Characteristics And Spatial Distribution Of An Earthquake

Explain the characteristics and spatial distribution of the following hazards using at least two  examples. [10 marks]

Every year the National Earthquake Information Center records an average of 20,000 earthquakes, however, this number still excludes the earthquakes that are too weak to be recorded. The natural hazard of earthquake can be caused by other hazards such as volcanic eruptions; however, the usual cause for earthquakes is still the shifting of the earth's plates. These plates include the mantle and the crust of the earth, the boundaries of the plates are formed by faults, and is the most quintessential locations of earthquakes. Tension is created as these plates move, the crust breaks as the tension reaches its maximum and releases energy in the form of waves, which eventually is the earthquake. The two more well-known earthquakes are the Sichuan Earthquake in 2008 and the Bohol Earthquake in 2013, which will be elaborated to demonstrate the characteristics and spatial distribution of earthquakes. 

Figure 1.

The Sichuan Earthquake in China killed around 70,000, 380,000 injured, 18,000 missing, caused a displacement of 9~12m and landslides. The earthquake is reported to occur along Longmenshan Fault as outlined by recorded shocks in Figure 1. The Longmenshan locates at border line of Indo-Australian Plate and Eurasian Plate, and is roughly in the middle of China. The earthquake happened in two stages as the Longmenshan fault broke twice, the first was around seven yards and the second was around four yards. As said by China Earthquake Administration, the earthquake took place for around 120 seconds (2 minutes), with the first 80 seconds being the strongest, with the most energy release. It was recorded to have an average speed of 3.1 km/s, in the direction of Northeast, and an estimation of 300km rupture range. The Shichuan Earthquake was said to be 8.0 Ms, which is a magnitude of an earthquake that only occurs once every five to ten years and can have detrimental effects to the surrounding communities. This earthquake also had over 42,000 after shocks in the region. Despite the damage it has done to the surrounding, this destructive earthquake was surprisingly not precisely predicted; however, it was concluded after it was actually an unpredictable earthquake in terms of size, place and time. 

Another well-known earthquake is the recent Bohol Earthquake that occurred in the Central Visayas, Philippines. The epicenter was at Bohol, but the action was felt throughout Visayas and Mindanao. This was a tectonic earthquake and discovered to be occurred in a previously undiscovered fault line, now named as - North Bohol Fault (Figure 2).

Figure 2 - Box outline of North Bohol Fault

This earthquake is recorded to be 7.2 Mw, with a depth of 12km and had a detrimental affect including: 200+ deaths, 8 missing, 1000 injured, 73,000 structures damaged and 14,500 destroyed. The magnitude of the earthquake can be illustrated with the description of its energy released being equivalent to 32 Hiroshima bombs, and is the strongest in 23 years. The earthquake lasted for about 34 seconds, and had around 4,000 aftershocks. The UP National Institute of Geological Sciences said that due to the lack of knowledge of the multitude of the Philippines' faults, the earthquakes can't be precisely predicted; as a result, the Bohol earthquake wasn't specifically identified before its occurrence. 

As seen from the examples of Sichuan and Bohol Earthquakes, earthquakes often are located at faults in the countries. Some faults are already discovered while others like the North Bohol Fault wouldn't be discovered until an earthquake happens. The two chosen earthquakes were both in the strong earthquake category, with a magnitude over 7.0. These earthquakes caused detrimental effects, yet couldn't be accurately predicted. After the main shock, there were still several after shocks that alarmed the citizens and may have caused injuries. Furthermore, it is evident that due to movements of plates (thrust plates in these cases), cause earthquakes.