f Induced seismicity: Setting the problem in perspective for EGS development

Induced seismicity is a recognised hazard in practically all engineering endeavours where<br>stress or pore pressure are altered. This can be taken as a reflection of the realisation that<br>has dawned in the past 20 years that the Earth's crust generally supports high shear stress<br>levels and is often close to failure. Historically, the most damaging events, which have<br>sometimes caused many fatalities, are associated with the impoundment of reservoirs.<br>However, earthquakes of sufficient size to cause damage to localities have also been<br>associated with mining activity, long-term fluid withdrawal wells, and long-term fluid injection<br>wells. Given that massive stimulation injections into crystalline rocks have routinely been<br>performed at EGS sites since the early 70s, it is perhaps surprising that the issue of the<br>seismic hazard associated with these operations has only recently come to the fore. Massive<br>injections of fluid have been conducted at Fenton Hill, Rosemanowes, Hijiori and Soultz (3.5<br>km reservoir) without producing events large enough to disturb the local population. More<br>recently, events approaching or exceeding 3.0 have occurred during or shortly following<br>injections at Soultz , Cooper Basin (Australia) and Basel, all of which were conducted at 4.5-<br>5.0 km. These events, particularly the event at Basel because of it proximity to a major<br>population centre and reports of damage, has galvanised attention on the seismic hazard<br>posed by EGS development.