Harrat Lunayyir Research Articles

Insights into the relationship between the Red Sea rift-related structures and the seismo-volcanic activity in Harrat Lunayyir, Saudi Arabia: A seismic tomography study

Harrat Lunayyir is one of the youngest Cenozoic volcanic fields in western Saudi Arabia, which in 2009 experienced a seismic swarm of about 34,000 events with a maximum magnitude of M5.4. Many studies debated about the relationship of a depicted shallow-level magma intrusion with the Red Sea rifting and the channelized northward flow from the Afar mantle plume. In this study, a detailed seismic tomography inversion for the P- and S-wave velocities, and the Poisson’s ratio structures beneath Harrat Lunayyir and the adjacent Red Sea offshore area is presented. The tomographic images showed evidence of magma intrusions beneath the Lunayyir Swarm Area (LSA) characterized by low P- and S-wave velocity and high Poisson’s ratio anomalies down to 21 km depth. These anomalies are in juxtaposition with some high-velocity anomalies attributed to remnants of solidified magma intrusions relevant to previous episodes of volcanicity. Beneath the Red Sea, the lithospheric mantle structure is clearly observed as a high P-wave velocity high-Poisson’s ratio anomaly at a depth of 20–40 km. Traces of high-Vp anomalies with anomalous Poisson’s ratio are observed in the area between the Red Sea and the LSA suggesting that a channel of hot lithospheric mantle material may link the Red Sea rift structure with the LSA. We suggest that the Red Sea drifting and its related processes in the Zabargad Shear Zone might have significantly contributed to the early formation of the Lunayyir volcanic field and could possibly have in an impact on any magmatic activity in the future.

Regional significance of crustal and sub-crustal rheological heterogeneities beneath the Harrat Lunayyir and their continuity into the neighboring harrats, Western Saudi Arabia – Perspectives of the Afar plume activity

The crustal and sub-crustal rheological parameters computed from the Receiver Function Analysis (REF) results on 24 seismic stations underlying the Harrat Lunayyir, Northern Rahat and Khaybar are investigated down to a depth of 150 km to gain an insight on the plume head and regional effects of the Afar Plume. The harrats are part of the Large Igneous Province with its spine along the Makkah-Madinah-Nafud (MMN) volcanic line. Rheological parameters are analyzed by individual plots to infer the variation of lithological composition introduced underneath the harrats by magmatic intrusions and metamorphism at crustal depths. The spatial mapping of the parameters indicates a pervasive and interconnected ‘Zone of Anomalous Rheological Parameters’ (ZARP) in the upper and lower crust with major features: (i) a clear continuity between the upper and lower crust, (ii) gap area between harrats Rahat and Lunayyir is a ZARP high, and (iii) the correlation between ZARP maps and detailed structural maps indicate a causal pathway of magmatic intrusion in crust connecting the harrats laterally. We investigate their vertical continuity by tracking the rheological parameters to lithospheric depths that unravels two anomalous zones at 30–50 km (the immediate sub-crust) and 80–150 km (sub-lithosphere) depths respectively. We speculate that the former acts as the zone of de-compressed crustal melting linked to crustal thinning that possibly acts as magma feeder zone for the harrats along MMN Line, and represent the plume head. The deeper zone at sub-lithospheric depth represents the extensive and persistent zone of horizontal mantle flow from the Afar region.

The key role of conjugate fault system in importing earthquakes into the eastern flank of the Red Sea

This study aims to synthesize seismic observations with gravity and magnetic data and to suggest a new scenario on the development of the Harrat Lunayyir (HL) tectonic system on the eastern Red Sea coastline, Saudi Arabia. Gravity and aeromagnetic anomalies distinctly mapped the NE and NW trends, while the InSAR data depict a small NW–SE graben and an NW–SE dyke. High-resolution relocations, which are well-consistent with the focal mechanism solutions for events with magnitudes greater than 3.0, admit two distinctly fault styles of different orientations. Thus, leading to the NE and NW fault planes’ reactivation related to the Precambrian basement faults and the Red Sea rift system, respectively. The spatiotemporal distributions of epicenters and focal mechanism solutions suggest a new seismic deformation scenario of the 2009 earthquake seismic activity. The low static frictions of 0.2–0.35 obtained from the stress inversion indicates reactivation of preexisting faults in the respective seismogenic zones. The obtained results give rise to a swarm-like sequence of tectonic implications, two activated fault styles differently oriented, and an NE conjugate fault system inherited in the region, which plays a vital role in transferring the ambient stress regime into the Red Sea’s eastern flank.

