Location
SLU Location
Because the moment tensor solution wanted a depth near 50 km and the ANSS location was 28 km, P and S first arrival times were read and the program elocate was run. The output of the program is elocate.txt. When started with a depth of 10 km, the solution converged to a depth of 39 km, but the first motion plot was incompatible with the moment tensor nodal planes. When the program was started with a depth of 100, it converged to a depth of 54 km with a marginally smaller RMS error in time. The P-wave first motion data for this latter solution was in agreement with the
moment tensor solution.
Location ANSS
The ANSS event ID is ak0182plgsrf and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0182plgsrf/executive.
2018/02/28 01:27:47 62.349 -148.729 39.5 4.3 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2018/02/28 01:27:47:0 62.35 -148.73 39.5 4.3 Alaska
Stations used:
AK.CAST AK.DHY AK.GHO AK.KLU AK.KNK AK.KTH AK.MLY AK.NEA2
AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AT.PMR TA.M22K
TA.M24K TA.POKR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 3.43e+22 dyne-cm
Mw = 4.29
Z = 52 km
Plane Strike Dip Rake
NP1 290 55 -55
NP2 59 48 -129
Principal Axes:
Axis Value Plunge Azimuth
T 3.43e+22 4 356
N 0.00e+00 28 88
P -3.43e+22 62 259
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.37e+22
Mxy -3.86e+21
Mxz 5.17e+21
Myy -7.27e+21
Myz 1.39e+22
Mzz -2.64e+22
#### T #######
######## ###########
############################
##############################
##################################
#######------#######################
##--------------------###############-
---------------------------##########---
------------------------------#######---
----------------------------------###-----
------------- --------------------------
------------- P -------------------##-----
------------- ------------------#####---
-------------------------------########-
-----------------------------##########-
--------------------------############
---------------------###############
##--------------##################
##############################
############################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-2.64e+22 5.17e+21 -1.39e+22
5.17e+21 3.37e+22 3.86e+21
-1.39e+22 3.86e+21 -7.27e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180228012747/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 = 290
DIP = 55
RAKE = -55
MW = 4.29
HS = 52.0
The NDK file is 20180228012747.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 2018/02/28 01:27:47:0 62.35 -148.73 39.5 4.3 Alaska
Stations used:
AK.CAST AK.DHY AK.GHO AK.KLU AK.KNK AK.KTH AK.MLY AK.NEA2
AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AT.PMR TA.M22K
TA.M24K TA.POKR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 3.43e+22 dyne-cm
Mw = 4.29
Z = 52 km
Plane Strike Dip Rake
NP1 290 55 -55
NP2 59 48 -129
Principal Axes:
Axis Value Plunge Azimuth
T 3.43e+22 4 356
N 0.00e+00 28 88
P -3.43e+22 62 259
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.37e+22
Mxy -3.86e+21
Mxz 5.17e+21
Myy -7.27e+21
Myz 1.39e+22
Mzz -2.64e+22
#### T #######
######## ###########
############################
##############################
##################################
#######------#######################
##--------------------###############-
---------------------------##########---
------------------------------#######---
----------------------------------###-----
------------- --------------------------
------------- P -------------------##-----
------------- ------------------#####---
-------------------------------########-
-----------------------------##########-
--------------------------############
---------------------###############
##--------------##################
##############################
############################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-2.64e+22 5.17e+21 -1.39e+22
5.17e+21 3.37e+22 3.86e+21
-1.39e+22 3.86e+21 -7.27e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180228012747/index.html
|
First motions and takeoff angles from an elocate run.
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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 +40
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 65 50 55 3.55 0.2865
WVFGRD96 4.0 240 70 50 3.64 0.3186
WVFGRD96 6.0 35 55 -40 3.69 0.3523
WVFGRD96 8.0 30 50 -45 3.77 0.3676
WVFGRD96 10.0 35 60 -40 3.78 0.3712
WVFGRD96 12.0 335 50 50 3.84 0.3874
WVFGRD96 14.0 330 55 45 3.85 0.3980
WVFGRD96 16.0 330 55 45 3.88 0.4036
WVFGRD96 18.0 145 60 35 3.88 0.4145
WVFGRD96 20.0 145 60 35 3.91 0.4278
WVFGRD96 22.0 145 60 35 3.93 0.4389
WVFGRD96 24.0 140 70 30 3.94 0.4497
WVFGRD96 26.0 140 70 25 3.96 0.4621
WVFGRD96 28.0 140 75 25 3.98 0.4728
WVFGRD96 30.0 310 60 -15 4.01 0.5008
WVFGRD96 32.0 310 60 -20 4.03 0.5275
WVFGRD96 34.0 305 60 -25 4.05 0.5531
WVFGRD96 36.0 305 60 -25 4.07 0.5775
WVFGRD96 38.0 300 60 -30 4.10 0.5999
WVFGRD96 40.0 295 50 -40 4.18 0.6234
WVFGRD96 42.0 295 50 -45 4.21 0.6363
WVFGRD96 44.0 295 55 -45 4.22 0.6472
WVFGRD96 46.0 295 55 -45 4.24 0.6604
WVFGRD96 48.0 290 55 -50 4.26 0.6699
WVFGRD96 50.0 290 55 -55 4.28 0.6771
WVFGRD96 52.0 290 55 -55 4.29 0.6810
WVFGRD96 54.0 290 55 -55 4.29 0.6807
WVFGRD96 56.0 290 55 -55 4.30 0.6763
WVFGRD96 58.0 295 60 -45 4.30 0.6712
WVFGRD96 60.0 295 60 -45 4.30 0.6643
WVFGRD96 62.0 290 60 -50 4.31 0.6566
WVFGRD96 64.0 290 60 -50 4.31 0.6486
WVFGRD96 66.0 295 65 -45 4.31 0.6392
WVFGRD96 68.0 295 65 -45 4.31 0.6320
WVFGRD96 70.0 295 65 -40 4.31 0.6260
WVFGRD96 72.0 295 65 -40 4.31 0.6194
WVFGRD96 74.0 295 65 -40 4.31 0.6124
WVFGRD96 76.0 295 70 -40 4.32 0.6079
WVFGRD96 78.0 295 70 -40 4.32 0.6031
WVFGRD96 80.0 295 70 -40 4.32 0.5977
WVFGRD96 82.0 295 70 -40 4.32 0.5916
WVFGRD96 84.0 295 70 -40 4.32 0.5857
WVFGRD96 86.0 295 70 -40 4.32 0.5796
WVFGRD96 88.0 295 70 -40 4.32 0.5735
WVFGRD96 90.0 295 75 -40 4.33 0.5678
WVFGRD96 92.0 295 75 -40 4.33 0.5632
WVFGRD96 94.0 295 75 -40 4.34 0.5580
WVFGRD96 96.0 295 75 -40 4.34 0.5530
WVFGRD96 98.0 300 75 -30 4.33 0.5488
The best solution is
WVFGRD96 52.0 290 55 -55 4.29 0.6810
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 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
|
|
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 09:28:29 PM CDT 2024
