Eos, Vol. 89, No. 34, 19 August 2008
MEETING Understanding Slow Slip, Tremor, and Quakes Aseismic Slip, Tremor, and Earthquakes Workshop; Sidney, British Columbia, Canada, 25–28 February 2008 PAGE 315 The frequent occurrence and importance of slow slip and tremor have only recently become apparent, owing largely to advances in seismic and geodetic monitoring. At some plate boundaries, slow slip relaxes a significant fraction of accumulated tectonic stress. The radiation of seismic waves as tremor, which often accompanies the geodetically detected slow slip, arises from underlying physical processes that are poorly understood but appear to differ from those governing earthquake- generated waves. To further understanding of aseismic slip, tremor, and earthquakes, more than 52 participants contributed to a workshop with the goals of improving research coordination, assessing the earthquake hazard implications, and identifying ways to capitalize on the education and outreach opportunities these phenomena present. Several potentially significant relationships among slow slip, tremor, and earthquakes emerged at this workshop. Slow slip and tremor appear to delineate the downdip limit of the locked zone, as indicated by megathrust rupture models in Japan and Alaska. Analyses of both steady plate motion and slow slip events from New Zealand map the locked por-
tions of the subduction interface with unprecedented resolution. In the Cascade Mountains, slow slip and tremor appear anticorrelated with crustal earthquakes, while in Hawaii and New Zealand slow slip is not accompanied by tremor but instead by increased seismicity rates. These variations within and between regions provide clues about the properties of faults in different environments, including those capable of producing damaging earthquakes. A consensus also emerged at the workshop that low effective stress prevails where tremor and slow slip occur. Passing surface waves and tidal forces can modulate tremor, and thermal and petrologic models and tomographic images indicate the presence of fluids in tremor and slow slip source regions. Theoretical frictional models require low effective stress to simulate slow slip observations. Frictional models that incorporate high fluid pressures and dilatant strengthening may explain the unique amplitude- duration scaling of slow slip. The practical implications of any direct causal relationship between slow slip, tremor, and damaging earthquakes remain unclear. While slow slip events transfer stress to the base of the locked zone,
book review Lagrangian Analysis and Prediction of Coastal Ocean Dynamics Annalisa Griffa, A. D. Kirwan Jr., Arthur J. Mariano,Tamay Özgökmen, and H.Thomas Rossby, Editors Cambridge University Press; 2007; 500 pp.; ISBN 978-0-521-87018-4; $170. PAGES 315–316 The basic governing equations of geophysical fluid dynamics have been known for many years, but how meaningful these equations are to our understanding of coastal and open oceans depends crucially on the accuracy of constitutive equations
that relate the internal and external forces acting on these bodies of water. Field measurements, whether Eulerian or Lagrangian, have played and continue to play a central role in two important aspects of geophysical fluid dynamics. First, the data collected often point to the presence of various stable and coherent structures, eddies, or streams,
thereby raising conditional weekly probabilities, the latter are still below thresholds that might motivate societal action. No observations to date confirm that large earthquakes can be triggered by slow slip. Nonetheless, the possibility of causal connections warrants discussions between the research community and emergency managers, as well as planning across agency and geographic boundaries. The frequency and regular recurrence of some slow slip and tremor also present the opportunity to share the excitement and enthusiasm that come with anticipated discovery, particularly in Cascadia, where pronounced episodes occur somewhere in the region approximately every 3 months. Conference attendees focused their recommendations for future studies and instrumentation deployment on areas where they expect great earthquakes and on broadening the range of tectonic environments examined for slow slip and tremor. This will entail improved monitoring of major fault zones globally, with continuous recording of seismic and geodetic data. The workshop was sponsored by the U.S. Geological Survey, the U.S. National Science Foundation’s Earthscope Program and University NAVSTAR Consortium (UNAVCO), and the Geological Survey of Canada. Abstracts, presentations, and other workshop information are posted at http://www .earthscope.org/science/cascadia. —JOAN GOMBERG, U.S. Geological Survey, University of Washington, Seattle; E-mail: gomberg@ usgs.gov; ANNE M. TREHU, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis; EVELYN ROELOFFS, Cascades Volcano Observatory, U.S. Geological Survey, Vancouver, Wash.; HERB DRAGERT, Pacific Geoscience Center, Geological Survey of Canada, Sidney, British Columbia, Canada