Asthenospheric Flow of Plume Material Beneath Arabia Inferred From S Wave Traveltime Tomography

AbstractWidespread Cenozoic volcanism in the Arabian Peninsula has been attributed to mantle plume activity and/or lithospheric thinning due to rift‐related extension. However, there is discrepancy between geochemical and geophysical studies about which mechanism is dominant over the other for post‐12 Ma volcanism. Plume signals in some volcanic fields in the Arabian shield are not evident in isotope analyses, but low‐velocity anomalies connected to Afar are found beneath the Arabian shield in tomographic studies and interpreted as asthenospheric flow from the Afar plume. To resolve this contradiction, we investigate the upper mantle beneath the Arabian Peninsula and northeastern Africa by inverting relative S and SKS wave arrival times recorded at dense seismic networks to derive a high‐resolution S wave velocity model. Our results clearly show a narrow, elongated low‐velocity anomaly along the Makkah‐Madinah‐Nafud (MMN) volcanic line beneath the Arabian shield at 100–300 km depth which extends northward to Harrat Ithnayn and Harrat Lunayyir, but most likely not further north. The limited extent of the low‐velocity anomaly and variations in lithospheric thickness of the Arabian shield may explain why geochemical studies did not find plume signals in some harrats. Therefore, the timing and plume signals of volcanism in western Arabia may not be age progressive from Afar. We also find a possible connection between the low‐velocity anomalies beneath Harrat Lunayyir and the MMN line, suggesting that the 2007–2009 seismic swarm may be associated with northward asthenospheric flow of plume material from Afar.

Thermal mapping of the 2009 dyke emplacement at Harrat Lunayyir, Saudi Arabia

The 2009 seismic episode at Harrat Lunayyir signalled a renewed geohazard and resulted in a regional dyke that propagated to a very shallow depth (a few hundred metres) below the surface. Since then, there has been an extensive research debate over the potential links between the volcanic/intrusive activity and tectonic processes, particularly because the earthquake swarm and dyke propagation did not eventually result in an eruption. The current study seeks to estimate the relative changes in surface temperatures as a means for detecting an impending dyke-fed eruption or, alternatively, dyke arrest at a shallow depth. An analysis of thermal (infrared) data with a focus on the spatial distribution of land surface temperatures over a longer period of observation may help reveal the link between volcanic activity and dyke propagation. Here, the land surface temperature changes in the centre of Harrat Lunayyir were recorded when the 2009 dyke was propagating toward the surface. The spatial distribution of the land surface temperatures in the area indicated the segmentation of the dyke and suggested the segments were arrested at somewhat different depths below the surface.

Structural Mapping of Dike-Induced Faulting in Harrat Lunayyir (Saudi Arabia) by Using High Resolution Drone Imagery

Dike intrusions produce faulting at the surface along with seismic swarms and possible eruptions. Understanding the geometry and kinematics of dike-induced fractures can provide relevant information on what controls magma emplacement and the associated hazards. Here, we focus on the Harrat Lunayyir volcanic field (western Saudi Arabia), where in 2009 a dike intrusion formed a NNW-SSE oriented, ten-kilometer-long and up to one-meter deep graben. This widens from ~2 km in the SSE to ~5 km in the NNW, showing a well-defined border normal fault to the west but a diffused fracture zone to the east. We conducted a fixed-wing drone survey to create high resolution (3.4 cm) ortho-rectified images and DEMs of the western fault and of a portion of the eastern fracture zone to determine the fracture geometry and kinematics. We then integrated these results with field observations and InSAR data from the 2009 intrusion. Both fault zones contain smaller segments (hundreds of meters long) consisting of normal faults and extension fractures, showing two dominant orientation patterns: NNW-SSE (N330° ± 10°) and NW-SE (N300° ± 10°). The NNW-ESE oriented segments are sub-parallel to the inferred 2009 dike strike (N340°), to pre-historical Harrat Lunayyir eruptive fissures (N330°) and to the overall Red Sea axis (N330°). This suggests that these segments reflect the present-day off-rift stress field close to the Red Sea shoulder. However, the NW-SE oriented segments are oblique to this pattern and exhibit en–echelon structures, suggesting different processes such as: (1) a transfer (or soft-linkage) between dike-parallel fault segments, (2) a topographic control on the fault propagation and (3) a possible reactivation of inherited regional faults. Vertical fault offsets obtained by the drone survey along the western fault vary with the local lithology and these data are not consistent everywhere with the offsets derived from the 2009 InSAR measurements. Field evidence within lava flows also shows the occurrence of previous slipping event(s) on the fault before the 2009 intrusion. Collectively, we suggest that the mismatch between the drone and InSAR datasets is related to the different spatial and temporal resolutions offered by the two techniques.

