Location
2009/08/18 02:50:16 40.62 -107.64 10.0 4.40 Colorado
Arrival Times (from USGS)
Arrival time list
SLU Location
After an initial inversion run, large time shifts were required. We picked arrivals form the permanent and the TA
network for use with the elocate programs and the CUS model. The SLU location isabout 0.1 degree west of the initial NEIC location. I prefer the SLU location because the waveform time shifts are no longer as large as -4 seconds at short distances.
Output of elocate
Felt Map
USGS Felt map for this earthquake
USGS Felt reports main page
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2009/08/18 02:50:16:7 40.62 -107.64 10.0 4.4 Colorado
Stations used:
IW.SMCO TA.L19A TA.L21A TA.L22A TA.L23A TA.M19A TA.M20A
TA.M21A TA.M22A TA.M23A TA.M24A TA.N20A TA.N22A TA.N23A
TA.N24A TA.N25A TA.O19A TA.O20A TA.O22A TA.O23A TA.O24A
TA.O25A TA.P19A TA.P20A TA.P21A TA.P22A TA.P23A TA.P24A
TA.P25A TA.Q21A TA.Q22A TA.Q23A TA.R21A TA.R22A TA.R23A
US.ISCO
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 4.32e+21 dyne-cm
Mw = 3.69
Z = 18 km
Plane Strike Dip Rake
NP1 247 74 -143
NP2 145 55 -20
Principal Axes:
Axis Value Plunge Azimuth
T 4.32e+21 12 12
N 0.00e+00 50 267
P -4.32e+21 37 111
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.58e+21
Mxy 1.79e+21
Mxz 1.62e+21
Myy -2.19e+21
Myz -1.75e+21
Mzz -1.39e+21
######### ##
############# T ######
--############## #########
--############################
----##############################
-----###############################
------################################
--------##################--------------
--------############--------------------
----------#######-------------------------
----------###-----------------------------
----------#-------------------------------
--------####-------------------- -------
----########------------------- P ------
--############----------------- ------
##############------------------------
###############---------------------
################------------------
#################-------------
####################--------
######################
##############
Global CMT Convention Moment Tensor:
R T P
-1.39e+21 1.62e+21 1.75e+21
1.62e+21 3.58e+21 -1.79e+21
1.75e+21 -1.79e+21 -2.19e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090818025016/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 = 145
DIP = 55
RAKE = -20
MW = 3.69
HS = 18.0
The location is the SLU location and not that of initial NEIC. The difference is a shift of about 7 km to the SW. All waveform inversion solutions, e.g., using CUS or WUS models, using the 0.02 - 0.10 or 0.02 - 0.06 Hz gave a depth near 16 km and about the same moment magnitude, the same orientations of the P and T axes. However the 0.02 - 0.10 Hz band gave an almost pure thrust event while the 0.02 - 0.06 Hz band gave an oblique solution. Because the location straddles two velocity regions, we use the lower frequency band and the WUS model, which gives a slightly better fit.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
FIRSTMOTION |
USGS/SLU Moment Tensor Solution
ENS 2009/08/18 02:50:16:7 40.62 -107.64 10.0 4.4 Colorado
Stations used:
IW.SMCO TA.L19A TA.L21A TA.L22A TA.L23A TA.M19A TA.M20A
TA.M21A TA.M22A TA.M23A TA.M24A TA.N20A TA.N22A TA.N23A
TA.N24A TA.N25A TA.O19A TA.O20A TA.O22A TA.O23A TA.O24A
TA.O25A TA.P19A TA.P20A TA.P21A TA.P22A TA.P23A TA.P24A
TA.P25A TA.Q21A TA.Q22A TA.Q23A TA.R21A TA.R22A TA.R23A
US.ISCO
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 4.32e+21 dyne-cm
Mw = 3.69
Z = 18 km
Plane Strike Dip Rake
NP1 247 74 -143
NP2 145 55 -20
Principal Axes:
Axis Value Plunge Azimuth
T 4.32e+21 12 12
N 0.00e+00 50 267
P -4.32e+21 37 111
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.58e+21
Mxy 1.79e+21
Mxz 1.62e+21
Myy -2.19e+21
Myz -1.75e+21
Mzz -1.39e+21
######### ##
############# T ######
--############## #########
--############################
----##############################
-----###############################
------################################
--------##################--------------
--------############--------------------
----------#######-------------------------
----------###-----------------------------
----------#-------------------------------
--------####-------------------- -------
----########------------------- P ------
--############----------------- ------
##############------------------------
###############---------------------
################------------------
#################-------------
####################--------
######################
##############
Global CMT Convention Moment Tensor:
R T P
-1.39e+21 1.62e+21 1.75e+21
1.62e+21 3.58e+21 -1.79e+21
1.75e+21 -1.79e+21 -2.19e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090818025016/index.html
|
First motion plot using elocate take-off angles and azimuths
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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 25 45 -90 3.27 0.2292
WVFGRD96 1.0 205 45 -85 3.31 0.2160
WVFGRD96 2.0 20 45 -90 3.40 0.2528
WVFGRD96 3.0 155 60 25 3.41 0.2514
WVFGRD96 4.0 150 50 10 3.46 0.2605
WVFGRD96 5.0 140 35 -15 3.51 0.2882
WVFGRD96 6.0 140 40 -15 3.52 0.3233
WVFGRD96 7.0 135 40 -25 3.54 0.3555
WVFGRD96 8.0 135 35 -25 3.60 0.3783
WVFGRD96 9.0 135 40 -30 3.62 0.4073
WVFGRD96 10.0 135 45 -35 3.63 0.4330
WVFGRD96 11.0 135 45 -35 3.64 0.4534
WVFGRD96 12.0 140 50 -30 3.64 0.4700
WVFGRD96 13.0 140 50 -30 3.65 0.4821
WVFGRD96 14.0 140 50 -25 3.66 0.4911
WVFGRD96 15.0 140 55 -25 3.67 0.4969
WVFGRD96 16.0 140 55 -25 3.68 0.5013
WVFGRD96 17.0 145 55 -20 3.68 0.5035
WVFGRD96 18.0 145 55 -20 3.69 0.5039
WVFGRD96 19.0 145 55 -20 3.69 0.5024
WVFGRD96 20.0 145 60 -20 3.70 0.4998
WVFGRD96 21.0 145 55 -20 3.71 0.4951
WVFGRD96 22.0 145 55 -15 3.72 0.4902
WVFGRD96 23.0 145 55 -15 3.72 0.4843
WVFGRD96 24.0 145 60 -15 3.73 0.4773
WVFGRD96 25.0 145 60 -15 3.74 0.4699
WVFGRD96 26.0 145 55 -10 3.74 0.4619
WVFGRD96 27.0 145 60 -15 3.75 0.4538
WVFGRD96 28.0 145 60 -15 3.76 0.4452
WVFGRD96 29.0 150 55 10 3.76 0.4357
The best solution is
WVFGRD96 18.0 145 55 -20 3.69 0.5039
The mechanism correspond 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 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.
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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.
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 Aug 18 12:15:45 CDT 2009
Last Changed 2009/08/18