Location
SLU Location
To check the ANSS location or to compare the observed P-wave first motions to the moment tensor solution, P- and S-wave first arrival times were manually read together with the P-wave first motions. The subsequent output of the program elocate is given in the file elocate.txt. The first motion plot is shown below.
Location ANSS
The ANSS event ID is nn00234666 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/nn00234666/executive.
2008/02/21 23:57:52 41.146 -114.931 13.2 4.6 Nevada
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2008/02/21 23:57:52:0 41.15 -114.93 13.2 4.6 Nevada
Stations used:
BK.CMB BK.WDC CI.CWC CI.FUR CI.GSC CI.ISA CI.MLAC CI.MPM
CI.RCT CI.TIN CI.VES IW.DCID1 IW.IMW IW.LOHW IW.MOOW
IW.REDW IW.RRI2 IW.SNOW IW.TPAW NN.BEK TA.D14A TA.E09A
TA.E14A TA.E16A TA.E17A TA.F10A TA.F13A TA.F15A TA.F17A
TA.G12A TA.G14A TA.H07A TA.H12A TA.H15A TA.I08A TA.I09A
TA.I11A TA.I13A TA.I14A TA.I15A TA.I18A TA.J09A TA.J10A
TA.J11A TA.J12A TA.J13A TA.J14A TA.J15A TA.J16A TA.J17A
TA.K07A TA.K09A TA.K10A TA.K12A TA.K13A TA.K15A TA.K17A
TA.K18A TA.K20A TA.L08A TA.L09A TA.L10A TA.L13A TA.L14A
TA.L15A TA.L19A TA.L21A TA.M07A TA.M08A TA.M13A TA.M14A
TA.M15A TA.M16A TA.M17A TA.N06A TA.N07B TA.N08A TA.N11A
TA.N14A TA.N15A TA.N16A TA.N17A TA.O06A TA.O08A TA.O11A
TA.O13A TA.O15A TA.O17A TA.O18A TA.O19A TA.O20A TA.P07A
TA.P14A TA.P15A TA.P16A TA.P17A TA.P18A TA.P19A TA.Q09A
TA.Q10A TA.Q11A TA.Q12A TA.Q13A TA.Q15A TA.Q16A TA.Q20A
TA.Q21A TA.R06C TA.R08A TA.R10A TA.R12A TA.R14A TA.R15A
TA.R18A TA.R19A TA.R20A TA.S09A TA.S10A TA.S12A TA.S13A
TA.S14A TA.S15A TA.S19A TA.T11A TA.T13A TA.T14A TA.T16A
TA.U11A TA.U12A TA.U14A TA.V11A TA.V12A TA.V13A TA.V15A
TA.W12A TA.W13A TA.W14A US.AHID US.BMO US.DUG US.ELK
US.HLID US.WVOR UU.BGU UU.CCUT UU.CTU UU.HVU UU.MPU UU.NLU
UU.NOQ UU.SPU UU.SRU UU.TCU UU.TMU
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.04e+23 dyne-cm
Mw = 4.61
Z = 9 km
Plane Strike Dip Rake
NP1 19 68 -118
NP2 255 35 -40
Principal Axes:
Axis Value Plunge Azimuth
T 1.04e+23 19 130
N 0.00e+00 26 31
P -1.04e+23 57 251
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.56e+22
Mxy -5.50e+22
Mxz -5.17e+21
Myy 2.69e+22
Myz 6.86e+22
Mzz -6.25e+22
##############
##################----
######################------
################----###-------
##########---------------####-----
########------------------#######---
######---------------------##########-
#####----------------------############-
####-----------------------#############
####------------------------##############
###------------------------###############
##--------- -------------###############
#---------- P ------------################
---------- -----------################
-----------------------#################
---------------------########## ####
-------------------########### T ###
-----------------############ ##
--------------################
-----------#################
-------###############
##############
Global CMT Convention Moment Tensor:
R T P
-6.25e+22 -5.17e+21 -6.86e+22
-5.17e+21 3.56e+22 5.50e+22
-6.86e+22 5.50e+22 2.69e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080221235752/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 = 35
RAKE = -40
MW = 4.61
HS = 9.0
The NDK file is 20080221235752.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 2008/02/21 23:57:52:0 41.15 -114.93 13.2 4.6 Nevada
Stations used:
BK.CMB BK.WDC CI.CWC CI.FUR CI.GSC CI.ISA CI.MLAC CI.MPM
CI.RCT CI.TIN CI.VES IW.DCID1 IW.IMW IW.LOHW IW.MOOW
IW.REDW IW.RRI2 IW.SNOW IW.TPAW NN.BEK TA.D14A TA.E09A
TA.E14A TA.E16A TA.E17A TA.F10A TA.F13A TA.F15A TA.F17A
TA.G12A TA.G14A TA.H07A TA.H12A TA.H15A TA.I08A TA.I09A
TA.I11A TA.I13A TA.I14A TA.I15A TA.I18A TA.J09A TA.J10A
TA.J11A TA.J12A TA.J13A TA.J14A TA.J15A TA.J16A TA.J17A
TA.K07A TA.K09A TA.K10A TA.K12A TA.K13A TA.K15A TA.K17A
TA.K18A TA.K20A TA.L08A TA.L09A TA.L10A TA.L13A TA.L14A
TA.L15A TA.L19A TA.L21A TA.M07A TA.M08A TA.M13A TA.M14A
TA.M15A TA.M16A TA.M17A TA.N06A TA.N07B TA.N08A TA.N11A
TA.N14A TA.N15A TA.N16A TA.N17A TA.O06A TA.O08A TA.O11A
TA.O13A TA.O15A TA.O17A TA.O18A TA.O19A TA.O20A TA.P07A
TA.P14A TA.P15A TA.P16A TA.P17A TA.P18A TA.P19A TA.Q09A
TA.Q10A TA.Q11A TA.Q12A TA.Q13A TA.Q15A TA.Q16A TA.Q20A
TA.Q21A TA.R06C TA.R08A TA.R10A TA.R12A TA.R14A TA.R15A
