Afar Depression Research Articles

Crustal Structure of the Flood Basalt Province of Ethiopia from Constrained 3-D Gravity Inversion

The Oligocene Afar mantle plume resulted in the eruption of a large volume of basaltic magma, including major sequences of rhyolitic ignimbrites, in a short span of time across Ethiopia. In order to assess the impact of these magmatic processes on the crust and to investigate the general crustal configuration beneath the Ethiopian plateau, northern part of the Main Ethiopian Rift and the Afar depression, analysis and modeling of the gravity field have been conducted. The Bouguer gravity map is dominated by long-wavelength anomalies that primarily arise from the isostatic compensation of the topography. Consequently, anomalies within the crust/upper mantle are masked and quantitative interpretation becomes difficult. The long-wavelength anomalies are approximated using admittance technique and subsequently removed from the Bouguer anomalies to obtain the residual isostatic anomalies. The residual map contains both short- and intermediate-wavelength anomalies related to geologic and tectonic features. The long-wavelength regional isostatic field is used to map the crust-mantle interface and the results are in good agreement with those determined by other geophysical methods. Seismic constrained gravity inversion was performed on the isostatic residual field and series of three-dimensional models have been constructed for the structures of the crust and upper mantle beneath the uplifted and rifted flood basalt province of northern Ethiopia. The inversion results have shown that the NW plateau has thick crust that rests on normal lithospheric mantle. Afar, On the other hand, is marked by thin stretched crust resting on a low-density upper mantle indicating a hotter thermal regime and partial melt. No lithospheric mantle is observed beneath Afar. The models further indicate the presence of an extensive sub-crustal thick (~12 km on average) and high-density (~3.06 gm/cc) mafic accreted igneous layer of fractionated cumulate (magmatic underplating) beneath the NW plateau. The study suggests that the underplate was fundamental to the accretion process and may have played a role in compensating most of the plateau uplift and in localizing stresses.

Counterclockwise block rotation linked to southward propagation and overlap of sub-aerial Red Sea Rift segments, Afar Depression: Insight from paleomagnetism

Thirty-four cooling units from the Pleistocene extrusive volcanic rocks exposed in regions bordered by the active Red Sea magmatic segments, in the northwestern central Afar Depression, were sampled for paleomagnetic study. Six to 12 samples were collected from each paleomagnetic site. Samples were demagnetized using Alternating Field (AF) and Thermal (TH) demagnetization techniques. The samples were then measured using the JR–6A spinner magnetometer available at the department of Earth Sciences of Addis Ababa University. The Natural Remanent Magnetization (NRM) direction reveals mostly one or two simple and straightforward components of magnetizations; the first, low-stability component is isolated by heating to 100°C–300°C or by AF of 10–30mT. The magnetization directions after these steps generally define straight lines that are directed towards the origin, and interpreted as primary NRM or ChRM (Characteristic Remanent Magnetization) and the direction of magnetization was determined. Results of the magnetization decay curve plots and rock magnetic analyses using a Variable Field Translation Balance (VFTB) indicate that magnetic mineralogy is dominated by Ti-poor titano-magnetite with magnetic grain sizes in the pseudo-single domain (PSD) range, with minor presence of goethite and maghemite in a few cases. An overall mean direction calculated for the 26 sites located within the overlap zone is obtained (D=354.4°, I=13.2°, N=26, K=43.5, α95=4.3°), and compared to the expected Geomagnetic Axial Dipole (GAD) field, based on the Apparent Polar Wander Path (APWP) Curve of the African plate (Besse and Courtillot, 1991, 2003), a difference in declination ΔD=−6.5°±4.0° is obtained. This declination difference is interpreted as counterclockwise rotation about a vertical axis, in agreement with rift propagation and right stepping overlap geometry of the Alayta–Dabbahu magmatic segments. The difference in inclination ΔI=7.8°±3.9° is believed to be related to the long-standing non-dipole field in Afar (Courtillot et al., 1984; Kidane et al., 2003).

Uppermost mantle (Pn) velocity model for the Afar region, Ethiopia: an insight into rifting processes

The Afar Depression, Ethiopia, offers unique opportunities to study the transition from continental rifting to oceanic spreading because the process is occurring onland. Using traveltime tomography and data from a temporary seismic deployment, we describe the first regional study of uppermost mantle P-wave velocities (VPn). We find two separate low VPn zones (as low as 7.2 km s−1) beneath regions of localized thinned crust in northern Afar, indicating the existence of high temperatures and, potentially, partial melt. The zones are beneath and off-axis from, contemporary crustal magma intrusions in active magmatic segments, the Dabbahu-Manda-Hararo and Erta'Ale segments. This suggests that these intrusions can be fed by off-axis delivery of melt in the uppermost mantle and that discrete areas of mantle upwelling and partial melting, thought to characterize segmentation of the uppermost mantle at seafloor spreading centres, are initiated during the final stages of break-up.

