About swarms of long-period earthquakes at volcano Nyamuragira of the Virunga region, Western Rift Valley of Africa (D.R. Congo)

Swarms of Long Period earthquakes generated by the Nyamuragira volcano for the period from January 2000 to November 2006 before 21 January 2000, 5 February 2001, 25 July 2002, 8 May 2004 and 27 November 2006 Nyamuragira eruptions have been analyzed. Interest is focused on the frequency distribution of these earthquakes and on the variation of the m-value of observed earthquake swarms. It is found that m-values which generally vary between 0.9 and 1.6, and shifts towards larger values of 1.7 to 3.23 two to four months prior to eruptions of the volcano.


Introduction
The Nyamuragira Volcano is situated in the Western Rift Valley of Africa, Virunga Province, at the northern edge of Lake Kivu. It covers an area of 800 km² (Pouclet and Villeneuve, 1971). The volcano Nyamuragira is characterized by frequent Hawaiian-type eruptions and highly potassic lavas (Hayashi et al., 1992;Tuluka et al., 2006). An active lava lake has persisted in its central crater from 1921 to 1938. Most eruptions, with the exception of the summit eruption of 1938, occur on the flanks of the volcano. This Volcano is also of interest due to its frequent, and potentially devastating fissural eruptions of highly fluidized lavas (Tedesco et al., 2007). In fact, Volcano Nyamuragira is among the most active volcanoes in the world as indicated, since 1901, by a sequence of 30 flanks eruptions (Pouclet, 1975;Smets et al, 2010, Kavotha et al., 2010. The most recent flank eruptions occurred on January 27, 2000;February 5, 2001;July 25, 2002;May 8, 2004 andNovember 27, 2006. At volcano Nyamuragira, seismic swarm composed mainly of long-period earthquakes precede by 2-4 months these flank eruptions (Zana et al., 1989;Tuluka, 2006).
However, not all earthquake swarms are followed by eruptions. Therefore, it is of great interest to draw characteristics of the swarms that are followed by eruptions in order to distinguish them from the swarms that are not followed by lava outbursts. This study deals with this aspect.
These Stations are each equipped with a shortperiod Kinemetrics vertical SS-1 ranger Seismometer (To=1sec) connected to a PS-2 portable seismic recorder instrument, three component short-period Lennartz(LE-3D/5s) seismometers and Nanometrics Trillium 40 broadband seismometer at KNN and KBB stations. Signals from the Kinemetrics seismometer are amplified and filtered in the amplifier module. The amplifier module has controls for amplifier gain and filter setting.
In this study, on the basis of waveform pattern, long-period earthquakes are defined as transient signals having weak P and emergent or no S phases with predominant frequency content between 1 and 3Hz . Long-period (LP) earthquakes are though to be caused by fluid pressurization and by shear failure, tensile fail or nonlinear flow processes at very shallow depths (McNutt, 2000).The LP are probably due to the excitation of some fixed cavity under the volcano and/or the migration of magmatic fluid consisting of hot water and/or magma (Fehler and Chouet, 1982;Nishimura et al., 2002;Tuluka et al., 2010) ( Figure 2).
The records (swarms) of one of these stations (Katale station) were analysed for the period from January 2000 to November 2006 and 44 swarms of long-period earthquakes were identified.
The Katale Station data have been chosen out of the seven stations taking part in the Goma Seismic network because this station is the nearest from the crater of volcano Nyamuragira and during the period from 2000 to 2006, the period when we made this study, the data used in it have only been recorded by Katale station without interuption as has been observed in almost the other stations comprising the seismic network.   All those records were reported to the same gain of 48db. We focussed our interest to swarms generated by Volcano Nyamuragira by analysing the amplitude distribution of the recorded earthquakes.
We considered 2006 and two examples were taken randomly to compile the two tables simply because it is the year where the last Nyamuragira eruption was observed during this study.
As noticed, the classification of the time interval in the two tables differs. It depends on the time of the beginning of a swarm. Thus, in the table 1, the swarm started on September 19 th , 2006 at 4Am and took end the following day (September 20 th ) at 4Am; whereas, in the table 2, it started at 9Am and took end at 3Am of the following day.
We used the Ishimoto and Iida relation (1939) who indicated that the number N of earthquakes whose amplitudes lie between A and A+dA vary as: Where A and N are the maximum amplitude and frequency, k and m are numerical constants.
k depends on the number of events, the recording station considered and the component used.
Suzuki (1953,1954,1958,1959) found that this formula was not influenced by the transmission of seismic waves and the seismometer used for observations.
Taking the logarithm of the equation (1), we obtain: With: N = Number of earthquakes recorded with amplitude A; A = Amplitude of seismogram. This relation has enabled us to determine the different values of « m » and with these values we have distinguished the different categories of swarms that are followed by eruptions and the swarms that are not followed by lava outbursts.
Standard error D is calculated by: Where: If the total number of earthquakes is N, and they are subdivided into n bins according to their amplitude, the constant k is calculated by: With all these formulas (2, 3, 4 and 5), we calculated the different values of "m" as those shown in the Table 3.

