February 28, 2011

Earthquake Basics

Posted by Austin Elliott

Before diving into the wealth of fascinating earthquake phenomena I plan to share with you, I’ll give an extremely brief basic overview of what earthquakes are and why they happen, so that we can all be on the same page.

Let’s start with the big picture: The Earth is a big rocky (and rather metallic) body, like a number of others in our solar system. Within this big orb of solid rock (and both liquid and solid metal), temperature and pressure generally increase with depth toward the center, partially because the original heat from Earth’s formation is insulated within its own massive rocky body.

Solid materials behave differently as their temperatures increase, becoming more pliable with increasing heat. The outside of our rocky orb is cold, crisp, and brittle–this is the Earth’s cracking crust, which we all are the most familiar with because it’s what we live on. Deeper within the Earth, the solid rock is hot enough that over huge distances and long spans of time it can be deformed like silly putty or very thick syrup. As heat from the center of the Earth radiates outward, the solid rocky “mantle” convects like a lava lamp. Heated material is less dense than cold material, so it rises, and vice versa. Over hundreds of millions of years the solid interior of the Earth churns, dragging the cold brittle crust with it. Imagine re-boiling a pot of stew that has formed a cooling skin. (Although the specifics of the manner in which the crust and mantle interact are the subject of ongoing research and controversy, that relationship is beyond the scope of this blog. For our purposes lateral motion of the crust is inherently related to convective motion of the mantle beneath it.)

The mantle’s churning and dragging breaks the brittle crust up into small pieces that grind together, spread apart, or override each other. These “small pieces” are the infamous tectonic plates, atop one [or two] of which you live. While at their centers the tectonic plates are all moving in different directions from each other, their edges are pressed together by the immense weight of rock and so they stick together in the same way that a dresser “sticks to” a carpeted floor, making harder to drag than to lift it up and transport free of carpeted friction. Eventually, of course, the centers of the plates have moved far enough relative to one another that their edges need to catch up. That’s when earthquakes happen.

The cold brittle crust is elastic, meaning that–just like your underwear waistband–it will deform a finite amount in response to an applied force, but once the force is let go it snaps back into place. So, as the rough edges of tectonic plates are held back by the adjacent plate(s), they can only withstand so much stretching before they have to snap back into shape, which they generally do catastrophically. This “elastic recovery” occurs along faults, giant cracks that penetrate the crust, colloquially known as “earthquake faults” or “fault lines”, a few of which are pictured in this blog’s header.

Plate edges are generally a crumpled, fractured mess, from millennia of being dragged along the ragged edges of their neighbors. Many of these fractures are faults, capable of slipping to produce a “fault rupture”, which radiates giant elastic waves as one side of the crack slides along the other. Imagine snapping a twig or a pencil; it bends before it breaks, but once it cracks you hear a loud snap and feel it sharply jolt your fingers. Those are compressional and elastic waves shooting through the air and the pencil: a tiny little analog to an earthquake, just like static shocks are tiny little analogs to lightning.

Every earthquake represents the elastic waves emanated from a small (or large) patch of a fault rupturing and slipping to recover from built-up strain, which is driven by motion of the brittle tectonic plates over a warm, slowly churning solid mantle. In each rupture, the brittle crust we live on deforms just a bit, but a finite amount. Every quake results in finite deformation, which accumulates over time to shape the surface of the planet. We’ll get to that in a later post. Large earthquakes can move the land surface by tens of meters within just a few seconds, forever altering the landscape. Small earthquakes barely affect the surface, moving a tiny patch of a fault by sometimes microscopic amounts. We’ll get to earthquake sizes and magnitudes some other time as well.

So that was your primer on earthquakes. Comments, questions, or clarifications are welcome.