Magnitude (sismologie) | EPFL Graph Search

vignette|Sismogramme enregistré par un sismographe à l'Observatoire Weston dans le Massachusetts, aux États-Unis. En sismologie, la magnitude est la représentation logarithmique du moment sismique, qui est lui-même une mesure de l'énergie libérée par un séisme déduite de l'amplitude de certaines ondes sismiques à des distances spécifiques (mesure de l'amplitude sur un sismogramme de l'onde P ou S). Plus le séisme a libéré d'énergie, plus la magnitude est élevée : un accroissement de magnitude de 1 correspond à une multiplication par 30 de l'énergie et par 10 de l'amplitude du mouvement. Les médias emploient souvent les termes d’échelle de Richter ou d’échelle ouverte de Richter, mais ces termes sont impropres : l'échelle de Richter, stricto sensu, est une échelle locale, surtout adaptée aux tremblements de terre californiens. Les magnitudes habituellement citées de nos jours sont en fait des magnitudes de moment (notées M ou M). La magnitude et l'intensité (comme l'échelle de Mercal

Laboratory Testing on Hydraulic Fracturing of Barre Granite: Studying Visual Observations, Acoustic Emissions and Radiated Energy related to Fracture Mechanisms

Michela Casanova

Hydraulic fracturing is frequently used to increase the permeability of rock formations. This can be done by creating new fractures as usually done for hydrocarbon extraction or extending and opening fractures as usually done in Enhanced Geothermal Systems (EGS). In both cases, only indirect information such as in-situ stress, pumping records and microseisms are recorded. However, understanding the exact processes of hydraulic stimulation remains very challenging as some of the fundamental aspects such as the fracturing processes and induced/triggered seismicity are still unknown. Unique equipment developed by the MIT CEE Rock Mechanics Group allows one to visually observe hydraulic fracturing while recording Acoustic Emissions (AE) simultaneously. Important information is obtained by applying different fluid pressure in individual pre-cut flaws with distinct flow rates and external stresses. This paper focuses on the main processes inducing hydraulic fracturing, and particularly hydro-shearing, by relating the AE data to visual observations, and investigating different experimental concepts in terms of focal mechanisms and normalized radiated seismic energy. In this work we show that (1) hydro-shearing may be induced by applying high external loads i.e. induce global shear failure; (2) local failure around the flaw tips is expected to occur in tensile mode; (3) the AE hypocenter location matches well with the visual crack observations; (4) some differences of focal mechanisms based on AE data and visual observations exist; (5) cyclic pressurization results in high normalized radiated seismic energy.

2020

Seismic Rehabilitation of Deficient Steel Braced Frames with Conventional and Innovative Retrofit Techniques

Valériane Matthey

Steel concentrically braced frames (CBFs) as lateral load resisting system are common in steel construction, for their high strength and architectural versatility. However, the brace connection details have to be well designed to ensure a ductility of the frame under seismic excitation. This Master Thesis studies the behavior of a steel CBF, located in Sendai (Japan). It experienced significant damage at brace connections after the 2011 Tohoku-Oki Earthquake in Japan. They are mainly due to the eccentricity created by the single-lap gusset plates along the long duration of the imposed ground motion hazard. The seismic response of the CBF is assessed by means of nonlinear analysis with the performance-based approach according to ASCE/SEI 41-13 (2014). As the braces do not meet any performance criteria, seismic retrofit is required. Three retrofit solutions are investigated and compared. Two of them are considered as conventional solutions. They consist to replace the braces and gusset plates, once with new Chevron braces and once with new X braces. The new braces are designed with the Capacity Design rules. The third solution modifies the CBF into a Rocking Braced Frame (RBF). This innovative high-performance system allows the column bases to uplift in order to dissipate the seismic energy and localize the damages in easily replaceable fuses to protect the primary members. The retrofit works are mainly located at the column bases. Dynamic analyses of the initial frame and its retrofits are computed for a seismic input recorded during the 2011 Tohoku-Oki Earthquake. The peak storey drift ratio (SDR), residual SDR and story shear forces are compared. The new Chevron CBF has the worst performance of the three retrofits. New X-bracing CBF and the RBF show a similar performance, the RBF being slightly better.

2020

Laboratory Investigation on Seismic Response of Rock Fractures Filled with Granular Materials

Wei Wu

Filling materials may exist in all scales of rock fractures, not only influencing seismic wave attenuation, but controlling rock mass instability. The objective of this study is to experimentally investigate the seismic response of rock fractures filled with granular materials. This experimental study simplifies the complex interaction between seismic waves and filled rock fractures and considers the seismic-induced compressive responses of a filled single fracture and a set of filled parallel fractures and the seismic-induced frictional slip on a filled fracture. The seismic-induced compressive response of a filled single fracture is investigated by a split Hopkinson rock bar (SHRB) test. The SHRB test simulates the P-wave propagation across the filled fracture and verifies the analytical solutions predicted by the displacement and stress discontinuity model. The seismic wave attenuation across the filled fracture is due to wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The specific fracture stiffness and the specific initial mass of the filling materials are two key parameters in interrelating the physical, mechanical and seismic properties of the single fracture filled with dry granular materials. The SHRB test is modified to study the seismic-induced compressive responses of a set of filled parallel fractures and to verify the analytical solutions predicted by the modified recursive method based on the displacement and stress discontinuity model. The seismic wave attenuation across the filled parallel fractures is due to multiple wave reflections between the filled fractures, as well as wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The dynamic compaction of the filling materials induces the decreases of loading rate and dominant frequency when the P-wave propagates across each filled fracture in a fracture set. The seismic-induced frictional slip on a filled fracture is performed by a dynamic-induced direct-shear (DIDS) test. A generated plane P-wave propagates as a dynamic shear load and induces the frictional slip along a layer of filling materials. The dynamic shear stress is non-uniformly accumulated in the filling materials. The shear stress at the trailing edge controls the dynamic triggering of frictional slip owing to the highly compacted filling materials. The dynamically triggered frictional slip is quickly arrested by the re-connection of sand contacts after the instantaneous disturbance, while the statically triggered frictional slip is unrecoverable during a short duration and contains a main slip and a few short slips before and after the main slip. The seismic response of rock fractures filled with granular materials is decomposed into the seismic-induced compressive responses and the seismic-induced frictional slip in this study. Both seismic-induced responses are strongly related to the dynamic response of the filling materials. The analytical models and the experimental experiences are possible to be numerically combined to study random wave propagation across rock fractures filled with granular materials and to be applied to estimate seismic wave attenuation and rock mass instability.

EPFL2013

Laboratory Investigation on Seismic Response of Rock Fractures Filled with Granular Materials

Wei Wu

EPFL2013