Doubledifference earthquake relocation of Charlevoix Seismicity Eastern Canada

Double-difference earthquake relocation of Charlevoix Seismicity, Eastern Canada implication for regional geological structures Meng Pang

Outlines • Introduction of the geological background of the Charlevoix region • Past seismological studies • Introduction to Hypo. DD • Some results of my current research • Future work • References

Why interested in CSZ? No surface rupture has ever been reported in the geological maps , scientific reports and a seismic reflection profile The EQ and rift faults taken into account by National Building Code of Canada (NBCC) in CSZ Earthquakes not distributed uniformly across the seismic zone, but concentrate in groups separated by less active areas

CSZ earthquakes in detail 1977~2003, recorded more than 3000 earthquakes with Mw-1. 0 ~ 5. 0. The epicenters define a 30 by 85 km ellipse with the major axis parallel to the St. Lawrence River From the magnitude-recurrence curves, the return period of magnitude 6 earthquakes is approximately 75 years

At depth, CSZ earthquakes occur solely in the Precambrian basement from the surface to 30 km depth, with two thirds between 7 and 15 km. with more than 80% at less than 15 km

Focal mechanisms of CSZ earthquakes

The St-Laurent, South Shore and Charlevoix faults bound active volumes

Complexity of CSZ seismicity • CSZ fault zones may be irregular surfaces, surrounded by highly fractured rocks. These highly fractured zones respond primarily to regional stresses • or some smaller events, they may respond to local changes in stress and/or strength • the anomalous CSZ earthquake activity must be due to inherent crustal weakness and/or high pore fluid pressure.

Several geophysics study on CSZ • Vlahovic et. developed a three-dimensional P wave velocity model for the Charlevoix seismic zone (CSZ) • The region of high velocity is surrounded by low velocities interpreted to be highly disrupted rocks. • Earthquakes avoid the high-velocity body and separate into two bands, one on either side of the feature.

What is Double difference earthquake ralocation?

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Some advantages for Hypo. DD • Residuals are minimized for pairs of earthquakes at each station while linking together all observed event -station pairs. • A least-squares solution is found by iteratively adjusting the vector difference between hypocentral pairs. • The double-difference algorithm minimizes errors due to unmodeled velocity structure without the use of station corrections.

Workflow of Hypo. DD

Data used in my research • Canadian National Seismic Network (2003. 052013. 06) • The region of latitude 47 N~48. 5 N and longitude 69 W~70. 5 W. • In total, there are 543 events with magnitudes varying from 0. 1 to 4. 4, recorded at the seven stations located on both shores of the St. Lawrence River.

Cross-section in Northeast

Cross-section in Center

Cross-section in Southwest

Conclusions Successfully relocated 331 events by using the 543 events catalog data period from May 2003 to June 2013. Earthquakes relocated using the catalog arrivals form a narrower zone of seismic activity than the catalog locations. From the cross-section images, I think there seems a northeast strike faulting and dipping about 50 degree.

Future work • Try to extend my station book and use more stations’ data to relocate again • Keep improving the parameters in the Hypo. DD processes and get more accurate results by reducing the residuals • Involved cross-correlation method of waveform data in Hypo. DD • Comparisons among only catalog data, only waveform data and combined with them

References • • • Adams, J. , Weichert, D. H. , Halchuk, S. , and Basham, P. W. , i Trial seismic hazard maps of Canada -1995: Final values for selected Canadian citiesi. Geological Survey of Canada Open File 3283, 1996. Lamontagne, M. , Rheological and Geological Constraints on the Earthquake Distribution in the Charlevoix Seismic Zone, Ph. D. Thesis, Carleton University, (Available on CD-ROM, Geological Survey of Canada Open File Report D- 3778); 1999. Lamontagne, M. , An overview of some significant Eastern Canadian earthquakes and their impacts on the geological environment, buildings and the public. Natural Hazards. 26; 2002. Lamontagne, M. Keating, P. , Toutin, T. Complex faulting con founds earthquake research in the Charlevoix Seismic Zone, Qu. E bec Eos, Transactions, American Geophysical Union, 81; 2000. Lamontagne, M. , Keating, P. and Perreault, S. , Seismotectonic characteristics of the Lower St. Lawrence Seismic Zone, Qu. E bec: Insights from geology, magnetics, gravity and seismics. Canadian Journal of Earth Science (in press). Vlahovic, G. , Powell, C. A. , Lamontagne, M. , Three-dimensional P-Wave Velocity Model for the Charlevoix Seismic Zone. Submitted to the Journal of Geophysical Research.

Thank You and Questions