2011/08/24 04:45:26 37.925 -77.994 5 3.40 Virginia
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution
ENS 2011/08/24 04:45:26:7 37.92 -77.99 5.0 3.4 Virginia
Stations used:
LD.MVL LD.SDMD US.BLA US.CBN US.MCWV
Filtering commands used:
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 7.94e+20 dyne-cm
Mw = 3.20
Z = 7 km
Plane Strike Dip Rake
NP1 8 51 98
NP2 175 40 80
Principal Axes:
Axis Value Plunge Azimuth
T 7.94e+20 82 322
N 0.00e+00 6 183
P -7.94e+20 5 92
Moment Tensor: (dyne-cm)
Component Value
Mxx 9.54e+18
Mxy 2.04e+19
Mxz 9.34e+19
Myy -7.80e+20
Myz -1.45e+20
Mzz 7.70e+20
---########---
-----############-----
------###############-------
------#################-------
-------###################--------
-------####################---------
--------#####################---------
--------######################----------
--------######## ###########----------
---------######## T ###########-----------
---------######## ###########--------
---------######################-------- P
---------######################--------
--------#####################-----------
---------####################-----------
--------###################-----------
--------#################-----------
--------###############-----------
-------#############----------
--------##########----------
-------######---------
--------------
Global CMT Convention Moment Tensor:
R T P
7.70e+20 9.34e+19 1.45e+20
9.34e+19 9.54e+18 -2.04e+19
1.45e+20 -2.04e+19 -7.80e+20
Details of the solution is found at
http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20110824044526/index.html
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STK = 175
DIP = 40
RAKE = 80
MW = 3.20
HS = 7.0
The waveform inversion is preferred.
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution
ENS 2011/08/24 04:45:26:7 37.92 -77.99 5.0 3.4 Virginia
Stations used:
LD.MVL LD.SDMD US.BLA US.CBN US.MCWV
Filtering commands used:
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 7.94e+20 dyne-cm
Mw = 3.20
Z = 7 km
Plane Strike Dip Rake
NP1 8 51 98
NP2 175 40 80
Principal Axes:
Axis Value Plunge Azimuth
T 7.94e+20 82 322
N 0.00e+00 6 183
P -7.94e+20 5 92
Moment Tensor: (dyne-cm)
Component Value
Mxx 9.54e+18
Mxy 2.04e+19
Mxz 9.34e+19
Myy -7.80e+20
Myz -1.45e+20
Mzz 7.70e+20
---########---
-----############-----
------###############-------
------#################-------
-------###################--------
-------####################---------
--------#####################---------
--------######################----------
--------######## ###########----------
---------######## T ###########-----------
---------######## ###########--------
---------######################-------- P
---------######################--------
--------#####################-----------
---------####################-----------
--------###################-----------
--------#################-----------
--------###############-----------
-------#############----------
--------##########----------
-------######---------
--------------
Global CMT Convention Moment Tensor:
R T P
7.70e+20 9.34e+19 1.45e+20
9.34e+19 9.54e+18 -2.04e+19
1.45e+20 -2.04e+19 -7.80e+20
Details of the solution is found at
http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20110824044526/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 0.03 n 3 lp c 0.10 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 295 80 -80 3.30 0.2342
WVFGRD96 1.0 300 80 -80 3.29 0.2368
WVFGRD96 2.0 295 75 -75 3.17 0.2323
WVFGRD96 3.0 345 75 60 3.08 0.2486
WVFGRD96 4.0 190 25 80 3.19 0.2917
WVFGRD96 5.0 180 35 80 3.20 0.3358
WVFGRD96 6.0 10 55 100 3.20 0.3608
WVFGRD96 7.0 175 40 80 3.20 0.3685
WVFGRD96 8.0 175 40 80 3.20 0.3622
WVFGRD96 9.0 165 40 70 3.20 0.3498
WVFGRD96 10.0 165 40 70 3.22 0.3359
WVFGRD96 11.0 165 40 70 3.21 0.3164
WVFGRD96 12.0 155 45 55 3.20 0.2975
WVFGRD96 13.0 155 45 55 3.20 0.2805
WVFGRD96 14.0 155 45 50 3.20 0.2644
WVFGRD96 15.0 150 50 45 3.19 0.2501
WVFGRD96 16.0 150 50 45 3.19 0.2363
WVFGRD96 17.0 155 50 45 3.19 0.2244
WVFGRD96 18.0 135 55 -50 3.18 0.2207
WVFGRD96 19.0 135 55 -50 3.19 0.2189
WVFGRD96 20.0 135 55 -50 3.21 0.2149
WVFGRD96 21.0 130 55 -55 3.22 0.2123
WVFGRD96 22.0 130 55 -55 3.23 0.2095
WVFGRD96 23.0 130 55 -55 3.24 0.2070
WVFGRD96 24.0 135 60 -50 3.23 0.2035
WVFGRD96 25.0 135 60 -50 3.23 0.1998
WVFGRD96 26.0 135 60 -50 3.24 0.1953
WVFGRD96 27.0 140 65 -50 3.23 0.1927
WVFGRD96 28.0 135 65 -50 3.24 0.1898
WVFGRD96 29.0 270 15 -95 3.32 0.1896
The best solution is
WVFGRD96 7.0 175 40 80 3.20 0.3685
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 0.03 n 3 lp c 0.10 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=Wed Aug 24 08:47:25 MDT 2011