Location
Location ANSS
The ANSS event ID is pr2017243002 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/pr2017243002/executive.
2017/08/31 07:53:22 19.101 -64.292 45.0 2.78 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2017/08/31 07:53:22:0 19.10 -64.29 45.0 2.8 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI AK.HDA
AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PPLA AK.PWL
AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AK.WRH AT.PMR
AT.TTA IM.IL31 IU.COLA TA.J20K TA.K20K TA.L19K TA.M19K
TA.M20K TA.M22K TA.O22K TA.POKR TA.TCOL
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 5.37e+22 dyne-cm
Mw = 4.42
Z = 96 km
Plane Strike Dip Rake
NP1 90 70 80
NP2 297 22 116
Principal Axes:
Axis Value Plunge Azimuth
T 5.37e+22 64 344
N 0.00e+00 9 93
P -5.37e+22 24 188
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.40e+22
Mxy -8.76e+21
Mxz 4.05e+22
Myy -7.66e+14
Myz -3.19e+21
Mzz 3.40e+22
--------------
----------------------
----###############---------
-#######################------
-###########################------
###############################-----
################ ###############----
################# T ################----
################# #################---
#######################################---
########################################--
--########################################
------##############################----##
-------------##############------------#
---------------------------------------#
--------------------------------------
------------------------------------
----------------------------------
----------- ----------------
---------- P ---------------
------- ------------
--------------
Global CMT Convention Moment Tensor:
R T P
3.40e+22 4.05e+22 3.19e+21
4.05e+22 -3.40e+22 8.76e+21
3.19e+21 8.76e+21 -7.66e+14
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170831075322/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 = 90
DIP = 70
RAKE = 80
MW = 4.42
HS = 96.0
The NDK file is 20170831075322.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 2017/08/31 07:53:22:0 19.10 -64.29 45.0 2.8 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI AK.HDA
AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PPLA AK.PWL
AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AK.WRH AT.PMR
AT.TTA IM.IL31 IU.COLA TA.J20K TA.K20K TA.L19K TA.M19K
TA.M20K TA.M22K TA.O22K TA.POKR TA.TCOL
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 5.37e+22 dyne-cm
Mw = 4.42
Z = 96 km
Plane Strike Dip Rake
NP1 90 70 80
NP2 297 22 116
Principal Axes:
Axis Value Plunge Azimuth
T 5.37e+22 64 344
N 0.00e+00 9 93
P -5.37e+22 24 188
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.40e+22
Mxy -8.76e+21
Mxz 4.05e+22
Myy -7.66e+14
Myz -3.19e+21
Mzz 3.40e+22
--------------
----------------------
----###############---------
-#######################------
-###########################------
###############################-----
################ ###############----
################# T ################----
################# #################---
#######################################---
########################################--
--########################################
------##############################----##
