1. DISCRIMINATING SMALL EARTHQUAKES FROM QUARRY BLASTS USING PEAK AMPLITUDE RATIO - Vmax/Hmax MA, S., EATON, D., & DINEVA, S. Department of Earth Sciences The University of Western Ontario London, Ontario, Canada, N6A 5B7

2. The key differences between blasts and earthquakes: (1)Focal mechanism – the type of source for blasts are explosive; -- the type of source for earthquakes are dislocation (2) Focal depths – the depths of blasts are on the surface or shallow; -- the depths of earthquakes are deeper (in southern Ontario earthquakes occur at depth ~ 3 km or deeper). (3) Source time function – in a blast generally there are several explosions; one rupture in a small earthquake generally. These basic differences can principally be used to distinguish blasts from small earthquakes.

3. Many scientists have studied blasts, for example: The following 4 slides produced by D. Craig Pearson, Brian W. Stump & David P. Anderson can help us clarify some concepts.

4. P hysical C onstraints on M ining E xplosions D. Craig Pearson D. Craig Pearson Los Alamos National Laboratory Brian W. Stump Brian W. Stump Southern Methodist University, Los Alamos National Laboratory David P. Anderson David P. Anderson Southern Methodist University Prepared for Seismological Research Letters Last update 15 May 1996 dpa@passion.isem.smu.edu

5. Blast model by D. Craig Pearson, Brian W. Stump & David P. Anderson

6. Produced by D. Craig Pearson, Brian W. Stump & David P. Anderson

8. Hourly distribution of seismic events in southern Ontario & western Quebec There are many seismic events in western Quebec and southern Ontario. We need to separate earthquakes and blasts for seismic related work & projects -- only around 10% of the 8000 events recorded from 2002 to 2005 have basic parameters. The red curve shows the distribution of the known blasts; the green curve shows the distribution of earthquakes alone.

9. Epicentre distribution for earthquakes from 2000-10-09 to 2005-07-23; blast sites: St. Mary’s Cement site, ACTON sites A1 & A2; stations WLVO & ACTO

10. Waveforms generated by a blast at St. Mary’s Cement; recorded at POLARIS station WLVO; station distance ~ 24 km.

11. Waveforms generated by a blast at ACTON site; recorded at POLARIS station ACTO; station distance ~ 13 km

12. Waveforms generated by a blast at St. Mary’s Cement; recorded at local station WLVO; station distance ~ 24 km.

13. Waveforms generated by a small earthquake (mN~1.1); recorded at POLARIS station STCO; the distance ~18 km. We are trying to use the ratio information to separate earthquakes from blasts.

14. We see that in general, ground motion produced by blasts is dominated by horizontal motion, whereas local earthquakes produce a relatively stronger vertical ground motion. The vertical ground motion from earthquakes arises from non-horizontal P waves, which arrive sooner than the strong surface-wave ground motion from blasts. Based on these observations, we are motivated to seek a simple blast discriminator based on the ratio of peak vertical to horizontal amplitude. We see the amplitudes are weaker along vertical waveforms generated by blasts at ACTON and St. Mary’s sites, so we are trying to identify blasts from small earthquakes using the ratio between the maximum amplitudes along vertical and horizontal components. Some people have used ratio information to study earthquakes and explosions, for example: Pg/Lg ratio has been used by W-Y Kim et al. (BSSA, 1997, 87, 569-588). Here we use Vertical(max)/Horizontal(max) ratio. We can use computer search the two factors automatically in a very short time. We made tests with waveforms generated by blasts at ACTON site and St. Mary’s site, as well as some small earthquakes.

15. In the formula: Vmax=the maximum amplitude along vertical record; The other part is the maximum amplitude along horizontal component

16. Ratios: the dark blue line was drawn with the ratios of blasts at ACTON sites; the yellow line was drawn with the ratios of blasts at St. Mary’s Cement site; the purple red line was drawn with the ratios of small earthquakes in southern Ontario (43-44N; 78-80W; 2000-10-09 to 2005-07-23). Quakes ACTON St Mary’s V/H

17. In the formula µ is the average; σ is the standard deviation. For the 8 blasts at ACTON sites, µ = 0.188584; σ = 0.05335464 For the 16 blasts at St. Mary’s Cement site, µ = 0.29861623; σ = 0.06166254 For the 15 small earthquakes in the southern Ontario region, µ = 0.452439; σ = 0.13809767

18. Normal distributions: the dark blue line was drawn with the ratios of blasts at ACTON sites; the yellow line was drawn with the ratios of blasts at St. Mary’s Cement site; the purple red line was drawn with the ratios of small earthquakes in southern Ontario (43- 44N; 78-80W). ACTON St Mary’s Quakes

19. Waveforms generated by a blast at St. Mary’s Cement; recorded at local station WLVO. The peak time positions along vertical and horizontal components are generally different!

20. Ratio_time difference: The dark blue diamond were drawn with the ratios of blasts at ACTON sites; the yellow triangles were drawn with the ratios of blasts at St. Mary’s Cement site; the purple red squares were drawn with the ratios of small earthquakes in southern Ontario (43-44N; 78-80W). Quakes ACTON St Mary’s

21. Normal distributions: the dark blue line was drawn with the time differences between Vmax and Hmax from blasts at ACTON sites; the yellow line was drawn with the time differences between Vmax and Hmax from blasts at St. Mary’s Cement site; the purple red line was drawn with the time differences between Vmax and Hmax from small earthquakes in southern Ontario (43-44N; 78-80W). ACTON St Mary’s Quakes

22. ACTON St Mary’s V/H Conclusion: The ratio averages for blasts in the 3 sites are small; for the studied earthquakes the average is large.

23. Synthetics generated using explosive source

24. The 3 component records at GAC, generated by a blast in western Quebec (2002-06-26)

25. The 3 component records at OTT, generated by the same blast at western Quebec (2002-06-26).(?)