Geophysical events are destructive phenomena. However, these are part of the normal functioning of our dynamic planet. These so called hazards are due to naturally occurring processes in the earth's interior.
Four hazards are considered under this category: Earthquakes, earthquake-induced landslides, tsunamis and volcanic eruptions. Sources of data for these hazards include the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the Earthquake and Natural Resource Atlas of the Philippines of 1998.
A natural process that is hazardous is the movement of lithospheric plates (the solid crust and a few kilometers of the upper mantle), which causes the tectonic earthquakes. US Geological Survey defines the term earthquake as "both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth (i.e. event by man made explosions)".
Furthermore, the resulting ground motion due to an earthquake produces another natural hazard such as landslides and tsunamis. Landslide is the downslope movement of soil and/or rock.
Tsunami is a sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands.
One other example of a hazard is the ascent of molten material called magma beneath the earth's surface, which results to eruptions of a volcano. A volcano is a vent at the Earth's surface through which magma (molten rock) and associated gases erupt, and also the cone built by effusive and explosive eruptions.
Reviewing the natural disaster record for the Philippines, volcano and earthquake disasters are frequent in the top ten. (EM-DAT: The OFDA/CRED International Disaster Database, www.em-dat.net - Université catholique de Louvain - Brussels -- Belgium)
As with the climate- and weather-related hazards and to generate four risk maps, the resulting normalized hazard maps are multiplied with maps of normalized gridded population density by city and municipality in 2000 as well as the normalized inverse of the HDI by province in 2000.