Joao F. B. D. Fonseca

A past giant lateral collapse and present-day flank instability of Fogo, Cape Verde Islands

Fogo island is a large and extremely steepsided oceanic island volcano in the Cape Verde archipelago. It has a large (ca. 9 km across) east facing summit collapse structure, the Monte Amarelo collapse, with a probable volume of at least 150–200 km3. For most of its history the Monte Amarelo volcano had a small but productive central vent complex and radial rift zones fed by laterally propagating dykes. Shortly before the collapse the latter were replaced by north–south-trending arrays of en echelon, vertically propagating dykes. Since the Monte Amarelo collapse the scar has partly filled with a new volcano, the Cha das Caldeiras volcano. The summit cone of this volcano, the Pico do Fogo, is a very young feature but has been abandoned in the most recent phase of activity, from the 18th century onwards. The same period has also seen the abandonment of earlier radial rift zones with laterally propagating dykes and their replacement with en echelon arrays of vents fed by vertically propagating dykes. These form an N–S-trending array within the older collapse structure and are associated with seaward displacement of the eastern flank of the volcano within the old collapse structure. The most recent eruptions, those of 1951 and 1995, appear to be associated with episodes of flank instability manifested in N–S surface fissuring and east facing normal faults. These recent structural changes in the volcano parallel those which took place in the Monte Amarelo volcano prior to its collapse.

Probabilistic Seismic-Hazard Assessment for Portugal

The probabilistic seismic hazard of Portugal is analyzed with a logic tree approach. A critical part of the work was the review of the seismic catalog and the moment magnitude (M) estimation for historical earthquakes. To produce a cat- alog with a uniform magnitude scale, the instrumental magnitudes were converted to M through empirical relations. Two seismic zonations were considered, each in- cluding two broad tectonic zones and a set of smaller seismicity zones. Catalog completeness and the b-value for the truncated exponential recurrence model were calculated for the broad sources defined by tectonic criteria. The smaller seismicity sources were used to calculate the a-value using the fixed b-value of the correspond- ing tectonic zone. This approach allows for a larger amount of data to estimate the most critical parameters by statistical methods, without compromising the spatial detail of hazard results. Three published attenuation relations were used in the logic tree, with weights that were based on tectonic considerations and on the comparison with macroseismic data converted to horizontal peak ground acceleration (PGA). The Ambraseys et al. (1996) attenuation model for PGA, used by most previous hazard studies of the region, seems to underestimate considerably the ground motion for mainland Portugal. A total of 96 hazard curves were calculated with SEISRISK III for each point of the map. The resulting mean hazard map for 10% exceedence probability in 50 years displays PGA values that range from 0.05g to 0.20g. These mean values are slightly higher than in previous PGA studies. The hazard patterns obtained display a maximum related to intraplate onshore seismicity, whereas pre- vious studies using intensities highlighted the southwest offshore contribution. Fur- ther work on ground-motion attenuation in western Iberia is necessary to improve the seismic-hazard assessment.

Lisbon 1755: A Case of Triggered Onshore Rupture?

It has been widely recognized, both in classical and in modern studies, that the Lisbon earthquake of 1755 was a multiple event, composed of three shocks separated by a few minutes (see, e.g., Reid, 1914). Attempts to constrain the location of the source have led to a diversity of proposals, reflecting apparent contradictions in the data. The tsunami and damage along the south and southwest Iberian coast and in Morocco favor an offshore source, whereas the presence of an additional zone of strong shaking in the Lower Tagus Valley (LTV), near Lisbon, favors a more northerly location. By combining the contemporary accounts with intensity data from other earthquakes, we favor a compound source with a large distance between the faults. We propose that, although the mainshock was offshore, the resulting stress changes induced the rupture of the LTV fault, at a distance on the order of 350 km (but subject to large uncertainty in the offshore location), a few minutes after the mainshock. We favor this model, rather than site effects causing high intensities in the Lisbon area, because the highest intensities show a negative correlation to soft soil. Several other phenomena described in the eyewitness accounts can also be explained by the local rupture now proposed, such as a tsunamilike wave in the Tagus River, ground deformation affecting the course of the Tagus River, and the spatial pattern of damaging aftershocks. Recognition of this “missing” episode of rupture on the LTV fault significantly changes the hazard estimate for the Lisbon area.

