Seismic hazard assessment and volcanogenic seismicity for the Democratic Republic of Congo and surrounding areas, Western Rift Valley of Africa

The Western Rift Valley of Africa has experienced several severe earthquakes with magnitude M≥6 and volcanic eruptions in recent historical times. Most of these earthquakes occurred in the Democratic Republic of Congo (DRC) and neighbouring countries such as Uganda and Tanzania. The largest earthquake on record, which occurred at Kasanga (Tanzania) on 13 December 1910 in the Lake Tanganyika area, had a magnitude of Ms7.3. The most recent significant earthquake occurred approximately 20 km north of Bukavu City (DRC) on 03 February 2008 with magnitude Mw=6.0 in the Basin of Lake Kivu. The Virunga volcano group, located at the northern edge of Lake Kivu, consists of eight major volcanoes (Muhavura, Gahinga, Sabinyo, Visoke, Karisimbi, Mikeno, Nyiragongo and Nyamuragira). The volcanoes Nyiragongo and Nyamuragira have been the most active since 1882. A probabilistic approach was used to map the seismic hazard in Democratic Republic of Congo and surrounding areas, and assess the seismic hazard level for 14 cities in the region. Seismic hazard maps for 2%, 5% and 10% chance of exceedance of the indicated ground accelerations in 50 years were prepared using a 90-year catalogue compiled for homogeneous magnitudes (Mw); the attenuation relations of Mavonga (for the Western Rift Valley of Africa), Atkinson and Boore (for eastern and North America) and Jonathan (for eastern and southern Africa); and the EZ-Frisk software package. A Poisson model of earthquake occurrence that assumes that events are independent was adopted. Therefore, foreshocks, aftershocks and earthquake swarms were removed from the initial catalogue of 2249 events. Furthermore Mw=4 was selected as the lower magnitude bound (Mmin) because smaller earthquakes are considered unlikely to cause damage, even to houses that are poorly designed and built. Thus, any remaining events with Mw<4 were also excluded from the catalogue, leaving a sub-catalogue of 821 events The highest estimated levels of seismic hazard were found in the Lake Tanganyika Rift seismic zone, where peak ground accelerations (PGA) in excess of 0.32g, 0.22g and 0.16g are expected to occur with 2%, 5% and 10% chance of exceedence in 50 years, respectively. The seismic hazard in the Congo basin diminishes with distance away from the Western Rift Valley until, at a distance of about 450 km, the chance of exceeding 0.05 g (the threshold value of engineering interest) is less than 10% in 50 years. Finally, from the probabilistic seismic hazard analysis of the DR Congo and surrounding areas, four seismic zones were identified and rated as follows: Zone A (very high hazard), Zone B (high hazard), Zone C ( moderate hazard), and Zone D (low hazard). The zone A includes the Lake Tanganyika Rift zone where PGA values of 0.32g, 0.22g and 0.16 g are expected to occur with probability 2%, 5% and 10% in 50 years, respectively. Zone B includes the Lake Kivu Basin, Mount Ruwenzori and Lake Edouard region. Zone C includes Rutsuru, Masisi, Upemba area and a part of the Congo basin close to the Western Rift. The remainder of the Congo basin constitutes the zone D. To understand how volcanoes work and reduce the risk due to the Virunga volcanic eruptions in the Western Rift Valley of Africa, the crustal structure beneath the Virunga volcanic area was investigated and studies of volcanogenic seismicity were carried out. From these studies, it was found: High velocity material (6.9 to 7.3 km/s) lies beneath the Kunene (KNN) and Kibumba (KBB) broadband stations at depths of 3-20 km and 3-10 km, respectively, which is indicative of magma cumulates within the volcanic edifice. A low velocity zone was observed below KNN and KBB at depths of 20-30 km and 18-28 km, respectively, and with average velocity 6.1 km/s and 5.9 km/s. This low velocity zone may sample the magma chamber or a melt-rich sill. The depth of the crust-mantle transition zone beneath the area sampled by the KNN and KBB in the Virunga area was determined to be about 39 to 43 km and 30 to 39 km, respectively. Swarm-type seismicity composed mainly of long-period volcanic earthquakes preceded the eruptions of Nyamuragira volcano and was probably enhanced by tectonic seismicity related to rifting. A steady increase in seismicity at a constant rate from a deep magma feeder (located at about 20 to 30 km depth) was observed ten or eleven months before eruption. In the last stage (1 or 2 months) before the eruption, the hypocenters of long-period volcanic earthquakes became shallower. The new model of the local crustal seismic velocity for the Virunga area is useful to map the migration of hypocenters of earthquakes accurately and reveals trends that could be precursors of volcanic eruptions This pattern of seismicity prior to the volcanic eruptions, integrated with other available data (e.g. INSAR, GPS), may be used to characterize the volcanic process and forecast volcanic eruptions in the Virunga area

