Location
Location ANSS
The ANSS event ID is se609212 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/se609212/executive.
2011/08/23 17:51:05 37.910 -77.936 6.0 5.8 Virginia
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2011/08/23 17:51:05:0 37.91 -77.94 6.0 5.8 Virginia
Stations used:
IU.HRV IU.SSPA IU.WCI IU.WVT LD.ACCN LD.FRNY LD.WVNY NM.BLO
NM.OLIL NM.USIN NM.UTMT PE.PSUB TA.Q44A TA.R44A TA.S44A
TA.S45A TA.SFIN TA.TIGA TA.Y47A US.AAM US.ACSO US.BINY
US.BLA US.CNNC US.ERPA US.GLMI US.GOGA US.LBNH US.LONY
US.LRAL US.MCWV US.TZTN
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 2.57e+24 dyne-cm
Mw = 5.54
Z = 7 km
Plane Strike Dip Rake
NP1 50 70 142
NP2 155 55 25
Principal Axes:
Axis Value Plunge Azimuth
T 2.57e+24 41 7
N 0.00e+00 48 206
P -2.57e+24 9 105
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.28e+24
Mxy 8.36e+23
Mxz 1.37e+24
Myy -2.30e+24
Myz -2.28e+23
Mzz 1.02e+24
##############
--####################
----########################
----########### ############
------########### T #############-
-------########### ############---
--------#########################-----
---------########################-------
---------#######################--------
-----------####################-----------
-----------##################-------------
------------################--------------
-------------#############------------ -
------------###########-------------- P
-------------#######-----------------
--------------##----------------------
------------##----------------------
-------#######--------------------
##############----------------
################------------
################------
##############
Global CMT Convention Moment Tensor:
R T P
1.02e+24 1.37e+24 2.28e+23
1.37e+24 1.28e+24 -8.36e+23
2.28e+23 -8.36e+23 -2.30e+24
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110823175105/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 = 155
DIP = 55
RAKE = 25
MW = 5.54
HS = 7.0
The NDK file is 20110823175105.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 |
GCMT |
USGS/SLU Moment Tensor Solution
ENS 2011/08/23 17:51:05:0 37.91 -77.94 6.0 5.8 Virginia
Stations used:
IU.HRV IU.SSPA IU.WCI IU.WVT LD.ACCN LD.FRNY LD.WVNY NM.BLO
NM.OLIL NM.USIN NM.UTMT PE.PSUB TA.Q44A TA.R44A TA.S44A
TA.S45A TA.SFIN TA.TIGA TA.Y47A US.AAM US.ACSO US.BINY
US.BLA US.CNNC US.ERPA US.GLMI US.GOGA US.LBNH US.LONY
US.LRAL US.MCWV US.TZTN
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 2.57e+24 dyne-cm
Mw = 5.54
Z = 7 km
Plane Strike Dip Rake
NP1 50 70 142
NP2 155 55 25
Principal Axes:
Axis Value Plunge Azimuth
T 2.57e+24 41 7
N 0.00e+00 48 206
P -2.57e+24 9 105
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.28e+24
Mxy 8.36e+23
Mxz 1.37e+24
Myy -2.30e+24
Myz -2.28e+23
Mzz 1.02e+24
##############
--####################
----########################
----########### ############
------########### T #############-
-------########### ############---
--------#########################-----
---------########################-------
---------#######################--------
-----------####################-----------
-----------##################-------------
------------################--------------
-------------#############------------ -
------------###########-------------- P
-------------#######-----------------
--------------##----------------------
------------##----------------------
-------#######--------------------
##############----------------
################------------
################------
##############
Global CMT Convention Moment Tensor:
R T P
1.02e+24 1.37e+24 2.28e+23
1.37e+24 1.28e+24 -8.36e+23
2.28e+23 -8.36e+23 -2.30e+24
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110823175105/index.html
|