Another look at the 2009 seismic activity, Harrat Lunayyir, Saudi Arabia

This study aims to recognize the influence of source- and path-effects on seismic spectra using the waveform data from the 2009 seismic activity, Harrat Lunayyir (HL), eastern flank of the Red Sea coastline, Saudi Arabia. This seismic activity is considered as a series of continuously evolving events reaching at some stage the maximum on 19 May 2009, with the largest shock of Mw 5.4, and then decayed. A total number of 218 events of magnitudes ML ≥ 2.0 were analyzed, which include the largest shock, 101 and 116 seismic events representing the seismic events within the early and decay stages, respectively. The data analysis using Frequency-time analysis revealed diversity patterns of frequency contents along the recorded seismic stations. Moreover, the modified WADATI diagram showed VP/VS variations of 1.7 ± 0.003 and 1.72 ± 0.002 before and after the occurrence of the largest-sized event among the HL seismic activity, respectively. Using the grid-search method to recover the space parameters representing the average source spectrum of each earthquake over the recorded seismic stations, estimates of source sizes and stress drops imply variations in self-similarity scaling relationship among the HL seismic activity. According to the results obtained from the current analyses, we propose a scenario began with an emplacement of seismic activity due to the local stress accumulation imports from an upward dike injection that is blocked by cooled and hardened magmatic materials in the upper crust from the earlier intrusions. The stress accumulation continued and was then released, at some stage, by the event having the maximum magnitude followed by seismic activities released due to stress relaxations.

Subsurface imaging of the Harrat Lunayyir 2007–2009 earthquake swarm zone, western Saudi Arabia, using potential field methods

The Harrat Lunayyir (HL) 2007–2009 earthquake swarm has attracted much scientific interest mainly due to its occurrence within the Large Igneous Province (LIP) of Saudi Arabia. Qualitative interpretation of aeromagnetic anomalies is suggestive of linear dike-like anomalies, striking NW-SE, across the earthquake swarm zone (ESZ). Residual Bouguer anomaly data for HL range between −4.3 and 1.5 mgal and show a 20 km-wide gravity low flanked by lateral gravity highs. The axial-zone of low gravity corresponds to the ESZ cluster. Employing a three-dimensional (3D) inversion of the residual gravity field with a one-dimensional (1D) P-wave velocity model as a proxy for background density variations with depth, we imaged a mid-to-upper crustal low density zone (>10 km depth). We interpret this zone as the source area of ascending magma that intruded into the upper crust underlying the HL during the 2009 seismic crisis. The inversion also highlights lower than average density rocks beneath the western parts of the HL whereas its eastern part is composed of rocks of higher density. The 3D magnetic inversions lead us to infer magmatic intrusion of lower magnetic susceptibility at this locality extending vertically from 1.57 km altitude (above sea level) to its bottom edge at 10.8 km depth (below sea level). Based on these results, the 2007–2009 ESZ is likely associated with rift-related extension associated with the Red Sea spreading, where a trans-crustal magmatic plumbing system extends to depths of about 10 km beneath the centre of HL.

Two-stage Red Sea rifting inferred from mantle earthquakes in Neoproterozoic lithosphere

We use earthquake geothermometry, measured heat flow, and structural constraints from P-wave receiver functions to model the thermal evolution of the lithosphere beneath Harrat Lunayyir. We suggest that the lithosphere thinned to its present 60-km thickness in a second stage of lithospheric thinning at 15–12 Ma following initial Red Sea extension at ∼27 Ma. Harrat Lunayyir is an active volcanic field located in the Arabian Shield >150 km east of the Red Sea rift axis. In the lithospheric mantle beneath Harrat Lunayyir we locate 64 high-frequency earthquakes at depths of 42–48 km, all with mL≤2.5. These brittle-failure earthquakes must have nucleated at relatively low temperatures, based upon global maximum nucleation depths and temperature-dependent-deformation experimental results. Therefore, the mantle earthquakes show that the upper mantle lithosphere is not in thermal equilibrium with the shallow (60 km) underlying asthenosphere. Our thermal modeling indicates that the lithosphere beneath Harrat Lunayyir thinned to its current 60-km thickness at 12 ± 2 Ma, as constrained by thermal modeling of: (1) surface heat-flow; (2) the depth to the mid-crustal brittle–ductile transition, and (3) the depth to the upper-mantle brittle–ductile transition.

