2009/05/29 18:16:01 48.516 -112.336 0.0 3.60 Montana
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution
ENS 2009/05/29 18:16:01:0 48.52 -112.34 0.0 3.6 Montana
Stations used:
TA.A12A TA.A14A TA.A15A TA.A16A TA.A18A TA.B12A TA.B13A
TA.B14A TA.B15A TA.B17A TA.B18A TA.B19A TA.C12B TA.C13A
TA.C15A TA.C19A TA.D12A TA.D13A TA.D14A TA.E14A TA.E15A
TA.F13A US.EGMT US.MSO US.NEW
Filtering commands used:
hp c 0.025 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 6.92e+20 dyne-cm
Mw = 3.16
Z = 2 km
Plane Strike Dip Rake
NP1 334 55 87
NP2 160 35 95
Principal Axes:
Axis Value Plunge Azimuth
T 6.92e+20 80 230
N 0.00e+00 3 336
P -6.92e+20 10 66
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.80e+19
Mxy -2.35e+20
Mxz -1.27e+20
Myy -5.50e+20
Myz -2.05e+20
Mzz 6.48e+20
--------------
--####----------------
---#########----------------
---############---------------
----###############---------------
-----#################--------------
-----###################-----------
------####################---------- P -
------######################-------- -
-------######################-------------
-------#######################------------
-------######### ############-----------
--------######## T ############-----------
-------######## #############---------
--------#######################---------
--------######################--------
--------#####################-------
---------###################------
--------##################----
----------##############----
----------###########-
------------##
Global CMT Convention Moment Tensor:
R T P
6.48e+20 -1.27e+20 2.05e+20
-1.27e+20 -9.80e+19 2.35e+20
2.05e+20 2.35e+20 -5.50e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090529181601/index.html
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STK = 160
DIP = 35
RAKE = 95
MW = 3.16
HS = 2.0
The waveform inversion is preferred. The is well defined because of the nearby USArray stations. The event is shallow because of the very short period Rayleigh waves seen in the bandpass filtered data. I changed the bandpass from the normal 0.02 - 0.10 Hz to 0.025 - 0.06 Hz to reduce the effect of the very short period surface waves. The odd this about this earthquake is that it a NS striking thrust fault and not the expected normal faulting. The following is from Mike Stickney:
Hi Bob:
I was thrilled to see that you were able to determine a focal mechanism for the May 29 earthquake in northern Montana and that your work indicates a very shallow focal depth. As I picked this event, it seemed clear to me that it was a very shallow event because of the impressive surface waves that are not typical of most local earthquakes I look at. I strongly suspect that this event may be related to oil and gas field activities (injection and flooding to enhance secondary recovery)—because the epicenter lies within the field and the focus is atypically shallow.
Although it may not be reported on the USGS information for this event, it was felt. I spoke with the Glacier County Disaster and Emergency Services coordinator and he did feel the earthquake
I was just curious to know if the shallow focus had anything to do with the fact that you were able to find a solution for such a small event. With an Mw of 3.16, this is much smaller than several previous events (mid-to-high 3’s) that apparently lacked sufficient low frequency energy for a solution.
Thanks for any thoughts. -Mike
Michael Stickney, Director
Earthquake Studies Office
Montana Bureau of Mines and Geology
Montana Tech of the University of Montana
1300 W Park St
Butte, MT 59701
The following compares this source inversion to others
USGS/SLU Moment Tensor Solution
ENS 2009/05/29 18:16:01:0 48.52 -112.34 0.0 3.6 Montana
Stations used:
TA.A12A TA.A14A TA.A15A TA.A16A TA.A18A TA.B12A TA.B13A
TA.B14A TA.B15A TA.B17A TA.B18A TA.B19A TA.C12B TA.C13A
TA.C15A TA.C19A TA.D12A TA.D13A TA.D14A TA.E14A TA.E15A
TA.F13A US.EGMT US.MSO US.NEW
Filtering commands used:
hp c 0.025 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 6.92e+20 dyne-cm
Mw = 3.16
Z = 2 km
Plane Strike Dip Rake
NP1 334 55 87
NP2 160 35 95
Principal Axes:
Axis Value Plunge Azimuth
T 6.92e+20 80 230
N 0.00e+00 3 336
P -6.92e+20 10 66
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.80e+19
Mxy -2.35e+20
Mxz -1.27e+20
Myy -5.50e+20
Myz -2.05e+20
Mzz 6.48e+20
--------------
--####----------------
---#########----------------
---############---------------
----###############---------------
-----#################--------------
-----###################-----------
------####################---------- P -
------######################-------- -
-------######################-------------
-------#######################------------
-------######### ############-----------
--------######## T ############-----------
-------######## #############---------
--------#######################---------
--------######################--------
--------#####################-------
---------###################------
--------##################----
----------##############----
----------###########-
------------##
Global CMT Convention Moment Tensor:
R T P
6.48e+20 -1.27e+20 2.05e+20
-1.27e+20 -9.80e+19 2.35e+20
2.05e+20 2.35e+20 -5.50e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090529181601/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 0.025 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 0.5 340 60 90 3.05 0.4161
WVFGRD96 1.0 155 40 85 3.06 0.4348
WVFGRD96 2.0 160 35 95 3.16 0.4821
WVFGRD96 3.0 155 40 85 3.21 0.4550
WVFGRD96 4.0 170 40 105 3.21 0.3686
WVFGRD96 5.0 310 70 45 3.17 0.3237
WVFGRD96 6.0 270 60 -15 3.23 0.3164
WVFGRD96 7.0 270 60 -15 3.24 0.3254
WVFGRD96 8.0 160 70 85 3.26 0.3333
WVFGRD96 9.0 350 20 100 3.26 0.3414
WVFGRD96 10.0 160 70 85 3.25 0.3460
WVFGRD96 11.0 345 20 95 3.25 0.3484
WVFGRD96 12.0 270 55 -25 3.32 0.3537
WVFGRD96 13.0 270 55 -30 3.33 0.3580
WVFGRD96 14.0 270 55 -30 3.34 0.3628
WVFGRD96 15.0 270 55 -30 3.34 0.3661
WVFGRD96 16.0 270 60 -35 3.35 0.3686
WVFGRD96 17.0 270 60 -35 3.36 0.3696
WVFGRD96 18.0 270 60 -35 3.37 0.3691
WVFGRD96 19.0 270 60 -35 3.37 0.3674
The best solution is
WVFGRD96 2.0 160 35 95 3.16 0.4821
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 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 0.025 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. |
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 L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.
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
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Thu Jun 4 09:49:10 CDT 2009