Book contents
- Frontmatter
- Contents
- Preface
- I Background and introductory material
- 1 The big, the bad, and the curious
- 2 What, where, how, and why?
- 3 The history of deep earthquakes
- II Properties of intermediate- and deep-focus earthquakes
- III The mechanism of deep earthquakes
- IV Why bother about deep earthquakes?
- V Geographic summary
- Earthquake index
- Index
1 - The big, the bad, and the curious
Published online by Cambridge University Press: 05 August 2013
- Frontmatter
- Contents
- Preface
- I Background and introductory material
- 1 The big, the bad, and the curious
- 2 What, where, how, and why?
- 3 The history of deep earthquakes
- II Properties of intermediate- and deep-focus earthquakes
- III The mechanism of deep earthquakes
- IV Why bother about deep earthquakes?
- V Geographic summary
- Earthquake index
- Index
Summary
If you asked most seismologists for a brief summary of what they know about intermediate- and deep-focus earthquakes, they might write something like:
Although nearly all of the world's earthquakes occur within the crust, there are a few with foci in the mantle having depths between 60 and 700 km. These so-called “deep earthquakes” occur near deep-ocean trenches within planar groups of hypocenters called Wadati–Benioff zones. In comparison with shallow-focus earthquakes, deep earthquakes tend to be smaller in size, have higher stress drops, more impulsive source-time functions, and few or no aftershocks. Generally, deep earthquakes aren't destructive because of their small size, distance from the surface, and tendency to occur in oceanic regions.
Unfortunately, most of the assertions in the above statement aren't strictly true; i.e., some are never true, some are true only sometimes, and some we aren't yet sure about. For example, in global earthquake catalogs about 25% of all earthquakes have reported focal depths exceeding 60 km (Table 1.1); the 75% that are shallower are hardly “nearly all.” Moreover, even for very large earthquakes with MW or mB larger than 8.0, those with focal depths of 60 km and more make up about a quarter of all events. Moreover, we shall see that not all deep earthquakes are associated with oceanic trenches; many important deep quakes occur as isolated events and not within planar groups. Whether deep quakes have higher stress drops and more impulsive source time functions is subject to debate.
- Type
- Chapter
- Information
- Deep Earthquakes , pp. 3 - 29Publisher: Cambridge University PressPrint publication year: 2006