Location
Location ANSS
The ANSS event ID is us2000bnw8 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/us2000bnw8/executive.
2017/11/15 11:52:02 55.085 -159.248 31.8 4.5 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2017/11/15 11:52:02:0 55.08 -159.25 31.8 4.5 Alaska
Stations used:
AK.CHN AT.CHGN AT.SDPT TA.S14K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 6.61e+22 dyne-cm
Mw = 4.48
Z = 36 km
Plane Strike Dip Rake
NP1 255 85 -15
NP2 346 75 -175
Principal Axes:
Axis Value Plunge Azimuth
T 6.61e+22 7 302
N 0.00e+00 74 57
P -6.61e+22 14 210
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.90e+22
Mxy -5.58e+22
Mxz 1.77e+22
Myy 3.20e+22
Myz 1.01e+21
Mzz -2.97e+21
###-----------
########--------------
############----------------
##############----------------
###############-----------------
T ################-----------------
# ################------------------
######################------------------
#######################------------#####
########################---###############
####################-----#################
#############------------#################
#######-------------------################
##-----------------------###############
-------------------------###############
------------------------##############
-----------------------#############
-----------------------###########
----- -------------#########
---- P -------------########
- ------------######
------------##
Global CMT Convention Moment Tensor:
R T P
-2.97e+21 1.77e+22 -1.01e+21
1.77e+22 -2.90e+22 5.58e+22
-1.01e+21 5.58e+22 3.20e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171115115202/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 = 255
DIP = 85
RAKE = -15
MW = 4.48
HS = 36.0
The NDK file is 20171115115202.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 2017/11/15 11:52:02:0 55.08 -159.25 31.8 4.5 Alaska
Stations used:
AK.CHN AT.CHGN AT.SDPT TA.S14K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 6.61e+22 dyne-cm
Mw = 4.48
Z = 36 km
Plane Strike Dip Rake
NP1 255 85 -15
NP2 346 75 -175
Principal Axes:
Axis Value Plunge Azimuth
T 6.61e+22 7 302
N 0.00e+00 74 57
P -6.61e+22 14 210
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.90e+22
Mxy -5.58e+22
Mxz 1.77e+22
Myy 3.20e+22
Myz 1.01e+21
Mzz -2.97e+21
###-----------
########--------------
############----------------
##############----------------
###############-----------------
T ################-----------------
# ################------------------
######################------------------
#######################------------#####
########################---###############
####################-----#################
#############------------#################
#######-------------------################
##-----------------------###############
-------------------------###############
------------------------##############
-----------------------#############
-----------------------###########
----- -------------#########
---- P -------------########
- ------------######
------------##
Global CMT Convention Moment Tensor:
R T P
-2.97e+21 1.77e+22 -1.01e+21
1.77e+22 -2.90e+22 5.58e+22
-1.01e+21 5.58e+22 3.20e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171115115202/index.html
|
|
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 +70
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 75 90 -5 3.83 0.2416
WVFGRD96 4.0 80 70 20 3.96 0.2874
WVFGRD96 6.0 255 85 10 4.03 0.3275
WVFGRD96 8.0 250 65 -25 4.12 0.3562
WVFGRD96 10.0 255 80 -20 4.16 0.3768
WVFGRD96 12.0 255 80 -20 4.20 0.3976
WVFGRD96 14.0 255 85 -20 4.23 0.4048
WVFGRD96 16.0 75 90 20 4.26 0.4001
WVFGRD96 18.0 75 90 20 4.29 0.3984
WVFGRD96 20.0 75 90 20 4.31 0.4019
WVFGRD96 22.0 75 90 20 4.33 0.4058
WVFGRD96 24.0 75 90 25 4.35 0.4147
WVFGRD96 26.0 255 90 -25 4.38 0.4433
WVFGRD96 28.0 75 90 25 4.40 0.4721
WVFGRD96 30.0 75 90 25 4.42 0.4992
WVFGRD96 32.0 255 85 -20 4.44 0.5199
WVFGRD96 34.0 255 90 -15 4.45 0.5308
WVFGRD96 36.0 255 85 -15 4.48 0.5367
WVFGRD96 38.0 75 90 10 4.50 0.5261
WVFGRD96 40.0 255 90 -15 4.53 0.5068
WVFGRD96 42.0 255 90 -15 4.56 0.5073
WVFGRD96 44.0 255 85 -15 4.59 0.5029
WVFGRD96 46.0 255 85 -15 4.61 0.4959
WVFGRD96 48.0 255 85 -15 4.62 0.4925
WVFGRD96 50.0 255 85 -10 4.63 0.4896
WVFGRD96 52.0 255 80 -15 4.66 0.4865
WVFGRD96 54.0 255 80 -10 4.66 0.4865
WVFGRD96 56.0 255 80 -10 4.67 0.4883
WVFGRD96 58.0 255 80 -10 4.68 0.4908
WVFGRD96 60.0 255 80 -10 4.69 0.4936
WVFGRD96 62.0 255 80 -10 4.70 0.4930
WVFGRD96 64.0 255 80 -10 4.70 0.4946
WVFGRD96 66.0 255 80 -5 4.70 0.4964
WVFGRD96 68.0 255 80 -5 4.70 0.4944
WVFGRD96 70.0 255 80 0 4.70 0.4968
WVFGRD96 72.0 255 80 0 4.70 0.4941
WVFGRD96 74.0 255 80 0 4.71 0.4967
WVFGRD96 76.0 255 80 0 4.71 0.4935
WVFGRD96 78.0 255 80 0 4.71 0.4923
WVFGRD96 80.0 255 80 0 4.71 0.4901
WVFGRD96 82.0 255 80 5 4.71 0.4864
WVFGRD96 84.0 255 80 5 4.71 0.4838
WVFGRD96 86.0 255 80 5 4.71 0.4830
WVFGRD96 88.0 255 80 5 4.71 0.4800
WVFGRD96 90.0 255 80 5 4.71 0.4753
WVFGRD96 92.0 260 75 5 4.71 0.4719
WVFGRD96 94.0 260 75 5 4.71 0.4696
WVFGRD96 96.0 260 75 5 4.71 0.4672
WVFGRD96 98.0 260 75 5 4.71 0.4640
The best solution is
WVFGRD96 36.0 255 85 -15 4.48 0.5367
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 +70
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 Sat Apr 27 08:45:47 PM CDT 2024