Results and discussion
We processed 44 seismic swarms consisting of 12720 micro earthquakes. As evidenced by Figu-re3A, several earthquake swarms characterized the period prior to the eruption of volcano Nyamuragira in 2004. This characteristic was com-      mon to the five successive eruptions that occurred from 2000 to 2006 at this volcano (2000; 2001; 2002; 2004 and 2006). According to Wafula et al. (2009), Mavonga et al. (2010 and Kavotha et al. (2010), these eruptions were characterized by precursory and post-eruption intense swarms of long-period earthquakes just in the time intervals of the seismicity increases as it is shown in each figure.
Clearly, the seismicity that precedes each eruption increases appreciably three to four months before the eruption. It is also clear in those figures that the number of long-period earthquakes increases when the volcanic eruption draws near.
However, some swarms of long period earthquakes have been observed before an eruption, but were not considered as precursors of the eruption. Indeed, by calculating their m-value, we find that they are not classified as swarms precursors. This is the case of the swarms of June 16, 2006 as well as from 05 May 2006 ( Figure 3B).
The most significant result in this analysis (Table 3) was that, the m-value was ranged from 0.9 to 3.23 with an interesting feature as follows: 1. For the 44 swarms, those that were not accompanied by an eruption, m-values were ranged from 0.9 to 1.6. 2. Particularly, the seismic swarms that just preceded the Nyamuragira eruptions (three or four months before an eruption) showed higher mvalues ranging from 1.7 to 3.23

Conclusion
In this work we have taken into account the daily frequency of long-period earthquakes. The major purpose of this analysis was to study the characteristics of swarms of long-period earthquakes observed at the Volcano Nyamuragira before, during and after the eruption. Through the analysis of 44 earthquakes swarms and the comparison of their m-values, we noticed that the frequency of the volcanic earthquakes generally increases when the volcanic eruption draws near. Figures3 (A-B-C-D) shows the history of the earthquake swarms for the period from 2000 to 2006: clearly, we notice that the Seismicity of volcano Nyamuragira significantly increases several months or weeks before the eruption. This seismicity decreases gradually towards the end of eruption.
The study of these earthquakes presents a particular interest in the prediction of the volcanic eruptions at volcano Nyamuragira.
Using the seismic data recorded at Katale station during the six years period from 2000 to 2006, we tried to characterize the volcanic earthquake swarms according to their m-values.
It was shown that the magnitude-frequencyrelation(Gutenberg-Richter/ishimoto-Lida relation) of swarms that forerun volcanic eruptions show higher m-values from 1.7 to 3.23 while other swarms are of lower m-values (0.9 -1.6).
It should also be noticed that a large number of volcanic earthquakes is recorded a few months or a few hours before the eruption. However, this seismic activity decreases rapidly and is followed by eruption tremors.