-------------##############------------#
---------------------------------------#
--------------------------------------
------------------------------------
----------------------------------
----------- ----------------
---------- P ---------------
------- ------------
--------------
Global CMT Convention Moment Tensor:
R T P
3.40e+22 4.05e+22 3.19e+21
4.05e+22 -3.40e+22 8.76e+21
3.19e+21 8.76e+21 -7.66e+14
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170831075322/index.html
|
Regional Moment Tensor (Mwr)
Moment 6.759e+15 N-m
Magnitude 4.5 Mwr
Depth 100.0 km
Percent DC 88 %
Half Duration –
Catalog US
Data Source US2
Contributor US2
Nodal Planes
Plane Strike Dip Rake
NP1 278 21 93
NP2 95 69 89
Principal Axes
Axis Value Plunge Azimuth
T 6.534e+15 N-m 66 3
N 0.429e+15 N-m 1 95
P -6.963e+15 N-m 24 186
|
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/3.5 -40 o DIST/3.5 +60
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 2.0 280 50 -80 3.59 0.2215
WVFGRD96 4.0 325 55 25 3.61 0.1933
WVFGRD96 6.0 330 55 30 3.67 0.2336
WVFGRD96 8.0 330 55 30 3.75 0.2553
WVFGRD96 10.0 330 55 25 3.79 0.2699
WVFGRD96 12.0 330 55 20 3.82 0.2751
WVFGRD96 14.0 330 60 20 3.85 0.2730
WVFGRD96 16.0 330 55 10 3.87 0.2674
WVFGRD96 18.0 330 55 10 3.89 0.2594
WVFGRD96 20.0 245 65 45 3.91 0.2574
WVFGRD96 22.0 245 75 45 3.94 0.2610
WVFGRD96 24.0 240 75 40 3.97 0.2676
WVFGRD96 26.0 240 75 40 3.99 0.2747
WVFGRD96 28.0 240 75 35 4.02 0.2815
WVFGRD96 30.0 240 75 35 4.03 0.2883
WVFGRD96 32.0 235 75 30 4.05 0.2925
WVFGRD96 34.0 235 75 30 4.07 0.2925
WVFGRD96 36.0 235 70 30 4.08 0.2951
WVFGRD96 38.0 230 75 25 4.11 0.3038
WVFGRD96 40.0 240 65 50 4.20 0.3509
WVFGRD96 42.0 245 65 55 4.24 0.3815
WVFGRD96 44.0 245 65 50 4.26 0.4025
WVFGRD96 46.0 245 65 50 4.28 0.4152
WVFGRD96 48.0 250 55 55 4.29 0.4218
WVFGRD96 50.0 250 55 55 4.30 0.4281
WVFGRD96 52.0 75 55 55 4.32 0.4402
WVFGRD96 54.0 75 55 55 4.33 0.4604
WVFGRD96 56.0 80 55 65 4.33 0.4795
WVFGRD96 58.0 85 55 70 4.34 0.4991
WVFGRD96 60.0 85 55 70 4.34 0.5155
WVFGRD96 62.0 85 60 70 4.36 0.5321
WVFGRD96 64.0 85 60 70 4.36 0.5471
WVFGRD96 66.0 90 60 80 4.36 0.5626
WVFGRD96 68.0 90 60 80 4.37 0.5793
WVFGRD96 70.0 90 60 80 4.37 0.5930
WVFGRD96 72.0 90 65 80 4.38 0.6066
WVFGRD96 74.0 90 65 80 4.39 0.6188
WVFGRD96 76.0 90 65 80 4.39 0.6308
WVFGRD96 78.0 90 65 80 4.39 0.6388
WVFGRD96 80.0 90 65 80 4.40 0.6463
WVFGRD96 82.0 90 65 80 4.40 0.6531
WVFGRD96 84.0 90 70 80 4.41 0.6577
WVFGRD96 86.0 90 70 80 4.42 0.6645
WVFGRD96 88.0 90 70 80 4.42 0.6677
WVFGRD96 90.0 90 70 80 4.42 0.6719
WVFGRD96 92.0 90 70 80 4.42 0.6736
WVFGRD96 94.0 90 70 80 4.42 0.6750
WVFGRD96 96.0 90 70 80 4.42 0.6758
WVFGRD96 98.0 90 70 80 4.42 0.6739
WVFGRD96 100.0 90 70 80 4.43 0.6748
WVFGRD96 102.0 90 70 80 4.43 0.6737
WVFGRD96 104.0 90 70 80 4.43 0.6728
WVFGRD96 106.0 90 70 80 4.43 0.6704
WVFGRD96 108.0 90 70 80 4.43 0.6672
WVFGRD96 110.0 90 70 80 4.43 0.6659
WVFGRD96 112.0 90 75 80 4.45 0.6628
WVFGRD96 114.0 90 75 80 4.45 0.6614
WVFGRD96 116.0 90 75 80 4.45 0.6570
WVFGRD96 118.0 255 15 65 4.45 0.6555
The best solution is
WVFGRD96 96.0 90 70 80 4.42 0.6758
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/3.5 -40 o DIST/3.5 +60
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 04:16:39 PM CDT 2024
