The Very Long Reach of Very Large Earthquakes
Dr. Susan Hough, U.S. Geological Survey

When the magnitude 7.3 Landers earthquake struck in 1992 in the desert north of Palm Springs, the earthquake map of the state of California lit up like a Christmas tree. These unprecedented observations led scientists to the discovery of remotely triggered earthquakes—earthquakes that follow large earthquakes but happen at much greater distances than the nearby aftershocks that are almost always associated with a big earthquake. This voyage of scientific discovery of remotely triggered earthquakes is part of a fundamental change in the way that scientists view earthquakes. Once thought to be isolated in both time and space, the “reach” of large earthquakes—their impact on not only the surrounding region but also the planet as a whole—is now known to be far longer, and far more interesting, than scientists realized just a few years ago.

Early studies linked the occurrence of these events to the stress changes caused by passing seismic waves. Scientists suggested the analogy of “shaking a soda can” to explain how earthquake waves might raise pressure in underground fluids, and thereby trigger other earthquakes. The initial view of remotely triggered earthquakes has expanded since the early 1990s as scientists have learned more about where and why these earthquakes happen. Looking back at accounts of earthquakes that struck before seismometers were invented, one finds compelling evidence that remotely triggered earthquakes occurred in the past, for example during the 1811-1812 New Madrid earthquake sequence in Missouri. The largest historic earthquake in New England may have also been triggered by the great Lisbon earthquake of 1755. These studies reveal that remotely triggered earthquakes occur commonly, and not just in volcanic regions where they were first observed—a result that requires earlier theories to be revised, or at the very least, expanded.

In California, a new hypothesis further suggests that large earthquakes may have had a very long reach through time as well as space, in the form of petroglyphs that may in fact chronicle some of California’s pre-historic earthquakes and volcanic events. Thus have scientists arrived at new paradigms for not only “earthquake interactions”—the interaction of earthquakes with each other—but also the interaction of earthquakes with human cultures, past and present.

About Dr. Hough

Education

Ph.D., Scripps Institution of Oceanography, UC San Diego, Earth Sciences, 1987
A.B., University of California, Berkeley, Geophysics, with honors, 1982

Professional Positions

Seismologist, Branch of Seismology, U.S. Geological Survey, Pasadena, California, 1992-Present
Editor in Chief, Seismological Research Letters, 2001-Present

Dr. Susan Hough is currently a seismologist with the U.S. Geological Survey in Pasadena and Editor-in-Chief of Seismological Research Letters. After completing her Ph.D. in 1987, she worked at Lamont-Doherty Earth Observatory of Columbia University in New York for 4 years before moving back to California. Her research interests include the nature of ground shaking produced by large earthquakes, historic earthquakes (the 1811—1812 New Madrid earthquakes in particular), earthquakes and earthquake hazard in India, and remotely triggered earthquakes.

Dr. Hough has published two books on earthquake science for a non-specialist audience: Earthshaking Science: What we know (and don’t know) about earthquakes, and Finding Fault in California: An earthquake tourist’s guide. A third book, Elastic Rebound: Past and future earthquakes on an urban planet (Susan Hough and Roger Bilham, a previous IRIS/SSA distinguished lecturer), is scheduled for publication in 2005. She has additionally published several feature articles in Natural History Magazine and American Scientist.

Books

Earthshaking Science: What we know (and don’t know) about earthquakes, Princeton University Press, 272 pp, 2002.

Finding Fault in California: An earthquake tourist’s guide, Mountain Press Publishers, 268 pp, 2004.

Elastic Rebound: Past and future earthquakes on an urban planet (Susan Hough and Roger Bilham), Oxford Press, expected publication date 2005.

Selected General Interest Publications

Hough, S.E., The aftershocks that weren't, Natural History Magazine, 64-69, March, 2001.

Hough, S.E. and R. Bilham, Shaken to the core, Natural History Magazine, 42- 48, February, 2003.

Ben-Avraham, Z. and S.E. Hough, Promised Land, Natural History Magazine, 44- 49, October, 2003.

Hough, S.E., Writing on the Walls, American Scientist, in press, August, 2004.

Selected Peer-Reviewed Publications

Hough, S.E., R.D. Borcherdt, P.A. Friberg, R. Busby, E.H. Field, and K.H. Jacob. Sediment-induced amplification and the collapse of the Nimitz Freeway, Nature, 344, 853-855, 1990.

Hough, S.E., Earthquakes in the Los Angeles Metropolitan region: A possible fractal distribution of rupture size, Science, 267, 211-213, 1995.

Hough, S.E., J.G. Armbruster, L. Seeber, and J.F. Hough, On the modified Mercalli intensities and magnitudes of the 1811-1812 New Madrid earthquakes, J. Geophysical Research 105, 23,839-23864, 2000.

Hough, S.E., Triggered earthquakes and the 1811-1812 New Madrid, central U.S. earthquake sequence, Bulletin Seismological Society America, 91, 1574- 1581, 2001.

Hough, S.E. and H. Kanamori, Source properties of earthquakes near the Salton Sea triggered by the 10/16/1999 M7.1 Hector Mine earthquake, Bull. Seismological Society of America, 92, 1281-1289, 2002.

Mueller, K., S.E. Hough, and R. Bilham, Analysing the 1811-1812 New Madrid earthquakes with recent instrumentally recorded aftershocks, Nature, 429, 284- 288, 2004.