Location
SLU Location
Because the initial ANSS location of 57 km is much less than the
moment tensor depth, first arrivals were hand picked and the
event located with the WUS model given below. The output of
elocate is given in elocate.txt. Although the WUS model
is not appropriate for this region, the SLU depth of 88 km is much closer to the RMT depth of 108 km than the initial 57 km depth.. in addition
the first motions selected agree well with the RMT nodal planes.
Location ANSS
The ANSS event ID is ak0187uygjsf and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0187uygjsf/executive.
2018/06/20 09:34:08 63.305 -148.123 71.9 3.7 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2018/06/20 09:34:08:0 63.31 -148.12 71.9 3.7 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.HDA AK.KLU AK.KNK AK.KTH AK.MCK
AK.NEA2 AK.PAX AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.WRH
IM.IL31 IU.COLA TA.J25K TA.J26L TA.M22K TA.M24K TA.N25K
Filtering commands used:
cut o DIST/3.3 -50 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.02e+22 dyne-cm
Mw = 3.94
Z = 108 km
Plane Strike Dip Rake
NP1 150 75 25
NP2 53 66 164
Principal Axes:
Axis Value Plunge Azimuth
T 1.02e+22 28 13
N 0.00e+00 61 179
P -1.02e+22 6 280
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.22e+21
Mxy 3.54e+21
Mxz 3.95e+21
Myy -9.38e+21
Myz 2.04e+21
Mzz 2.16e+21
##############
-#####################
----############ #########
-----############ T ##########
--------########### ############
---------#########################--
-----------#######################----
-------------#####################------
-----------####################-------
P ------------##################---------
-------------###############-----------
-----------------############-------------
------------------#########---------------
------------------######----------------
--------------------#-------------------
-----------------###------------------
-------------#######----------------
------###############-------------
####################----------
#####################-------
#####################-
##############
Global CMT Convention Moment Tensor:
R T P
2.16e+21 3.95e+21 -2.04e+21
3.95e+21 7.22e+21 -3.54e+21
-2.04e+21 -3.54e+21 -9.38e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180620093408/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 150
DIP = 75
RAKE = 25
MW = 3.94
HS = 108.0
The NDK file is 20180620093408.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 |
SLUFM |
USGS/SLU Moment Tensor Solution
ENS 2018/06/20 09:34:08:0 63.31 -148.12 71.9 3.7 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.HDA AK.KLU AK.KNK AK.KTH AK.MCK
AK.NEA2 AK.PAX AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.WRH
IM.IL31 IU.COLA TA.J25K TA.J26L TA.M22K TA.M24K TA.N25K
Filtering commands used:
cut o DIST/3.3 -50 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.02e+22 dyne-cm
Mw = 3.94
Z = 108 km
Plane Strike Dip Rake
NP1 150 75 25
NP2 53 66 164
Principal Axes:
Axis Value Plunge Azimuth
T 1.02e+22 28 13
N 0.00e+00 61 179
P -1.02e+22 6 280
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.22e+21
Mxy 3.54e+21
Mxz 3.95e+21
Myy -9.38e+21
Myz 2.04e+21
Mzz 2.16e+21
##############
-#####################
----############ #########
-----############ T ##########
--------########### ############
---------#########################--
-----------#######################----
-------------#####################------
-----------####################-------
P ------------##################---------
-------------###############-----------
-----------------############-------------
------------------#########---------------
------------------######----------------
--------------------#-------------------
-----------------###------------------
-------------#######----------------
------###############-------------
####################----------
#####################-------
#####################-
##############
Global CMT Convention Moment Tensor:
R T P
2.16e+21 3.95e+21 -2.04e+21
3.95e+21 7.22e+21 -3.54e+21
-2.04e+21 -3.54e+21 -9.38e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180620093408/index.html
|
First motions and takeoff angles from an elocate run.
