Rupture process of the 2001 May 7 Mw 4.3 Ekofisk induced earthquake

Abstract

S U M M A R YOn 2001 May 7, following unintentional water injection, a moderate size induced earthquake struck the Ekofisk oil field, North Sea. Despite of its relatively moderate magnitude, clear low-frequency waveforms could be recorded up to more than 2000 km epicentral distance, suggesting a slow rupture at very shallow depth and wave propagation through low-velocity shallow structures. The event poses a rare opportunity to constrain rupture velocity, duration and rise time of a superficial M > 4 event occurring on a horizontal plane in soft, water-saturated sediments. Two previous studies discussed the earthquake point source finding vertical dipslip focal mechanisms with opposite senses of P and T axes. A further investigation was thus required to provide a basis for a deeper discussion of the failure dynamics. We significantly improve the used data set, test different earth models and derive a point source as well as a kinematic rupture model. We carefully discuss parameter uncertainties and effects related to shallow sources and wave propagation through different crustal structures to resolve the previous controversy. We additionally provide a kinematic rupture model, based on apparent source times derived from Rayleigh and Love waves. The waveforms resolve a predominant unilateral rupture along a horizontal plane at about 2 km depth. We derive an unusually slow rupture, consequence of a slow rupture velocity of about 500 m s -1 and a long rise time of about 7 s. An independent modelling of GPS-based static displacements allows to confirm the focal mechanism polarity and to locate the centroid at the eastern side of the field, resulting in a much larger seismic moment in comparison with dynamic seismic moment. The rupture directivity is confirmed by the relative location of the centroid with respect to the epicentre, which is set at the site of water injection.

Discrimination between induced, triggered, and natural earthquakes close to hydrocarbon reservoirs: A probabilistic approach based on the modeling of depletion‐induced stress changes and seismological source parameters

“…Especially the near‐field GPS data with observed uplift could demonstrate, in combination with the retrieved source mechanism, that the seismic source centroid was shallow above the field formation at the eastern border of the field [ Cesca et al , ]. A kinematic inversion for parameters of the extended source and rupture further indicated that the subhorizontal plane ruptured unilaterally over a length of about 6 km in azimuthal direction of about 133° [ Cesca et al , ]. The kinematic solution further indicated a very slow rupture velocity of only about 500 m s −1 (equal to 0.26 times the shear wave velocity) and a long rise time of about 7 s. The unusual slow and long rupture explains that such a moderate‐strong earthquake was able to radiate energetic low‐frequency Rayleigh and Love waves well observed up to 2000 km epicentral distance.…”

“…The kinematic solution further indicated a very slow rupture velocity of only about 500 m s −1 (equal to 0.26 times the shear wave velocity) and a long rise time of about 7 s. The unusual slow and long rupture explains that such a moderate‐strong earthquake was able to radiate energetic low‐frequency Rayleigh and Love waves well observed up to 2000 km epicentral distance. As suggested by Ottemöller et al [] and supported by the kinematic source inversion of Cesca et al [], the rupture was likely to be triggered by an unintended water injection during about 2 years of 1.9·10 6 m 3 in 2 km depth near the northern central part of the field [ Ottemöller et al , ] (see Figure a for location). The triggering of the earthquake rupture may have been realized by the formation and slow growth of a large horizontal hydrofracture within the overburden in about 2 km depth within the shale and mud rocks.…”

“…From the reservoir depletion model we calculate the stress rate for each stress component at each half‐space grid point with a spacing of 500 m. Figure b shows the Coulomb stress rate in direction of the slip vector of the earthquake on a horizontal plane at 2 km depth. It is noteworthy that the highest Coulomb stress rates up to 120 kPa/yr occur along the northern and eastern borders of the field where the rupture was located by Cesca et al []. The Coulomb stress rate in Figure b is associated with , a parameter needed in the discriminatory equation .…”

Current challenges in monitoring, discrimination, and management of induced seismicity related to underground industrial activities: A European perspective

“…Even a low‐aftershock productivity evidenced, e.g., by Dahm et al [] for an induced earthquake in Germany, in comparison to tectonic earthquake of similar magnitude in the same region, could be finally linked to the shallow source location within different geological units. Still linked to shallow depth and near‐surface processes, Cesca et al [] found anomalous slow rupture of about 500 m/s for an induced earthquake, taking place along a weakened shallow subhorizontal failure plane, at the Ekofisk gas field, North Sea. Recently, Folesky et al [] studied the rupture directivity of fluid‐induced microseismic events and analyzed the induced seismicity sequence related to the EDS project in Basel (Switzerland).…”

Full Waveform Seismological Advances for Microseismic Monitoring

“…In mining environments, the analysis of microseismicity is important for mines stability monitoring (e.g., Cesca, Sen, & Dahm, 2014;Gharti, Oye, Roth, & Kuehn, 2010;Sen, Cesca, Bischoff, Meier, & Dahm, 2013). In particular, different human operations, including mining (e.g., Bischoff, Cete, Fritschen, & Meier, 2010;Feignier & Young, 1992;Fletcher & McGarr, 2005;McGarr, 1992;Sen et al, 2013;Trifu, Angus, & Shumila, 2000), water reservoirs impoundment (Do Nascimento, Lunn, & Cowie, 2005;Gough & Gough, 1970;Guha, 2000;Gupta & Rastogi, 1976), wastewater or fluid injection also including brine from hydraulic fracturing of shales (Ake, Mahrer, O'Connell, & Block, 2005;Cornet & Jianmin, 1995;Ellsworth, 2013;Healy, Rubey, Griggs, & Raleigh, 1968;Kim, 2013;Sasaki, 1998;Sileny, Hill, Eisner, & Cornet, 2009;Tadokoro, Ando, & Nishigami, 2000), geothermal fields operation (e.g., Deichmann & Giardini, 2009;Brodsky & Lajoie, 2013;Ross et al, 1999), and oil and gas field exploitation (e.g., Bardainne, Dubos-Sallée, Sénéchal, Gaillot, & Perroud, 2008;Cesca, Dahm, Juretzek, & K€ uhn, 2011;Dahm et al, 2007;Grasso & Wittlinger, 1990;Rutledge, Phillip, & Schuessler, 1998) and gas injection (Cesca et al, in press) can trigger or induce seismicity. Modern full waveform analysis methods can significantly contribute to monitor and understand the dynamics of rupturing processes and the perturbation to the background stress field, for example, in order to detect weakened regions and evaluate risks of further dangerous ruptures or chan...…”