Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

01/1988 (SEAN 13:01) No fumarolic activity; lake temperature measured

No fumarolic activity or hot springs were observed 18 January 1988 at locations previously reported active. Lake surface temperature was 28.2-29.1°C at 0940.

Information Contacts: B. van Wyk de Vries, H. Rymer, and G.C. Brown, Dept of Earth Sciences, Open Univ; P. Hradecky and H. Taleno, INETER.

02/1991 (BGVN 16:02) No fumarolic or seismic activity

"No fumarolic or seismic activity was detected in 1990."

Information Contacts: B. van Wyk de Vries, O. Castellón, A. Murales, and V. Tenorio, INETER.

06/1997 (BGVN 22:06) On 8 June, a M 3.7 tectonic earthquake struck this reposing caldera

A swarm of seismic events was observed in June 1997, centered on the E flank of the Laguna de Apoyo. The strongest event was M 3.7 on 8 June. This and many other events were felt with maximum modified Mercalli intensity of IV in nearby villages and Granada city (10 km from Apoyo). The events were of tectonic character.

Information Contacts: Wilfried Strauch, Department of Geophysics, and Marta Navarro C., Department of Volcanoes, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email: wil@ibw.com.ni).

07/2000 (BGVN 25:07) Tectonic seismicity between Apoyo and Masaya in July 2000

[The following was originally included within the Masaya report, not as a stand-alone report about Apoyo.]

July 2000 seismicity near Masaya and Laguna de Apoyo. During July 2000 there were over 300 earthquakes near Laguna de Apoyo (Apoyo volcano) and Masaya. The earthquakes, determined to be of tectonic rather than volcanic origin, caused surficial damage at both volcanoes.

At 1329 on 6 July a small M 2 earthquake occurred near the N rim of Laguna de Apoyo that was followed at 1330 by a M 5.4 earthquake (figure 1). It was located ~32 km SE of Managua, at 11.96°N, 86.02°E, with a focal depth less than 1 km (figure 2). The earthquake was felt in most of Nicaragua and was most strongly felt in the cities of Managua (Modified Mercalli V-VI) and Masaya (VI), and in the region near Laguna de Apoyo (maximum intensity of VII or VIII). The earthquake caused numerous landslides down the volcano's crater walls and surface faulting was observed. In towns located in the epicentral zone, trees and electric lines fell and many houses were partially or totally destroyed. About 70 people were injured and four children were killed by collapsing walls or roofs of homes. At Masaya volcano, ~8 km from the epicenter, there were minor collapses of Santiago crater's walls. No change in degassing was observed at the volcano.

Figure 1. Seismogram showing the M 2 and M 5.4 earthquakes near the Masaya volcano station on 6 July 2000. Courtesy of INETER.

Figure 2. Epicenters near Masaya for the M 5.4 earthquake on 6 July, and the M 4.8 earthquake on 25 July 2000 (stars). The aftershocks from these earthquakes are also shown (small circles). Courtesy of INETER.

Immediately after the earthquake there were many smaller, shallow earthquakes in a zone that includes the area between Masaya, Laguna de Apoyo, and W of Granada (figure 2). In the epicentral zone property was destroyed, cracks opened in the ground, landslides occurred, and trees fell. Several landslides occurred at the edges and steep walls of Laguna de Apoyo. A large number of earthquakes continued until 10 July (figure 3 and table 1). The number of earthquakes then diminished until 1554 on 25 July when a M 4.8 earthquake took place, initiating a series of smaller earthquakes that lasted until about 27 July.

Figure 3. Graph showing the number of earthquakes in the Masaya region between 4 and 30 July 2000. Courtesy of INETER.

Table 1. A summary of earthquakes in vicinity of Masaya and Laguna de Apoyo in early July 2000. Courtesy of INETER.

    Date     Time  Number of daily earthquakes     Maximum magnitude

    07 July  1330             180                         5.2
    08 July  1100              70                         3.8
    09 July  1200              81                         3.6
    10 July  1800              27                         3.1
    11 July  1800               6                         3.3
    13 July  1800              16                         2.8

The July earthquakes were the most destructive seismic events since the 1972 Managua earthquake. The epicentral zone of the July 2000 earthquakes correlates with the same active zones of past earthquakes, which are caused by fault movement between the Cocos and Caribbean plates.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/, Email: wil.gf@ineter.gob.ni).