Chapter 5: Earthquakes and Earth's Interior
Seismographs/Seismometers
An instrument designed to amplify ground motion in order to detect very weak earthquakes or a great earthquake that has occurred in another part of the world. Has a weight freely suspended from a support that is securely attached to bedrock. When vibrations from an earthquake reach the instrument, the inertia of the weight keeps it relatively stationary while Earth and the support move.
Richter scale
Calculated by measuring the amplitude of the largest seismic wave recorded on a seismogram.
Tsunami
Caused by major undersea earthquakes that set in motion a series of large ocean waves that are known by this Japanese name.
Modified Mercalli Intensity Scale
Developed using California buildings as its standard. Based on the 12-point Mercalli Intensity scale, an area in which some well-built wood structures and most masonry buildings are destroyed by an earthquake would be assigned a Roman numeral X.
Intraplate Earthquakes
Earthquakes that do not take place along traditional mid-ocean ridges, fracture zones, or ocean trenches but rather in the middle of plates. Troublesome earthquakes.
Precursors
Events or changes that precede an earthquake and thus might provide a warning.
Moment magnitude
Favored scale for measuring medium and large earthquakes, which estimates the total energy released during an earthquake.
Megathrust fault
Forms when convergence entails the subduction of oceanic lithosphere under another plate, and the area of contact between the two plates forms an extensive fault zone, that can be several thousand kilometers long.
Earthquake
Ground shaking caused by the sudden and rapid movement of one block of rock slipping past another along fractures in Earth's crust, called faults.
Fracking
Injection of water, sand, and chemicals down and across into horizontally drilled wells, so that natural gas can flow up into the well.
NEPEC
National Earthquake Prediction Evaluation Council
Inertia
Objects at rest tend to stay at rest, and objects in motion tend to remain in motion, unless acted upon by an outside force.
Seismic gap
A segment that has not experienced an earthquake within the past one to several centuries.
Earthquake Prediction
A short-term (hours to days) statement that an earthquake will occur at a given location.
Hypocenter
The location where slippage begins is called the _____________ , or focus.
Liquefaction
The phenomenon of transforming a somewhat stable soil into mobile material capable of rising toward Earth's surface.
Epicenter
The point on Earth's surface directly above the hypocenter is called the _______________ .
Seismograms
The records obtained by seismographs that provide useful information about the nature of seismic waves. Reveal that two main types of seismic waves are generated by the slippage of a rock mass.
Seismology
The study of earthquake waves.
Paleoseismology
The study of the timing, location, and size of prehistoric earthquakes.
Circum-Pacific belt
The zone of greatest seismic activity which encompasses the coastal regions of Chile, Central America, Indonesia, Japan, and Alaska, including the Aleutian Islands.
Secondary waves
These waves shake the particles at right angles to their direction of travel. These waves change the shape of the material that transmits them. Liquids and gasses do not transmit these waves.
Magnitude (scales)
Use data from seismographs to estimate the amount of energy released at an earthquake's source.
Intensity (scales)
Uses observed property damage to estimate the amount of ground shaking at a particular location.
Surface waves
Waves that travel in the rock layers just below Earth's surface. There are two types of surface waves. One causes surface and anything resting on it to move up and down and the second causes Earth's surface to move from side to side.
Body waves
Waves that travel through Earth's interior. Further divided into two types - primary waves (P Waves) and secondary waves (S Waves) which are identified by their mode of travel through intervening materials.
Earthquake Hazard Analysis
What is the probability that a given amount of ground shaking will occur during a given period of time at some location?
Elastic rebound
When slippage allows the deformed (bent) rock to "snap back" to its original, stress-free, shape; a series of earthquake waves radiate outward as it slides.
Earthquake Forecasting
A long-term (years to decades) statement of the probability of an earthquake in a region (or the probability of one or more earthquakes in a region).
Probability
A number expressing the likelihood that a specific event will occur, expressed as the ratio of the number of actual occurrences to the number of possible occurrences.
Seismic waves
A form of energy that travels through the lithosphere and Earth's interior. The energy carried by these waves causes the material that transmits them to shake.
Primary waves
Push/pull waves; they momentarily push (compress) and pull (stretch) rocks in the direction the waves are traveling. This wave motion is similar to that generated by striking a drum, which moves air back and forth to create sound. Solids, liquids, and gasses resit stresses that change their volume when compressed and, therefore, elastically spring back once the stress is removed. These waves can travel through all these materials.
Pressure Axis
Reflects the maximum compressive stress direction.
Tension Axis
Reflects the minimum compressive stress direction.
Locked Faults
Result when the confining pressure exerted by the overlying crust is enormous, causing these fractures in the crust to be "squeezed shut".
Fault creep
Slow, gradual displacement, exhibited by some sections of the San Andreas fault that produce little seismic shaking.
Foreshocks
Small earthquakes that often, but not always, precede major earthquakes by days or, in some cases, several years.
Aftershocks
Strong earthquakes are followed by numerous earthquakes of lesser magnitude, called ____________ , which result from crust along the fault surface adjusting to the displacement caused by the main shock.
