Location
SLU Location
Because of the difference between the ANSS depth of 20 km and the moment tensor depth of 42 km, P and S arrival times were
read and the event was relocated using the WUS model using the program elocate. The processing
results are given in the file elocate.txt. The SLU epicenter is the same within 1 km, but the depth is 36 km, which is closer to the RMT depth.
Location ANSS
The ANSS event ID is ak0182cn8ti1 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0182cn8ti1/executive.
2018/02/20 18:20:14 61.019 -147.602 25.1 3.6 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2018/02/20 18:20:14:0 61.02 -147.60 25.1 3.6 Alaska
Stations used:
AK.BERG AK.CRQ AK.FID AK.GLB AK.GLI AK.HIN AK.HMT AK.KLU
AK.KNK AK.RC01 AK.SAW AK.SCM AK.SWD TA.M22K TA.N25K
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 5.13e+21 dyne-cm
Mw = 3.74
Z = 42 km
Plane Strike Dip Rake
NP1 260 80 -40
NP2 358 51 -167
Principal Axes:
Axis Value Plunge Azimuth
T 5.13e+21 19 315
N 0.00e+00 49 68
P -5.13e+21 35 211
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.30e+20
Mxy -3.83e+21
Mxz 3.17e+21
Myy 1.36e+21
Myz 1.34e+20
Mzz -1.13e+21
########------
##############--------
###################---------
# #################---------
### T ##################----------
#### ###################----------
############################----------
#############################-----------
#############################-----------
######################---------###########
#############------------------###########
#######------------------------###########
##-----------------------------###########
------------------------------##########
-----------------------------###########
----------------------------##########
---------- -------------##########
--------- P -------------#########
------- ------------########
--------------------########
---------------#######
---------#####
Global CMT Convention Moment Tensor:
R T P
-1.13e+21 3.17e+21 -1.34e+20
3.17e+21 -2.30e+20 3.83e+21
-1.34e+20 3.83e+21 1.36e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180220182014/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 = 260
DIP = 80
RAKE = -40
MW = 3.74
HS = 42.0
The NDK file is 20180220182014.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 2018/02/20 18:20:14:0 61.02 -147.60 25.1 3.6 Alaska
Stations used:
AK.BERG AK.CRQ AK.FID AK.GLB AK.GLI AK.HIN AK.HMT AK.KLU
AK.KNK AK.RC01 AK.SAW AK.SCM AK.SWD TA.M22K TA.N25K
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 5.13e+21 dyne-cm
Mw = 3.74
Z = 42 km
Plane Strike Dip Rake
NP1 260 80 -40
NP2 358 51 -167
Principal Axes:
Axis Value Plunge Azimuth
T 5.13e+21 19 315
N 0.00e+00 49 68
P -5.13e+21 35 211
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.30e+20
Mxy -3.83e+21
Mxz 3.17e+21
Myy 1.36e+21
Myz 1.34e+20
Mzz -1.13e+21
########------
##############--------
###################---------
# #################---------
### T ##################----------
#### ###################----------
############################----------
#############################-----------
#############################-----------
######################---------###########
#############------------------###########
#######------------------------###########
##-----------------------------###########
------------------------------##########
-----------------------------###########
----------------------------##########
---------- -------------##########
--------- P -------------#########
------- ------------########
--------------------########
---------------#######
---------#####
Global CMT Convention Moment Tensor:
R T P
-1.13e+21 3.17e+21 -1.34e+20
3.17e+21 -2.30e+20 3.83e+21
-1.34e+20 3.83e+21 1.36e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180220182014/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 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 265 90 -5 3.10 0.3310
WVFGRD96 4.0 85 75 -10 3.22 0.4002
WVFGRD96 6.0 90 70 20 3.28 0.4363
WVFGRD96 8.0 100 55 40 3.38 0.4592
WVFGRD96 10.0 90 65 20 3.37 0.4704
WVFGRD96 12.0 90 70 25 3.38 0.4817
WVFGRD96 14.0 90 70 25 3.41 0.4945
WVFGRD96 16.0 90 70 25 3.43 0.5085
WVFGRD96 18.0 90 70 25 3.45 0.5210
WVFGRD96 20.0 85 85 25 3.47 0.5344
WVFGRD96 22.0 85 85 30 3.50 0.5513
WVFGRD96 24.0 85 90 30 3.52 0.5676
WVFGRD96 26.0 265 90 -35 3.54 0.5814
WVFGRD96 28.0 265 85 -35 3.56 0.5918
WVFGRD96 30.0 265 85 -35 3.58 0.5992
WVFGRD96 32.0 265 85 -40 3.59 0.6023
WVFGRD96 34.0 265 85 -40 3.61 0.6018
WVFGRD96 36.0 265 85 -40 3.63 0.5983
WVFGRD96 38.0 260 80 -35 3.64 0.5979
WVFGRD96 40.0 260 80 -40 3.72 0.6019
WVFGRD96 42.0 260 80 -40 3.74 0.6058
WVFGRD96 44.0 260 80 -40 3.75 0.6054
WVFGRD96 46.0 265 90 -35 3.77 0.6026
WVFGRD96 48.0 265 90 -30 3.77 0.6008
WVFGRD96 50.0 265 90 -30 3.79 0.5999
WVFGRD96 52.0 85 90 30 3.80 0.5984
WVFGRD96 54.0 85 90 30 3.81 0.5964
WVFGRD96 56.0 85 90 30 3.81 0.5929
WVFGRD96 58.0 85 85 30 3.83 0.5902
WVFGRD96 60.0 90 80 30 3.84 0.5871
WVFGRD96 62.0 90 80 30 3.85 0.5855
WVFGRD96 64.0 90 75 25 3.87 0.5818
WVFGRD96 66.0 90 75 25 3.87 0.5786
WVFGRD96 68.0 90 75 25 3.88 0.5750
WVFGRD96 70.0 90 75 25 3.89 0.5699
WVFGRD96 72.0 90 75 25 3.89 0.5638
WVFGRD96 74.0 90 75 25 3.90 0.5578
WVFGRD96 76.0 90 75 25 3.90 0.5513
WVFGRD96 78.0 90 75 25 3.90 0.5448
WVFGRD96 80.0 90 75 25 3.91 0.5379
WVFGRD96 82.0 90 75 25 3.91 0.5304
WVFGRD96 84.0 95 70 30 3.93 0.5247
WVFGRD96 86.0 95 70 30 3.94 0.5189
WVFGRD96 88.0 95 70 30 3.94 0.5119
WVFGRD96 90.0 95 70 30 3.94 0.5041
WVFGRD96 92.0 95 70 30 3.94 0.4954
WVFGRD96 94.0 95 70 30 3.94 0.4854
WVFGRD96 96.0 95 70 30 3.95 0.4748
WVFGRD96 98.0 95 75 25 3.93 0.4633
The best solution is
WVFGRD96 42.0 260 80 -40 3.74 0.6058
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
|
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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 09:22:26 PM CDT 2024