USGS/SLU Regional Moment Solution
11/08/23 17:51:04.59
Epicenter: 37.936 -77.933
MW 5.7
USGS/SLU REGIONAL MOMENT TENSOR
Depth 6 No. of sta: 25
Moment Tensor; Scale 10**17 Nm
Mrr= 4.20 Mtt=-0.15
Mpp=-4.05 Mrt= 1.52
Mrp= 0.96 Mtp=-0.79
Principal axes:
T Val= 4.73 Plg=73 Azm=344
N -0.35 15 196
P -4.39 9 104
Best Double Couple:Mo=4.6*10**17
NP1:Strike=177 Dip=39 Slip= 66
NP2: 26 55 108
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USGS WPhase Moment Solution
11/08/23 17:51: 3
Epicenter: 37.974 -77.968
MW 5.8
USGS/WPHASE CENTROID MOMENT TENSOR
11/08/23 17:51: 3.00
Centroid: 37.974 -77.715
Depth 11 No. of sta: 73
Moment Tensor; Scale 10**17 Nm
Mrr= 4.85 Mtt= 0.29
Mpp=-5.13 Mrt= 1.03
Mrp=-2.13 Mtp=-1.37
Principal axes:
T Val= 5.60 Plg=71 Azm= 42
N = 0.19 15 188
P = -5.79 10 281
Best Double Couple:Mo=5.7*10**17
NP1:Strike= 30 Dip=37 Slip= 117
NP2: 177 57 71
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August 23, 2011, VIRGINIA, MW=5.8
Meredith Nettles
Howard Koss
CENTROID-MOMENT-TENSOR SOLUTION
GCMT EVENT: C201108231751A
DATA: II DK IU CU MN G GE LD
L.P.BODY WAVES: 65S, 107C, T= 40
MANTLE WAVES: 18S, 19C, T=125
SURFACE WAVES: 82S, 142C, T= 50
TIMESTAMP: Q-20110823145453
CENTROID LOCATION:
ORIGIN TIME: 17:51:07.3 0.2
LAT:37.84N 0.01;LON: 77.96W 0.01
DEP: 12.0 FIX;TRIANG HDUR: 1.9
MOMENT TENSOR: SCALE 10**24 D-CM
RR= 5.200 0.084; TT= 0.326 0.084
PP=-5.520 0.089; RT= 2.080 0.222
RP=-0.542 0.274; TP=-0.880 0.079
PRINCIPAL AXES:
1.(T) VAL= 6.025;PLG=69;AZM= 11
2.(N) -0.364; 21; 187
3.(P) -5.655; 1; 278
BEST DBLE.COUPLE:M0= 5.84*10**24
NP1: STRIKE= 28;DIP=47;SLIP= 119
NP2: STRIKE=169;DIP=50;SLIP= 62
--#########
----##############-
-----################--
------#################----
-------#################-----
-------######## ########-----
------######## T ########-----
P ------######## #######-------
------##################-------
---------################--------
---------################--------
--------###############--------
---------############----------
---------##########----------
---------#######-----------
---------###-----------
------###----------
#######----
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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.
mLg Magnitude
Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 150 85 -5 5.39 0.5906
WVFGRD96 1.0 150 85 -5 5.41 0.6013
WVFGRD96 2.0 325 55 -10 5.49 0.6087
WVFGRD96 3.0 155 50 20 5.52 0.6222
WVFGRD96 4.0 155 50 25 5.53 0.6533
WVFGRD96 5.0 160 50 35 5.55 0.6826
WVFGRD96 6.0 160 50 35 5.55 0.6989
WVFGRD96 7.0 155 55 25 5.54 0.7036
WVFGRD96 8.0 155 55 25 5.54 0.7028
WVFGRD96 9.0 155 60 20 5.54 0.6986
WVFGRD96 10.0 155 55 20 5.55 0.6969
WVFGRD96 11.0 155 55 20 5.56 0.6879
WVFGRD96 12.0 155 60 15 5.56 0.6800
WVFGRD96 13.0 155 60 15 5.56 0.6710
WVFGRD96 14.0 150 60 5 5.57 0.6622
WVFGRD96 15.0 150 60 5 5.57 0.6545
WVFGRD96 16.0 150 60 5 5.58 0.6485
WVFGRD96 17.0 150 60 5 5.59 0.6422
WVFGRD96 18.0 150 65 5 5.60 0.6355
WVFGRD96 19.0 150 65 5 5.61 0.6284
WVFGRD96 20.0 150 60 5 5.62 0.6203
WVFGRD96 21.0 150 60 5 5.63 0.6130
WVFGRD96 22.0 150 65 5 5.64 0.6048
WVFGRD96 23.0 150 65 5 5.64 0.5965
WVFGRD96 24.0 150 65 0 5.65 0.5879
WVFGRD96 25.0 150 65 5 5.66 0.5789
WVFGRD96 26.0 150 65 0 5.67 0.5701
WVFGRD96 27.0 150 65 0 5.67 0.5601
WVFGRD96 28.0 150 65 0 5.68 0.5489
WVFGRD96 29.0 150 65 0 5.69 0.5387
The best solution is
WVFGRD96 7.0 155 55 25 5.54 0.7036
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 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 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= 49.64
DIP= 67.48
RAKE= 135.90
OR
STK= 159.99
DIP= 50.00
RAKE= 30.00
DEPTH = 6.0 km
Mw = 5.74
Best Fit 0.8344 - 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
