The diversity of the physics of earthquakes

Abstract

Earthquakes exhibit diverse characteristics. Most shallow earthquakes are "brittle" in the sense that they excite seismic waves efficiently. However, some earthquakes are slow, as characterized by tsunami earthquakes and even slower events without any obvious seismic radiation. Also, some earthquakes, like the 1994 Bolivian deep earthquake, involved a large amount of fracture and thermal energy and may be more appropriately called a thermal event, rather than an earthquake. Some earthquakes are caused by processes other than faulting, such as landslides. This diversity can be best understood in terms of the difference in the partition of the released potential energy to radiated, fracture, and thermal energies during an earthquake. This approach requires detailed studies on quantification of earthquakes and estimation of various kinds of energies involved in earthquake processes. This paper reviews the progress in this field from historical and personal points of view and discusses its implications for earthquake damage mitigation.Key words: Quantification of earthquakes; energy budget of earthquakes; radiation efficiency; rupture speed; earthquake rupture pattern; earthquake early warning. Reviewamplitude of seismic waves observed on the standard Wood-Anderson seismograph at a distance ∆ from the epicenter. The function f(∆) is determined empirically so that M L = 3 if A is 1 mm at a distance of 100 km. Although this is a purely empirical parameter and cannot be directly related to any specific physical parameter of the earthquake, it proved to be a very useful quantification parameter and has long been used widely not only in California but also worldwide.The scale has been extended so that it can be determined with different types of instruments and different kinds of seismic waves. Yet, most of the scales were still empirical in the sense that they were determined from the observed amplitude and it was difficult to relate them to the physical parameters of the earthquake. Gutenberg and Richter (1942, 1955), Båth (1966) and many others attempted to relate the magnitude to more meaningful seismic source parameters such as the radiated energy, E R . The relation obtained by Gutenberg (1956),where M S is the surface-wave magnitude (the magnitude scale computed from the amplitude of seismic surface waves at a period of about 20 sec) has been used for a long time as a useful relationship, which converts the empirical parameter M S to a physical parameter E R . This practice is meaningful only if the 20 sec wave represents the overall energy spectrum. It turned out that the 20 sec wave represents the energy spectrum reasonably well for earthquakes up to M S = 7.5. For events larger than M S = 7.5, the peak of the energy spectrum is at a period much longer than 20 sec. The 20 sec wave used for M S determination can no longer represent the total energy of the radiated seismic wave; as a result, the M S scale saturates beyond M S = 8. For earthquakes with very large fault areas, for which the total amount of...

Tomography and Dynamics of Western-Pacific Subduction Zones

“…Complex physical and chemical reactions take place in the source zone of future earthquakes, causing heterogeneities in the material properties and stress field, which can be detected using seismic tomography and other geophysical methods. The source zone of an M 6 to 9 earthquake extends from about 10 km to 500 km (Kanamori, 2004). The spatial resolution of recent tomographic images is close to that scale, which has enabled us to image earthquake-related heterogeneities (e.g., earthquake volume) in the crust and uppermost mantle.…”

“…So far, seismologists have been quite successful in using seismic waveforms radiated by a large earthquake to estimate the coseismic slip distribution on the fault plane (see the review by Kanamori, 2004). Although seismic tomography has been applied successfully to various tectonic environments on Earth since the late 1970s, the application of seismic tomography to the study of earthquake faults has not been as successful as for the other tectonic environments.…”

Energy partitioning during an earthquake

“…The width ofthe gouge layer, T, has been measured by various investigators [Robertson, 1982;Otsuki, 1978]. We can estimate the total fracture energy used to form the gouge layer as follows [Kanamori, 2004].…”

A way of estimating the characteristic slip displacement

“…For a Model-III crack, g R is a function of v R =b in the following form (cf. Kanamori and Heaton 2000;Kanamori 2004;Kanamori and Brodsky 2004):…”