Continental Flood Basalts and Rifting: Geochemistry of Cenozoic Yemen Volcanic Province

Rift formation is a crucial topic in global tectonics. The Yemen rift-related area is one of the most important provinces, being connected to the rifting processes of the Gulf of Aden, the Red Sea and Afar Triangle. In this paper, a review of the Yemen volcanic province and its relations with the Red Sea rifting are presented. Tertiary continental extension in Yemen resulted in the extrusion of large volumes of effusive rocks. This magmatism is divided in the Oligo-Miocene Yemen Trap Series (YTS) separated by an unconformity from the Miocene-Recent Yemen Volcanic Series (YVS). Magmas of the YTS were erupted during the synrift phase and correlate with the first stage of sea-floor spreading of the Red Sea and the Gulf of Aden (30 - 15 Ma), whereas the magmas of the YVS were emplaced during the post rift phase (10 - 0 Ma). A continental within plate character is recognized for both the YTS and YVS basalts. The YTS volcanic rocks are contemporaneous with, and geochemically similar to, the Ethiopian rift volcanism, just as the volcanic fields of the YVS are geochemically alike to most of the Saudi Arabian volcanics. YTS and YVS have analogous SiO2 ranges, but YVS tend to have, on average, higher alkalis and MgO contents than YTS. Fractional crystallization processes dominate geochemical variations of both series. Primitive magmas (MgO > 7.0%) are enriched in incompatible elements and LREEs with respect to primitive mantle, but YVS are more enriched than YTS. To first order, the different geochemical patterns agree with different degrees of partial melting of an astenospheric mantle source: 25% - 30% of partial melting for YTS and 10% - 3% for YVS. Secondly, the higher degree of enrichment in incompatible elements of YVS reflects also greater contribution of a lithospheric mantle component in their source region.

Gas chemistry of the Dallol region of the Danakil Depression in the Afar region of the northern-most East African Rift

A combination of noble and major gas composition and isotope geochemistry provides a window into the source of volatiles and the mechanisms of transport associated with a series of hot springs located near the Dallol volcano within the Danakil Depression along the Red Sea arm of the Afar triple junction. The helium isotopic composition of these gases range up to 11.9 times the atmospheric ratio (11.9R/Ra), which suggests that the Afar plume interacts with the Afar depression across at least the 300km transect from Tendaho-Gabo basin to Dallol within the Danakil Depression. The 4He/40Ar* of ~14 in the mantle-rich end-member at Dallol indicates significant degassing prior to emplacement at Dallol either during basaltic dyke intrusions beneath the Danakil Depression or during the release and transport of fluids from a degassed subsolidus source in the upper mantle along high permeability fracture zones. The CO2/3He of the magmatic end-member is ~2× higher (7.7×109) and more positive δ13C (CO2) (−2.1‰) than other archetypal plumes (e.g. Hawaii, Iceland, etc.). The Dallol composition is consistent with a hypothetical model that assumes a plume-type starting composition and experiences ~92% degassing (where helium is preferentially degassed with respect to CO2) and the addition of CO2 from the thermal degradation of carbonate. Non-atmospheric excess N2 with a δ15N (N2) of +3.5 to +4‰ dominates the Dallol volatiles and suggests interaction between mantle fluids and Proterozoic meta-sediments. By comparing and modeling the range in atmospherically (e.g. 20Ne, 36Ar, 84Kr) and mantle-derived (e.g. 4He/40Ar* and CO2/3He) components in Dallol volatiles, we propose that the coherent variations in these gases result from mixing of magmatic volatiles with extremely degassed remnant fluids present within the hydrothermal reservoir.

