INETER- Geofisica - Vulcanología - Volcanes - Casita

The first scientific reports on the

Desastrous Casita Mudflow, October 30, 1998


From: Smithsonian Institution's Global Volcanism Program's World-Wide Activity Notices, November 2, 1998
GVN Disclaimer-- "Notices of volcanic activity posted on these pages are preliminary and subject to change as events are studied in more detail. Messages posted directly by observers have not been edited or confirmed. Messages forwarded or posted by GVN staff may have been edited, but also have not been confirmed."

Mud avalanche from Casita Volcano, Nicaragua

Date: Mon, 02 Nov 1998 01:05:27
From: Wilfried Strauch

(GVP Note: Casita Volcano is part of the San Cristobal Volcanic Complex)

Report to Volcano Network, Managua, Sunday, 01 of November, 1998

Nicaragua suffers the worst natural disaster since the 1972 Managua earthquake. Heavy rain, inundations and mud avalanches cause widespread destruction, during the last week. A mud avalanche at Casitas volcano killed 1000 people, on Friday.


Heavy rainfalls related to Hurricane Mitch and lasting more than one week, have caused inundations of large parts of Central and North-Western Nicaragua. Rivers destroyed several important bridges, the Panamerican Highway between Honduras/El Salvador and Nicaragua is interrupted at many places. About 200 people were reported beeing killed in the floods, in their collapsed houses or in minor mud flows.

Traffic between Managua and the northern and northwestern departments, as Matagalpa, Esteli, Jinotega, Leon, Chinandega is interrupted. Many of the isolated places suffer shortages of food. At several places people had to seek salvation on their roofs or on trees as the water level increased very fast. Tens of thousands lost their homes.Dramatic rescue operations took place as for instance in Malacatoya where a group of people was saved by a ship,coming from Granada city. Malacatoya river had raised its level by more than 15 m, the highways were overflooded and destroyed.

At some places the landscape changed completely - rivers broadened their bed, or united themselves with other rivers, as occurred in the area of Sebaco. New lakes were formed and mountains were washed away, collapsed and disappeared. Crops were largely destroyed.

Helicopter based rescue operations of the Nicaraguan Army were possible since yesterday when the meteorological conditions improved.

Only today the dimension of a mud avalanche became clear which ocurred already on Friday afternoon at Casitas volcano. The mud covered an area of about 20 km length and 2-3 km width, southwest of the volcano. Numerous villages, settlements and houses between Casitas volcano and the town of Posoltega were destroyed.

Exact informations about population density in this area do not exist but it is assumed that more than 1000 people, maybe even 2000, could have died. Today, Nicaraguan Army and Red Cross reported having found 400 cadavers. Rescue is very difficult because of the mud, and the rain continues.

The Nicaraguan Government declared Natural Disaster Emergency for the most affected regions of the country. The main tasks for the next days are the rescue of people who are still in danger, helicopter transports of food to the isolated places and the preliminary reparation of the communication lines.

Due to this extreme disaster the the Nicaraguan economy has certainly suffered a sensitive drawback. Thus, the reparation of highways is now of extraordinary importance as the coffee harvest should begin in a few days.


Wilfried Strauch
Geophysical Department
Instituto Nicaraguense de Estudios Territoriales
Managua, Nicaragua

From: Smithsonian Institution's Global Volcanism Program's World-Wide Activity Notices, November 16, 1998
GVN Disclaimer-- "Notices of volcanic activity posted on these pages are preliminary and subject to change as events are studied in more detail. Messages posted directly by observers have not been edited or confirmed. Messages forwarded or posted by GVN staff may have been edited, but also have not been confirmed."

Report on the October 30, 1998 avalanche and breakout flow of Casita Volcano, Nicaragua, triggered by Hurricane Mitch

November 14, 1998

On October 30, 1998 a disastrous event (called a "mudflow" in the newspapers) occurred on the south flank of Casita Volcano. According to official reports, the incident killed between 1560 and 1680 people, displaced hundreds more, destroyed several towns and settlements, and disrupted the Pan American Highway at numerous bridges. For several days there was uncertainty about the origin of the natural event. On November 11 and 12 the first scientific team visited the volcano to determine the causes and effects of the disaster. The team examined the summit area on the first day and made a complete traverse of the devastated zone as far south as the Pan American Highway on the second day. This report presents the conclusions of the team and provides some recommendations regarding future risks at this volcano.

Background

Casita Volcano (1405 masl) is located within the Cordillera Maribios, a 70 km long volcanic chain, that extends from the northern shore of Lake Managua to the vicinity of Chinandega. Casita is part of the San Cristobal volcanic complex that consists of 5 principal volcanic edifices (Hazlett, 1987). San Cristobal, 4 km WNW of Casita, is the largest volcano in Nicaragua (1745 masl). It has exhibited frequent episodes of historic activity and at the present it is emitting a vigorous fumarolic plume. For these reasons San Cristobal has been studied in more detail than Casita.

Casita is a composite volcano with deeply dissected morphology. The top of the volcano consists of a cluster of dacite (?) domes. At its summit is a 1-km diameter crater that could be reached by a road, now impassable, servicing telecommunication towers. A set of prominent NW trending normal faults cut the summit area bounding each side of the crater. Explosion craters on the southern plain are aligned along a conjugate set of fractures trending NW-SE. The domes of the summit area are autobrecciated and exhibit strong hydrothermal alteration, which is consistent with low temperature fumarolic activity observed during past decades (Sapper, 1913; Hazlett, 1987). There has been no historic volcanic activity reported here.

