 80 170
P -1.19e+23 7 305
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.00e+22
Mxy 1.10e+23
Mxz 3.58e+21
Myy -4.00e+22
Myz 2.03e+22
Mzz -1.80e+15
----##########
--------#############
-----------############# T #
-------------############ ##
-------------###################
P -------------####################
- --------------####################
-------------------#####################
-------------------#####################
---------------------###################--
---------------------##############-------
---------------------########-------------
--------------------#---------------------
#####################-------------------
#####################-------------------
####################------------------
####################----------------
###################---------------
#################-------------
#################-----------
##############--------
##########----
Global CMT Convention Moment Tensor:
R T P
-1.80e+15 3.58e+21 -2.03e+22
3.58e+21 4.00e+22 -1.10e+23
-2.03e+22 -1.10e+23 -4.00e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190702181707/index.html
|
Regional Moment Tensor (Mwr)
Moment 1.271e+16 N-m
Magnitude 4.67 Mwr
Depth 8.0 km
Percent DC 82%
Half Duration -
Catalog US
Data Source US 2
Contributor US 2
Nodal Planes
Plane Strike Dip Rake
NP1 262 88 177
NP2 352 87 2
Principal Axes
Axis Value Plunge Azimuth
T 1.327e+16 N-m 4 217
N -0.121e+16 N-m 86 50
P -1.207e+16 N-m 1 307
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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.
mLg Magnitude
Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 350 80 -5 4.37 0.5143
WVFGRD96 4.0 350 85 -5 4.47 0.6417
WVFGRD96 6.0 170 90 0 4.54 0.7022
WVFGRD96 8.0 170 90 5 4.61 0.7453
WVFGRD96 10.0 170 90 10 4.65 0.7489
WVFGRD96 12.0 350 85 5 4.68 0.7414
WVFGRD96 14.0 350 85 10 4.71 0.7286
WVFGRD96 16.0 350 85 5 4.73 0.7094
WVFGRD96 18.0 350 85 5 4.75 0.6857
WVFGRD96 20.0 350 85 0 4.77 0.6586
WVFGRD96 22.0 350 80 5 4.78 0.6300
WVFGRD96 24.0 350 80 5 4.79 0.6003
WVFGRD96 26.0 350 80 0 4.80 0.5702
WVFGRD96 28.0 350 80 0 4.81 0.5404
WVFGRD96 30.0 170 90 10 4.82 0.5204
WVFGRD96 32.0 170 90 10 4.83 0.5042
WVFGRD96 34.0 170 90 10 4.84 0.4860
WVFGRD96 36.0 170 90 10 4.86 0.4689
WVFGRD96 38.0 170 90 10 4.88 0.4575
WVFGRD96 40.0 170 90 20 4.93 0.4537
WVFGRD96 42.0 350 90 -15 4.95 0.4524
WVFGRD96 44.0 170 90 15 4.97 0.4495
WVFGRD96 46.0 350 90 -15 4.98 0.4446
WVFGRD96 48.0 170 90 15 5.00 0.4393
WVFGRD96 50.0 350 90 -15 5.01 0.4344
WVFGRD96 52.0 350 90 -15 5.02 0.4306
WVFGRD96 54.0 170 90 15 5.03 0.4255
WVFGRD96 56.0 350 85 -10 5.04 0.4228
WVFGRD96 58.0 350 85 -10 5.05 0.4194
WVFGRD96 60.0 350 85 -10 5.06 0.4158
WVFGRD96 62.0 350 85 -10 5.06 0.4110
WVFGRD96 64.0 170 90 10 5.07 0.4066
WVFGRD96 66.0 350 90 -10 5.07 0.4028
WVFGRD96 68.0 170 90 10 5.08 0.3980
WVFGRD96 70.0 350 90 -10 5.08 0.3907
WVFGRD96 72.0 350 90 -5 5.08 0.3825
WVFGRD96 74.0 170 90 5 5.08 0.3738
WVFGRD96 76.0 350 90 0 5.08 0.3658
WVFGRD96 78.0 350 90 5 5.08 0.3589
WVFGRD96 80.0 170 85 -5 5.08 0.3541
WVFGRD96 82.0 350 90 5 5.09 0.3529
WVFGRD96 84.0 170 85 -5 5.09 0.3517
WVFGRD96 86.0 170 85 -5 5.10 0.3496
WVFGRD96 88.0 170 85 -5 5.10 0.3473
WVFGRD96 90.0 170 85 -5 5.10 0.3444
WVFGRD96 92.0 170 85 -5 5.11 0.3423
WVFGRD96 94.0 350 90 5 5.11 0.3407
WVFGRD96 96.0 170 90 -5 5.12 0.3388
WVFGRD96 98.0 350 90 5 5.12 0.3369
The best solution is
WVFGRD96 10.0 170 90 10 4.65 0.7489
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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 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 Thu Apr 25 02:20:14 PM CDT 2024
