When considering the potential effects of an earthquake there are several factors that must be taken into consideration. These will be listed, described and their effects on the geological event taken into account independently and then followed up by applying them to a situation that may affect the city of Southampton (don’t worry, this is incredibly unlikely!).
Firstly, a basic understanding of earthquakes is required. An earthquake is caused by movements within the earth’s crust, typically around tectonic plate boundaries. These movements lead to build ups of potential energy due to the friction which is released suddenly as seismic waves. This is an earthquake. The seismic waves released can be separated into two types, body waves and surface waves. Body waves can be further split into two types, P or primary waves, and S or secondary waves. Primary waves are compressional so can travel through solids and liquids; these arrive first. Secondary waves are what is called a “transverse” wave, arrive second and can only travel through solids. Surface waves do not penetrate the earth but can also be split into two types. Lowe waves have a horizontal displacement and Rayleigh waves show a vertical displacement. In an earthquake situation, it is the S-waves and the surface waves that cause the most damage.
Back to the original question, some of the factors of earthquakes that should be considered are Magnitude, source, Epicentre and Hypocentre, as well as any fore-shocks and after-shocks.
Magnitude: An earthquake’s magnitude is a measure of its intensity/size. There are several scales that we can use, these include; the Richter scale (A logarithmic scale ranging from 1-10), the moment scale (A theoretically infinite scale, also logarithmic) and the severity scale (based on the number of deaths or injuries and the cost of any damages). We mostly use the moment scale as it encompasses the size of the fault and the amount of energy required to move a given distance. Each step increase in the scale represents a 30x increase in energy released. The magnitude is in part controlled by the environment in which it is caused: the largest earthquakes (Mag. 8) occur at subduction zones.
Epicentre: An earthquake’s epicentre is the point at the surface which the earthquake displacement occurs. This is a 2D consideration. It can influence the earthquake’s effect on built up areas: if the epicentre is in a city centre, a large number of buildings and people will be effects, however, if it is 10 miles out, the effects won’t be as severe due to the waves losing energy and intensity as they travel.
Hypocentre: An earthquake’s hypocentre is the point within the earth that the earthquake originates. The principal difference between this and the epicentre is that this is 3D so takes depth into account. The deeper the earthquake the more time the waves have to deplete in energy, and the strongest earthquakes tend to occur at depth: 0-70km = small, 70-300km = large, 300+km = intermediate.
Fore/after-shocks: these shocks are preliminary and subsequent shocks of a smaller magnitude than the main shock. Fore-shocks are caused by the initial shifting of rock before the main shock, and after-shocks the re-adjustment of the moved area. These, despite having a lower magnitude, can be just as damaging.
Earthquakes also have a number of associated hazards that can occur at various intervals relating to the main shock. These include Tsunamis, Landslides, Liquefaction, Sink holes, volcanic eruptions in some cases, secondary fire, burst water mains, collapsed buildings and many more.
Another important aspect of the effects an earthquake can have is the affected area’s geology. Loose soil does not withstand seismic waves very well. However, strong, highly competent, strongly consolidated rocks such as granites and high grade metamorphic rocks can withstand the affects very well.
Attached here is a geological map of Southampton. If a sizeable enough earthquake struck Southampton (this is unlikely on account of the location of the UK being far from any plate boundaries and there being no significant faults in the area) and the epicentre was within the city, the effects would be as follows:
As it is a built up area one of the greatest threats would be from collapsing buildings. The Southampton building codes are unlikely to be up to earthquake standard as this is not a present threat.
In the geological map you will see the Southampton is mostly covered in river deposits which consist of relatively poorly consolidated sands and gravels. These particular sediments are likely to develop cracks and fractures themselves leading to damage of water and gas mains as well as creating physical hazards.
The area down by the docks and encompassing ocean village is made up of reclaimed land. This consists of dredged sediments and sands. The unconsolidated nature and high water content would make these areas at high risk of liquefaction. Liquefaction is the process by which water rises to the surface through sediments and causes massive instability. Areas affected by this hazard will bear capsized buildings and mass collapses.
The areas further north are made up of laminated sediments, mostly clays and sands. These are likely to slip, fault and potentially experience some liquefaction depending on the water content. This is still quite likely as the mineral structure of a clay allows it to hold a lot of water.
Other secondary hazards that are likely to occur include secondary fires and the bursting of water mains (this makes it harder to put out said fires). Even given the proximity to water a tsunami is unlikely given that the water wouldn’t be travelling over a large distance and it is comparatively shallow. A landslide is likely given the relatively poorly consolidated sediment, however, there are not a large number of topographic slopes in the area so slumping is much more likely.
In summary, provided a magnitude 5 or higher earthquake (this is the magnitude typically required to damage buildings and induce liquefaction) hit Southampton, the main risk would be from building collapse induced by liquefaction. The area surrounding the dock would be significantly damaged and would lead to a loss of industry as a long term effect. Large cracks and fractures would likely form disrupting transport and basic services such as water and gas.
References and further reading:
An overview of earthquake hazards:
US Geological Survey FAQs - Earthquakes, plate tectonics, Earth structure:
Earthquake magnitude scale:
British Geological survey - what is an earthquake?