Spring 2018

April 6, 2018

Speaker: Christine Ruhl, UC Berkeley

Title: Spatiotemporal Variation of Stress Drop During the 2008 Mogul, Nevada Earthquake Swarm: Implications for the Interplay of Fluid Flow, Faulting, and Aseismic Slip

Abstract: After approximately 2 months of swarm-like earthquakes in the Mogul neighborhood of west Reno, NV, seismicity rates and event magnitudes increased over several days culminating in an Mw 4.9 (ML 5.1) dextral strike- slip earthquake on 26 April 2008. Rapid temporary instrument deployment directly above the sequence provides high-resolution coverage of microseismicity, enabling a detailed analysis of swarm behavior and faulting geometry. Double-difference waveform-based relocations reveal an internally clustered sequence in which foreshocks evolved on multiple structures surrounding the eventual main shock rupture [Ruhl et al., 2016, JGR]. Seismicity also defines a fault-fracture mesh and detailed fault structure from approximately 26 km depth on the previously unknown Mogul fault that may be an evolving incipient strike-slip fault zone in a zone of distributed deformation (the Walker Lane). The seismicity volume expands before the main shock, consistent with pore pressure diffusion related to fluids [Ruhl et al., 2016, JGR]. We estimate well-constrained, independent P and S wave corner frequencies and stress drop for 148 earthquakes (2.2ML≤5.1) using EGF-derived spectral ratios. Resulting stress drops vary over two orders of magnitude and enable investigation of stress drop variation within a well-recorded sequence. During the fluid-driven foreshock period, stress drops increase away from the initiation point similar to observations of induced earthquakes. Along the mainshock fault plane, high stress drop foreshocks cluster around an absence of seismicity which is only ruptured by low stress drop foreshocks and is not re-ruptured in the aftershock period. The area of the “seismicity void” is approximately equal to the expected rupture dimension for the mainshock earthquake, which also has a relatively high stress drop. These observations are best explained by a difference in rheology along the fault plane where a velocity-weakening region is surrounded by velocity-strengthening regions. As stress accumulates in the seismic cycle, aseismic slip occurs in the transitional areas on the periphery of the asperity causing numerous high-stress drop foreshocks just prior to the high-stress drop rupture of the asperity (i.e., the mainshock). The unprecedented detail achieved for these shallow, low magnitude earthquakes confirms that stress drop, when measured precisely, is a physically-meaningful observation that has implications for fault zone properties as well as rupture nucleation and propagation.

April 13, 2018

Speaker: Robin Matoza, UC Santa Barbara

Title: Seismo-acoustic signatures of explosive volcanic eruptions

April 20, 2018

Speaker: Jeff McGuire, Woods Hole Oceanographic Institution (WHOI)

Title: Offshore Adventures in the Cascadia Subduction Zone: Examining Stress and Slip on the Quietest Plate Boundary

April 27, 2018

Speaker: Andrew Poppe, UC Berkeley

Title: Interplanetary dust: the view from near and far

Abstract: Interplanetary dust is present throughout the solar system and provides a key connection to fundamental planetary building blocks. IDP grains are produced via several processes, including asteroidal disruption, cometary outgassing, and grain-grain mutual collisions and arise from several parent sources, such as the asteroid belt, Jupiter-family, Halley-type, and Oort Cloud comets, and Edgeworth-Kuiper Belt objects. The relative density and flux of IDPs from any individual source at a given location in the solar system is a complex function of both dust production rates and subsequent gravitational and non-gravitational interactions (i.e., radiation pressure, Poynting-Robertson drag, Lorentz force, etc.). Here, I describe recent modeling and observational efforts directed at understanding and constraining both the individual components and overall morphology of the interplanetary dust cloud throughout the solar system. In particular, I will touch on new results from the New Horizons Student Dust Counter and the Lunar Dust Experiment (LDEX) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE).

We also briefly discuss two new mission concepts regarding interplanetary dust. The first, i2DUNE, would measure interplanetary and interstellar dust from an Earth-orbiting (or near-Earth) spacecraft making use of next-generation trajectory and mass composition dust detectors. The second, Interstellar Probe, is a mission to leave the Solar System in order to gain a unique vantage point for look-back imaging of the interplanetary dust complex. IP would provide global imaging of our solar system’s debris disk in analogy with the plethora of observations of exozodiacal disks around other stars. Both of these missions present exciting opportunities to revolutionize our view and understanding of dust both near and far.

May 4, 2018

Speaker: Noah Randolph-Flagg, UC Berkeley

Title: Hydrothermal systems at Long Valley Caldera

Abstract: At Long Valley Caldera, continuously monitored water wells and hydrothermally altered rocks provide insights into hydrothermal processes. First, we show how columns of hydrothermal alteration are the products of pressure diffusion instabilities as boiling meteoric water infiltrated into the still-cooling Bishop Tuff. Second, we show data from the modern Long Valley hydrothermal system where one meter water level drops are coincident with the passage of teleseismic Rayleigh waves. Some local, triggered earthquakes are also coincident with these water level changes. We suggest that dilatational waves may cause flow, and pressure diffusion, into and out of nearby fractures.

May 11, 2018

Speaker: Paul Betka, Lamont-Doherty Earth Observatory.

Title: Structure of the IndoBurman Ranges: mechanical stratification during extreme sediment accretion

May 18, 2018

Speaker: Abram H. Clark, Naval Postgraduate School

Title: Yielding in granular materials: from riverbeds to renormalization group

May 25, 2018

Speaker: Isamu Matsuyama, University of Arizona


Tides and the interior structure of the Moon and icy satellites

Abstract: The interior structure of a satellite determines its response to rotational and tidal forcing, which in turn affects its equilibrium shape and tidal heating. The unusual shape of the Moon given its present rotational and orbital state has been explained as due to a fossil figure preserving a record of remnant rotational and tidal deformation. Previous studies assume a fossil figure preserved by an interior with infinite rigidity, ignoring the deformation due to changes in the rotational and orbital potentials as the Moon evolves to the present state. In the first part of the talk, I will present a lunar model that can explain the present lunar figure as due to fossil figure preserved by an interior with finite rigidity.

Icy satellites of the outer solar system have emerged as potential habitable worlds due to the presence of subsurface oceans. As a long-term energy source, tidal heating in these subsurface can influence the thermal, rotational, and orbital evolution of icy satellites, and the sustainability of oceans. Previous studies considering ocean tidal heating ignore the presence of an overlying shell or use a thin shell approximation. In the second part of the talk, I will present a new theoretical treatment for ocean tidal heating in thin subsurface oceans with overlying shells of arbitrary thickness.

June 1, 2018

Speaker: Chris McKay, NASA Ames Research Center

Title: Terraforming Mars: Science, ethics and current events

Summary: Scientific and public interest in terraforming Mars is growing, spurred mostly by Elon Musk and his company SpaceX and their plans for Mars settlement. I will review the scientific basis for terraforming Mars and the environmental ethics considerations.

June 8, 2018

Speaker: Myriam Telus, UCSC

Title: Carbonates in meteorites: tracing fluid and ice composition in planetesimals