Earthquakes are an example of seismic activity created by plate boundaries. They can be caused by the subduction of oceanic crust which is densest at 2.9 g/cm3 under continental crust which weighs 2.7g/m3 at destructive plate boundaries. Earthquakes can also occur along conservative plate boundaries such as that shared by the Pacific and North American plates which move at 5-9 cm/year and 2-3 cm/year respectively causing the 1994 Los Angeles earthquake along the San Andreas fault alongside which lies the San Gregorio and Hayward faults. Earthquakes have different impacts dependent on the location of their foci, the point at which they originate from underground, the presence of land in the surrounding areas, but also the human factors such
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The Boxing Day tsunami could not have occurred if the foci of the earthquake had not been 160km south west of the Sumatran city of Banda Aceh in the Indian Ocean. If it had been further towards the land then the tsunami waves are unlikely to have been able to reach 28m in height and thereby cause the devastation they did when encroaching 800m inland. Although sizeable damage would have been inflicted by an event of its size if located under or nearer to land, this is not possible due to the location of the plate boundaries which are entirely a natural occurrence. Similarly, the foci of the Kobe earthquake was 20km off Osaka Bay and subsequently its shockwaves led to great damage resulting in 20% of the buildings in Kobe’s CBD becoming unusable. If it had been further out to sea, the power of the seismic waves may have depleted due to the distance travelled and so less damage inflicted.
One could argue however that these points do not have merit due to the human use of the land that is known to be in the locale of the earthquake zones. If no one inhabited Sumatra there would not have been deaths, nor would there have been the 6300 in Kobe had it not been so densely populated. Simply by living one could argue that human factors are causes of impacts, especially when assessing fatalities.
Further, the human factors can be criticised for not doing enough to limit impacts or earthquake hazards especially events such as death as a result