The risk areas in the municipality are the slopes of Bulusan Volcano, the riverbanks, the catchment basin of Cadac-an River and the hills and mountains with steep slopes.
The active crater of Bulusan Volcano is located 7.5 kilometers north from the urban center and straddling the northeastern boundary between barangay Cogon and the municipality of Bulusan. The volcano is part of the Bicol Volcanic Chain with a height of 1,559 meters and a base diameter of 15 kilometers. It is characterized as a stratovolcano formed inside a caldera. Eruption types of the volcano are caldera-forming which occurred 40,000 years ago, strombolian eruptions (1918-1919) and phreatic type (1918-1922, 1980, 2006-2007 and 2010-2011). The hazards associated with Bulusan Volcano are pyroclastic flows, lava flows, lahars and ash fall. A four-kilometer radius around the volcano is declared a permanent danger zone.
The pyroclastic flow hazard zone is about 1,632.72 hectares which encompasses the BVNP forest areas and reaches up to the built-up area of Cogon and Sito Talistison in Mapaso. On the other hand, the lava flow hazard zone overlaps the pyroclastic flow hazard zone and extends further up to five kilometers radius and includes the built-up area of Cogon, Mapaso and portions of the national highway. The lava flow hazard zone covers 2,855.87 hectares of mostly forest and agricultural use lands.
Lahar flows from 2006-2007 ash explosions are mainly confined to gullies at the slopes of the volcano which flows down to the dry creeks of Cogon, Gulang-gulang, Monbon, Mapaso and Patag. A lahar flow in October 2007 prompted the evacuation of 1,596 persons from near and downstream of the gullies. However, in a worst-case scenario of a cataclysmic eruption of Bulusan Volcano, the areas to be affected by lahar will cover the whole Irosin valley. In this scenario, lahar will inundate the urban areas as well as built-up areas of the rural barangays except for Cawayan, Bagsangan, Salvacion, Casini and Liang. Lahar will affect 4,372 hectares or 29% of the total land area.
Recent activities of Bulusan Volcano are phreatic or steam driven which results to explosion of ash, rock and volcanic materials. The area affected by ash fall is largely determined by the volume of material ejected and the prevailing wind direction. An ash explosion in February 21, 2011 reached 3 kilometers high above the summit and ash blanketed areas to the southwest reaching as far as Masbate Island.
Irosin is listed by the MGB-DENR as among the 48 municipalities in the Bicol Region considered as geologically hazardous and highly prone to flooding and landslides. The municipality is usually affected by tropical storms and typhoons that pass through the Bicol Region. These events triggers landslides, flash floods, mudslides, widespread floods that together with high winds causes destruction to houses, buildings, roads, infrastructures and agriculture.
An estimated 792 hectares of land is frequently flooded. These are areas where heavy torrential rains of one to two days could bring about flooding. Moderate to strong typhoons could submerge these areas in 0.5 to 2.0 meters in flood waters for a few days to a few weeks. Development of urban settlements in these areas is not recommended. These areas are mostly confined to the valley floor near the convergence point of Cadac-an River, Monbon River and Buenavista River. Frequent flooding is also observed near the banks of creeks and rivers of the riparian system of the municipality. A flood assessment report indicates that 21 out of the 28 barangays are flood-prone, namely San Isidro, Salvacion, Batang, Monbon, San Juan, Bagsangan, Tabon-tabon, Sto. Domingo, Gabao, Bulawan, Tongdol, Gumapia, Carriedo, Buenavista, Macawayan, San Pedro, Patag and Mapaso. A recent flooding incident on January 10-11, 2011 triggered by torrential rains submerged 520 hectares of rice lands and prompted the evacuation of 2,180 persons. The earliest recorded flooding happened on December 24, 1933 when the Cadac-an River burst its banks and several people perished. A concrete river control was constructed in 1937 to keep the violent surge of the river from eroding the poblacion.
From the hazard maps of MGB-DENR, an additional 1,336 hectares of lands are considered as occasionally to rarely flooded areas. These are areas that become inundated during moderate to strong typhoons. Flood depths vary from a few centimeters to one meter and floods may last from a few hours to a few days. The rarely flooded area nearly encompasses the whole of Irosin valley and there is no historical reference that flooding of such extent has occurred in the past.
Areas with high susceptibility to landslides are areas with high probability of occurrence of mass movements particularly rock and debris slides, slumps and debris flow. The crater walls surrounding the Irosin valley, the volcanic ravines and gullies of Bulusan Volcano and the very steep and nearly vertical slopes underlain by Tabon-tabon volcanic are rated high susceptibility areas and are unsuitable for urban land use.
Twenty-two out of the 28 barangays of Irosin have zones with landslide hazard and a total of 102 landslides were mapped. Barangays located at undulating hills and mountains have the highest number of landslide mapped where Mapaso has 11 landslide mapped, Salvacion has 15, Liang has 13, Casini has 6, Batang has 11, Gabao has 9 and Bagsangan has 7 landslide mapped. Five landslides were mapped in Patag, four each in Monbon, Bulawan, Cogon, Bolos, three in barangay Tongdol, two in Cawayan, and one each in San Isidro, Gulang-gulang, San Julian and Buenavista.
There are no known active faults traversing the municipality of Irosin. The most active fault in the area is the Masbate Fault located 45 kilometers southwest in Masbate and Ticao Islands. Another fault in the vicinity is the Northern Samar Lineament which is located southeast about 55 kilometers in Samar Island. The nearest fault is an unnamed fault traversing the Bicol Region that ends in Sorsogon Bay located just 30 north of the municipality. All faults in the area are in a northwest to southeast orientation. The Philippine Trench is 180 kilometers to the west at the Philippine Sea.
Recorded earthquake epicenters from 1600 to 2005 in the vicinity of Irosin are mostly clustered in the Masbate Fault and in San Bernardino Strait. Past earthquakes of significant strength include the 1954 magnitude 6.7 earthquake with epicenter in Sorsogon City which is 30 kilometers away. A magnitude 7.4 earthquake also occurred in 1943 with an epicenter at San Bernardino Strait about 35 kilometers away.