Monday, 16 July 2018  


 

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I. Selwyn Sacks

Selwyn Sacks


Selwyn Sacks, a fellow of the American Geophysical Union, has held a number of committee appointments over the years, including chairman of the National Research Council Panel on Real Time Earthquake Warning of the National Academy of Sciences and cochairman of Lithosphere - Asthenosphere Sounding of the International Association of Geomagnetism and Aeronomy.

 

Sacks and collegues

More than 30 years ago, Carnegie's Selwyn Sacks and his colleague Dale Evertson of the University of Texas invented the Sacks-Evertson borehole strainmeter — an extremely sensitive instrument hat, when cemented in place about 200 meters below the Earth's surface, is able to detect minute changes in rock deformation. Borehole trainmeters have been installed worldwide and are advancing our understanding of what small stresses deep inside the Earth can tell us about earthquakes and volcanic eruptions. Sacks (far right) and colleagues are standing at a hole for a strainmeter they installed on Trizonia, an island in the Gulf of Corinth, in 2002.

 

The interaction of the mobile tectonic plates, which make up the Earth's surface, causes stresses that result in deformation and rock failure. When these stresses are released rapidly, earthquakes can result. The stresses then diffuse slowly away from the source. Geophysicist Selwyn Sacks studies the slow deformations that are a result of strain diffusion from large earthquakes, volcanic eruptions, and spreading events at the plate boundaries. Sacks's analyses have helped determine the viscosity of the crust and uppermost mantle for Japan, California, and Iceland. Some of the deformations resulting from great earthquakes can be measured hundreds of kilometers from the source even after a century. In fact, it appears that the present-day strain field cannot be reliably estimated without allowing for the effects of past powerful earthquakes. As an example, Sacks and colleagues showed that the devastating earthquake in Kobe, Japan, in 1995 was probably triggered by strain diffusing from large earthquakes in 1944 and 1946.

Strain pulses are also capable of inhibiting earthquakes in instances where faults have many different orientations. Because of the orientation sensitivity, it is possible to calculate the probability of fault failures in these circumstances. In a region in south central Japan, for instance, Sacks and collaborators found that all earthquakes during the period 1901-1969 occurred where the probability of fault failure was enhanced by strain diffusion that mostly resulted from the 1891 Nobi earthquake. No earthquakes occurred where the strain pulses increased fault clamping.

Although the physics of strain diffusion is reasonably well understood, the physics of slow earthquake rupture is not. Recent observations using highly sensitive borehole strainmeters - instruments coinvented by Selwyn Sacks - enabled the discovery of slow events that were not seen on instrumentation available earlier. Strainmeters can measure tiny deformations within the Earth. Sacks recognized and analyzed the slower events and deformations. He and longtime colleague Alan Linde of DTM discovered that slow events can occur on the same faults that fail rapidly at other times. One of the most dramatic examples of this phenomenon occurs off northeast Japan, where there are subduction events as large as magnitude 8. Most of the plate motion there is released as slow, nondestructive events that could not even be detected until recently. To study this phenomenon more, Sacks and his team installed measuring instruments below the seafloor in 1999.

SELECTED PUBLICATIONS
  • Rydelek, P. A., and I. S. Sacks. 2003. Triggering and inhibition of great Japanese earthquakes: the effect of Nobi 1891 on Tonankai 1944, Nankaido 1946, and Tokai, Earth Planet. Sci. Lett. 206, 289-296.

  • Rydelek, P. A., and I. S. Sacks. 2001. Migration of large earthquakes along the San Jacinto fault; stress diffusion from the 1857 Fort Tejon earthquake, Geophys. Res. Lett. 28, 3079-3082.

  • Sacks, I. S., K. Suyehiro, G. D. Acton, et al. 2000. Western Pacific Geophysical Observatories, Sites 1150 and 1151, Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 186, College Station, ODP/Texas A&M University.

  • Suyehiro, K., I. S. Sacks, G. Acton, and the Leg 186 Scientific Party. 2000. Japan Trench geophysical observatories: ODP Leg 186, JOIDES Journal 26 (no. 1), 10-16.

  • Takanami, T., I. S. Sacks, and A. Hasegawa. 2000. Attenuation structure beneath the volcanic front in northeastern Japan from broad-band seismograms, Phys. Earth Planet. Inter. 121, 339-357.

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