Meteorological conditions

Hurricane Mitch was a major factor in the disaster of Casita. Abnormal rainfall related to Mitch, recorded at the nearby city of Chinandega, began on October 25. By October 27 the precipitation reached 100 mm per day and continuously increased to its maximum of almost 500 mm per day on October 30, the day of the avalanche. Subsequently precipitation continuously dropped to normal levels within three days. The rainfall on October 30 was exceptional. The normal monthly average for October is 328 mm. In 1998 the October rainfall was 1984 mm, which is more than 6 times the normal average.

Source zone

The main source of the avalanche was 200 m SW of the volcano summit, 60 to 80 m below the telecommunication towers. A secondary source was located at the same elevation 100 m SE of the summit. The rock in this area is a hydrothermally altered and brecciated dacite dome. The principal rupture occurred along an approximately 500-m segment of a NE- trending fault that intersects the summit. A plate measuring approximately 20 m thick, 60 m high, and 150 m long detached first and then slid down the fault plane that was inclined about 45 degrees towards the SE. The volume of source block for the first rockslide was approximately 200,000 cubic meters.

Avalanche event

Inhabitants of the lower plains heard the avalanche as a sound described as a helicopter. Multiple witnesses gave the time as between 10:30 and 11:00 am on October 30. The main slide mass (autobrecciated dome cemented by hydrothermal mineralization) immediately shattered into its original breccia blocks coated by vein precipitants. The initial SE movement of the avalanche blocks was deflected to the SW along a deep gully oriented parallel to the fault. A smaller part of the avalanche surmounted a small ridge and continued to flow to the SE towards the village of Argelia.

For the first two km the main avalanche was confined to a narrow valley. The top of the flow was 150 to 250 m wide and the depth of flow was 30 to 60 m. A typical cross section of the peak flow was 7500 to 9000 meters squared. The flow swashed back and forth on its downward course. Super elevation calculations at locations of overbank flow give a velocity of about 15 m/s in the upper reaches. Deposits high on the volcano consist of altered dacite block up to m-size. There is essentially no matrix and the finest particles are cm sized. The margin of the avalanche is sharp and flying rocks up to 2 or 3 m height scars the adjacent trees. A few trees were decapitated at heights of several meters.

At a distance of between 2 and 3 km from the source large ramps of avalanche materials formed imbricate ridges at a prominent break in slope. Here the deposits were 4 to 6 m thick, still with no matrix. The avalanche materials were essentially clast supported. The avalanche scoured clay-rich soil as well as blocks of lava from the walls and up to 10 m deep in the base of the valley where it passed.

Lahar runout flow

At approximately 2:00 PM a lahar runout flow as defined in Scott (1988) initiated from the major accumulation zone of the primary avalanche. The source of the lahar runout flow was in the thickest accumulation of debris at the mouth of the avalanche valley, 3 km from the summit and 3 km above the towns of El Porvenir (formerly Augusto Cesar Sandino) and Rolando Rodriguez. The population of these two towns was respectively 600 and 1250 according to the last census. When we visited the sites of El Porvenir and Rolando Rodriguez we could only find our position by GPS. There was almost no evidence of former human habitation.

Apparently the lahar runout flow resulted from either a dam breakout or dewatering of the debris pile. The flood surge moved as a hyperconcentrated flow depositing a thin (about 40 cm thick) layer of gravel with some clay matrix on the overbank zones and transporting meter-size blocks within the incised channels. The peak height of the flood surge was 3 meters as it entered El Porvenir, as evidenced by stripped bark from the few standing trees. Nearly all vegetation and soil was removed by the leading edge of the wave. However, a few islands of vegetation were spared on some hills. The width of the flood surge in its upper reaches was about 1500 m. Assuming an average peak depth of about 3 m, the cross sectional area of the flood surge would be 4,500 meters squared.

Casualties and Damage

Based on our personal observations in the field, the two towns mentioned above (El Porvenir and Rolando Rodriguez) were completely destroyed beyond recognition. About 2,000 people lived there. We do not know how many people survived, probably only a few. We saw many cadavers and dead livestock lying on the overbank and burnt for sanitary reasons. We do not know who was responsible for these sanitary measures because we have not yet received the official report in this regard. Many other smaller hamlets, isolated residences, and farms were destroyed.

Future hazard potential

The disaster of October 30, 1998 was produced by the coincidence of two discrete events, an avalanche and extraordinarily heavy rains. Neither of these alone would have produced such extensive damage to the surrounding area. In this respect it should also be noted that the towns of El Porvenir and Rolando Rodriguez were only established a few decades ago in an area of high geologic risk. To reduce the risk for new settlements near volcanoes, a comprehensive geologic hazard study should be made.

In the absence of another episode of heavy rainfall, the deposit seems to be stable. In fact, there is little mud or silt within the deposits at higher elevations to facilitate remobilization. However, the conditions near the summit that favored the rockslide avalanche still exist. An altered and fractured dacite occurs on steep slopes at a high elevation. Destabilizing events, such as an earthquake or torrential rains, could produce another avalanche in an adjacent area. The probability of such an extreme avalanche seems remote. However, an assessment of the associated hazards and risks should be undertaken.

There have been press reports concerning a new eruption of Cerro Negro Volcano that is located to the southeast of Casita. A report by Julio Alvarez Garcia (INETER) on November 10, 1998 and observations made by two of our group (MFS and CS) during a short visit on Friday, November 13 have determined that this report is false. Apparently the most recent lava flows of 1995 contained enough heat to produce small steam clouds which were mistaken for a new lava flow.


Information contacts:

Michael F. Sheridan, SUNY at Buffalo
Claus Siebe, UNAM, Mexico
Christophe Bonnard, EPFL Lausanne, Switzerland
Wilfried Strauch, INETER, Nicaragua
Martha Navarro, INETER, Nicaragua
Jorge Cruz Calero, INETER, Nicaragua
Nelson Buitrago Trujillo, INETER, Nicaragua