Seismic Waves and the Earth’s Interior Structure Modified from: Walter D. Mooney, USGS Menlo Park, CA.

What is an Earthquake? A sudden movement of the Earth’s crust, usually caused by the release of elastic potential energy stored in rocks. An earthquake is caused by sudden movement on a sub-surface fault. The energy which is released (which was stored as strain in the rock) is converted to seismic waves which radiate from the earthquake focus. These seismic waves cause ground shaking and can be measured using seismometers. Seismogram of the 1906 earthquake recorded in Germany San Francisco 1906 (USGS)

Energy released Seismic waves USGS The energy released comes from elastic potential energy stored in rocks. That energy becomes kinetic energy which travels as waves radiating from the focus of the earthquake. Seismic waves USGS

Types of Seismic Waves There are two types of seismic waves: body waves and surface waves. Body waves travel through the body of the Earth and can be detected by seismometers all over the world. There are two types of body waves: Primary waves or P waves, and Secondary waves or S waves. P waves are faster than S waves The lag time between P and S waves is determined by distance to the epicenter. Two categories of seismic wave exist – body waves (which travel through the entire volume of the Earth) and surface waves (which are restricted to the near-surface). Each category consists of several types of seismic wave which all propagate through the earth with different velocity, amplitude, and particle motion.

Body Waves Primary waves behave exactly like sound waves. They move as pressure waves through the rocks. This kind of motion is possible in liquids and solids because they can both be compressed. These waves move at about 8.1 km per second. Secondary waves move at right angles to the direction of motion. This kind of motion is not possible in liquids, so S waves are blocked by liquids. S waves move at about 4.5 km per second. Body waves are those which travel through the entire volume of the Earth. There are two types: P-waves (Primary, or first arriving) are the quickest and have a compressional particle motion parallel to the direction of travel. S-waves (Shear) are quicker than surface waves, and have a shearing particle motion perpendicular to the direction of travel. The velocity of a P or S wave is related to the bulk or shear moduli (respectively) of the material through which they travel.

Surface Waves There are two types of surface waves. Rayleigh waves and Love waves are not used for epicenter location. They affect the surface region near the epicenter. They are the main cause of earthquake damage. Surface waves are those which are restricted to propagation close to the free surface. There are two main types: Love waves have a shearing particle motion but only in the horizontal plane (parallel to the ground surface). Rayleigh waves have a reverse retrograde ellipse particle motion parallel to the direction of propagation.

Ray theory Seismic waves can be represented as rays This is what we did when we drew straight lines in our paper model of Earth’s interior. Seismic waves travel through the Earth according to the laws of optics.

When waves move from one material into another, their speed changes When waves move from one material into another, their speed changes. This causes them to change direction. q1 q2 Faster q1 slower q2 Slower Faster When velocity increases with depth within the Earth the rays a refracted back towards the Earth’s surface (left hand example), when the velocity decreases within the earth the rays are refracted away from the surface (right hand example).

Ray Paths in a Layered Medium The changes in speed and direction of seismic waves as they move through the body of the Earth can be used to infer the thickness and density of Earth’s internal layers. 1/a1 1/a2 1/a3 Distance Time With several layers of increasing velocity this effect is repeated. The time taken for the ray to travel from the source to the receiver is the sum of the distance traveled in each layer multiplied by the velocity of that layer. If we have a number of recording stations in a simple patch of ground and we plot the time of arrival against the distance from the source we create a plot like the one on the right. Courtesy J. Mori

Structure in the Earth results in complicated paths From these complicated paths, we can reconstruct the depth and thickness of the regions that the waves are moving through. The Earth has a complicated structure. The changes in velocity with depth result in refraction and reflection of the seismic energy. So the ray paths through the Earth are complicated.

Seismic Tomography The use of seismic waves to determine the internal structure of the body of the planet is called seismic tomography. The concept is exactly the same as the use of sound waves to make images of structures inside our own bodies to produce sonograms like this one. P waves are basically sound waves. Copywrite Tasa Graphic Arts

Because the outer core is a liquid, S waves do not propagate through it (this is because a liquid cannot support shearing) and P waves are slower. So the ray that enters the inner core bends away from the surface, deeper into the earth. Courtesy J. Mori

Forward Branch Courtesy J. Mori Steeper incident rays are less affected and so we end up with deeper ray paths arriving nearer the source. Courtesy J. Mori

Forward Branch Shadow Zone Courtesy J. Mori We then have another short forward branch. However, no direct P energy arrives in a zone ~105-~143 degrees from the source – the P wave shadow zone because of the refractive properties of the mantle-core boundary. Courtesy J. Mori

Locating an Earthquake Epicenter Earthquakes begin underground at a point called the focus of the earthquake. The point on the surface directly above the focus is the first to feel the earthquake. This point is the epicenter. The epicenter is the point on the surface from which the earthquake seems to originate. Courtesy J. Mori

Locating an Earthquake Epicenter To find the epicenter, we need to have distance data from at least three different seismograph stations. With this data, we can triangulate to locate the epicenter. Triangulation is how GPS and cell phone tracking work.

Locating an Earthquake Epicenter To calculate distance for one station, we need the lag time between the arrival of the P and S waves. The lag time corresponds to distance from the epicenter. The greater the lag time, the greater the distance. P and S lag time corresponds to distance.

Locating an Earthquake Epicenter Once we know the distance to the epicenter from the seismograph station, we can draw a circle with that radius on a map. The earthquake epicenter is always somewhere on the circle.

Locating an Earthquake Epicenter If we have data from two stations, we know the earthquake epicenter must be at one of two possible locations. The earthquake epicenter must be in both circles at the same time.

Locating an Earthquake Epicenter If we have data from three stations, we know the exact location of the epicenter. This is the only point contained in all three circles.