Modelling of surface stresses and fracturing during dyke emplacement: Application to the 2009 episode at Harrat Lunayyir, Saudi Arabia

Correct interpretation of surface stresses and deformation or displacement during volcanotectonic episodes is of fundamental importance for hazard assessment and dyke-path forecasting. Here we present new general numerical models on the local stresses induced by arrested dykes. In the models, the crustal segments hosting the dyke vary greatly in mechanical properties, from uniform or non-layered (elastic half-spaces) to highly anisotropic (layers with strong contrast in Young's modulus). The shallow parts of active volcanoes and volcanic zones are normally highly anisotropic and some with open contacts. The numerical results show that, for a given surface deformation, non-layered (half-space) models underestimate the dyke overpressure/thickness needed and overestimate the likely depth to the tip of the dyke. Also, as the mechanical contrast between the layers increases, so does the stress dissipation and associated reduction in surface stresses (and associated fracturing). In the absence of open contacts, the distance between the two dyke-induced tensile and shear stress peaks (and fractures, if any) at the surface is roughly twice the depth to the tip of the dyke. The width of a graben, if it forms, should therefore be roughly twice the depth to the tip of the associated arrested dyke. When applied to the 2009 episode at Harrat Lunayyir, the main results are as follows. The entire 3–7km wide fracture zone/graben formed during the episode is far too wide to have been generated by induced stresses of a single, arrested dyke. The eastern part of the zone/graben may have been generated by the inferred, arrested dyke, but the western zone primarily by regional extensional loading. The dyke tip was arrested at only a few hundred metres below the surface, the estimated thickness of the uppermost part of the dyke being between about 6 and 12m. For the inferred dyke length (strike dimension) of about 14km, this yields a dyke length/thickness ratio between 2400 and 1200, similar to commonly measured ratios of regional dykes in the field.

Seismic hazard assessment for Harrat Lunayyir – A lava field in western Saudi Arabia

Distribution of earthquakes along the western part of the Arabian Peninsula is related to the Red Sea floor spreading and volcanism. The Arabian Peninsula contains Cenozoic lava fields (harrats) extending from Yemen in the south up to Syria in the north. Harrat Lunayyir that is a very young volcanic area has suffered from two earthquake swarms with maximum magnitude MW 5.7 caused by the dike intrusion. The area was not considered as a seismic source zone when constructing seismic hazard maps used in the current issue of Saudi Building Code (SBC-2007). In this work, probabilistic seismic hazard is estimated for the Harrat Lunayyir area and the town of Alays located a few kilometers to south-east of the Harrat. Two models of seismic source zones describing seismic process in Harrat Lunayyir are considered as alternatives in the logic tree scheme, namely: the areal source model and the fault (characteristic earthquake) source model. The results of probabilistic estimations of seismic hazard show that the area is characterized by significant level of hazard. The expected peak ground amplitudes at rock site and for return period 2475 yrs are larger than 200cm/s2 and 140cm/s2 for the central part of the area and the town of Alays respectively. The level of uncertainty quantified in terms of “relative uncertainty” is the largest in the central part of the area.

Analyze the spatial distribution of lava flows in Al-Ays Volcanic Area, Saudi Arabia, using remote sensing

The Al-Ays Volcanic Area (AAVA) experienced a massive earthquake swarm in 2009 under the Harrat Lunayyir (HL). To examine the space distribution of basaltic lava flows in AAVA, we present here an analysis of satellite images. This has helped to identify three generations of lava flows within AAVA for the first time, their spatial extent and relationship to geological features. Various remote sensing treatments were applied to the Landsat 8 image: (a) color composites, (b) band ratioing, (c) principal component analysis, and (d) a mathematical index. Spectral analysis of the maps assists with charting the spatial configuration of the lava flows. Total area occupied by basalts is ~677 km2, of which the oldest lava flows occupy almost 52% (354 km2) and the lavas of intermediate age share 43% (295 km2) while the recent lava flows merely share 5% (28 km2), including 6.8 km2 referring to the volcanic cones. It should be noted that the generation of lava flows outlines a rather small area, of ≤20 km radius, circumventing the swarm epicenter zone. These maps may prove useful in volcanic hazard mapping for the rugged terrain.