The development of extension and magmatism in the Red Sea rift of Afar

Despite the importance of continental breakup in plate tectonics, precisely how extensional processes such as brittle faulting, ductile plate stretching, and magma intrusion evolve in space and time during the development of new ocean basins remains poorly understood. The rifting of Arabia from Africa in the Afar depression is an ideal natural laboratory to address this problem since the region exposes subaerially the tectonically active transition from continental rifting to incipient seafloor spreading. We review recent constraints on along-axis variations in rift morphology, crustal and mantle structure, the distribution and style of ongoing faulting, subsurface magmatism and surface volcanism in the Red Sea rift of Afar to understand processes ultimately responsible for the formation of magmatic rifted continental margins. Our synthesis shows that there is a fundamental change in rift morphology from central Afar northward into the Danakil depression, spatially coincident with marked thinning of the crust, an increase in the volume of young basalt flows, and subsidence of the land towards and below sea-level. The variations can be attributed to a northward increase in proportion of extension by ductile plate stretching at the expense of magma intrusion. This is likely in response to a longer history of localised heating and weakening in a narrower rift. Thus, although magma intrusion accommodates strain for a protracted period during rift development, the final stages of breakup are dominated by a phase of plate stretching with a shift from intrusive to extrusive magmatism. This late-stage pulse of decompression melting due to plate thinning may be responsible for the formation of seaward dipping reflector sequences of basalts and sediments, which are ubiquitous at magmatic rifted margins worldwide. ► We review along-axis variations in extensional processes in the Red Sea rift of Afar. ► Change in rift morphology is coincident with crust thinning. ► Spatial variations in proportion of extension by ductile plate stretching. ► A pulse of decompression melting forms seaward dipping reflector sequences.

Morphological landscapes of ethiopia

We give an outline of the topographic features of the Main Ethiopian Rift and the Afar Triangle as well as of the Ethiopian Highlands. We highlight the role played by the active tectonics and young volcanism which have largely determined the appearance and morphological diversity of contemporary landscapes of Ethiopia. A brief description of several unique natural sites is provided.

Morpho-tectonic analysis of the Tekeze River and the Blue Nile drainage systems on the Northwestern Plateau, Ethiopia

This study examines the morpho-tectonic evolution of the drainage system in the Northwestern Plateau in Ethiopia dominated by the Tekeze River and the Blue Nile. The Northwestern Plateau is underlain by Precambrian crystalline rocks, followed by Mesozoic sedimentary section and topped with Oligocene–Quaternary volcanic rocks. The plateau is bounded in the east and southeast by the Afar Depression and the Main Ethiopian Rift, respectively. Digital Elevation Models (DEMs) from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the Shuttle Radar Topography Mission (SRTM) data are analyzed to extract morpho-tectonic parameters including the Normalized Steepness Index (Ksn), the Concavity (θ) and the Regression Fit (r2) between the observed and predicted channel profiles from the sub-basins and the tributaries of the Tekeze River and the Blue Nile. Analysis of these morpho-tectonic parameters has shown that the evolution of the drainage systems on the Northwestern Plateau was influenced by three tectonic and geological events. The first event resulted in a broad and regional uplift of the plateau, most likely due to the rise of the Afar mantle plume ∼30Ma. This regional uplift was accompanied by moderate incision rate of the Tekeze River and the Blue Nile drainage systems within the entire Northwestern Plateau. The second event, which was in the form of shield volcanoes build-up, occurred at ∼22Ma and resulted in localized increase in the incision rate around these volcanoes. The third event is manifested by rift-flank uplift at ∼11Ma on the western escarpments of the Afar Depression and the northwestern escarpments of the Main Ethiopian Rift. This event resulted in an increase in the incision rate of the Tekeze River and the Blue Nile drainage systems, but this increase seems to diminish towards the west and northwest leaving the drainage systems in the lower reaches of the two rivers relatively tectonically undisturbed, hence allowing for the establishment of a long-lived hydrological stability.

Time independent seismic hazard analysis of the afar depression

A Bayesian approach is applied to conduct a seismic hazard analysis in Afar Triple junction, resulting in a spatial distribution of the probability of exceedance in 100 years for earthquakes of MSK intensities VI and VII. The highest values of the exceedance probability for intensity VII are relatively scattered, showing spatial distribution that conforms with the RRR (rift-rift-rift) relative translational and rotational motions of the three bounding (Main Ethiopian Rift-Southern Red Sea Rift-Gulf of Aden Rift). The spatial distribution of the probability of exceedance for intensity VI is similar, but in scope. Iso-curves of the probability of exceedance for intensity VI seem to strike in a NW and NE direction while the intensity VII distribution is characterized by NW, NE and E-W directions. The map of spatial distributions of the probability of exceedance indicates specific areas of high seismic hazard in the future in the study area. Key words: Afar depression, Bayesian seismic hazard assessment, rift.

