The focal depth of an earthquake is how far beneath the earth's surface the hypo-center of the earthquake.Because of its location in the Indian Ocean, however, its focal depth is measured as 30 kilometers below mean sea level, rather than being measured at the seabed.

The distance in front of and behind the focal point that is in relative focus

There is no way to answer this. The magnitude of an earthquake does not correlate with its depth.

The closer the object is to the camera, the shorter the "focal distance". The depth of field gets shallower as the focal length decreases, up until the lens has reached its minimum focusing distance (closer than that and the image will be blurry).

If you photograph something at F1.8 that is 5 feet away, your depth of field (area in focus) might be 3 feet (1.5 feet each side of focal point), where if you took a shot at F1.8 but 30 feet away from your subject, the depth of field could be 20 feet (10 feet each side of focal point). I've made up these numbers, but there are charts you can use to calculate DoF based on focal length and aperture.

False. As the subducted slab descends into the mantle, the focal depth increases.