Location
2010/02/27 22:22:27 35.54 -96.75 4.0 4.40 Oklahoma
SLU Location
Error Ellipse X= 0.3690 km Y= 0.5651 km Theta = 71.8577 deg
RMS Error : 0.110 sec
Travel_Time_Table: CUS
Latitude : 35.5585 +- 0.0049 N 0.5491 km
Longitude : -96.7383 +- 0.0044 E 0.3924 km
Depth : 4.17 +- 0.73 km
Epoch Time : 1267309347.889 +- 0.09 sec
Event Time : 20100227222227.889 +- 0.09 sec
Event (OCAL) : 2010 02 27 22 22 27 889
HYPO71 Quality : CC
Gap : 79 deg
The SLU location is made using the program elocate and arival time picks from the NetQuake instruments in the source region and adjacent TA stations. The output is given in elocate.txt.
The CUS model provides a smaller RMS errors (0.11 sec vs 0.17 sec) and the WUS model gives a depth of 17 km. Thus depth is highly model dependent. The waveform solution is 7 km.
USGS Location
MAG DATE TIME LAT LON H(km) State
4.1Mw 2010/02/27 22:22:27 35.623 -96.762 3.8 Oklahoma
Arrival Times (from USGS)
Arrival time list
Felt Map
USGS Felt map for this earthquake
USGS Felt reports main page
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2010/02/27 22:22:27:0 35.54 -96.75 4.0 4.4 Oklahoma
Stations used:
NM.MGMO NM.UALR TA.129A TA.131A TA.133A TA.134A TA.135A
TA.232A TA.233A TA.234A TA.332A TA.333A TA.334A TA.335A
TA.434A TA.435B TA.ABTX TA.O31A TA.O32A TA.P29A TA.P30A
TA.P31A TA.P32A TA.P33A TA.Q29A TA.Q30A TA.Q32A TA.Q33A
TA.R28A TA.R29A TA.R30A TA.R31A TA.R32A TA.R33A TA.S28A
TA.S29A TA.S30A TA.S31A TA.S32A TA.S33A TA.T28A TA.T29A
TA.T30A TA.T31A TA.T32A TA.T33A TA.TUL1 TA.U27A TA.U29A
TA.U30A TA.U31A TA.U32A TA.U34A TA.V31A TA.V32A TA.V33A
TA.V34A TA.W28A TA.W29A TA.W30A TA.W31A TA.W32A TA.W33A
TA.W34A TA.WHTX TA.X29A TA.X31A TA.X32A TA.X33A TA.X34A
TA.Y30A TA.Y31A TA.Y32A TA.Y33A TA.Y34A TA.Z31A TA.Z32A
TA.Z33A TA.Z34A TA.Z35A US.AMTX US.KSU1 US.MIAR
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 2.11e+22 dyne-cm
Mw = 4.15
Z = 4 km
Plane Strike Dip Rake
NP1 40 80 -160
NP2 306 70 -11
Principal Axes:
Axis Value Plunge Azimuth
T 2.11e+22 7 172
N 0.00e+00 68 66
P -2.11e+22 21 265
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.03e+22
Mxy -4.61e+21
Mxz -1.72e+21
Myy -1.78e+22
Myz 7.42e+21
Mzz -2.47e+21
##############
######################
###########################-
###########################---
---##########################-----
----------###################-------
---------------#############----------
-------------------#########------------
----------------------#####-------------
------------------------------------------
--- -------------------###--------------
--- P ------------------######------------
--- ----------------##########----------
-------------------#############--------
------------------################------
---------------###################----
------------######################--
---------#########################
-----#########################
-###########################
############ #######
######## T ###
Global CMT Convention Moment Tensor:
R T P
-2.47e+21 -1.72e+21 -7.42e+21
-1.72e+21 2.03e+22 4.61e+21
-7.42e+21 4.61e+21 -1.78e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100227222227/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is
STK = 40
DIP = 80
RAKE = -160
MW = 4.15
HS = 4.0
This earthquake is east of the other earthquakes located in this part of Oklahoma in January and February. The low pass filtered seismogram fits show short period surface waves which is indicative of the shallow velocity structure that is not built into the WUS model. The surface-wave analysis was performed because of the well dispersed surface waves at short distance. This was especially observed in the Love waves. Such information can be used to refine the local velocity model and Q. The dispersion will be incorporated in the continental surface-wave tomography study. The surface waves like a 4 km depth, which the waveform inversion results depend on the frequency band used, which was purposely chosen to be less sensitive to the short-period surface waves, which are indicative of a shallow depth. For this event, the surface-wave solution is used since the waveforms do not have great depth sensitivity.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
SLUFM |
USGS/SLU Moment Tensor Solution
ENS 2010/02/27 22:22:27:0 35.54 -96.75 4.0 4.4 Oklahoma
Stations used:
NM.MGMO NM.UALR TA.129A TA.131A TA.133A TA.134A TA.135A
TA.232A TA.233A TA.234A TA.332A TA.333A TA.334A TA.335A
TA.434A TA.435B TA.ABTX TA.O31A TA.O32A TA.P29A TA.P30A
TA.P31A TA.P32A TA.P33A TA.Q29A TA.Q30A TA.Q32A TA.Q33A
TA.R28A TA.R29A TA.R30A TA.R31A TA.R32A TA.R33A TA.S28A
TA.S29A TA.S30A TA.S31A TA.S32A TA.S33A TA.T28A TA.T29A
TA.T30A TA.T31A TA.T32A TA.T33A TA.TUL1 TA.U27A TA.U29A
TA.U30A TA.U31A TA.U32A TA.U34A TA.V31A TA.V32A TA.V33A
TA.V34A TA.W28A TA.W29A TA.W30A TA.W31A TA.W32A TA.W33A
TA.W34A TA.WHTX TA.X29A TA.X31A TA.X32A TA.X33A TA.X34A
TA.Y30A TA.Y31A TA.Y32A TA.Y33A TA.Y34A TA.Z31A TA.Z32A
TA.Z33A TA.Z34A TA.Z35A US.AMTX US.KSU1 US.MIAR