TA.R18A TA.R19A TA.R20A TA.S09A TA.S10A TA.S12A TA.S13A
TA.S14A TA.S15A TA.S19A TA.T11A TA.T13A TA.T14A TA.T16A
TA.U11A TA.U12A TA.U14A TA.V11A TA.V12A TA.V13A TA.V15A
TA.W12A TA.W13A TA.W14A US.AHID US.BMO US.DUG US.ELK
US.HLID US.WVOR UU.BGU UU.CCUT UU.CTU UU.HVU UU.MPU UU.NLU
UU.NOQ UU.SPU UU.SRU UU.TCU UU.TMU
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.04e+23 dyne-cm
Mw = 4.61
Z = 9 km
Plane Strike Dip Rake
NP1 19 68 -118
NP2 255 35 -40
Principal Axes:
Axis Value Plunge Azimuth
T 1.04e+23 19 130
N 0.00e+00 26 31
P -1.04e+23 57 251
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.56e+22
Mxy -5.50e+22
Mxz -5.17e+21
Myy 2.69e+22
Myz 6.86e+22
Mzz -6.25e+22
##############
##################----
######################------
################----###-------
##########---------------####-----
########------------------#######---
######---------------------##########-
#####----------------------############-
####-----------------------#############
####------------------------##############
###------------------------###############
##--------- -------------###############
#---------- P ------------################
---------- -----------################
-----------------------#################
---------------------########## ####
-------------------########### T ###
-----------------############ ##
--------------################
-----------#################
-------###############
##############
Global CMT Convention Moment Tensor:
R T P
-6.25e+22 -5.17e+21 -6.86e+22
-5.17e+21 3.56e+22 5.50e+22
-6.86e+22 5.50e+22 2.69e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080221235752/index.html
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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:
hp c 0.02 n 3
lp c 0.06 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 90 75 -25 4.27 0.3872
WVFGRD96 1.0 90 75 -20 4.28 0.3977
WVFGRD96 2.0 90 70 -20 4.37 0.4472
WVFGRD96 3.0 265 35 -15 4.50 0.4774
WVFGRD96 4.0 265 35 -20 4.52 0.5335
WVFGRD96 5.0 265 35 -20 4.53 0.5799
WVFGRD96 6.0 260 35 -30 4.54 0.6120
WVFGRD96 7.0 260 40 -35 4.54 0.6307
WVFGRD96 8.0 255 35 -40 4.61 0.6518
WVFGRD96 9.0 255 35 -40 4.61 0.6550
WVFGRD96 10.0 260 40 -35 4.60 0.6491
WVFGRD96 11.0 260 40 -30 4.59 0.6359
WVFGRD96 12.0 265 45 -25 4.59 0.6223
WVFGRD96 13.0 270 50 -10 4.58 0.6078
WVFGRD96 14.0 270 50 -10 4.59 0.5933
WVFGRD96 15.0 270 50 -10 4.59 0.5768
WVFGRD96 16.0 275 55 5 4.60 0.5613
WVFGRD96 17.0 275 55 5 4.60 0.5457
WVFGRD96 18.0 275 55 5 4.60 0.5293
WVFGRD96 19.0 275 55 5 4.60 0.5125
WVFGRD96 20.0 275 55 5 4.61 0.4958
WVFGRD96 21.0 275 55 5 4.61 0.4791
WVFGRD96 22.0 275 55 5 4.62 0.4626
WVFGRD96 23.0 275 55 5 4.62 0.4467
WVFGRD96 24.0 275 55 5 4.62 0.4311
WVFGRD96 25.0 275 55 5 4.62 0.4161
WVFGRD96 26.0 275 55 5 4.63 0.4016
WVFGRD96 27.0 275 60 10 4.64 0.3881
WVFGRD96 28.0 275 60 10 4.64 0.3751
WVFGRD96 29.0 275 60 10 4.64 0.3626
The best solution is
WVFGRD96 9.0 255 35 -40 4.61 0.6550
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
hp c 0.02 n 3
lp c 0.06 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.
Surface-Wave Focal Mechanism
The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 15.00
DIP= 74.99
RAKE= -125.00
OR
STK= 264.71
DIP= 37.70
RAKE= -25.05
DEPTH = 7.0 km
Mw = 4.62
Best Fit 0.9154 - P-T axis plot gives solutions with FIT greater than FIT90
Surface-wave analysis
Surface wave analysis was performed using codes from
Computer Programs in Seismology, specifically the
multiple filter analysis program do_mft and the surface-wave
radiation pattern search program srfgrd96.
Data preparation
Digital data were collected, instrument response removed and traces converted
to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.
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Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection
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Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the
possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here.
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns.
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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled.
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Love-wave radiation patterns
Rayleigh-wave radiation patterns
