2011/08/24 00:04:36 37.912 -77.950 7 4.20 Virginia
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution
ENS 2011/08/24 00:04:36:0 37.91 -77.95 7.0 4.2 Virginia
Stations used:
CO.HODGE CO.JSC IU.SSPA LD.BRNJ LD.LUPA LD.MVL LD.PAL
LD.SDMD LD.WVNY PE.PSUB TA.KMSC US.ACSO US.BINY US.BLA
US.CBN US.CNNC US.ERPA US.MCWV US.NHSC US.TZTN
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 5.69e+21 dyne-cm
Mw = 3.77
Z = 6 km
Plane Strike Dip Rake
NP1 40 60 109
NP2 185 35 60
Principal Axes:
Axis Value Plunge Azimuth
T 5.69e+21 69 350
N 0.00e+00 17 210
P -5.69e+21 13 116
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.18e+20
Mxy 2.01e+21
Mxz 2.47e+21
Myy -4.31e+21
Myz -1.48e+21
Mzz 4.63e+21
-----#########
------################
-------####################-
------#######################-
-------########################---
-------########################-----
-------#########################------
--------########## ###########--------
-------########### T ###########--------
--------########### ##########----------
-------########################-----------
-------#######################------------
-------#####################--------------
-------###################--------------
-------#################----------- --
-------##############------------- P -
------############---------------
------#########-------------------
-----####---------------------
--####----------------------
####------------------
###-----------
Global CMT Convention Moment Tensor:
R T P
4.63e+21 2.47e+21 1.48e+21
2.47e+21 -3.18e+20 -2.01e+21
1.48e+21 -2.01e+21 -4.31e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110824000436/index.html
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STK = 185
DIP = 35
RAKE = 60
MW = 3.77
HS = 6.0
The waveform inversion is preferred.
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution
ENS 2011/08/24 00:04:36:0 37.91 -77.95 7.0 4.2 Virginia
Stations used:
CO.HODGE CO.JSC IU.SSPA LD.BRNJ LD.LUPA LD.MVL LD.PAL
LD.SDMD LD.WVNY PE.PSUB TA.KMSC US.ACSO US.BINY US.BLA
US.CBN US.CNNC US.ERPA US.MCWV US.NHSC US.TZTN
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 5.69e+21 dyne-cm
Mw = 3.77
Z = 6 km
Plane Strike Dip Rake
NP1 40 60 109
NP2 185 35 60
Principal Axes:
Axis Value Plunge Azimuth
T 5.69e+21 69 350
N 0.00e+00 17 210
P -5.69e+21 13 116
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.18e+20
Mxy 2.01e+21
Mxz 2.47e+21
Myy -4.31e+21
Myz -1.48e+21
Mzz 4.63e+21
-----#########
------################
-------####################-
------#######################-
-------########################---
-------########################-----
-------#########################------
--------########## ###########--------
-------########### T ###########--------
--------########### ##########----------
-------########################-----------
-------#######################------------
-------#####################--------------
-------###################--------------
-------#################----------- --
-------##############------------- P -
------############---------------
------#########-------------------
-----####---------------------
--####----------------------
####------------------
###-----------
Global CMT Convention Moment Tensor:
R T P
4.63e+21 2.47e+21 1.48e+21
2.47e+21 -3.18e+20 -2.01e+21
1.48e+21 -2.01e+21 -4.31e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110824000436/index.html
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The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
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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 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 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 185 10 45 3.99 0.4378
WVFGRD96 1.0 190 15 50 3.91 0.4465
WVFGRD96 2.0 185 20 50 3.84 0.4691
WVFGRD96 3.0 185 25 55 3.81 0.4980
WVFGRD96 4.0 185 30 55 3.79 0.5239
WVFGRD96 5.0 185 30 60 3.79 0.5361
WVFGRD96 6.0 185 35 60 3.77 0.5362
WVFGRD96 7.0 185 35 60 3.76 0.5257
WVFGRD96 8.0 180 35 55 3.75 0.5123
WVFGRD96 9.0 175 35 45 3.73 0.4983
WVFGRD96 10.0 175 35 45 3.75 0.4998
WVFGRD96 11.0 150 40 -20 3.71 0.4893
WVFGRD96 12.0 150 45 -20 3.70 0.4903
WVFGRD96 13.0 150 45 -20 3.71 0.4907
WVFGRD96 14.0 150 45 -15 3.71 0.4895
WVFGRD96 15.0 150 45 -15 3.72 0.4869
WVFGRD96 16.0 150 45 -15 3.72 0.4831
WVFGRD96 17.0 150 45 -15 3.73 0.4782
WVFGRD96 18.0 150 50 -15 3.72 0.4735
WVFGRD96 19.0 150 50 -15 3.73 0.4680
WVFGRD96 20.0 150 45 -15 3.75 0.4623
WVFGRD96 21.0 150 45 -15 3.76 0.4556
WVFGRD96 22.0 150 45 -15 3.77 0.4483
WVFGRD96 23.0 150 50 -15 3.76 0.4413
WVFGRD96 24.0 150 50 -15 3.77 0.4351
WVFGRD96 25.0 150 50 -15 3.77 0.4286
WVFGRD96 26.0 150 50 -15 3.78 0.4214
WVFGRD96 27.0 150 50 -15 3.79 0.4141
WVFGRD96 28.0 150 50 -15 3.80 0.4081
WVFGRD96 29.0 150 50 -15 3.80 0.4015
The best solution is
WVFGRD96 6.0 185 35 60 3.77 0.5362
The mechanism correspond to the best fit is
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The best fit as a function of depth is given in the following figure:
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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 and because the velocity model used in the predictions may not be perfect. 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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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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. |
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:
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.
Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.
Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.
The CUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
MODEL.01 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Thu Aug 25 12:53:39 CDT 2011