Location

2009/07/07 19:11:45 75.275 -72.201 10.0 5.9

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2009/07/07 19:11:45:0 75.28 -72.20 10.0 5.9 Stations used: .-1 Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.26e+25 dyne-cm Mw = 6.00 Z = 13 km Plane Strike Dip Rake NP1 15 55 120 NP2 150 45 55 Principal Axes: Axis Value Plunge Azimuth T 1.26e+25 65 343 N 0.00e+00 24 176 P -1.26e+25 5 84 Moment Tensor: (dyne-cm) Component Value Mxx 1.84e+24 Mxy -1.91e+24 Mxz 4.42e+24 Myy -1.22e+25 Myz -2.55e+24 Mzz 1.03e+25 #############- ##################---- --####################------ ---#####################------ ----######################-------- -----######################--------- ------########## ##########--------- -------########## T ##########---------- -------########## ##########-------- --------#######################-------- P ---------#####################--------- ---------#####################------------ ----------###################------------- ----------##################------------ -----------################------------- -----------##############------------- ------------###########------------- ------------#########------------- -------------#####------------ ---------------------------- ----------######------ ---########### Global CMT Convention Moment Tensor: R T P 1.03e+25 4.42e+24 2.55e+24 4.42e+24 1.84e+24 1.91e+24 2.55e+24 1.91e+24 -1.22e+25 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090707191145/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 = 150
      DIP = 45
     RAKE = 55
       MW = 6.00
       HS = 13

Since the purpose of studying this event was to get more dispersion data and since the event is not on land, the surface wave solution is used. The orientation of the Pressure and tension axes is the same as the published moment tensor soltuions.

Moment Tensor Comparison

The following compares this source inversion to others

SLU USGSMT GCMT USGSCMT

USGS/SLU Moment Tensor Solution ENS 2009/07/07 19:11:45:0 75.28 -72.20 10.0 5.9 Stations used: .-1 Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.26e+25 dyne-cm Mw = 6.00 Z = 13 km Plane Strike Dip Rake NP1 15 55 120 NP2 150 45 55 Principal Axes: Axis Value Plunge Azimuth T 1.26e+25 65 343 N 0.00e+00 24 176 P -1.26e+25 5 84 Moment Tensor: (dyne-cm) Component Value Mxx 1.84e+24 Mxy -1.91e+24 Mxz 4.42e+24 Myy -1.22e+25 Myz -2.55e+24 Mzz 1.03e+25 #############- ##################---- --####################------ ---#####################------ ----######################-------- -----######################--------- ------########## ##########--------- -------########## T ##########---------- -------########## ##########-------- --------#######################-------- P ---------#####################--------- ---------#####################------------ ----------###################------------- ----------##################------------ -----------################------------- -----------##############------------- ------------###########------------- ------------#########------------- -------------#####------------ ---------------------------- ----------######------ ---########### Global CMT Convention Moment Tensor: R T P 1.03e+25 4.42e+24 2.55e+24 4.42e+24 1.84e+24 1.91e+24 2.55e+24 1.91e+24 -1.22e+25 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090707191145/index.html

USGS Body-Wave Moment Tensor Solution 09/07/07 19:11:45.58 BAFFIN BAY Epicenter: 75.325 -72.317 MW 5.9 USGS MOMENT TENSOR SOLUTION Depth 19 No. of sta: 77 Moment Tensor; Scale 10**17 Nm Mrr= 4.87 Mtt= 3.99 Mpp=-8.86 Mrt= 2.23 Mrp= 0.28 Mtp=-1.92 Principal axes: T Val= 6.77 Plg=49 Azm= 5 N 2.40 40 190 P -9.17 2 98 Best Double Couple:Mo=8.2*10**17 NP1:Strike= 41 Dip=60 Slip= 139 NP2: 155 56 38 ####### -################ ---################## ----####################- ------######## ########---- -------######## T ########----- -------######## #######------ --------##################------- ---------################-------- ----------##############------- ----------############--------- P -----------##########---------- -----------#######------------- ------------####--------------- ----------------------------- -------######------------ #############-------- ##############--- #######

July 7, 2009, BAFFIN BAY, MW=6.0 Goran Ekstrom CENTROID-MOMENT-TENSOR SOLUTION GCMT EVENT: C200907071911A DATA: II IU CU IC G GE L.P.BODY WAVES: 93S, 195C, T= 40 MANTLE WAVES: 70S, 98C, T=125 SURFACE WAVES: 95S, 208C, T= 50 TIMESTAMP: Q-20090707221723 CENTROID LOCATION: ORIGIN TIME: 19:11:49.4 0.1 LAT:75.35N 0.01;LON: 72.64W 0.02 DEP: 20.8 0.2;TRIANG HDUR: 2.5 MOMENT TENSOR: SCALE 10**25 D-CM RR= 1.230 0.012; TT= 0.206 0.009 PP=-1.430 0.010; RT= 0.332 0.020 RP=-0.306 0.019; TP= 0.105 0.008 PRINCIPAL AXES: 1.(T) VAL= 1.353;PLG=73;AZM= 20 2.(N) 0.130; 15; 173 3.(P) -1.477; 7; 265 BEST DBLE.COUPLE:M0= 1.42*10**25 NP1: STRIKE= 12;DIP=40;SLIP= 114 NP2: STRIKE=162;DIP=54;SLIP= 71 ##########- ---#############--- ----###############---- ------################----- ------##################----- -------##################------ -------######## #######------ --------######## T ########------ ------####### ########------ P ------#################------- -------################------- ---------###############------- ----------##############------- ----------############------- -----------#########------- ----------######------- -----------#------- ----######-

USGS Centroid Moment Tensor Solution 09/07/07 19:11:45.50 BAFFIN BAY Epicenter: 75.340 -72.306 MW 6.1 USGS CENTROID MOMENT TENSOR 09/07/07 19:11:58.90 Centroid: 74.987 -72.685 Depth 18 No. of sta:121 Moment Tensor; Scale 10**18 Nm Mrr= 1.19 Mtt= 0.09 Mpp=-1.28 Mrt= 0.84 Mrp= 0.50 Mtp= 0.03 Principal axes: T Val= 1.71 Plg=61 Azm=341 N -0.32 25 192 P -1.39 12 96 Best Double Couple:Mo=1.5*10**18 NP1:Strike= 27 Dip=62 Slip= 119 NP2: 157 39 47 ####### -###############- --################--- ---##################---- -----#################------- -----####### ########-------- -----####### T ########-------- ------####### #######---------- ------#################---------- ------################------- - -------##############-------- P - --------############--------- - --------##########------------- ---------########-------------- ---------######-------------- ---------##-------------- -------##------------ --#########------ #######

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.

Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=      14.72
  DIP=      54.61
 RAKE=     119.84
  
             OR
  
  STK=     149.99
  DIP=      45.00
 RAKE=      55.00
 
 
DEPTH = 13.0 km
 
Mw = 6.00
Best Fit 0.8373 - 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.

Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

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.

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:

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=Thu Jul 9 09:18:41 CDT 2009

Last Changed 2009/07/07