On 26 November 2009, a 2.8-magnitude earthquake (henceforth referred to in this paper as the De Hoeve tremor) occurred in southwestern Friesland in the Netherlands at an approximate depth of 2 km. This relatively shallow depth indicated that the event was induced. The closest mining activities were believed to be in the Weststellingwerf field. A study was required to determine the origin of the tremor, evaluate if it could be followed by other tremors in the future, and estimate its magnitude.

Introduction

The tremor’s epicenter was identified by the KNMI (Koninklijk Nederlands Meteorologisch Instituut: Royal Netherlands Meteorological Institute) (Fig. 1). Three wells are located near the epicenter: Weststellingwerf-01 (WSF-01), Noordwolde-01 (NWD-01), and De Hoeve-01 (DHV-01). They all are producing from formations at a depth of approximately 2 km.

• Well WSF-01 was a gas producer between 1998 and 2004; -cold‑water injection started in 2008 with no tremor reported during the gas-production phase.
• Well NWD-01 is immediately outside of the epicenter perimeter. It was producing gas at the time of the event, with no tremor measured before November 2009 and none after.
• Well DHV-01 is located in the middle of the estimated epicenter area, but had not been drilled at the time of the event.