Department of Mechanical Engineering
PRESENTS
A.J. Rosakis
Graduate Aeronautical Laboratories
ACalifornia Institute of Technology, Pasadena, CA 91125
LABORATORY EARTHQUAKES:
(1) The Sub-Rayleigh to intersonic transition
(2) The influence of fault bends on rupture growth
ABSTRACT
The goal of the present study is to create model laboratory experiments mimicking the dynamic shear rupture process. We hope to use such experiments to observe new physical phenomena and to create benchmark comparisons with existing analysis and numerics and field observations. The experiments use high-speed photography, photoelasticity, and infrared thermography as diagnostics. The fault systems are simulated using two photoelastic plates (Homalite) held together by friction. The far field tectonic loading is simulated by pre-compression and the triggering of dynamic rupture (spontaneous nucleation) is achieved by an exploding wire technique. The fault forms an acute angle with the compression axis to provide the shear driving force necessary for continued rupturing. We investigate the dependence the characteristics of rupturing, such as rupture speed, rupture mode on experimental conditions such as far-field biaxial compression, tilt angle of the fault to the compression axis, as well as on the frictional properties of the fault interface. Results on both homogeneous and bimaterial interfaces are reported. For bimaterial interfaces, various combination of dissimilar materials, including Homalite/polycarbonate pairs, are chosen to mimic wave speed mismatch conditions that are reported to exist across mature, crustal faults.
In the first part of the present lecture we concentrate on the experimental observation of the phenomenon of, spontaneously nucleated, supershear rupture and on the visualization of the mechanics of the Sub-Rayleigh to supershear rupture transition in such frictionally held interfaces. The results suggest that under certain conditions supershear rupture propagation can be facilitated during large earthquakes (e.g. the 2001 central Kunlunshan earthquake in Tibet or the 2002 Denali earthquake in Alaska).
In the second part of this lecture earthquake ruptures are modeled as dynamically propagating shear cracks with the aim of gaining insight into the physical mechanisms governing their arrest or, otherwise, the often observed variations in rupture speeds. Fault bends have been proposed as being a major cause for these variations. Following this line of reasoning, the existence of deviations from fault planarity is embraced as the main focus of this study. Asymmetric impact is used to generate shear loading and to propagate dynamic mode-II cracks along the bonded interfaces of two otherwise identical homogeneous constituents. Secondary planes inclined at various angles are also introduced to represent fault bends or kinks. The experiments show that certain fault bend inclinations are favored as alternate paths for rupture continuation, whereas others suppress further motion of the incoming rupture. The asymptotic elastodynamic stress fields at the tip of the growing rupture are used to develop two criteria for rupture propagation or arrest at the kinked interfaces. These criteria correlate very well with the experimental results.
BIOSKETCH
Professor of Aeronautics and Mechanical Engineering at Caltech. B.A. and a M.A. degree in Engineering Science - Oxford University; Sc.M. and Ph.D. degree in Solid Mechanics - Brown University. Author of over 120 works on quasi-static and dynamic failure of metals, composites, and interfaces with emphasis on dynamic fracture and dynamic localization. Recent interests include shear dominated intersonic rupture of inhomogeneous solids, rupture mechanics of crustal earthquakes, and reliability of thin films. His awards include: IBM Faculty Development Award; NSF Presidential Young Investigator Award; Rudolf Kingslake Medal and Prize from SPIE; Hetenyi, Lazan and Frocht awards from SEM; Excellence in Teaching Award from the Caltech Graduate Student Council. He is a past Chairman of the Fracture & Failure Mechanics Committee of the Applied Mechanics Division, a Fellow of the ASME and the New York Academy of Sciences.
Wednesday, May 19, 2004
Bourns Hall A265
10:10 a.m.-11:00 p.m.
(Refreshments will be served at 10:00 a.m.)
