Location
Location ANSS
The ANSS event ID is usc000s84p and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/usc000s84p/executive.
2014/08/29 18:37:29 65.629 -122.650 9.0 4.6 Canada
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2014/08/29 18:37:29:0 65.63 -122.65 9.0 4.6 Canada
Stations used:
AK.BAL AK.BARN AK.BCP AK.BESE AK.CCB AK.CRQ AK.CTG AK.DOT
AK.GLB AK.HDA AK.ISLE AK.JIS AK.MCAR AK.MDM AK.MESA AK.PIN
AK.TABL AK.TGL AK.WRH AK.YAH AT.MENT AT.SIT AT.SKAG AT.YKU2
CN.DHRN CN.DLBC CN.HYT CN.INK CN.KUKN CN.NOWN CN.YKW3
IM.IL31 IU.COLA TA.C36M US.EGAK US.WRAK
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +100
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 8.71e+22 dyne-cm
Mw = 4.56
Z = 16 km
Plane Strike Dip Rake
NP1 305 90 -105
NP2 215 15 0
Principal Axes:
Axis Value Plunge Azimuth
T 8.71e+22 43 50
N 0.00e+00 15 305
P -8.71e+22 43 200
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.12e+22
Mxy 7.71e+21
Mxz 6.89e+22
Myy 2.12e+22
Myz 4.83e+22
Mzz 0.00e+00
--------------
-----#################
-----#######################
----##########################
----##############################
########################## #######
###----#################### T ########
###--------################# #########
###----------###########################
###--------------#########################
###-----------------######################
###-------------------####################
###----------------------#################
##-------------------------#############
##---------------------------###########
##-----------------------------#######
#----------- ------------------###
#---------- P --------------------
--------- ------------------
#---------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
0.00e+00 6.89e+22 -4.83e+22
6.89e+22 -2.12e+22 -7.71e+21
-4.83e+22 -7.71e+21 2.12e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140829183729/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 = 215
DIP = 15
RAKE = 0
MW = 4.56
HS = 16.0
The NDK file is 20140829183729.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 |
USGSMT |
USGS/SLU Moment Tensor Solution
ENS 2014/08/29 18:37:29:0 65.63 -122.65 9.0 4.6 Canada
Stations used:
AK.BAL AK.BARN AK.BCP AK.BESE AK.CCB AK.CRQ AK.CTG AK.DOT
AK.GLB AK.HDA AK.ISLE AK.JIS AK.MCAR AK.MDM AK.MESA AK.PIN
AK.TABL AK.TGL AK.WRH AK.YAH AT.MENT AT.SIT AT.SKAG AT.YKU2
CN.DHRN CN.DLBC CN.HYT CN.INK CN.KUKN CN.NOWN CN.YKW3
IM.IL31 IU.COLA TA.C36M US.EGAK US.WRAK
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +100
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 8.71e+22 dyne-cm
Mw = 4.56
Z = 16 km
Plane Strike Dip Rake
NP1 305 90 -105
NP2 215 15 0
Principal Axes:
Axis Value Plunge Azimuth
T 8.71e+22 43 50
N 0.00e+00 15 305
P -8.71e+22 43 200
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.12e+22
Mxy 7.71e+21
Mxz 6.89e+22
Myy 2.12e+22
Myz 4.83e+22
Mzz 0.00e+00
--------------
-----#################
-----#######################
----##########################
----##############################
########################## #######
###----#################### T ########
###--------################# #########
###----------###########################
###--------------#########################
###-----------------######################
###-------------------####################
###----------------------#################
##-------------------------#############
##---------------------------###########
##-----------------------------#######
#----------- ------------------###
#---------- P --------------------
--------- ------------------
#---------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
0.00e+00 6.89e+22 -4.83e+22
6.89e+22 -2.12e+22 -7.71e+21
-4.83e+22 -7.71e+21 2.12e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140829183729/index.html
|
|
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 -40 o DIST/3.3 +100
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 145 45 90 4.32 0.5217
WVFGRD96 2.0 145 40 90 4.38 0.5439
WVFGRD96 3.0 145 30 90 4.45 0.4933
WVFGRD96 4.0 150 25 95 4.50 0.4511
WVFGRD96 5.0 240 10 15 4.53 0.4923
WVFGRD96 6.0 235 10 10 4.51 0.5620
WVFGRD96 7.0 235 10 15 4.50 0.6117
WVFGRD96 8.0 240 10 20 4.57 0.6448
WVFGRD96 9.0 240 10 20 4.57 0.6881
WVFGRD96 10.0 230 10 10 4.56 0.7199
WVFGRD96 11.0 230 10 10 4.56 0.7437
WVFGRD96 12.0 225 10 10 4.56 0.7610
WVFGRD96 13.0 225 10 10 4.56 0.7727
WVFGRD96 14.0 220 10 5 4.56 0.7800
WVFGRD96 15.0 220 10 5 4.56 0.7835
WVFGRD96 16.0 215 15 0 4.56 0.7838
WVFGRD96 17.0 215 15 0 4.56 0.7822
WVFGRD96 18.0 215 15 0 4.56 0.7781
WVFGRD96 19.0 215 15 5 4.57 0.7724
WVFGRD96 20.0 215 15 5 4.57 0.7651
WVFGRD96 21.0 215 15 5 4.59 0.7565
WVFGRD96 22.0 215 15 5 4.59 0.7460
WVFGRD96 23.0 215 15 5 4.59 0.7343
WVFGRD96 24.0 215 15 5 4.60 0.7216
WVFGRD96 25.0 215 15 5 4.60 0.7078
WVFGRD96 26.0 205 15 -5 4.60 0.6932
WVFGRD96 27.0 205 15 -5 4.61 0.6780
WVFGRD96 28.0 205 15 -5 4.61 0.6621
WVFGRD96 29.0 205 15 -5 4.61 0.6455
The best solution is
WVFGRD96 16.0 215 15 0 4.56 0.7838
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 -40 o DIST/3.3 +100
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 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 10:43:59 AM CDT 2024
