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Abstract

 

The Nicaraguan Seismic Network localized in 1996 a number of 2,501 seismic events. The majority of them were located in the subduction zone off coast of the Pacific Ocean. Seismic events occurred also in the volcanic structures of Cosigüina, El Chonco, Telica, Rota, Cerro Negro, Momotombo, Apoyeque, Masaya, Lake Apoyo, Concepcion and Maderas

 

17 seismic events were felt by the population. The majority of  them were located in the subduction zone off coast  of the Pacific Ocean. Only few of the felt events were located in volcanic chain of Nicaragua and along local faults.

 

Telica volcano presented a high level of seismic activity. A mean number of 160 volcano-seismic events was recorded daily at seismic station TELN located near the crater. The events have frequencies between 3 and 5 Hz and their waveforms indicate volcanic origin, possibly are they caused by degassing in the crater or in the conduits of the volcanic system. A monochromatic tremor with a frequency of 8 Hz is observed nearly continously. The crater walls are collapsing and some seismic events might be related to this process.

 

At Momotombo volcano a strong swarm of seismic events occurred during March - April, 1996. A number of 2000 tectonic-seismic events was recorded in this time at seismic station MOMJ located near the crater. The strongest event with magnitude 3.5 ML was felt in the settlements near Momotombo and by workers of the geothermal power plant situated about 3 km south of the crater, at the shore of lake Managua Due to the danger of landslides provocated by the seismic activity the plant was partially evacuated during the maximum of the swarm activity.

 

Other swarms of seismic events but with much less events occurred near volcanoes Cerro Negro and El Hoyo.

 

Masaya volcano during 1996 presented a continuous seismic tremor but the level of local seismicity was low. The volcano maintained a high level of gas emanation (several hundred of SO2 per day) through the year which not lowered when the open vent in Santiago crater was partially closed in the beginning of December by collapsed material from the crater walls. The collapse was part of a process which intensified at the end of September when park guards of National Park of Masaya volcano informed INETER that they had observed explosions in Santiago crater. In the following weeks nothing important occurred but October 11 the personnel of Masaya Volcano Park again reported strong explosions. These explosions were recorded by the seismic station MASJ, at this time the amplitude of the tremor was higher and bursts of seismic oscillations were observed frequently, probably signals of fluctuations of the degassing in the lava lake below Santiago crater.

 

In Maderas volcano occurred a lahar, which destroyed the -village El Corozal and killed 5 people on September 27. During the rainy season lahars are expected at Ometepe island coming down from Concepcion an Maderas volcanoes, but the rainfall intensity had been unusually high in 1996. About 250 people affected by the lahar of El Corozal were living several days in the local schoolhouse under very difficult conditions.

 

In the months of March and April, 1996, The Geophysical department of INETER cooperated with  scientists from Switzerland, United States and Germany to carry out an important geoscientific experiment in Nicaragua and the Pacific Ocean. The objective of the experiment was to obtain data for the investigation of the structutre of the earths crust beneath Nicaragua, especially in the Pacific coast region and beneath the volcanic chain. In addition to the 20 stations of the National Seismic network INETER in cooperation with foreign scientists from Switzerland and the USA maintained 20 mobile digital seismic stations obtained temporally from GeoForschungszentrum Potsdam, Germany, 8stations from ETH/Zuerich, Switzerland, 12 stations from  Lawrence Livermore Laboratory (USA).These devices were installed in the area between Telica and Momotombo volcanoes. Additionally 10 ocean bottom seismographs recorded the natural seismic activity and air gun shots fired by the german research ship SONNE which at the same time carried out seismic, geological and geophysical research off shore of Nicaragua.

In cooperation with the Nicaraguan Army two explosions were carried out  in lake Managua, near Momotombo volcano to get informations additionally to the natural seismicity. The data processing of the data obtained during this experiment is still going on. Results are expected to publisheds in 1998. A description of the research done by the research ship SONNE can be found in ....

 

The Nicaraguan Seismic Network counts with 21 telemetric seismic stations. The data are digitally recorded and processed at Managua data center. Any seismic event detected by the data acquisition routine is processed regularly within a short time by the seismologist on duty. In case of a strong event the system emits an acoustic signal and the seismologist determines immediately the parameters of the event and sends within minutes a report to the Nicaraguan authorities and the mass media.

 

This Annual Bulletin is the first published with a abstract in english. It is distributed in Nicaragua to governmental organizations and mass media, in Central America to cooperating institutions in Guatemala, Honduras, Costa Rica, Panama, furthermore it is sent to several cooperating institutions in South America, North America and Europe. Phase data and waveforms can be obtained writing (see address on first page) or sending an email to the editor (email vtenorio.gf@inetr.gob.ni).