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 2.11e+22 dyne-cm
Mw = 4.15
Z = 4 km
Plane Strike Dip Rake
NP1 40 80 -160
NP2 306 70 -11
Principal Axes:
Axis Value Plunge Azimuth
T 2.11e+22 7 172
N 0.00e+00 68 66
P -2.11e+22 21 265
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.03e+22
Mxy -4.61e+21
Mxz -1.72e+21
Myy -1.78e+22
Myz 7.42e+21
Mzz -2.47e+21
##############
######################
###########################-
###########################---
---##########################-----
----------###################-------
---------------#############----------
-------------------#########------------
----------------------#####-------------
------------------------------------------
--- -------------------###--------------
--- P ------------------######------------
--- ----------------##########----------
-------------------#############--------
------------------################------
---------------###################----
------------######################--
---------#########################
-----#########################
-###########################
############ #######
######## T ###
Global CMT Convention Moment Tensor:
R T P
-2.47e+21 -1.72e+21 -7.42e+21
-1.72e+21 2.03e+22 4.61e+21
-7.42e+21 4.61e+21 -1.78e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100227222227/index.html
|
The first motion plot agrees with the waveform inversion. Triangles are strong dilations, - is a weak dilatation, + a weak compression and a circle is a strong compression. The X indicates a nodal arrival.
The takeoff angles and picks are
in the elocate.txt file.
|
Waveform Inversion
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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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 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 from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 305 70 -15 3.86 0.3594
WVFGRD96 1.0 310 90 5 3.86 0.3856
WVFGRD96 2.0 310 90 0 3.95 0.4713
WVFGRD96 3.0 310 85 5 3.99 0.5059
WVFGRD96 4.0 310 85 5 4.02 0.5236
WVFGRD96 5.0 310 90 -5 4.05 0.5312
WVFGRD96 6.0 310 80 5 4.07 0.5337
WVFGRD96 7.0 310 75 5 4.10 0.5351
WVFGRD96 8.0 130 85 15 4.12 0.5359
WVFGRD96 9.0 130 85 20 4.14 0.5297
WVFGRD96 10.0 310 70 5 4.15 0.5224
WVFGRD96 11.0 310 70 0 4.16 0.5163
WVFGRD96 12.0 130 90 20 4.17 0.5100
WVFGRD96 13.0 310 85 25 4.18 0.5048
WVFGRD96 14.0 310 85 20 4.19 0.5002
WVFGRD96 15.0 310 80 20 4.20 0.4951
WVFGRD96 16.0 310 80 20 4.20 0.4891
WVFGRD96 17.0 310 80 20 4.21 0.4823
WVFGRD96 18.0 310 80 15 4.22 0.4749
WVFGRD96 19.0 310 80 15 4.22 0.4672
WVFGRD96 20.0 310 80 15 4.23 0.4588
WVFGRD96 21.0 310 80 15 4.24 0.4497
WVFGRD96 22.0 310 80 15 4.24 0.4409
WVFGRD96 23.0 310 80 15 4.25 0.4318
WVFGRD96 24.0 310 80 15 4.25 0.4226
WVFGRD96 25.0 310 80 -10 4.26 0.4129
WVFGRD96 26.0 310 80 -10 4.26 0.4041
WVFGRD96 27.0 310 80 -10 4.27 0.3953
WVFGRD96 28.0 310 80 -10 4.28 0.3865
WVFGRD96 29.0 310 80 -10 4.28 0.3779
The best solution is
WVFGRD96 8.0 130 85 15 4.12 0.5359
The mechanism correspond to the best fit is
|
|
Figure 1. Waveform inversion focal mechanism
|
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
|
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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 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
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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.
|
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= 40.00
DIP= 79.99
RAKE= -160.00
OR
STK= 306.38
DIP= 70.32
RAKE= -10.63
DEPTH = 4.0 km
Mw = 4.15
Best Fit 0.8729 - P-T axis plot gives solutions with FIT greater than FIT90
First motion data
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az Dist First motion
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.
|
Love-wave radiation patterns
Rayleigh-wave radiation patterns
Broadband station distribution
The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used
Waveform comparison for this mechanism
Since the analysis of the surface-wave radiation patterns uses only spectral
amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were
emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute
forward synthetics and compare the observed and predicted waveforms.
The fits to the waveforms with the given mechanism are show below:
This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the
observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:
hp c 0.02 n 3
lp c 0.06 n 3
Discussion
The Future
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.
Acknowledgements
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 L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.
Appendix A
Spectra fit plots to each station
Velocity Model
The WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
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
Quality Control
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Tue Mar 2 11:56:44 CST 2010
Last Changed 2010/02/27
