Reports from March 2002 through September 2003 were provided by Instituto Nicaraguense de Estudios Territoriales (INETER). Activity has been generally constant from 2001 through 2003, with tremor and very low magnitude earthquakes, usually detected by the station on the N side of the volcano (CONN). Throughout the summary period, there were occasionally technical difficulties at the Mombacho station, so no activity was registered on those days. Periods of noticeably high seismicity occurred between June and October 2002, in April 2003, and during June-August 2003 (table 2).

Table 2. Monthly count of earthquakes registered at Concepción, February 2002-September 2003. Courtesy of INETER.

Seismicity between April 2002 and February 2003. In April 2002 there were 1,433 microearthquakes detected, a significant increase over the total of 33 recorded during February-March; the majority of the seismicity was recorded on 5, 9, and 10 April. The majority of activity was classified as long-period (LP) events with frequencies between 1 and 4 Hz; some events related to rock fracturing had frequencies between 8 and 10 Hz. Activity in May was similar, with low-magnitude earthquakes and tremor. However, due to problems with CONN, only 346 earthquakes were detected. On the day of the highest activity, 19 May, 76 microearthquakes were recorded. One earthquake, only recorded at CONN, occurred on 28 May with an S-P time difference of 0.8 seconds, suggesting the hypocenter was at ~ 6.4 km depth.

June-August activity was consistent with previous months. June recorded 865 microearthquakes, while July recorded 1,229 events, mostly early in the month. CONN registered 1,219 earthquakes in August. Seismicity was heaviest on 29 and 30 August, with 116 and 139 earthquakes, respectively. The earthquakes were classified as mainly LP. On 4 August an earthquake of M 2.7 occurred ~ 15 km S of the volcano at a depth of 12.5 km. On 14 August another seismic station (URBN) was installed around Concepción, this one in the community of Urbaite, on the S flank.

In September activity levels were again generally stable. Reception problems continued but by 2 September the signal was reestablished. There were 1,250 earthquakes recorded, the majority at the end of the month, with highs of 149 on 26 September and 152 on 27 September. In October, technical problems prevented recordings until after 21 October. However, in those ten days 1,031 microearthquakes registered, with 161 and 172 on 28 and 31 October, respectively. Both CONN and URBN detected lahars on the N flank on 28 and 31 October, during a time of moderate rainfall. Activity declined in November, although 784 earthquakes were still recorded. Activity was highest on 1 and 2 November, with 115 and 129 earthquakes respectively.

Activity declined further in December, with 389 microearthquakes, although no recordings were obtained on five days due to technical problems. Similar to the past several months, activity was classified as generally LP or degassing events. Only 179 microearthquakes were recorded in January (data was not received on four days). In February, only 108 microearthquakes were detected. All events ranged between 1.5 and 3.5 Hz frequency and were classified as LP or degassing events.

Seismicity between March and June 2003. Beginning in March 2003 and continuing through April and May, activity increased to unusual levels. Between 2 and 18 March CONN registered a series of 31 earthquakes with considerable amplitude; they were not felt by residents in the area. Because the stations at Urbaite (URBN) and Maderas (MADN) were not working, only CONN recorded the activity. However, the difference in arrival times between the S and P waves indicated a depth of 15-16 km. The seismic signals began at low frequencies, followed by an increase in the spectral frequency content.

On 19 March the volcano entered a new period of increased activity. By the end of March more than 700 events were registered by the seismic station. Although during the first week of April very few earthquakes were recorded, by 11 April the station began to register a series of earthquakes of considerable amplitude, similar to the series in March. More than 1,400 events were recorded, mainly LP events. Only 476 events were recorded in May, also mainly LP events. A total of 1,298 events were recorded in June.

Seismicity between July and September 2003. Unusual seismic activity, including harmonic tremor that began at the end of June, continued in July. Starting 1 July, CONN began to register a series of LP events accompanied by low-frequency harmonic tremor and a saturated seismic signal like the one that occurred in March. Harmonic tremor occurred throughout July, with episodes of 7 minutes on 2 July, 45 minutes on 4 July, and about 60 minutes on 13 July. Long-period earthquakes and harmonic tremor increased between 23 July and the end of the month.

A total of 43 earthquakes with saturated amplitudes were registered only by CONN in July, but it was not possible to determine locations or magnitudes. The time difference in the S-P arrivals implied hypocenters 15-16 km beneath the volcano. They lasted a little over a minute and had a combination of high and low frequencies. The earthquakes with saturated signals had frequencies of 2-4 Hz; some were accompanied by a low-energy high-frequency signal. The majority of these events (7) occurred on 15 and 16 July, and had ceased by 23 July. Taking the spectral content into account, these appear to be LP events; however, it is not very common for LP events to begin with low frequencies followed by high. No data were recorded on 18, 21, and 22 July due to technical problems at Mombacho, but a total of more than 1,100 earthquakes were recorded by seismic stations.

With 1,586 earthquakes registered, seismicity was unusually high in August. Harmonic tremor also increased. Starting 1 August, CONN began to register a series of LP earthquakes accompanied by low-frequency harmonic tremor and earthquakes with saturated signals, as in previous months. Frequency ranged from 1 to 2.5 Hz, with occasionally higher values. On 16 August tremors were registered that lasted for four minutes; on 22 August, after two days with no tremor and few earthquakes, there was more unusual activity consisting of seven hours of intermittent tremor episodes.

Seismicity continued in September with 828 total events, the majority on 12 and 13 September. Seismic tremor was present throughout September, with frequency levels similar to those of the previous months.

Geological Summary. Volcán Concepción is one of Nicaragua's highest and most active volcanoes. The symmetrical basaltic-to-dacitic stratovolcano forms the NW half of the dumbbell-shaped island of Ometepe in Lake Nicaragua and is connected to neighboring Madera volcano by a narrow isthmus. A steep-walled summit crater is 250 m deep and has a higher western rim. N-S-trending fractures on the flanks have produced chains of spatter cones, cinder cones, lava domes, and maars located on the NW, NE, SE, and southern sides extending in some cases down to Lake Nicaragua. Concepción was constructed above a basement of lake sediments, and the modern cone grew above a largely buried caldera, a small remnant of which forms a break in slope about halfway up the N flank. Frequent explosive eruptions during the past half century have increased the height of the summit significantly above that shown on current topographic maps and have kept the upper part of the volcano unvegetated.

Information Contacts: Emilio Talavera, Instituto Nicaraguense de Estudios Territoriales (INETER), Dirección General de Geofísica, Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/ geofisica).