A novel apparatus to study the mechano-chemical processes active during the nucleation and propagation of earthquakes (MEERA).

We present a novel apparatus designed to investigate the mechanical and chemical processes active during the nucleation and the subsequent propagation of a seismic rupture. The earthquake is experimentally represented by the sudden frictional sliding of two blocks caused by either: i) the passage of a rupture front from a nearby seismogenic source at prescribed slip velocity, or ii) by the sudden release of strain energy cumulated during the slow (tectonic) loading stage preceding the nucleation of seismic rupture. M.E.E.R.A. (Mechanics of Earthquake and Extended Rupture Apparatus) is a biaxial horizontal machine installed at the laboratories of the Istituto Nazionale di Geofisica e Vulcanologia of Rome (Italy) thanks to a grant funded by the Italian Dipartimento di Protezione Civile. MEERA works on two blocks sized 320x80x50 mm³ put in frictional contact under a normal load up to 30 MPa. Blocks can be either rocks or analogue materials. The normal load and the shear stress are supplied by 6 hydraulic piston cylinders. One piston applies the tangential force up to 150 kN and up to 40 mm/s of slip rate. The other 5 cylinders modulate the normal force on the 320 x 50 mm² contact surface. The 6 pistons are mounted on a rigid stainless-steel vessel that can be closed by a top built in plexiglass, which enables the environmental chamber for fluid confinement. The plexiglass top resists up to 6 MPa of fluid pressure exerted and controlled by using two ISCO pumps.  MEERA is designed following the outline described in McLaskey and Yamashita (2017) and introduces three novelties: the control in displacement and displacement rate of the tangential piston up to 1kS/s; the environmental chamber; the rigid stainless-steel frame. MEERA is designed to study how the tectonic loading of a frictional interface composed of natural rocks determine the stress state and shear stress evolution governing seismogenic processes. To this end, the simulated fault in MEERA is equipped with acoustic sensors, strain gauges, optical fibers and high velocity cameras to measure and constrain rupture nucleation processes and earthquake source parameters, including directivity and rupture velocity, the dynamics of seismic ruptures and the earthquake energy budget at different scales. We aim at comparing the laboratory observations and the signals collected by MEERA with those collected by the newly developed on-fault observatory of the ERC FEAR project in the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, Swiss Alps) to provide novel insights in earthquake mechanics.