Chapter 11 Flashcards
Define the term earthquake.
An earthquake is the shaking caused by the release of energy that takes place when rocks under stress within Earth break and then the two sides slide past each other.
How does elastic rebound theory help to explain how earthquakes happen?
Rocks under stress will deform elastically until they reach the point where the stored elastic energy exceeds the rock strength. At that point the rock breaks and an earthquake is produced.
What is a rupture surface, and how does the area of a rupture surface relate to earthquake magnitude?
The rupture surface is the surface over which there is displacement of rock during an earthquake. The magnitude of an earthquake is proportional to the area of the rupture surface and the average amount of displacement over that surface.
What is an aftershock and what is the relationship between aftershocks and stress transfer?
An aftershock is any earthquake that is considered to have been caused by a previous earthquake as a result of the transfer of stress from the original earthquake.
Episodic slip on the middle part of the Cascadia subduction zone is thought to result in an increase in the stress on the upper part where large earthquakes take place. Why?
Episodic slip on the middle part of the Cascadia subduction zone decreases stress within that area, but some of that stress is transferred to the locked zone up dip along the plate boundary, there increasing the level of stress on the locked part.
Explain the difference between magnitude and intensity as expressions of the size of an earthquake.
Magnitude is the amount of energy released by an earthquake. Each earthquake has only one magnitude, although there are different ways of measuring it, and they may give slightly different results. Intensity is a measure of the amount of damage done or what people felt. Intensity varies depending on the distance to the epicentre and the type of rock or sediment underlying an area.
How much more energy is released by an M7.3 earthquake compared with an M5.3 earthquake?
An M7.3 earthquake releases 1,024 times as much energy as an M5.3 earthquake.
Earthquakes are relatively common along the mid-ocean ridges. At what type of plate boundary do most such quakes occur?
Both divergent and transform boundaries are associated with mid-ocean ridges. Most earthquakes take place on the transform boundaries.
The northward motion of the Pacific Plate relative to the North America Plate takes place along two major transform faults. What are they called?
The northward motion of the Pacific Plate relative to the North America Plate takes place along the San Andreas Fault in California and along the Queen Charlotte Fault off the coast of British Columbia and southern Alaska.
Why is earthquake damage likely to be more severe for buildings built on unconsolidated sediments as opposed to solid rock?
Unconsolidated sediments, especially if they are saturated with water, can lose strength when subjected to earthquake shaking. This can cause buildings to subside or tilt. Unconsolidated sediments can also amplify the vibrations of an earthquake.
Why are fires common during earthquakes?
Gas lines and electrical transmission wires are typically damaged during an earthquake, and this can lead to serious fires.
What type of earthquake is likely to lead to a tsunami?
A large subduction earthquake (greater than M7.5) can generate a tsunami because they typically result in vertical displacement of the sea floor.
What did we learn about earthquake prediction from the 2004 Parkfield earthquake?
The 2004 Parkfield earthquake showed that we cannot rely on foreshocks to predict earthquakes, or on any of the many other parameters that were being carefully measured around Parkfield in the years leading up to the quake.
What are some of the things we should know about an area in order to help minimize the impacts of an earthquake?
We should know about the history of past large earthquakes, the typical locations of small earthquakes, the types of geological materials beneath the surface (especially soft water-saturated sediments), the types of infrastructure that is present, and the various ways that people can be evacuated from an area or assistance can be brought in.
What is the difference between earthquake prediction and forecasting?
Forecasting involves estimating the risk of an earthquake happening in a region within a period of time (usually expressed in decades). Prediction involves stating that an earthquake is likely to happen at a certain location on a specific day or month or year in the future. With our current state of knowledge of earthquakes, prediction is not possible.