|
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 +30
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 2.0 240 85 0 2.95 0.1370
WVFGRD96 4.0 240 65 10 3.07 0.1603
WVFGRD96 6.0 240 70 10 3.12 0.1727
WVFGRD96 8.0 240 70 10 3.20 0.1797
WVFGRD96 10.0 240 70 5 3.24 0.1824
WVFGRD96 12.0 240 70 5 3.27 0.1799
WVFGRD96 14.0 140 85 -25 3.31 0.1886
WVFGRD96 16.0 325 85 25 3.35 0.1998
WVFGRD96 18.0 140 90 -25 3.38 0.2069
WVFGRD96 20.0 325 80 20 3.42 0.2133
WVFGRD96 22.0 325 80 20 3.45 0.2187
WVFGRD96 24.0 330 80 15 3.47 0.2234
WVFGRD96 26.0 330 75 15 3.49 0.2262
WVFGRD96 28.0 330 75 20 3.51 0.2270
WVFGRD96 30.0 330 75 15 3.53 0.2319
WVFGRD96 32.0 330 75 15 3.55 0.2364
WVFGRD96 34.0 330 75 15 3.58 0.2416
WVFGRD96 36.0 330 75 10 3.60 0.2475
WVFGRD96 38.0 330 75 10 3.64 0.2581
WVFGRD96 40.0 330 75 10 3.70 0.2773
WVFGRD96 42.0 330 75 5 3.73 0.2862
WVFGRD96 44.0 330 75 10 3.76 0.2909
WVFGRD96 46.0 330 80 5 3.78 0.2960
WVFGRD96 48.0 330 80 0 3.80 0.3019
WVFGRD96 50.0 330 80 0 3.82 0.3093
WVFGRD96 52.0 330 80 -5 3.84 0.3196
WVFGRD96 54.0 330 80 -5 3.85 0.3300
WVFGRD96 56.0 330 80 -5 3.86 0.3423
WVFGRD96 58.0 330 80 -5 3.87 0.3545
WVFGRD96 60.0 150 90 10 3.87 0.3645
WVFGRD96 62.0 330 85 -10 3.89 0.3757
WVFGRD96 64.0 150 90 15 3.89 0.3858
WVFGRD96 66.0 330 90 -10 3.89 0.3953
WVFGRD96 68.0 150 90 15 3.90 0.4029
WVFGRD96 70.0 150 85 15 3.90 0.4098
WVFGRD96 72.0 330 90 -15 3.90 0.4169
WVFGRD96 74.0 150 85 15 3.90 0.4230
WVFGRD96 76.0 150 85 20 3.91 0.4287
WVFGRD96 78.0 150 80 20 3.91 0.4338
WVFGRD96 80.0 150 80 20 3.91 0.4374
WVFGRD96 82.0 150 80 20 3.91 0.4418
WVFGRD96 84.0 150 80 25 3.92 0.4442
WVFGRD96 86.0 150 80 25 3.92 0.4471
WVFGRD96 88.0 150 80 25 3.92 0.4498
WVFGRD96 90.0 150 80 25 3.93 0.4508
WVFGRD96 92.0 150 80 25 3.93 0.4535
WVFGRD96 94.0 150 80 25 3.93 0.4555
WVFGRD96 96.0 150 75 25 3.93 0.4561
WVFGRD96 98.0 150 75 25 3.93 0.4573
WVFGRD96 100.0 150 75 25 3.94 0.4592
WVFGRD96 102.0 150 75 25 3.94 0.4593
WVFGRD96 104.0 150 75 25 3.94 0.4598
WVFGRD96 106.0 150 75 25 3.94 0.4600
WVFGRD96 108.0 150 75 25 3.94 0.4607
WVFGRD96 110.0 150 75 25 3.94 0.4605
WVFGRD96 112.0 150 75 25 3.95 0.4596
WVFGRD96 114.0 150 75 25 3.95 0.4586
WVFGRD96 116.0 150 75 25 3.95 0.4581
WVFGRD96 118.0 150 75 25 3.95 0.4575
WVFGRD96 120.0 150 75 25 3.95 0.4567
WVFGRD96 122.0 150 75 25 3.95 0.4559
WVFGRD96 124.0 150 75 25 3.96 0.4550
WVFGRD96 126.0 150 75 25 3.96 0.4504
WVFGRD96 128.0 150 75 25 3.95 0.4413
WVFGRD96 130.0 150 80 25 3.95 0.4306
WVFGRD96 132.0 150 80 25 3.95 0.4209
WVFGRD96 134.0 150 80 30 3.95 0.4159
WVFGRD96 136.0 150 80 30 3.96 0.4144
WVFGRD96 138.0 150 80 30 3.96 0.4130
WVFGRD96 140.0 150 80 30 3.96 0.4115
WVFGRD96 142.0 150 80 30 3.96 0.4105
WVFGRD96 144.0 150 80 30 3.96 0.4095
WVFGRD96 146.0 150 80 30 3.96 0.4081
WVFGRD96 148.0 150 80 25 3.96 0.4050
The best solution is
WVFGRD96 108.0 150 75 25 3.94 0.4607
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 +30
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 Thu Apr 25 11:38:23 PM CDT 2024
