Rupture directivity of fluid‐induced microseismic events: Observations from an enhanced geothermal system

Abstract

The rupture process of fluid‐induced microseismic events is still poorly understood, mainly due to usually small magnitudes and sparse monitoring geometries. The high‐quality recordings of the earthquake sequence 2006–2007 at the enhanced geothermal system at Basel, Switzerland, constitute a rare exception, allowing a systematic directivity study of 195 events using the empirical Green's function method. We observe clear directivity signatures for about half the events which demonstrates that rupture directivity persists down to small magnitudes (ML∼1). The predominant rupture behavior is unilateral. We further find evidence that directivity is magnitude dependent and varies systematically with distance from the injection source. Whereas pore pressure seems to play the dominant role close to the injection source and no preferred rupture direction is observable, directivity aligns parallel to the event distribution with increasing distance ( ≳100 m) and is preferably oriented away from the injection point. The largest analyzed events (ML∼2) show a distinct behavior: They rupture toward the injection source, suggesting that they nucleate in the vicinity of the pressure front and propagate backward into the perturbed volume. This finding is of particular relevance for seismic hazard assessment of georeservoirs, since it implies that maximum event size is related to dimension of the fluid‐perturbed volume. Our study also resolves rupture complexities for a small group of events. This shows that small fault heterogeneities exist down to a scale of a few tens of meters. The observation of directivity and complexity in induced microseismic events suggest that future source studies account for these phenomena.

Earthquake Directivity, Orientation, and Stress Drop Within the Subducting Plate at the Hikurangi Margin, New Zealand

“…More detailed modeling is to be preferred, but for many earthquakes of interest, the data are insufficient to resolve more complex sources. Folesky et al () found that the azimuthal variations in amplitude were less affected by filtering than the duration and so could also resolve orientation and rupture velocity. Their approach requires the same EGFs to be used at each station to preserve relative amplitude, limiting the input data.…”

“…Lengliné and Got (), Kane et al (), and Wang, Rubin, and Ampuero () used azimuthal variation in spectral amplitudes to infer that the predominant rupture direction of earthquakes on the San Andreas Fault at Parkfield is consistent with the material contrast across the fault. Folesky et al () found that rupture direction within a cluster of fluid‐induced earthquakes depends on location and magnitude. There is still considerable uncertainty in the results of directivity studies due to lack of resolution of the data.…”

Patterns of Rupture Directivity of Subduction Zone Earthquakes in Northern Chile

“…This is repeated station‐wise for the entire station network. We fit the amplitude variation model described by equation to the amplitudes obtained from the station‐wise reconstructed RSTFs in order to find the rupture direction of the event, that is, we solve for the parameters and ϕ 0 (for more details, see Folesky et al, ). In the target area, the vast majority of events occur at the plate interface (Sippl et al, ) exhibiting similar thrust mechanisms (Cesca et al, ) with one fault plane oriented in good agreement with the dipping angle of the subducting slab.…”

“…It is current understanding that rupture initiation, propagation, and termination are complex phenomena which are affected by multiple properties such as the material contrast of the juxtaposed rock types at the two sides of a fault (Ben‐Zion, ; Weertman, ). Also, among other aspects, stress state (e.g., Ampuero & Ben‐Zion, ) and perturbations of the stress field, for example, by pore pressure changes due to fluid injection, were proposed to affect rupture directivity (Folesky et al, ; Shapiro et al, ).…”

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

“…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). They found that the largest events of the sequence ( M l about 2) nucleate close to the pressure front and propagate backward into the stimulated volume, implying that maximum event size is related to dimension of the fluid‐perturbed volume [ Folesky et al , , ].…”