We performed time lapse measurements of velocity variations using empirical Green's functions reconstructed by autocorrelation of seismic noise recorded during a period of 17 years in the region of l'Aquila, Italy. The time lapse approach permitted us to evaluate the spatial (depth) dependence of velocity variation (dv/v). By quantitatively comparing the 17 years of dv/v time series with independent data (e.g. strain induced by earthquakes, hydrological loading) we unravel a group of physical processes inducing velocity variations in the crust over multiple time and spatial scales. We find that rapid shaking due to three magnitude 6+ earthquakes mainly induced near surface velocity variations. On the other hand, Slow strain perturbation (period 5 years) associated with hydrological cycles, induced velocity changes primarily in the middle-crust. The observed behavior suggests the existence of a large volume of fluid filled cracks exist deep in the crust. Our study, beyond shedding new light into the depth dependent rheology of crustal rocks in the region or l'Aquila, highlights the possibility of using seasonal and multiyear perturbations to probe the physical properties of seismogenic fault volumes. 1. Introduction Detailed laboratory protocols exist to estimate how rocks respond to strain perturbations, and show that a variety of non-linear responses exists for variable rock types with different physical properties (e.g. cracks density, microstructure, presence of fluid, temperature and pressure effects

Understanding the physical parameters that modulate seismicity and that control the properties of earthquakes remains an ongoing challenge in earthquake mechanics. Several factors play a key role, one of which is the fault strength, which in the upper crust is determined by the product of the friction coefficient and the effective normal stress (i.e., the total normal stress minus fluid pressure). Much attention has been focused on how fault strength varies due to changes in the friction coefficient (

<p class="paragraph"><span class="normaltextrun"><span lang="EN-US">The strength and sliding behavior of faults in the upper crust are largely controlled by friction and effective stress, which is itself modulated by the fluid pressure. However, while many studies have investigated the role of friction on the earthquake cycle, relatively little effort has gone into understanding the effects linked to dynamic changes in fluid pressure. Here, we explore coupled interactions between slow tectonic loading and fluid pressure generation during the interseismic period with rapid sliding and elastic stress transfer during earthquakes on a plane strain thrust fault in two dimensions. Our models incorporate rate- and state-dependent friction along with dramatic changes in the fault permeability during sliding. In these modes, earthquakes are nucleated where fluid pressures are locally high and then propagated as slip pulses onto stronger parts of the fault. For the model without overpressure, the ruptures are more crack-like. Our model produces a wide range of sliding velocities from rapid to slow earthquakes, which occur due to the presence of high pore pressures prior to rupture. The models also show evidence for aftershocks that are driven by fluid transfer along the fault plane after the mainshock. Overall, we find that the presence of relatively modest fluid overpressures tends to reduce coseismic slip, stress drop, maximum sliding velocity, rupture velocity, and the earthquake recurrence time relative to ruptures in a dry crust. This study shows that fluids can exert an important influence on earthquakes in the crust, which is mostly due to modulation of the effective stress and variations in permeability, and to a lesser extent to poroelastic coupling. </span></span></p>

<p>Fluvial stratigraphy is the product of changes in Earth’s history and inverting this record has often resulted in interpretations associated with changes in base-level caused by sea-level and/or basement subsidence (downstream drivers).  Similarly, environmental perturbations occurring in the upstream reaches of a fluvial system (i.e., the source region), such as climate driven changes of water discharge and/or perturbations of sediment supplied to rivers (upstream drivers), can also drive river bed evolution.  Therefore, both changes in upstream and downstream drivers can cause a river’s equilibrium profile to respond and adjust through aggradation or degradation and hence generate stratigraphy.  Furthermore, it is likely to have changes in both upstream and downstream drivers simultaneously because both drivers may be themselves driven by the same factors e.g., astronomical cycles can drive both sea-level variation at the downstream end of fluvial systems and water discharge variations in the upstream end.  Deciphering the effects of the two drivers is vital to be able to comprehensively interpret the narrative of Earth’s history preserved in fluvial successions.  We explore this issue with river flume experiments, where we are able to test the influence of both upstream and downstream drivers in isolation.  Furthermore, the small scale of physical modelling reduces the spatial and temporal timescales compared to natural systems and allows us to investigate how quickly the system responds.</p><p>We use a narrow (0.05 m), long (2.25 m) flume with an initial gradient of zero.  Side-profile photos are taken throughout the experiment run, which are then analysed and fitted to monitor river bed evolution.  Top view photos record channel dimensions.  We use low flow rates (~<600 ml/min) delivered by a peristaltic pump, to avoid turbulence and ensure bedload transport.  We have three aims with our experiments. Firstly, to investigate the role of changes in water discharge and sediment supply on equilibrium river profiles and the timescales associated.  Secondly, to carry out a series of perturbation experiments varying downstream drivers (i.e., sea-level), which theoretically produce the same amount of aggradation as the upstream parameters we have used in order to compare.  Thirdly to vary both upstream and downstream parameters simultaneously to investigate the effects. Results to date suggest that the growing wedge maintains a relatively constant slope and that the slope of the wedge is dependent on sediment concentration (sediment discharge/water discharge), when using the same grain-size distribution for each experiment.  Furthermore, results imply that the system is highly sensitive to perturbations when the setup of the system is with relatively low sediment concentrations. Therefore, a greater magnitude of response is seen than with setups of higher sediment concentrations. Currently we are undergoing perturbation experiments and downstream perturbations, the results of which will also be presented here. Ultimately we will use our findings to upscale our experiments into a fully 3-D flume tank that will grow as an unconfined fan in order to observe any similarities and differences.</p>

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