Palaeoseismology of the Vilariça Segment of the Manteigas-Bragança Fault in northeastern Portugal

Abstract The Manteigas-Bragança fault is a major, 250-km-long, NNE-striking, sinistral strike-slip structure in northern Portugal. This fault has no historical seismicity for large earthquakes, although it may have generated moderate (M5+) earthquakes in 1751 and 1858. Evidence of continued left horizontal displacement is shown by the presence of Cenozoic pull-apart basins as well as late Quaternary stream deflections. To investigate its recent slip history, a number of trenches were excavated at three sites along the Vilariça segment, north and south of the Douro River. At one site at Vale Meão winery, the occurrence of at least two and probably three events in the past 14.5 ka was determined, suggesting an average return period of about 5–7 ka. All three events appear to have occurred as a cluster in the interval between 14.5 and 11 ka, or shortly thereafter, suggesting a return period of less than 2 ka between events within the cluster. In the same area, a small offset rill suggests 2–2.5 m of slip in the most recent event and about 6.1 m after incision below a c. 16 ka alluvial fill event along the Douro River. At another site along the Vilariça River alluvial plain, NE of the Vale Meão site, several trenches were excavated in late Pleistocene and Holocene alluvium, and exposed the fault displacing channel deposits dated to between 18 and 23 ka. In a succession of closely spaced parallel cuts and trenches, the channel riser was traced into and across the fault to resolve c. 6.5 m of displacement after 18 ka and c. 9 m of slip after c. 23 ka. These observations yield a slip rate of 0.3–0.5 mm/a, which is consistent with earlier estimates. Combining the information on timing at Vale Meão winery and displacement at Vilariça argues for earthquakes in the M7+ range, with coseismic displacements of 2–3 m. This demonstrates that there are potential seismic sources in Portugal that are not associated with the 1755 Lisbon earthquake or the Tagus Valley, and, although rare, large events on the Vilariça fault could be quite destructive for the region. This work provides an analogue for the study of active faulting in intracontinental settings and supports the view that earthquakes within intracontinental settings tend to cluster in time. In addition, this study highlights the usefulness and application of multiple field, remote sensing and geochronological techniques for seismic hazard mitigation.

Multiparameter monitoring of Fogo Island, Cape Verde, for volcanic risk mitigation

Fogo Island in the Cape Verde Archipelago (North Atlantic) is a stratovolcano of nearly conical shape that rises 2829 m above sea level and V6000 m above the surrounding seafloor. With a population of 40 000, the island has known intense historical volcanic activity since AD 1500, with an average interval between eruptions of the order of 20 years. Twentieth-century rates were more subdued, with only two flank eruptions in 1951 and 1995. Following the 1995 eruption, increased awareness of the volcanic hazard affecting the population of the island led to the deployment of the permanent VIGIL Network. Seismographic stations (both broadband and short-period), tiltmeters and a CO2 sensor where installed in Fogo, together with a telemetry infrastructure to allow remote real-time monitoring. A broadband seismographic station was installed in neighbour Brava Island. The operation of the network was complemented by the introduction of routine geodetic and microgravity surveying and the operation of an automatic meteorological station. In this paper, we describe the methodology adopted to monitor the volcanic activity, combining real-time data analysis (volcanotectonic and volcanic earthquakes, volcanic tremor and tilt) with repeated surveying at intervals of several months (GPS, microgravity). Examples of data from the first years of operation are presented. In particular, the data pertaining to a period of anomalous activity in September^October 2000 are discussed, in the context of the risk mitigation strategy currently being developed.

Fogo Volcano, Cape Verde Islands : seismicity-derived constraints on the mechanism of the 1995 eruption

Seismic data recorded in the vicinity of the active vent of the 1995 Fogo eruption [Global Volcanism Network Bulletin, 1995, 20(3) 2–4.] is used to constrain the associated stress field and deformation. Using the frequency content of the seismograms to distinguish between brittle fracture of cold host rock and deformation in the vicinity of the intruding magma, a sub-vertical dyke with 060° strike is identified as the feeder of the eruption, and delineated down to a depth of about 4 km. The local stress field during the eruption is estimated from composite focal mechanisms. Besides the expected σ3 direction normal to the dyke, a group of focal solutions point to a stress field with nearly dyke-parallel σ3, which is interpreted as a re-adjustment of the edifice following the eruption, maybe partially controlled by gravity. This effect may have increased the instability of the steep eastern flank of the island, since the deformation detected during the magma drainback is compatible with predominantly dip-slip motion on east-dipping surfaces.

Investigations unveil Holocene thrusting for onshore Portugal

A current paleoseismological investigation of the Lower Tagus Valley (LTV), near Lisbon, Portugal, brought to light unprecedented information about the seismotectonics of this area of important onshore intraplate seismicity Through a combination of geomorphic, geological, and geophysical tools, a site with evidence for recent deformation was tracked down and selected for detailed study. Upon trenching, a very clear exposure of a thrust fault can be observed. Its last episode of deformation was constrained with carbon 14 to postdate A.D. 365.

