The rocks and sediments of the region have been divided into four units: metamorphic rocks of unknown but great age; granites of Jurassic-Cretaceous age; fossiliferous sediments of Cretaceous and Tertiary age; Quaternary sands, silts, clays, gravels, and artificial fill. The metamorphic and intrusive rocks are deeply weathered.
In general, damage is clearly related to the type of materials of construction, independently of the subsoil. Nonetheless, damage was over soft alluvium and artificial fill. The overpass on the north of the railway bridge on Rio Bio Bio sunk into artificial fill and was damaged. The houses that survived best were built of reinforced concrete or wood. Houses of adobe or bricks held together with poor mortar or mud failed miserably. In the steeper zones, danger from landslides menaces populous areas in the hills La Pólvora, Chepe, Chacabuco, and others.