Aeromagnetic data over Harrat Lunayyir and surrounding areas, western Saudi Arabia

Harrat Lunayyir is one of the smaller lava fields in western Saudi Arabia that is of current interest due to a dike intrusion episode in 2009, an ongoing swarm of earthquakes and the possible hazard that pose. In addition to seismology, other geophysical data have been used to study the structure of the area, and the available aeromagnetic information is shown and discussed here. The reduced to the pole (RTP) magnetic grid and its enhancements have been used to define some of the main anomalies, and these have been correlated with the known geology, including the Red Sea coastal dike system. There are numerous linear features that are possibly related to the dikes or faulting, but within the area of the harrat, details due to the underlying structure are largely obscured by the magnetic surface lavas. Northeasterly trends in the magnetic data may indicate old zones of weakness that intersect one of the main coastal dikes at the location of the recent seismic activity and surface fissure, suggesting that this is a point of weakness resulting in the volcanism and seismicity that also appears to be largely limited to two of the NE trends. The association of the recent seismicity with a known geological and aeromagnetic feature is important in determining the seismic hazard for the region, especially if the location of future activity can be used to reduce overall uncertainty in the analysis by identifying potential fault sources. Here, the seismicity appears to lie on one of the NNW-trending coastal dikes that have been reactivated recently along a section between two NE-trending older faults.

Mapping b-value for 2009 Harrat Lunayyir earthquake swarm, western Saudi Arabia and Coulomb stress for its mainshock

The Harrat Lunayyir (HL) earthquake swarm of 2009 originated in the HL volcanic field and attracted global attention mainly due to three factors: (i) its relatively short life span that ushered a large frequency of the swarm population (30,000 events in <2years), (ii) the swarm epicenter zone was contained within a small crustal volume under the HL and (iii) the migratory nature of the swarm following the tectonic trend of a normal fault zone beneath HL. The HL belongs to the Large Igneous Province of Saudi Arabia (LIP-SA) where it correlates to the Great Dikes locally. Our aim in this study is to describe the spatial distribution of the hypocenters, b-value character, and Coulomb stress failure (CSF) in an attempt to analyze the underlying geodynamic process that caused the swarm. We utilize the relocated hypocenters monitored by local networks to examine the b-value characteristics for the swarm. This is best represented in a cross section showing two domains of higher b-value anomalies: two patches occurring at shallow depth and at the deeper crust to the SE from the mainshock originated at the shallower depth northwestward. Consistently positive ΔCFF pattern with a large percentage of aftershocks imply how the mainshock rupture controlled the aftershocks activity. This implies that the failure along the NNW fault trend is due to the prevailing ambient stress field imparted to the swarm. We model this by CSF associated with the mainshock for three time dependent situations: (a) foreshock and aftershock epicenters, (b) foreshock hypocenters, and (c) aftershock hypocenters. In actuality, multiple factors might have controlled the aftershock activity as we speculate that positive Coulomb stress was associated in an area where the higher b-value prevails. The CSF produced by the mainshock illustrates how the stress dissipated along the NNW normal fault zone that interrupts the Great Dykes along the Red Sea coast. These results further suggest that the crustal heterogeneity under HL act as an asperity in the epicentral area, whose origin may relate to magma intrusion into upper crust. However, seismic survey is required for detailing this geologic inference.

Pathways of volatile migration in the crust beneath Harrat Lunayyir (Saudi Arabia) during the unrest in 2009 revealed by attenuation tomography

Harrat Lunayyir is a relatively young basaltic field in Saudi Arabia located at the western margin of the Arabian Peninsula. In April–June 2009, strong seismic activity and ground deformations at this site marked the activation of the magma system beneath Harrat Lunayyir. In this study, we present new three-dimensional models of the attenuation of P and S waves during the unrest in 2009 based on the analysis of t*. We measured 1658 and 3170 values of t* for P and S waves, respectively, for the same earthquakes that were previously used for travel time tomography. The resulting anomalies of the P and S wave attenuation look very similar. In the center of the study area, we observe a prominent high-attenuation pattern, which coincides with the most active seismicity at shallow depths and maximum ground deformations. This high-attenuation zone may represent a zone of accumulation and ascending of gases, which originated at depths of 5–7km due to the decompression of ascending liquid volatiles. Based on these findings and previous tomography studies, we propose that the unrest at Harrat Lunayyir in 2009 was triggered by a sudden injection of unstable liquid volatiles from deeper magma sources. At some depths, they were transformed to gases, which caused the volume to increase, and this led to seismic activation in the areas of phase transformations. The overpressurized gases ultimately found the weakest point in the rigid basaltic cover at the junction of several tectonic faults and escaped to the surface.

