2010 Chile Earthquake Case Study

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Earthquake Case History:
2010 Mw = 8.8 Mega-thrust Earthquake, Maule, Chile
5/2/13
By Travis Eddy

1. Introduction
In early 2010 central south Chile experienced a Mw = 8.8 earthquake and large tsunami waves that devastated areas on the Chilean Pacific coast, nearby offshore islands, and areas near the epicenter. In addition to the tsunami, the earthquake had many other geological consequences including aftershocks, terrestrial and submarine land-sliding, elevation changes, and a gravity shift. The purpose of this paper is to discuss and analyze the earthquake, its consequences, the resulting damages, and mitigation.

2. Geologic Setting
Figure 1: Profile of area around the epicenter (Moscoso, et.al., 2011) The site of the main event lies ~ 10 kilometers offshore of the Chilean coast, beneath the broad flat 30 – 40 kilometer wide canyon cut continental shelf, and adjacent to the 4.7 kilometer deep sediment filled trench [Figure 1] (Voelker, et.al., 2011). The region is tectonically characterized by the ~6 kilometer thick oceanic Nazca plate subducting underneath the continental South American plate at a convergence azimuth of ~ 78 ͦ and at a rate of 6.6 centimeters per year making the area one of the most active convergent margins on earth, typically experiencing a Mw > 8 earthquake every 10 – 20 years (Moscoso, et.al., 2011). Notable quakes in recent history include the 1960 Mw = 9.5 Valdivia earthquake which was the largest ever recorded, and quakes occurring in 1985 [Mw = 8.0], 1939 [7.8], 1928 [8.0], 1906, and 1835 which was documented by Darwin (Moscoso, Figure 2: Locking Degree in 2010 rupture area (Moreno, et.al., 2012) et.al., 2011; Tanimoto, Ji, 2010). Significantly the 1835 rupture plane has been plate locked since then, resulting in the accumulation of stress for 175 years prior to the main event [Figure 2] (Moreno, et.al., 2012). Other geologic features that could have possibly contributed to the triggering of the main event include lateral plate interface frictional property variations, fault dip morphology, oceanic plate features, fore-arc behavior, the abundance of splay faults (Moreno, et.al., 2012).

3. The Main Event
Figure 3: Location of epicenter off the Pacific coast of Chile with the Chile Trench in white and slip contours in yellow (Melnick, et.al., 2012) At 6:34 UTC on February 7th 2010 the 6th largest earthquake to ever be recorded, with Mw = 8.8, occurred off the coast of central south Chile. Only weeks earlier, an earthquake occurred relatively nearby in Haiti, however the event in Chile was 500 times larger than the Haitian event (Chinn, 2011). The epicenter was determined to lie ~10 kilometers offshore Chile in the Pacific Ocean at ~35.97 ͦ S, ~72.87 ͦ W [Figure 3], with the hypocenter located ~22 kilometers down [Figure 1](Moscoso, et.al., 2011). The dipping mega-thrust rupture plane extended bilaterally for >500 kilometers and considered to be a seismic gap along the 1835 rupture plane, most of the 1906 rupture plane, and the 1928/1985 inter- plate interface (Moscoso, et.al., 2011). Various models suggest the seismic moment lies between 1.8x1022 – 2.6x1022 Newton-meters (Pollitz, et.al., 2011). Broadband surface waves and back projection methods yield average rupture velocities ranging from 2.0 – 2.5 kilometers per second (Lay, et.al., 2010). The 130 second moment rate is characterized by an increase for 90 seconds and then a decrease for the final 40 seconds (Lay, et.al., 2010). Consequences of the main event included aftershocks, tsunami waves, land sliding, ground changes, a gravity shift, and damages, which are further discussed below.

4. Aftershocks [Figure 4]
Figure 4 : Location of aftershocks [blue circles] and epicenter [red star] (Rietbrock, et.al., 2012) After the occurrence of the main event, coulomb stress defined as the change in sheer stress plus the product of a frictional coefficient and the change in the normal stress: Δσc = Δτ + μ Δσ...
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