Stochastic Modeling of the Eruption History of Nyiragongo Volcano in the Virunga Volcanic Province, Western Branch of the East African Rift System

The modeling of statistical distribution of the eruptive frequency provides basic information to quantitatively assess the volcanic hazard and constrain the physics of the eruptive process. Here, we discuss the statistics of the time series of lateral eruptions of the Nyiragongo volcano in the Virunga Volcanic Province, western branch of the East African Rift System. We examine eruption data with a volcanic explosivity index of at least 1 listed in the Global Volcanism Network Bulletins. After investigating the completeness, stationarity, and independence of the eruption time series, we employ five distribution models (Brownian passage time, gamma, log-logistic, lognormal, and Weibull) to fit the repose time. First, we identify a clear tendency for events to cluster in time. We hypothesize two clusters, the ‘pre-1927’ cluster related to the intracraterial and volcanic activity of the lava lake, and the ‘post-1977’ cluster mainly related to lateral eruptions (i.e. those potentially generating lava flows). Using the maximum likelihood estimations, we evaluate the model parameters with a 95% confidence interval. Next, we use the Akaike Information Criterion to determine the most suitable distribution and we perform the Bayesian Model Averaging approach to assess uncertainty issues in model selection process. The results suggest that the BPT distribution provides the best fit for data of lateral eruptions (post-1977). Then we estimate the time-dependent probability of the occurrence of a lateral eruption for the 50-year period between 2022 and 2072. The estimates reach 50.79%, 88.61%, 97.59%, 99.50%, and 99.89% for 2032, 2042, 2052, 2062, and 2072 years, respectively

Stochastic Modeling of the Eruption History of Nyiragongo Volcano in the Virunga Volcanic Province, Western Branch of the East African Rift System

The modeling of statistical distribution of the eruptive frequency provides basic information to quantitatively assess the volcanic hazard and constrain the physics of the eruptive process. Here, we discuss the statistics of the time series of lateral eruptions of the Nyiragongo volcano in the Virunga Volcanic Province, western branch of the East African Rift System. We examine eruption data with a volcanic explosivity index of at least 1 listed in the Global Volcanism Network Bulletins. After investigating the completeness, stationarity, and independence of the eruption time series, we employ five distribution models (Brownian passage time, gamma, log-logistic, lognormal, and Weibull) to fit the repose time. First, we identify a clear tendency for events to cluster in time. We hypothesize two clusters, the ‘pre-1927’ cluster related to the intracraterial and volcanic activity of the lava lake, and the ‘post-1977’ cluster mainly related to lateral eruptions (i.e. those potentially generating lava flows). Using the maximum likelihood estimations, we evaluate the model parameters with a 95% confidence interval. Next, we use the Akaike Information Criterion to determine the most suitable distribution and we perform the Bayesian Model Averaging approach to assess uncertainty issues in model selection process. The results suggest that the BPT distribution provides the best fit for data of lateral eruptions (post-1977). Then we estimate the time-dependent probability of the occurrence of a lateral eruption for the 50-year period between 2022 and 2072. The estimates reach 50.79%, 88.61%, 97.59%, 99.50%, and 99.89% for 2032, 2042, 2052, 2062, and 2072 years, respectively

Seismic activity related to the June 2014 New lava Lake apparition at Nyamulagira volcano in the Western Branch of the East African Rift

peer reviewedA lava lake activity is observed at Nyamulagira volcano during this last decade. The training process of this lava lake began in 2012 by the release of gas fumes that was been continuously observed in the crater of the volcano. However no change in seismic activity was observed compared to the usual activity of the volcano until April 2014. On 22 June 2014, an activity of glow was observed by the Goma Volcano Observatory and the inhabitants of the city of Goma. On July 3rd 2014, the United States's organization NASA (National Aeronautics and Space Administration) noted this situation by its satellite detection and published on his Web site the apparition of a new lava lake in the crater of Nyamulagira. Nyamulagira volcano (in its known history) logged again a lave lake from 1921 to 1938. Here are analyzed the seismic activity which preceded this new event at mount Nyamulagira. It was found that this event was been preceded by a significant swarm activity of Long Period earthquakes in April 2014, six hybrid earthquakes and volcanic tremors in June 2014. The April 2014 swarm of low frequency earthquakes lasted about four days and was been located in the NorthEastern part of the Nyamulagira crater (at the place where appeared the new lava lake) and was been interpreted as expressed by the precursor movements of the opening of the crater. The six hybrid earthquakes were been interpreted also as the events that led to a falling movement of the land masses and the opening of the crater. After the visibility of the lava lake in June 2014, the activity of LP events reduced, no swarm and hybrid events were been recorded from this period, but the number of Volcano-Tectonic events remained constant