U12A-01 INVITED 10:20h
Earthquake Source Physics
Understanding the physics of earthquakes requires observational, theoretical, numerical, experimental, and field investigations. Seismological observations are generally on average macroscopic parameters (i.e., integrated quantities such as seismic moment, radiated energy, average stress drop etc), while most of the experimental and field observations are often on local microscopic parameters (i.e., friction, stress intensity factor, fracture energy, grain size of fault gouge, porosity etc). To construct a comprehensive earthquake source model, we need to link the macroscopic and microscopic source parameters. In this process spatial heterogeneities of fault properties and the vast difference (at least a factor of 1010) in time and length scales between earthquakes and laboratory observations need to be properly taken into account. The last decade has seen significant progress in linking them together with theory. Two main difficulties are: (1) no direct seismological method is available to determine the absolute level of crustal stresses and (2) the accuracy of macroscopic source parameters is not good enough to allow definitive interpretations, despite the increase in the quality and quantity of seismic data. In this tutorial presentation, I will summarize the recent progress in this field with an emphasis on recent seismological observations. Because of the two difficulties, the conceptual model still has a wide range of possibilities from very high shear stress on faults with low seismic efficiency to low shear stress with high seismic efficiency. However, regardless of these opposing views on the state of stress on faults, recent observations of the spatial-temporal energy release patterns of large earthquakes suggest that the earthquake process involves substantial chaotic elements and accurate predictions of its behavior are difficult. Nevertheless, the improved understanding of the physics of earthquake source suggests an effective damage mitigation measures through "earthquake early warning". This concept relies on a better understanding of the earthquake nucleation process and the recent advancement of numerical methods. Long term processes leading up to an earthquake have attracted seismologists' attention, especially those involving fluid motion in the crust. In particular, as one of the few cases in nature where the immediate cause of an earthquake is apparent, triggering provides a fundamental clue to initiation. I will briefly summarize the recent progress in this field.
U12A-02 INVITED 11:20h
Titan: Explorations of a Frontier with Terrestrial Allusions
Saturn's largest moon, Titan, distinguishes itself from other moons by sporting an atmosphere with ten times the column abundance of Earth's. The moon's atmosphere is also similar to Earth's atmosphere in that it is nitrogen-based, rich in hydrocarbons, and hypothesized have a liquid cycle like the hydrological cycle, with clouds, rain and seas. (However, on Titan methane plays the role of terrestrial water.) Recent ground-based and Cassini observations indicate that large methane cloud systems concentrate at Titan's south pole. Radar images of Titan's surface suggest the presence of surface liquids, and near-IR measurements suggest that Titan's icy bedrock is exposed, despite the expected kilometer of organic photochemically-produced sediments. Nonetheless, at the time of writing this abstract little is known about Titan's lower atmosphere and surface, which are obscured by the thick veil of Titan's stratospheric haze. For example, we do not know the methane humidity of the atmosphere, or the surface composition, Nor do we understand the origin and evolution of the atmosphere, or the formation of clouds in an atmosphere with a radiative time constant that exceeds a Titan year by a factor of several. This year Cassini has begun to unveil Titan; it is equipped with an orbiter which began its close flybys of Titan on October 26, 2004, and with a probe which will descend into Titan's atmosphere on January 14 next year. Here I will provide a background to our understanding of Titan's atmosphere and surface, and underscore key open questions in the field. Frequent comparisons will be made between the structures of Titan's and Earth's lower atmospheres.