Seismic Waves

Large strain energy released during an earthquake travels as seismic waves in all directions through the Earth’s layers, reflecting and refracting at each interface. These waves are of two types body waves and surface waves; the latter are restricted to near the Earth’s surface. Body waves consist of Primary Waves (P-waves)and Secondary Waves (S-waves), and surface waves consist of Love waves andRayleigh waves. Under P waves, material particles undergo extensional and compressional strains along direction of energy transmission, but under S-waves, oscillate at right angles to it Love waves cause surface motions similar to that by S waves, but with no vertical component. Rayleigh wave makes a material particle oscillate in an elliptic path in the vertical plane (with horizontal motion along direction of energy transmission).

P-waves are fastest, followed in sequence by S Love and Rayleigh waves. For example, in granites, P- and S-waves have speeds ~4.8 km/sec and 3.0km/sec, respectively. S-waves do not travel through liquids. S-waves in association with effects of Love waves cause maximum damage to structures by their racking motion on the surface in both vertical and horizontal directions. When P- and S-waves reach the Earth’s surface, most of their energy is reflected back. Some of this energy is returned back to the surface by reflections at different layers of soil and rock. Shaking is more severe (about twice as much) at the Earth’s surface than at substantial depths. This is often the basis for designing structures buried underground for smaller levels of acceleration than those above the ground.

Measuring Instruments

The instrument that measures earthquake shaking, a seismograph, has three components – the sensor, the recorder and the timer. The principle on which it works is simple and is explicitly reflected in the early seismograph a pen attached at the tip of an oscillating simple pendulum (a mass hung by a string from a support) marks on a chart paper that is held on a drum rotating at a constant speed. A magnet around the string provides required damping to control the amplitude of oscillations. The pendulum mass, string, magnet and support together constitute thesensor; the drum, pen and chart paper constitute the recorder; and the motor that rotates the drum at constant speed forms the timer.

One such instrument is required in each of the two orthogonal horizontal directions. Of course, for measuring vertical oscillations, the string pendulum is replaced with a spring pendulum oscillating about a fulcrum. Some instruments do not have a timer device the drum holding the chart paper does not rotate). Such instruments provide only the maximum extent (or scope) of motion during the earthquake; for this reason they are called seismoscopes.

The analog instruments have evolved over time, but today, digital instruments using modern computer technology are more commonly used. The digital instrument records the ground motion on the memory of the microprocessor that is in-built in the instrument.

Strong Ground Motions

Shaking of ground on the Earth’s surface is a net consequence of motions caused by seismic waves generated by energy release at each material point within the three-dimensional volume that ruptures at the fault. These waves arrive at various instants of time, have different amplitudes and carry different levels of energy. Thus, the motion at any site on ground is random in nature with its amplitude and direction varying randomly with time.

Large earthquakes at great distances can produce weak motions that may not damage structures or even be felt by humans. But, sensitive instruments can record these. This makes it possible to locate distant earthquakes. However, from engineering viewpoint, strong motions that can possibly damage structures are of interest. This can happen with earthquakes in the vicinity or even with large earthquakes at reasonable medium to large distances.

Characteristics of Strong Ground Motions

The motion of the ground can be described in terms of displacement, velocity or acceleration. The variation of ground acceleration with time recorded at a point on ground during an earthquake is called an accelerogram. The nature of accelerograms may vary depending on energy released at source, type of slip at fault rupture, geology along the travel path from fault rupture to the Earth’s surface, and local soil They carry distinct information regarding ground shaking; peak amplitude, duration of strong shaking, frequency content (e.g., amplitude of shaking associated with each frequency) and energy content (i.e., energy carried by ground shaking at each frequency) are often used to distinguish them.

Peak amplitude (peak ground acceleration, PGA) is physically intuitive. For instance, a horizontal PGA value of 0.6g (= 0.6 times the acceleration due to gravity) suggests that the movement of the ground can cause a maximum horizontal force on a rigid structure equal to 60% of its weight. In a rigid structure, all points in it move with the ground by the same amount, and hence experience the same maximum acceleration of PGA. Horizontal PGA values greater than 1.0g were recorded during the 1994 Northridge Earthquake in USA. Usually, strong ground motions carry significant energy associated with shaking of frequencies in the range 0.03-30Hz (i.e., cycles per sec).

Generally, the maximum amplitudes of horizontal motions in the two orthogonal directions are about the same. However, the maximum amplitude in the vertical direction is usually less than that in the horizontal direction. In design codes, the vertical design acceleration is taken as  to  of the horizontal design acceleration. In contrast, the maximum horizontal and vertical ground accelerations in the vicinityof the fault rupture do not seem to have such a correlation.