Earthquakes are one of the most destructive forces of nature. Structures can suffer severe damage when an earthquake strikes. That’s why, seismic loadings must be taken into account when designing structures, especially skyscrapers.
How do earthquakes happen?
As an engineer, in order to make solving a problem easier, it’s important to understand what it is. So, what exactly is an earthquake and how do they happen?
Everyone knows about the existence of tectonic plates and how they influence the movement of the Earth’s crust. Earthquakes happen when these tectonic plates move or collide with each other and release large amounts of energy. This is measured using the Richter scale.
The movement of these tectonic plates can generally be attributed to mantle convection, a phenomenon where warm mantle currents carry plates of lithosphere along like a conveyor belt. It may also be caused by the buoyant upwelling of the mantle at mid-ocean ridges. In this case, gravity causes the higher plate at the ridge to push away the lithosphere that lies further from the ridge. Another cause may be slab pull; it’s a phenomenon where the older, colder plates sink at subduction zones. Subsequently, the cooler sinking plate pulls the rest of the warmer plate along behind it.
Professor Iain Stewart, a geologist from Plymouth University, explains how earthquakes happen and how they affect structures in this short clip. Knowing that waves radiate from the base of a structure throughout its whole body is essential in designing earthquake-resistant buildings.
One of the main elements of any earthquake-resistant building is base isolation. Let’s take a quick look at what exactly this is.
What is base isolation?
The so-called base isolation is a technique developed by engineers to prevent – or at least minimize – the damage to buildings when exposed to earthquakes. These kinds of systems are used all over the world and are most prevalent in New Zealand, India, Japan, Italy, and the United States.
More traditional constructions, like fixed-base buildings, tend to be built directly onto the ground. While this is a sound practice for places that do not experience frequent earthquakes, it is highly advised against if not.
When an earthquake hits, the ground (and the building attached to it) moves with the quake’s motion, causing massive damage to the building. To counteract this, most earthquake-proof buildings are isolated from the ground in some manner.
This usually involves using flexible bearings or pads known as base isolators. These kinds of systems move during a quake, but they move to counteract the forces generated by the movement of the building.
Base isolators work in a similar fashion to car suspension systems, which allow a vehicle to travel over rough ground by isolating the interior and absorbing the shock of uneven ground, without throwing the passengers inside around.
This means that taller buildings have a very real risk of overturning or toppling during earthquakes if they have base isolators installed. For these kinds of buildings, other measures are required.
Base isolators are also not suitable for some sites due to other geotechnical and geographical reasons. For example, there may not be enough space to install them.
They also require hard soil, not soft soil, to operate at peak efficiency.
What are the different types of earthquakes?
Earthquakes generally fall under one of a few distinct categories. These are:
- Tectonic earthquakes.
- Volcanic earthquakes.
- Collapse earthquakes.
- Explosion earthquakes.
Tectonic earthquakes: occur at plate tectonic boundaries. Sometimes, friction between tectonic plates causes them to lock together and become unable to move. However, the rest of the plate carries on moving, which leads to increased pressure on the locked section. Eventually, the locked section succumbs to the pressure and shatters, the plates move rapidly, releasing energy and causing an earthquake.
Volcanic earthquakes: are quakes that result whenever tectonic activity also causes volcanic activity.
Collapse earthquakes: are minor earthquakes that occur whenever something like a mine or underground cavern collapses.
Explosive earthquakes: are any form of an earthquake that is caused by a massive explosion, like a nuclear weapon detonation. Like collapse earthquakes, these tend to be very minor.
Earthquakes are also sometimes caused by human activity such as the injection of fluids into deep wells, the excavation of mines, and the filling of large reservoirs.
All earthquakes’ relative strength is measured using the Richter Scale. According to Michigan Tech, typical ranges for various magnitudes of earthquakes include: –
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