whose study naturally becomes the focus of analytical and computational efforts. Second, the availability of good data serves to calibrate how the response of bodies of water to forces should be modeled. This book provides a substantial collection of well-written articles on how Lagrangian data are collected, analyzed, and eventually assimilated into models. The book consists of 13 chapters written by more than 40 contributors with backgrounds in physical oceanography, biology, and mathematics, and with expertise in experimentation, theory, and computation. The majority of the contributors have been working closely together since the initial Lagrangian Analysis and Prediction of Coastal Ocean Dynamics (LAPCOD) meeting, held in October 2000 in Ischia, Italy. Some of the chapters are reviews by LAPCOD researchers providing comprehensive summaries of their efforts in the past 10 or so years. Other chapters include tech-
Eos, Vol. 89, No. 34, 19 August 2008 nical content, introducing approaches intended to address challenges formulated by various members of the LAPCOD community. Several chapters deserve special mention. In chapter 1 (“Evolution of Lagrangian methods in oceanography”), T. Rossby gives a historical account of the advent of floats and drifters, which, as a nonexpert, I found enormously instructive as well as inspiring. Rossby’s account is a vivid reminder of the truly ingenious techniques used by so many pioneering oceanographers who made today’s data collection a reality. This chapter includes a number of photographs (some of which are reproduced in color plates elsewhere in the book) of floats and drifters, and of their inventors, in action. The next two chapters focus on the characteristics of a variety of drifters (including the Surface Velocity Program drifters) and on a collection of favorite trajectories observed by LAPCOD contributors. The middle chapters introduce a series of technical mathematical issues. Chapter 4, “Particle motion in a sea of eddies,” looks at how Lagrangian data may reveal the presence of coherent vortices and how these eddies affect the dynamics of turbulent oceans. This chapter is followed by “Predictability of Lagrangian motion in the upper ocean” (chapter 6). Here Lagrangian stochastic models (LSM) are the key tool; in
Correction PAGE 314 In the article “Exploration Station Brings AGU Science to Children and Parents,” published in the 5 August 2008 issue of Eos (89(32), 293), one of the science event’s cosponsors should have been correctly identified as the American Astronomical Society. Eos regrets this error.
an LSM, one views the velocity of a fluid particle as a large-scale deterministic component and models its fluctuations by a stochastic differential equation. Chapters 7 and 8, “Lagrangian data assimilation in general ocean circulation models” and “Dynamic consistency and Lagrangian data in oceanography: Mapping, assimilation and optimization schemes,” respectively, continue to take up the mathematical challenges faced when trying to assimilate Lagrangian data into models, especially when the models have an inherently Eulerian basis. These challenges, which are currently under intense investigation in the mathematics community, aim to apply stochastic and optimization methods to allow for combining Eulerian state vectors with Lagrangian drifter coordinates. These mathematical challenges are further developed in chapter 9 (“Observing turbulence regimes and Lagrangian dispersal properties in the ocean,” by V. Rupolo), which presents in detail the analysis of several velocity data sets obtained in the Atlantic and Pacific oceans. Chapters 10–12 are dedicated to ocean biology. In these chapters, the connection between having knowledge about Lagrangian trajectories and gleaning information about the biophysical dynamics and lifespan of planktons is made abundantly clear. While all chapters in this book have excellent reference sections, these three
chapters are particularly distinguished by providing the interested reader—especially a starting graduate student in search of a thesis topic—with an exquisite account of current literature on the subject. In the last chapter, chapter 13, A. Mariano and E. Ryan summarize the LAPCOD group’s findings in the three LAPCOD meetings that have taken place since 2000. What is striking about this chapter is its connectivity with the book’s first chapter: The authors make a special effort to explain how the data collected by the various floats described in chapter 1 affect our understanding of currents and basins. Equally significant, the authors introduce a collection of methods and tools that rely on analyzing data to address estimating the mean flow, the influence of topography on ocean motion, and dispersion and mixing, as well as address biological applications. The material presented in this book should be eminently accessible to a large number of readers, including graduate students and advanced undergraduates with a reasonable background in analysis and statistics. Because of the significance of the subject matter and the clarity of the exposition, this book likely will become a standard reference in the near future. —REZA MALEK-MADANI, Department of Mathematics, U.S. Naval Academy, Annapolis, Md.; E-mail:
[email protected]
