Physics of Earthquakes

During an earthquake, one side of the fault moves suddenly with respect to the other. This process radiates energy as seismic waves, and generates heat due to friction and other non-linear processes. The study of the recent deep Bolivian earthquake suggests that only 4 % of the total strain energy was released as seismic waves, and most of the energy was converted to heat. The total amount of heat energy released is 1 to 10 times more than that released during the major volcanic eruptions such as the 1980 Mt St Helens eruption. This result suggested that melting can be an important mechanism for promoting seismic slip, especially for deep-focus earthquakes.

With the advent of modern broad-band seismic networks we can study seismic wave radiation in detail, from which we can understand how an earthquake nucleates, ruptures, and stops. The goal is to understand the deterministic as well as "complex" aspect of earthquake physics.

• Kanamori, H., and M. Kikuchi, The 1992 Nicaragua Earthquake: a slow tsunami earthquake associated with subducted sediments, Nature, 361, 714-716, 1993.
• Kikuchi, M., and H. Kanamori, The mechanism of the deep Bolivia earthquake of June 9, 1994, Geophys. Res. Lett, 21, 2341-2344, 1994.
• Kanamori, H., D. L. Anderson, and T. H. Heaton, Frictional melting during the rupture of the 1994 Bolivian earthquake, Science, 1998.

Interaction of Atmosphere and Lithosphere

The earth's surface is covered by atmosphere, and energy coupling occurs between atmosphere and lithosphere. For example, during the major eruption of Mount Pinatubo in the Philippines on June 15, 1991, an unusually long (at least two hours) seismic wave train having periods of about 230 sec was recorded at seismic stations throughout the world. This oscillation has a very sharp spectral peak at a period of 228 sec. We found that this wave train is the seismic Rayleigh wave excited by atmospheric oscillations near the volcano that were set off by continuous thermal energy flux from the volcano. This study demonstrated that modern seismological networks can be used to study the physics of volcanic eruptions; it provides new information about how volcanoes affect the Earth's atmosphere and a way to quantify volcanic eruptions. Study of acoustic and gravity waves has interesting applications for understanding unusual waves excited by space shuttles and the impact of the Shoemaker-Levy comet.

• Kanamori, H., J. Mori, D. Anderson, and T. Heaton, Seismic Excitation by the Space Shuttle Columbia, Nature, 349, 781-782, 1991.
• Kanamori, H., and J. Mori, Harmonic Excitation of Mantle Rayleigh Waves by the 1991 eruption of Mount Pinatubo, Philippines, Geophys. Res. Lett., 19, 721-724, 1992.
• Kanamori, H., Excitation of Jovian normal modes by an impact source, Geophys. Res. Lett., 20, 2921-2924, 1993.
• Ingersoll, A. P., and H. Kanamori, Waves from the collisions of Shoemaker-Levy 9 with Jupiter, Nature, 374, 706-708, 1994.
• Kanamori, H., J. Mori, and D. G. Harkrider, Excitation of atmospheric oscillations by volcanic eruptions, J. Geophys. Res., 99, 21,947-21,961, 1994.

Application of Real-Time Seismology to Hazard Mitigation

Advances in seismic sensor and data acquisition systems, digital communications, and computers make it possible to build reliable real-time earthquake information systems. Such systems provide a means for modern urban regions to cope effectively with the aftermath of major earthquakes. While accurate earthquake predictions are difficult, these systems aid in the post-earthquake response and recovery phases, and thus are socially beneficial in modern industrialized urban and suburban regions. In the long term these systems also provide basic data for mitigation strategies such as improved building codes. We are developing a modern seismic network, TriNet, to accomplish this goal. TriNet is a joint project between Caltech, U.S. Geological Survey, and the State of California.

• Kanamori, H., Locating Earthquakes with Amplitude: Application to Real- Time Seismology, Bull. Seismol. Soc. Am., 83, 264-268, 1993.
• Kanamori, H., E. Hauksson, and T. Heaton, Real-time seismology and earthquake hazard mitigation, Nature 390, 461-464, 1997.

Physics of Long-Term Crustal Processes Associated with Earthquakes

The physical properties of the Earth's crust are likely to change as a function of time. However, physics of such changes is not well understood. For example, after the 1992 Landers, California, earthquake, seismic activity in many places in California increased significantly. One interpretation is that the strength of crust was suddenly reduced when it was shaken by passing seismic waves from the Landers earthquake. We have explored a mechanism for such weakening. Although this field is at an early stage of development, a better understanding of the physics of fluid-filled crust would lead to unraveling many interesting, but mysterious, phenomena associated with earthquakes. These phenomena include changes in seismicity, seismicity patterns, electric-magnetic field disturbances, and changes in ground-water level and chemistry.

• Sturtevant, B., H. Kanamori, and E. Brodsky, Seismic triggering by rectified diffusion in geothermal systems, J. Geophys. Res. 101, 25269, 1996.