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 in principle 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. Epicentre distribution for earthquakes from 2000-1009 to 2005-0723; blast sites: St. Mary’s Cement site, ACTON sites A1 & A2; stations WLVO & ACTO

9. Waveforms generated by a blast at St Marys Cement; recorded at Polaris station WLVO; station distance ~ 24 km.

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

11. Waveforms generated by a small earthquake (mN~1.1); recorded at Polaris station STCO; the distance ~18 km

12. Waveforms generated by a small earthquake (mN~1.3); recorded at Polaris station WLVO; distance~12 km

13. Waveforms generated by a blast at St Marys Cement; recorded at Polaris station WLVO; station distance ~ 24 km.

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 Marys sites, so we are trying to identify blasts from small earthquakes using the ratio between the maximum amplitudes along vertical and horizontal components. 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 very short time. We made tests with waveforms generated by blasts at ACTON site and St Marys 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 Marys Cement site; the purple red line was drawn with the ratios of small earthquakes in southern Ontario (43-44n; 78-80w). Quakes ACTON St Mary’s V/H

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 Marys 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 Marys Cement; recorded at Polaris station WLVO; station distance ~ 24 km.

20. Ratio_time: 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 Marys 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 Marys 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

24. We analyzed the waveforms generated by 16 blasts at St. Marys Cement in 2004 and 2005, recorded at Polaris station WLVO. We can see visually that the amplitudes at vertical component are consistently weaker than those at horizontal components for all 16 blasts. The ratios between the maximum amplitude at vertical component and the maximum amplitude at horizontal component (the vector sum of N-S and E-W components) for those 16 blasts at station WLVO are in table 1. The average of the ratios in the table is 0.29861623; the standard deviation is 0.06166254. Table 1. The ratios and time differences from 16 blasts at St. Marys Cement DateTimeStationDistance (km) (km) Vmax_t- Hmax_t Vmax/Hmax(ratio) 2004010217:15:29WLVO24 0.90 0.900.258513 2004010919:20:24WLVO24 0.18 0.180.258404 2004050517:28:58WLVO24 2.75 2.750.285460 2004051716:01:53WLVO24-0.520.325624 2004061016:00:38WLVO24 0.33 0.330.271461 2004081916:20:17WLVO24-3.500.323975 2004110316:58:27WLVO24-1.020.322012 2004111717:07:37WLVO24-0.860.393929 2004112417:05:36WLVO24 0.76 0.760.259568 2004121717:12:39WLVO24-0.140.232295 2004122217:21:05WLVO24 0.34 0.340.286718 2004122817:12:42WLVO24 1.37 1.370.463455 2005010717:07:16WLVO24 0.21 0.210.221667 2005020117:08:48WLVO24-0.140.310529 2005030217:04:45WLVO24 0.09 0.090.247845 2005051016:30:44WLVO24 0.98 0.980.313686 2005082416:28:22WLVO24 1.05 1.050.261232