Incorporating Descriptive Metadata into Seismic Source Zone Models for Seismic-Hazard Assessment: A Case Study of the Azores-West Iberian Region

In probabilistic seismic-hazard analysis (PSHA), seismic source zone (SSZ) models are widely used to account for the contribution to the hazard from earth- quakes not directly correlated with geological structures. Notwithstanding the impact of SSZ models in PSHA, the theoretical framework underlying SSZ models and the criteria used to delineate the SSZs are seldom explicitly stated and suitably docu- mented. In this paper, we propose a methodological framework to develop and docu- ment SSZ models, which includes (1) an assessment of the appropriate scale and degree of stationarity, (2) an assessment of seismicity catalog completeness-related issues, and (3) an evaluation and credibility ranking of physical criteria used to delin- eate the boundaries of the SSZs. We also emphasize the need for SSZ models to be supported by a comprehensive set of metadata documenting both the unique character- istics of each SSZ and the criteria used to delineate its boundaries. This procedure ensures that the uncertainties in the model can be properly addressed in the PSHA and that the model can be easily updated whenever new data are available. The pro- posed methodology is illustrated using the SSZ model developed for the Azores-West Iberian region in the context of the Seismic Hazard Harmonization in Europe project (project SHARE) and some of the most relevant SSZs are discussed in detail. Online Material: Tables describing characteristics and boundaries of the seismic source zones.

Comparison and cross-checking of historical, archaeological and geological evidence for the location and type of historical and sub-historical eruptions of multiple-vent oceanic island volcanoes

Abstract Oceanic island volcanoes, like many others, have many small vents scattered over their flanks in addition to, or in place of, large summit vents. These small vents are commonly monogenetic and many eruptions of this type of volcano involve activity at more than one such vent: lines of volcanic cones are often produced by eruptions fed by dykes, and if more than one dyke is emplaced during an eruption these vents can be along different alignments, and many kilometres apart. Identifying which vents were produced in which eruptions is an important problem in reconstructing the development of multiplevent volcanoes. Reconstruction of historical and sub-historical eruptions of two oceanic island volcanoes, the Cumbre Vieja volcano on La Palma in the Canary Islands and Cha das Caldeiras volcano on Fogo, Cape Verde Islands, has indicated that historical eyewitness accounts and archaeological evidence can be extremely valuable adjuncts to detailed geological studies. In contrast, secondary accounts including folk memories and earlier accounts in the scientific literature are commonly inconsistent both with the eyewitness accounts and with the results of detailed geological mapping, archaeological evidence or other historical documents. A common source of error is confusion of the vents of historical eruptions with older but larger or more prominent volcanic vents that lie along the same line of sight as viewed from adjacent settlements or from convenient viewpoints. Examples of this are provided by mis-locations of the vents of the AD 1677 eruption on La Palma and of some of the vents of the AD 1951 eruption on Fogo. Another problem arises when the location or style of eruption on a volcano has changed in early historical time, as has occurred on Fogo. The differences between early historical accounts of eruptions on this volcano and the more detailed accounts of more recent eruptions has led to the discrediting of the former by some researchers, whereas geological studies have supported the early historical accounts.

Damage from lava flows: insights from the 2014–2015 eruption of Fogo, Cape Verde

Fast-moving lava flows during the 2014–2015 eruption of Fogo volcano in Cape Verde engulfed 75% (n = 260) of buildings within three villages in the Chã das Caldeiras area, as well as 25% of cultivable agricultural land, water storage facilities and the only road into the area. The eruption had a catastrophic impact for the close-knit communities of Chã, destroying much of their property, land and livelihoods. Volcanic risk assessment typically assumes that any object - be it a building, infrastructure or agriculture - in the path of a lava flow will be completely destroyed. Vulnerability or fragility functions for areas impacted by lava flows are thus binary: no damage in the absence of lava and complete destruction in the presence of lava. A pre-eruption field assessment of the vulnerability of buildings, infrastructure and agriculture on Fogo to the range of volcanic hazards was carried out in 2010. Many of the areas assessed were subsequently impacted by the 2014–2015 eruption and, shortly after the eruption ended, we carried out a post-eruption field assessment of the damage caused by the lava flows. In this paper, we present our findings from the damage assessment in the context of building and infrastructural vulnerability to lava flows. We found that a binary vulnerability function for lava flow impact was appropriate for most combinations of lava flow hazard and asset characteristics but that building and infrastructure type, and the flow thickness, affected the level of impact. Drawing on these observations, we have considered potential strategies for reducing physical vulnerability to lava flow impact, with a focus on buildings housing critical infrastructure. Damage assessments for lava flows are rare, and the findings and analysis presented here are important for understanding future hazard and reconstruction on Fogo and elsewhere.