Magnetic stripes of a transitional continental rift in Afar

Magnetic stripes parallel to mid-ocean ridges are one of the most signifi cant consequences of seafl oor spreading, and have played an essential role in the establishment of the plate tectonics theory and the determination of seafl oor spreading rates. Similar magnetic anomaly patterns have not been well documented subaerially in continental rifts transitioning into seafl oor spreading centers. Here, using high-resolution magnetic data that were collected across the Tendaho Graben in the Afar Depression, Ethiopia, we document one of the fi rst examples of subaerial magnetic lineations similar in pattern and amplitude to those that characterize seafl oor spreading centers. The ~50-km-wide graben is the southernmost structural and geomorphological expression of the on-land continuation of the Red Sea propagator, which is taken to represent the Arabian-Nubian plate boundary within Afar. The graben is bounded by northwest-trending border faults, with the footwalls dominated by ca. 1.7 Ma basalts and the downthrown blocks constituting progressively younger basalts toward the center of the graben, reaching ca. 35 ka. The Tendaho magnetic fi eld is characterized by an ~10-km-wide linear negative magnetic anomaly that corresponds to a normal-polarity zone that is fl anked by two parallel, ~20-km-wide linear positive magnetic anomalies of reversed polarity. This work shows that magnetic stripes can be developed in transitional continental rifts before the development of oceanic spreading centers. The common assumption that magnetic stripes can be used to date the onset of seafl oor spreading may need to be re-evaluated in light of the evidence provided here.

Geoheritage conservation in Ethiopia: the case of the Simien Mountains

Geoheritage conservation in Ethiopia: the case of the Simien Mountains Ethiopia constitutes one of the most significant environmental and cultural reserves on Earth. Ethiopia's natural and cultural tourist attractions are mostly associated with geological features: the active Ethiopian and Afar rifts as well as the Simien and Bale massifs are few examples. Ethiopia's cultural history, religious manifestations and civilization, like the rock-hewn churches of Lalibela and the stelae of Axum, are also imprinted in rock. Geomorphological and geological features, notably the isolation of the north-western highlands from the external world by the harsh Afar depression close to the sea, determined the route of Ethiopian history. Though tourism has been identified as a major sustainable development sector, systematic geoheritage evaluation and conservation strategies are lacking in the country. I this paper the Simien Mountains are presented as major geoheritages which should be prioritized for geoconservation in order to develop sustainable tourism (geotourism) in the area.

Surface wave tomography across Afar, Ethiopia: Crustal structure at a rift triple-junction zone

[1] The Afar Depression in northeast Africa contains the rift triple-junction between the Nubia, Arabia and Somalia plates. We analyze Rayleigh wave group velocity from 250 regional earthquakes recorded by 40 broadband stations to study the crustal structure across Afar and adjacent plateau regions in northern Ethiopia. The dispersion velocities are inverted to obtain surface wave tomographic maps for periods between 5 and 25 seconds, sensitive to approximately the top 30 km of the lithosphere. The tomographic maps show a significant low dispersion velocity anomaly (>20%) within the upper crust, below the site of recent dyke intrusions (2005–present) in the Dabbahu and Manda-Hararo magmatic segments. Similar low velocity regions are imaged where magma intrusion in the Afar crust has been inferred over the last decade from seismicity or volcanic eruptions. We invert two group velocity curves to compare the S-wave velocity structure of the crust within an active magmatic segment with that of adjacent areas; the active region has a low velocity zone (Vs ∼ 3.2 km/s), between about 6–12 km, which we infer to be due to the presence of partial melt within the lower crust.

Selective variance reduction of multi-temporal LST imagery in the East Africa Rift System

A new method for elevation and latitude decorrelation stretch of multi-temporal land surface temperature (LST) in the East Africa Rift System (EARS) from MODIS 2008 monthly average night imagery and Globe digital elevation model (DEM) is presented. Multiple linear regression analysis of principal components images (PCAs) quantifies the variance explained by elevation and latitude. Selective variance reduction (SVR) reconstructs the multi-temporal LST imagery from the residual images and selected PCAs by taking into account the portion of variance not related to elevation and latitude. Clustering of the reconstructed imagery identifies two major thermal anomalies a) in the Afar Triangle, and b) a new one in between the Ethiopia and Kenya. These regions present LST values higher than the elevation and latitude predicted ones through out the year. It is assumed that the new thermal anomaly corresponds to a triple junction formed in between the Ethiopian Rift and the Eastern and the Western branches of the EARS, in an area where active volcanoes and mantle plume activity concentrate. SVR is expected to assist tectonic and volcanic zones characterization on the basis of their thermal response.

