Physically, it involves alternating compression and rarefaction of the medium. The P-wave is always the first disturbance from an earthquake or explosion which arrives first at every station. Rift zones are associated with basaltic volcanism, and have associated shallow earthquakes which give a real-time record of the rifting activity. The rift zone seen in the USGS glossary , along the summit of Mauna Loa, is so full of erupted lavas that there is no topographic depression, but a slight ridge. The possibility or likelihood that something bad will happen to you, your house, your town, or your family because of your exposure to a hazard.
People assume risk by building on a beachfront.
Volcano A broad, gently sloping mountain built by the eruption of many basaltic lava flows accompanied by very little explosive activity. Shield volcanoes have a central summit caldera and are usually elongated along rift zones, typically two, which are marked by many small cones, pit craters, and fissures. The shape is caused by the low viscosity of the basaltic lava, which permits lava to flow long distances down a slight grade. See the USGS web site . Physically, it involves shearing distortion of the medium.