What is an Earthquake?

When two blocks of the earth slip past one another suddenly earthquake takes place. The slipping surface is called the fault plane (or the fault) and the location below the surface of the earth where the earthquake begins is termed as the hypocenter, whereas the location right above the surface of the earth is called the epicenter. Sometimes smaller earthquakes or foreshocks can happen in the same place as the larger earthquakes. The scientist can’t confirm whether an earthquake is a forestock until a bigger earthquake takes place. Mainshock is the main earthquake which is the largest one and is always followed by aftershocks (comparatively smaller ones) and occurs afterwards almost in the same place as the mainshock.

What causes an Earthquake?

As we know our earth consists of four layers: the inner core, outer core, mantle and the crust. The top of the mantle and the crust make a thinner skin on our planet’s surface.

The skin is made up of many pieces just like a puzzle that covers our earth’s surface. These puzzle pieces keeps moving around slowly, constantly sliding past one another and sometimes also bumping in to each other.

The energy that would usually allow the blocks to flow past one another is being stored up while the edges of faults are stuck together and the rest of the block is moving. All that stored energy is released when the force of the sliding blocks ultimately overcomes the friction of the fault’s sharp edges and it unsticks. Similar to ripples on a pond, the energy emanates from the fault in all directions as seismic waves. Seismic waves travel through the earth, shaking everything in their path, including our homes and ourselves, when they reach the surface!

What is the Richter scale?

The most widely used scale for measuring earthquakes is the Richter magnitude scale, sometimes abbreviated as Richter scale. Charles F. Richter of the California Institute of Technology created it in 1935 as a mathematical tool to compare earthquake sizes. An earthquake’s magnitude, or the amount of energy released during an earthquake, is rated using the Richter scale.

The Mercalli Scale, which is based on a number of variables such depth, geography, and population at the epicenter, is used to evaluate earthquake damage instead of the Richter scale. An earthquake in a heavily populated region that kills a lot of people and does a lot of damage could be the same size as one in a rural place that only frightens the local wildlife. Humans may not even be able to sense large-magnitude earthquakes that occur beneath the oceans.

The highest wave ever recorded at a particular distance from the seismic source is measured for amplitude (height) to determine the Richter magnitude. The differences in length between the several seismographs and the earthquake epicenter are taken into account.

Since the Richter scale is a base-10 logarithmic scale, the intensity of each order of magnitude increases by a factor of ten over its predecessor. Put differently, a two has ten times the intensity of a one, while a three has 100 times the intensity. The Richter scale illustrates an increase in wave amplitude. In other words, the wave amplitude increases 100 times between a level 7 and level 9 earthquake, and it is 10 times larger in a level 6 earthquake than in a level 5 earthquake. Between whole number values, the energy emitted increases 31.7 times.

History of large Earthquakes

Tectonic earthquakes can occur at magnitudes less than zero, which indicate fault slippage of a few centimeters, or at magnitudes more than nine, which indicate fault displacements of several meters. An earthquake’s magnitude depends on both the area of the fault plane that ruptures and the amount of displacement. As a result, an earthquake will always be larger in the rupture area. A fault region measuring roughly 1000 km³, or 50 km long and 20 km wide, is ruptured by an earthquake of magnitude 7.

Additionally, depth has a significant impact on the intensity of earthquakes. It is well known that the Earth’s solid core may produce earthquakes at different depths. An earthquake’s force increases with depth, but its likelihood of reaching the surface also decreases significantly. Because an earthquake can cause more damage to surface structures the shallower it is, shallow earthquakes are more frequent and dangerous.

While there is no upper limit to the theoretical magnitude, the largest known earthquakes have been somewhat larger than 9. As of March 2011, the greatest earthquake to ever strike Japan since records began, the most recent major earthquake with a magnitude of 9.0 or higher was in 2011.

Seismic Analysis and why it is Important

Seismic analysis is a subfield of structural analysis that focuses on estimating the seismic response of a building or non-structure. It is a component of structural engineering, earthquake engineering, or structural evaluation and retrofit (also structural engineering) in earthquake-prone places.

The fundamental tenet of seismic analysis in the last thirty years or so has been that structures should be built to withstand powerful earthquakes without collapsing in order to save lives, but to accept irreversible damage in the event that an earthquake occurs with a given “low” probability within the building’s anticipated lifespan.

Methods used for Seismic analysis of structures

The following five types of structural analysis approaches can be classified. They are as follows:

Equivalent static analysis

In order to replicate the effects of ground motion during an earthquake, this technique identifies a set of forces acting on a structure. These forces are often described by a seismic design response spectrum. It is presumed that the structure responds in a fundamental way. For this to be true, the building needs to be low-rise and not twist significantly as the earth shifts. The response is taken from a design response spectrum (specified or estimated by the building code) given the natural frequency of the structure. By include elements to account for higher buildings with specific higher modes and small degrees of twisting, several building standards improve the utility of this idea.

Response spectrum analysis

This approach makes it possible to take into account the reactions that a building may have (in the frequency domain). Numerous construction regulations require this, except extremely simple or complex structures. The response of a structure can be described as an assemblage of several distinct specific shapes, or modes, which are analogous to the “harmonics” present in a vibrating string. It is possible to identify these modes for a structure using computer analysis.

Linear dynamic analysis

Static approaches are appropriate when higher mode effects are negligible. Usually, this holds true for short, regular structures. Therefore, for tall buildings, buildings with torsional anomalies, or non-orthogonal systems, a dynamic approach is required. In the linear dynamic process, the building is seen as a Multi-Degree-Of-Freedom (MDOF) system with a linear elastic stiffness matrix and an analogous viscous damping matrix. Either modal spectrum analysis or time history analysis is utilized to simulate the seismic input; in both cases, linear elastic analysis is employed to determine the relevant internal forces and displacements. One advantage of these linear dynamic approaches over linear static processes is that higher modes can be taken into consideration.

Nonlinear static analysis

When the design produces a nearly equal distribution of nonlinear response across the structure, or when the structure is expected to remain virtually elastic given the amount of ground motion, linear approaches are applied. As the structure’s performance aim implies more inelastic demands, the uncertainty associated with linear approaches increases. To prevent unexpected performance, a high degree of conservatism is needed in both demand assumptions and acceptance criteria. Thus, procedures including inelastic analysis can aid in reducing ambiguity and conservatism. One type of nonlinear static analysis is pushover analysis.

Nonlinear dynamic analysis

Low-uncertainty results can be obtained by nonlinear dynamic analysis, even if it requires a detailed structural model and ground motion measurements. Estimates of component deformations are provided for each degree of freedom in the complete structural model when it is exposed to a ground-motion record. The modal responses are then combined using methods similar to the square-root sum-of-squares in nonlinear dynamic investigations.

Codes referred for Seismic analysis and design of Structures

• India -IS 1893, IS 4326, IS 13920, IS 13827, IS 13828, IS 13935
• S – ASCE 7, ACI 350.3, AISC 341, AISC 360, ACI 318
• Europe – EN 1998/Eurocode 8,

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