Earthquake Tip 2: How the Ground Shakes PDF

Learning 2 Earthquake Design Earthquake Tip and...

Learning 2 Earthquake Design Earthquake Tip and Construction How the ground shakes? Seismic Waves Large strain energy released during an earthquake P-Waves Push and pull 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 (Figure 1). Body waves consist of Extension Compression Primary Waves (P-waves) and Secondary Waves (S- S-Waves waves), and surface waves consist of Love waves and Up and down Rayleigh 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 (Figure 2). Love waves cause surface motions similar to that by S-waves, but Side to side with no vertical component. Rayleigh wave makes a Direction of material particle oscillate in an elliptic path in the Energy Transmission vertical plane (with horizontal motion along direction of energy transmission). Love Waves Sideways in horizontal plane Structure Surface Waves Rayleigh Waves Elliptic in vertical plane Soil Body Waves Fault EQ Rupture Geologic Strata Figure 2: Motions caused by Body and Surface Waves (Adapted from FEMA 99, Non-Technical Figure 1: Arrival of Seismic Waves at a Site Explanation of the NEHRP Recommended Provisions) P-waves are fastest, followed in sequence by S-, Love and Rayleigh waves. For example, in granites, P- Measuring Instruments and S-waves have speeds ~4.8 km/sec and The instrument that measures earthquake shaking, ~3.0km/sec, respectively. S-waves do not travel a seismograph, has three components – the sensor, the through liquids. S-waves in association with effects of recorder and the timer. The principle on which it works Love waves cause maximum damage to structures by is simple and is explicitly reflected in the early their racking motion on the surface in both vertical seismograph (Figure 3) – a pen attached at the tip of an and horizontal directions. When P- and S-waves reach oscillating simple pendulum (a mass hung by a string the Earth's surface, most of their energy is reflected from a support) marks on a chart paper that is held on back. Some of this energy is returned back to the a drum rotating at a constant speed. A magnet around surface by reflections at different layers of soil and the string provides required damping to control the rock. Shaking is more severe (about twice as much) at amplitude of oscillations. The pendulum mass, string, the Earth's surface than at substantial depths. This is magnet and support together constitute the sensor; the often the basis for designing structures buried drum, pen and chart paper constitute the recorder; and underground for smaller levels of acceleration than the motor that rotates the drum at constant speed those above the ground. forms the timer. 3 IITK-BMTPC Earthquake Tip 2 How the ground shakes? page 2 local soil (Figure 1). They carry distinct information regarding ground shaking; peak amplitude, duration of strong shaking, frequency content (e.g., amplitude of String shaking associated with each frequency) and energy Magnet content (i.e., energy carried by ground shaking at each frequency) are often used to distinguish them. Pendulum Bob Peak amplitude (peak ground acceleration, PGA) is Pen physically intuitive. For instance, a horizontal PGA Rotating value of 0.6g (= 0.6 times the acceleration due to Support Drum 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 Chart Paper Direction of points in it move with the ground by the same Ground Shaking Recorded amount, and hence experience the same maximum acceleration of PGA. Horizontal PGA values greater than 1.0g were recorded during the 1994 Northridge Figure 3: Schematic of Early Seismograph Earthquake in USA. Usually, strong ground motions One such instrument is required in each of the two carry significant energy associated with shaking of orthogonal horizontal directions. Of course, for frequencies in the range 0.03-30Hz (i.e., cycles per sec). measuring vertical oscillations, the string pendulum 1985 Mexico Earthquake (SCT 1A; N90E) (Figure 3) is replaced with a spring pendulum oscillating about a fulcrum. Some instruments do not 1940 Imperial Valley Earthquake (El Centro; S00E) have a timer device (i.e., the drum holding the chart paper does not rotate). Such instruments provide only 1971 San Fernando Earthquake (Pacoima Dam; N76W) the maximum extent (or scope) of motion during the earthquake; for this reason they are called seismoscopes. 0 10 20 30 40 50 60 Time (sec) The analog instruments have evolved over time, but today, digital instruments using modern computer 0.5g 1991 Uttarkashi Earthquake (Uttarkashi, N75E) technology are more commonly used. The digital instrument records the ground motion on the memory Figure 4:: of the microprocessor that is in-built in the instrument. Some typical recorded accelerograms Strong Ground Motions Shaking of ground on the Earth’s surface is a net Generally, the maximum amplitudes of horizontal consequence of motions caused by seismic waves motions in the two orthogonal directions are about the generated by energy release at each material point same. However, the maximum amplitude in the within the three-dimensional volume that ruptures at vertical direction is usually less than that in the the fault. These waves arrive at various instants of horizontal direction. In design codes, the vertical time, have different amplitudes and carry different design acceleration is taken as 1 2 to 2 3 of the levels of energy. Thus, the motion at any site on ground is random in nature with its amplitude and horizontal design acceleration. In contrast, the direction varying randomly with time. maximum horizontal and vertical ground accelerations Large earthquakes at great distances can produce in the vicinity of the fault rupture do not seem to have weak motions that may not damage structures or even such a correlation. be felt by humans. But, sensitive instruments can Reading Material record these. This makes it possible to locate distant Bolt,B.A., (1999), Earthquakes, Fourth Edition, W. H. Freeman and earthquakes. However, from engineering viewpoint, Company, New York, USA strong motions that can possibly damage structures are of interest. This can happen with earthquakes in Authored by: the vicinity or even with large earthquakes at C.V.R.Murty reasonable medium to large distances. Indian Institute of Technology Kanpur Characteristics of Strong Ground Motions Kanpur, India The motion of the ground can be described in Sponsored by: terms of displacement, velocity or acceleration. The Building Materials and Technology Promotion variation of ground acceleration with time recorded at Council, New Delhi, India a point on ground during an earthquake is called an accelerogram. The nature of accelerograms may vary This release is a property of IIT Kanpur and BMTPC New Delhi. It may be reproduced without changing its contents (Figure 4) depending on energy released at source, and with due acknowledgement. Suggestions/comments type of slip at fault rupture, geology along the travel may be sent to: [email protected]. Visit www.nicee.org or path from fault rupture to the Earth’s surface, and www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips. 4