1. Constraints on Seismogenesis of Small Earthquakes from the Natural Earthquake Laboratory in South African Mines (NELSAM) Margaret S. Boettcher (USGS Mendenhall Fellow) Art McGarr and Malcolm Johnston (USGS) Thomas H. Jordan (SCEC/USC) Vincent Heesakkers and Ze’ev Reches (Oklahoma U.)

2. Gold Mine Seismic Network TauTona Mine: 25 Geophones Mponeng Mine: 20 Geophones 1-3 components, 200 - 3000 Hz overburden ~3.6 km Gold Reef NELSAM Network 6 sites with 13 instruments accelerometers, geophones, & a strain meter 6-12 kHz sampling rates

3. ~300,000 Earthquakes within 200m of the Gold-bearing Reef -2 ≤ M ≤ 3

4. Motivating Questions: 1.Do physically-controlled minimum magnitude earthquakes exist? 2.What are nucleation zone sizes? 3.Are the source properties of very small earthquakes the same as for large damaging events? Are they the same as those of laboratory experiments? 4.What implications do very small earthquakes have for earthquake prediction?

5. M min Gutenberg-Richter Distribution Richardson & Jordan, BSSA, 2002 Background Information: previous M min studies in South African Gold Mines- Richardson & Jordan, 2002; McGarr, 2005; Spottiswoode et. al “Directly Mining-Induced” M < 0 explosive components high frequency source spectra affinity to active mining (space and time) “Tectonic-Like” shear + implosive components failure on pre-existing planes of weakness distributed in space and time follow Gutenberg-Richter distribution M min Multiple populations of mining-induced seismic events

6. Weekly Recorded Seismicity 6-7 pm Ore Production Blasting

7. Are there physically-controlled minimum magnitude earthquakes? Detection limit, not physical limit, controls the size of the minimum observed earthquakes M min ≈ -1.2 Gutenberg-Richter Distribution

8. b-value ≈ 0.85, independent of strain rate a-value is proportional to strain rate (e.g. McGarr, 1976) Ore Production Hours Not During Production Sundays # M W = 0 events per hour: 0.3 (Sundays) 0.9 (Not During Production) 5.0 (Ore Production) Properties of the Magnitude-Frequency Distribution, N =10 (a-bM)

9. We currently have no observations of a physically-controlled minimum magnitude. What is the current minimum detectable magnitude? What implications does this size have for earthquake nucleation and prediction?

10. Natural Earthquake Laboratory in South African Mines (NELSAM)

11. M W ≈ -3.4 foreshock(s) prior to a M W = -2.7 hypocentral distance of 28 m P S P S P S ?

12. M W ≈ -3.4 foreshock(s) prior to a M W = -2.7 hypocentral distance of 28 m 100-1000 Hz Foreshock Mainshock S spectra P spectra S spectra P spectra

13. Source Parameters of very small earthquakes: radius ≤ 0.5 m shear + volumetric components apparent stress in the range of previous observations

14. What constraints can we place on a nucleation patch size? Minimum Observed Earthquake: M min ≈ -3.4 (M complete = -1.2) radius ≈ 0.5 m (~2 m) slip ≈ 30 µm (~180 µm) Nucleation Patch:  even smaller!  we can estimate a theoretical nucleation size from laboratory results… assuming a circular rupture model and a constant stress drop of 3 MPa

15. Estimates of the Nucleation Patch Size (from laboratory results of frictional failure)  r ≈ 5 cm, M 0 ≈ 1000 Nm, M W ≈ -4  observations of very small mining-induced earthquakes are consistent with laboratory results Laboratory Observations: (Lockner & Okubo, 1983; Okubo & Dieterich, 1984)  d ≈ 0.73   ≈ 0.1  n  D C ≥ 5  m Mine Parameters: G = 36 GPa;  n ≈ 80 MPa  d  slope of the slip weakening curve stiffness of the seismic source

16. Detection limitations, not earthquake source physics, control the observed catalog completeness threshold (M W ≤ -1.2) in TauTona Mine, South Africa. There is no evidence for minimum-sized earthquakes. With decreasing seismic moment, earthquakes exhibit shear + tensile failure. If earthquake nucleation processes are dependent on fault zone properties, then using surface monitoring it is very unlikely that we will be able to detect precursory signals of earthquake processes (r ≈ 5 cm, M W ≈ -4). Apparent stress calculated for a number of very small mining induced earthquakes is in the same range as those previously determined for small mining events at much shallower depths as well as for large tectonic earthquakes. The Gutenberg-Richter b-value ≈ 0.85 characterizes seismicity in TauTona mine, independent of strain rate, while the a-value is directly proportionally to strain rate. Implications for Earthquake Physics and Prediction