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First, let's understand what these ridges are. Both the East Pacific Rise and the Mid-Atlantic Ridge are underwater mountain ranges formed by plate tectonics. They are divergent boundaries, where the tectonic plates are moving away from each other, causing magma from the mantle to rise and solidify, creating new oceanic crust.

Marine magnetic anomalies are variations in the Earth's magnetic field that are detected by measuring magnetic properties of the ocean floor. These anomalies are related to the process of seafloor spreading and the formation of new oceanic crust. As new crust forms, magnetic minerals within the rocks align themselves with the Earth's magnetic field, resulting in these anomalies.

One of the main factors affecting the width of marine magnetic anomalies is the rate at which the tectonic plates are moving apart, also known as the spreading rate. The faster the spreading rate, the wider the anomalies will be. To find out why the anomalies around the East Pacific Rise are wider than those around the Mid-Atlantic Ridge, we need to compare their spreading rates.

Based on the available data, it is known that the spreading rate at the East Pacific Rise is much faster than at the Mid-Atlantic Ridge. In fact, the spreading rate at the East Pacific Rise can be around 150 mm/year, while the spreading rate at the Mid-Atlantic Ridge is only around 25 mm/year.

Due to the higher spreading rate at the East Pacific Rise, the newly formed oceanic crust moves away from the ridge more quickly, giving more time for magnetic minerals to align with the Earth's magnetic field and causing the anomalies to be wider. On the other hand, the slower spreading rate at the Mid-Atlantic Ridge results in narrower anomalies, as the crust moves away more slowly and there is less time for the magnetic minerals to align. In conclusion, the marine magnetic anomalies bordering the East Pacific Rise in the southeastern Pacific Ocean are wider than those bordering the Mid-Atlantic Ridge in the South Atlantic Ocean due to the differences in their spreading rates, with the East Pacific Rise having a much faster spreading rate than the Mid-Atlantic Ridge.