The nature of the crust beneath the Afar triple junction: Evidence from receiver functions

The Afar depression is an ideal locale to study the role of extension and magmatism as rifting progresses to seafloor spreading. Here we present receiver function results from new and legacy experiments. Crustal thickness ranges from ∼45 km beneath the highlands to ∼16 km beneath an incipient oceanic spreading center in northern Afar. The crust beneath Afar has a thickness of 20–26 km outside the currently active rift segments and thins northward. It is bounded by thick crust beneath the highlands of the western plateau (∼40 km) and southeastern plateau (∼35 km). The western plateau shows VP/VS ranging between 1.7–1.9, suggesting a mafic altered crust, likely associated with Cenozoic flood basalts, or current magmatism. The southeastern plateau shows VP/VS more typical of silicic continental crust (∼1.78). For crustal thicknesses 2.0) can only be explained by significant amounts of magmatic intrusions in the lower crust. This suggests that melt emplacement plays an important role in late stage rifting, and melt in the lower crust likely feeds magmatic activity. The crust between the location of the Miocene Red Sea rift axis and the current rift axis is thinner ( 2.0) than beneath the eastern part of Afar (>26 km, VP/VS < 1.9). This suggests that the eastern region contains less partial melt, has undergone less stretching/extension and has preserved a more continental crustal signature than west of the current rift axis. The Red Sea rift axis appears to have migrated eastward through time to accommodate the migration of the Afar triple junction.

Geological summary of the Busidima Formation (Plio-Pleistocene) at the Hadar paleoanthropological site, Afar Depression, Ethiopia

The Hadar paleoanthropological site in Ethiopia preserves a record of hominin evolution spanning from approximately 3.45Ma to 0.8Ma. An angular unconformity just above the ca. 2.95Ma BKT-2 complex divides the sediments into the Hadar Formation (ca. 3.8–2.9Ma) and the Busidima Formation (ca. 2.7–0.15Ma). The unconformity is likely a response to a major tectonic reorganization in the Afar Depression, and activation of the As Duma fault near the Ethiopian Escarpment (west of Hadar) created a half-graben in which the Busidima Formation was deposited. The pattern and character of sedimentation in the region changed dramatically above the unconformity, as cut-and-fill channel conglomerates and silt-dominated paleosols that comprise the Busidima Formation stand in sharp contrast to the underlying deposits of the Hadar Formation. Conglomerate deposition has been related to both the perennial, axial paleo-Awash and ephemeral, escarpment-draining tributaries. Overbank silts have yielded fossils attributed to early Homo and Oldowan stone tools. Numerous tuffaceous deposits exist within the Busidima Formation, but they are often spatially limited, fine-grained, and reworked. Recent work on the tephrostratigraphic framework of the Busidima Formation at Hadar has identified at least 12 distinct vitric tephras and established the first geochemical-based correlations between Hadar and the neighboring project areas of Gona and Dikika. Compared to Gona and Dikika, where Busidima Formation sediments are exposed over large areas, the highly discontinuous sediments at Hadar comprise less than 40m in composite section and are exposed over an area of <20km2, providing only snapshots into the 2.7–0.15Ma window. The stratigraphic record at Hadar confirms the complex depositional history of the Busidima Formation, and also provides important details on regional stratigraphic correlations and the pattern of deposition and erosion in the lower Awash Valley reflective of its tectonic history.

Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression

Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture.

Young rift kinematics in the Tadjoura rift, western Gulf of Aden, Republic of Djibouti

The Tadjoura rift forms the westernmost edge of the westerly propagating Sheba ridge, between Arabia and Somalia, as it enters into the Afar depression. From structural and remote sensing data sets, the Tadjoura rift is interpreted as an asymmetrical south facing half‐graben, about 40 km wide, dominated by a large boundary fault zone to the north. It is partially filled up by the 1–3 Myr old Gulf Basalts which onlapped the older Somali Basalts along its shallower southern flexural margin. The major and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basaltic types, namely the Gulf, Somali, Goumarre, and Hayyabley Basalts. These results, together with radiometric age data, lead us to propose a revised volcano‐stratigraphic sketch of the two exposed Tadjoura rift margins and to discriminate and date several distinct fault networks of this oblique rift. Morphological and statistical analyses of onshore extensional fault populations show marked changes in structural styles along‐strike, in a direction parallel to the rift axis. These major fault disturbances are assigned to the arrest of axial fault tip propagation against preexisting discontinuities in the NS‐oriented Arta transverse zone. According to our model, the sinistral jump of rifting into the Asal‐Ghoubbet rift segment results from structural inheritance, in contrast with the en échelon or transform mechanism of propagation that prevailed along the entire length of the Gulf of Aden extensional system.

