Modified Mercalli Intensity Scale

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1. Ground shaking intensity is defined as the severity of ground motion, i.e. displacement, during an earthquake and is assessed using the Modified Mercalli Intensity Scale (MMI). This scale is a good indicator on the effects and earthquake has on the environment and its population. It is based on three features:  People’s perception  Building performance  Changes in the natural environment The MMI is often associated to Peak Ground Acceleration (PGA), which is used to quantify the severity of ground shaking as shown in Table 1. The intensity of an earthquake is a qualitative measure of the shaking at a location and it usually depends on:  The Magnitude of an earthquake  Distance from the fault  Local Geology These will all affect the way in which seismic waves will propagate through the ground and therefore will be responsible for all the subsequent event which will occur as a result of the earthquake. Subsidence is a result of earthquake shaking. As the ground shakes, unconsolidated sediments lose their bearing strengths and the ground “shifts” downwards relative to sea level and this resulted in liquefaction and landslides and flooding. Subsidence can also occur as a result of downward displacement on one side of a fault which may sometimes affect large areas of land. The 4th of September 2010 Darfield event generated a magnitude 7.1 earthquake with a MMI of 9. This earthquake resulted in extensive liquefaction and differential subsidence which was located close to major streams and rivers around Christchurch. The 22nd of February 2011 Christchurch City event created a magnitude 6.3 earthquake with a MMI of 9. This earthquake occurred within 10 kilometres of the city at a shallow depth of 5 kilometres. This cau... ... middle of paper ... ...l change and liquefaction in the Avon-Heathcote Estuary. Report No. U11/13, Christchruch: Environment Canterbury Regional Council, 2011. Quigley, M. Van Dissen, R., Villamor, P., Litchfield, N., Barrell, D., Furlong, K., Stahl, T., Duffy, B., Bilderback, E,. Noble, D., Townsend, D., Begg, J., Jongens, R., Ries, W., Claridge, J., Klahn, A., Mackenzie, H., Smith, A., Hornblow, S., Nicol, R. “Surface rupture of the Greendale Fault during the Mw 7.1 Darfield (Canterbury) earthquake, New Zealand: Initial Findings.” Bulletin of the New Zealand Society for Earthquake Engineering 43:4 (2010): 236-242. SC Cox, HK Rutter , A Sims , M Manga , JJ Weir , T Ezzy , PA White , TW Horton & D Scott (2012). “Hydrological effects of the Mw 7.1 Darfield (Canterbury) earthquake, 4 September 2010, New Zealand.” New Zealand Journal of Geology and Geophysics 55:3 (September 2012): 231-247.

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