The physics of earthquakes

Earthquakes occur as a result of global plate motion. However, this simple picture is far from complete. Some plate boundaries glide past each other smoothly, while others are punctuated by catastrophic failures. Some earthquakes stop after only a few hundred metres while others continue rupturing for a thousand kilometres. Earthquakes are sometimes triggered by other large earthquakes thousands of kilometres away. We address these questions by dissecting the observable phenomena and separating out the quantifiable features for comparison across events. We begin with a discussion of stress in the crust followed by an overview of earthquake phenomenology, focusing on the parameters that are readily measured by current seismic techniques. We briefly discuss how these parameters are related to the amplitude and frequencies of the elastic waves measured by seismometers as well as direct geodetic measurements of the Earth's deformation. We then review the major processes thought to be active during the rupture and discuss their relation to the observable parameters. We then take a longer range view by discussing how earthquakes interact as a complex system. Finally, we combine subjects to approach the key issue of earthquake initiation. This concluding discussion will require using the processes introduced in the study of rupture as well as some novel mechanisms. As our observational database improves, our computational ability accelerates and our laboratories become more refined, the next few decades promise to bring more insights on earthquakes and perhaps some answers