Location
2009/08/17 00:22:12 38.471 -102.728 10.0 3.90 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
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090817002212/index.html
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is
STK = 70
DIP = 50
RAKE = -80
MW = 3.85
HS = 10.0
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
FIRST_MOTION |
USGS/SLU Moment Tensor Solution
ENS 2009/08/17 00:22:11:6 38.47 -102.73 10.0 3.9 Colorado
Stations used:
TA.KSCO TA.P26A TA.P27A TA.P28A TA.P29A TA.P30A TA.Q25A
TA.Q26A TA.Q28A TA.Q29A TA.R23A TA.R25A TA.R26A TA.R27A
TA.R28A TA.R29A TA.R30A TA.R31A TA.S25A TA.S26A TA.S27A
TA.S28A TA.S29A TA.S30A TA.T25A TA.T26A TA.T27A TA.T28A
TA.T29A TA.U25A TA.U26A TA.U27A TA.U28A TA.U29A
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 7.50e+21 dyne-cm
Mw = 3.85
Z = 10 km
Plane Strike Dip Rake
NP1 70 50 -80
NP2 235 41 -102
Principal Axes:
Axis Value Plunge Azimuth
T 7.50e+21 5 153
N 0.00e+00 8 244
P -7.50e+21 81 33
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.78e+21
Mxy -3.10e+21
Mxz -1.49e+21
Myy 1.49e+21
Myz -3.48e+20
Mzz -7.27e+21
##############
######################
############################
################------------##
#############---------------------
###########-------------------------
##########----------------------------
#########------------------------------#
#######------------- ----------------#
#######-------------- P ---------------###
######--------------- --------------####
#####--------------------------------#####
#####------------------------------#######
###-----------------------------########
###--------------------------###########
-#-----------------------#############
-###---------------#################
##################################
##############################
###################### ###
################### T
##############
Global CMT Convention Moment Tensor:
R T P
-7.27e+21 -1.49e+21 3.48e+20
-1.49e+21 5.78e+21 3.10e+21
3.48e+20 3.10e+21 1.49e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090817002212/index.html
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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 0.10 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 0.5 100 85 -70 3.85 0.3432
WVFGRD96 1.0 100 85 -70 3.87 0.3437
WVFGRD96 2.0 100 85 -60 3.76 0.3576
WVFGRD96 3.0 100 85 -55 3.73 0.3759
WVFGRD96 4.0 70 65 -80 3.79 0.4107
WVFGRD96 5.0 70 60 -80 3.80 0.4487
WVFGRD96 6.0 70 55 -80 3.81 0.4732
WVFGRD96 7.0 70 55 -80 3.81 0.4908
WVFGRD96 8.0 70 55 -80 3.81 0.4982
WVFGRD96 9.0 70 50 -80 3.82 0.4991
WVFGRD96 10.0 70 50 -80 3.85 0.4995
WVFGRD96 11.0 70 50 -80 3.84 0.4900
WVFGRD96 12.0 75 50 -75 3.84 0.4762
WVFGRD96 13.0 75 50 -75 3.84 0.4599
WVFGRD96 14.0 85 55 -65 3.83 0.4433
WVFGRD96 15.0 90 55 -55 3.83 0.4271
WVFGRD96 16.0 90 55 -55 3.83 0.4111
WVFGRD96 17.0 95 60 -50 3.83 0.3949
WVFGRD96 18.0 95 60 -50 3.83 0.3792
WVFGRD96 19.0 95 60 -45 3.84 0.3632
WVFGRD96 20.0 95 60 -50 3.86 0.3544
WVFGRD96 21.0 95 60 -45 3.87 0.3418
WVFGRD96 22.0 95 60 -45 3.87 0.3299
WVFGRD96 23.0 95 60 -45 3.87 0.3194
WVFGRD96 24.0 95 60 -45 3.88 0.3098
WVFGRD96 25.0 95 60 -45 3.88 0.3018
WVFGRD96 26.0 95 60 -45 3.89 0.2943
WVFGRD96 27.0 275 55 -45 3.92 0.2892
WVFGRD96 28.0 275 55 -45 3.92 0.2858
WVFGRD96 29.0 275 55 -45 3.93 0.2821
The best solution is
WVFGRD96 10.0 70 50 -80 3.85 0.4995
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 0.10 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 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
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=Sun Aug 16 21:48:45 CDT 2009
Last Changed 2009/08/17