NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by the National Science Foundation (NSF) under Contract No. EAR-0126428. The opinions, findings, conclusions, and recommendations expressed herein are those of the authors and do not necessarily reflect the views of the National Science Foundation.

Cover: Both images (SRTM image of fault-bounded Antelope Valley, CA, with Landsat overlay; and fully-lit composite hemisphere image showing North America) are non-copyrighted, courtesy of National Aeronautics and Space Administration.