Seismic anisotropy beneath the Afar Depression and adjacent areas: Implications for mantle flow

Shear wave splitting is a robust tool to infer the direction and strength of seismic anisotropy in the lithosphere and underlying asthenosphere. Previous shear wave splitting studies in the Afar Depression and adjacent areas concluded that either Precambrian sutures or vertical magmatic dikes are mostly responsible for the observed anisotropy. Here we report results of a systematic analysis of teleseismic shear wave splitting using all the available broadband seismic data recorded in the Afar Depression, Main Ethiopian Rift (MER), and Ethiopian Plateau. We found that while the ∼450 measurements on the Ethiopian Plateau and in the MER show insignificant azimuthal variations with MER‐parallel fast directions and thus can be explained by a single layer of anisotropy, the ∼150 measurements in the Afar Depression reveal a systematic azimuthal dependence of splitting parameters with a π/2 periodicity, suggesting a two‐layer model of anisotropy. The top layer is characterized by a relatively small (0.65 s) splitting delay time and a WNW fast direction that can be attributed to magmatic dikes within the lithosphere, and the lower layer has a larger (2.0 s) delay time and a NE fast direction. Using the spatial coherency of the splitting parameters obtained in the MER and on the Ethiopian Plateau, we estimated that the optimal depth of the source of anisotropy is centered at about 300 km, i.e., in the asthenosphere. The spatial and azimuthal variations of the observed anisotropy can best be explained by a NE directed flow in the asthenosphere beneath the MER and the Afar Depression.

Monogenetic vent self-similar clustering in extending continental crust: Examples from the East African Rift System

The spatial clustering of fracture networks and vents in basaltic volcanic fields has been analyzed in three sectors of the East African Rift System, the classical example of an active continental rift. Fracture trace maps and monogenetic basaltic vents have been thus collected in the Afar Depression, in the Main Ethiopian Rift, and in the Virunga Belt (Western Rift). The mapped vents are generally younger than 2 Ma, and most are of Holocene age. All the analyzed fracture networks have self-similar clustering with fractal exponents ( D f ) varying in the 1.54−1.85 range. Also, vents show a self-similar clustering with fractal exponents ( D v ) in the 1.17−1.50 range. For all the studied sectors, the relationship D f > D v has been observed. The fractal exponents for vents ( D v ) of power-law distributions are computed in a range of lengths with a lower and an upper cutoff. The upper cutoff ( Uco ) for the fractal clustering of vents in the studied sectors of the East African Rift System are compared with the respective crust thickness derived by independent geophysical data. The computed Uco s for the studied sectors well match the crust thickness in the volcanic fields. A preliminary conceptual model to explain the relationships between the upper cutoffs of the fractal distribution of vents and the thickness of the crust in the volcanic fields is thus proposed in the light of the percolation theory.

Low-Frequency Hybrid Earthquakes near a Magma Chamber in Afar: Quantifying Path Effects

Areas of active volcanism contain elaborate velocity structures that complicate interpretations of earthquake source mechanisms. We examine the spectral characteristics of 805 earthquakes that immediately followed a large volume basaltic dike intrusion and associated silicic flank eruption of Dabbahu volcano in the Afar Depression as recorded on near-source seismometers. We use these results to quantify the contribution of scattering and attenuation to the observed spectra of low-frequency hybrid and volcano-tectonic earthquake clusters from beneath Dabbahu volcano and around the dike zone. We find strong variations in the signal amplitude and frequency content of earthquakes recorded at stations separated by as little as 2 km, caused by preferential attenuation of high frequencies depending on the vantage point. These observations imply that there are large impedance contrasts near the cooling, solidifying, and recently intruded dike. We estimate the intrinsic absorption attenuation coefficient, QI, and inverse scattering length, g0, averaged over a 300-sq-km area beneath Dabbahu. Our results are consistent with the highest attenuation coefficients from studies of volcanic provinces in Italy (QI-1 ? 0.02, g0?0.1 km-1 for a signal at 2 Hz). The magnitude of these two parameters indicates there are large impedance contrasts present in the area due to the recent intrusion of magma and associated fracturing.