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Speaker: Emily Warren-Smith, Seismologist, Geological Society of New Zealand

Host: Shuoshuo Han

Title: Stress and Fluid Processes During Northern Hikurangi SSEs: What Can We Learn From Microseismicity?

Abstract: The occurrence of slow motion earthquakes, or slow slip events (SSEs), in subduction zones has been proposed to be linked to the presence of, and fluctuations in near-lithostatic fluid pressures (P_f) within the megathrust shear zone and subducting oceanic crust. In particular, the 'fault-valve' model is used to describe occasional, repeated breaching of a low-permeability interface shear zone barrier, which caps an overpressured hydrothermal fluid reservoir. In this model, a precursory increase in fluid pressure may therefore be anticipated to precede megathrust rupture. However, physical observations of such increases, and subsequent decreases, particularly within the subducting slab where hydrothermal fluids are sourced, remain elusive. Here we use earthquake focal mechanisms recorded on an ocean-bottom seismic network to identify changes in the stress tensor within subducting oceanic crust during four slow-slip events in New Zealand’s Northern Hikurangi subduction zone. We propose that these changes represent precursory accumulation and subsequent release of fluid pressure within overpressured subducting oceanic crust via a ‘valving’ model for megathrust slip behaviour. We also considermicroseismicity datasets alongside geophysical observations to identify spatially heterogeneous hydrological coupling between the two plates which we propose may be the source of fluids triggering shallow SSEs. We identify patches of co-located, differing seismic behavior, including: interface micro-seismicity, pre-SSE swarms, lower plate strike-slip earthquakes, long-term repeating earthquakes and elevated Vp/Vs zones. These patches correlate well with conductive interface regions constrained by magnetotelluric studies and thermal springs exhibiting mantle helium isotopic signatures. We propose these patches represent ‘vent’ sites where hydrological coupling between the plates is increased, facilitating episodic fluid valving which may be linked to SSE timing. These observations are important for future targeted efforts to monitor localised changes in fault zone properties during SSE cycles, in a bid to understand their physical behaviour and episodicity.

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Speaker: Kennda Lynch, Lunar and Planetary Institute

Host: Duncan Young

Title: S^3 (Subsurface, Subaqueous, and Salty): Looking For Life in All the Right Places

A seminar from the Center for Planetary Systems Habitability

Speaker: Prof. Ryan P. Russell, University of Texas at Austin

Title: Spacecraft Dynamics for Outer Planet Moons Missions: Challenges of Searching for Life at Icy Worlds

Abstract: Finding life beyond the Earth within our solar system is an age old quest. The icy moons of the outer planets are among the leading candidate search sites due to the abundance of water and internal heat sources driven by strong tides. In many observed cases, water transports to the surface and beyond via cracks in the dynamically shifting ice crust. Planetary scientists and other life hunters have long dreamed of sending in-situ robots to these exotic locations. While NASA has successfully executed and is currently planning flyby missions to icy world moons, orbiters and landing missions have not yet made it past the proposal stage. In this talk, we will discuss the challenges of designing such missions, primarily from a spacecraft dynamics perspective. Multiple phases of the mission will be discussed including the heliocentric planetary tour phase, the planetary capture and moon tour phases, the moon science orbit phase and optional landing excursions. The icy world moons lie deep in the gravity well of their gas giants parents. Exploration with current technology, therefore requires the extensive use of gravity assists in lieu of large propulsive maneuvers. The design space is extraordinarily large, as typical decade-long itineraries include hundreds of gravity assists and thousands of revolutions, while traversing every level of the sun-planet-moon solar system hierarchy. The open dynamical questions include which body to flyby and when, and where should the precious reserve of propellant be expelled. Intense radiation and perturbations due to third-body and non-spherical gravity make for a hostile environment near the planetary moons. Such constraints require hard limitations on radiation exposure and careful exploitation of the flow and equilibria of the natural dynamical system. Several recent and ongoing advances in the design of such missions will be discussed, including new algorithms to assess reachability, to solve the pathfinding combinatorial optimization problem, to locally optimize promising solutions in high fidelity, to find long-life science orbits, to solve and archive databases of periodic orbits, to identify feasible landing options, and to automate the whole process.

https://habitability.utexas.edu/events/

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Speaker: Jeanine Ash, Rice University

Host: Chris Lowery

Title: Making and Breaking Molecules

Abstract: Methane is fundamentally significant to Earth’s habitability and the evolution of life. Its concentration in the atmosphere is the result of constant interplay between biological and geological process. My work focuses on the enzyme-level chemical reactions that make and break methane molecules, and how recent advances in isotope ratio mass spectrometry can provide new tools for tracing these reactions at the global level. In this talk I’ll introduce the concept of multiply-substituted isotopologues (commonly called “clumped” isotopes), and share experimental and case studies that show how these tools can be used to illuminate methane creation and destruction processes in the deep.