The lithospheric shear-wave velocity structure of Saudi Arabia: Young volcanism in an old shield

We investigate the lithospheric shear-wave velocity structure of Saudi Arabia by conducting H-κ stacking analysis and jointly inverting teleseismic P-receiver functions and fundamental-mode Rayleigh wave group velocities at 56 broadband stations deployed by the Saudi Geological Survey (SGS). The study region, the Arabian plate, is traditionally divided into the western Arabian shield and the eastern Arabian platform: The Arabian shield itself is a complicated mélange of crustal material, composed of several Proterozoic terrains separated by ophiolite-bearing suture zones and dotted by outcropping Cenozoic volcanic rocks (locally known as harrats). The Arabian platform is primarily covered by 8 to 10km of Paleozoic, Mesozoic and Cenozoic sedimentary rocks. Our results reveal high Vp/Vs ratios in the region of Harrat Lunayyir, which are interpreted as solidified magma intrusions from old magmatic episodes in the shield. Our results also indicate slow velocities and large upper mantle lid temperatures below the southern and northern tips of the Arabian shield, when compared with the values obtained for the central shield. We argue that our inferred patterns of lid velocity and temperature are due to heating by thermal conduction from the Afar plume (and, possibly, the Jordan plume), and that volcanism in western Arabia may result from small-scale adiabatic ascent of magma diapirs.

Graben formation and dike arrest during the 2009 Harrat Lunayyir dike intrusion in Saudi Arabia: Insights from InSAR, stress calculations and analog experiments

AbstractDetailed spatial and temporal accounts of propagating dikes from crustal deformation data, including their interplay with faulting, are rare, leaving many questions about how this interplay affects graben formation and the arrest of dikes unanswered. Here we use interferometric synthetic aperture radar (InSAR) observations, stress calculations, and analog experiments to investigate the interaction between an intruding dike and normal faulting during the 2009 Harrat Lunayyir dike intrusion in western Saudi Arabia. We generated five displacement maps from InSAR data to unravel the temporal evolution of deformation covering the majority of the intrusion. We find that the observed surface displacements can be modeled by a ~2 m thick dike with an upper tip ~2 km below the surface on 16 May 2009, 4 weeks after the onset of seismic unrest. In the following three days, the dike propagated to within ~1 km of the surface with graben‐bounding normal faulting dominating the near‐field deformation. The volume of the dike doubled between mid‐May and mid‐June. We carried out analog experiments that indicate that the wedge‐shaped graben grew outward with the faulting style changing progressively from normal faulting to oblique. Coulomb failure stress change calculations show that the intruding dike caused two zones of shallow horizontal tension on both sides of the dike, producing two zones of fissuring and normal faulting at the surface. In return, the faulting provoked compression around the upper tip of the dike, holding back its vertical propagation.

Structural cause of a missed eruption in the Harrat Lunayyir basaltic field (Saudi Arabia) in 2009

Harrat Lunayyir is one of the most volcanically active recent basaltic fields in western Saudi Arabia. A period of substantial seismic unrest, featuring more than 30,000 local events, occurred in Harrat Lunayyir in April–June 2009. Although this crisis was presumably related to ongoing magma activity, it ended without any surface volcanic activity. We create new tomographic models of P and S velocities (V P and V S ) and use them to explain the causes of this unrest and the reasons the eruption failed. A large seismic anomaly of high V P /V S ratio below 7 km depth coincides with the locations of more than 50 recent cinder cones with ages of older than 100 ka, and is interpreted as a steady-state magma reservoir. We also identify another seismic anomaly at depths below 15 km, which is interpreted as a conduit for fluids and melts from deeper sources. Because the location of this conduit is slightly outside the main reservoir, some of the incoming material was dispersed. As a result, the activation of the crustal reservoir was not sufficiently strong to pierce the rigid basaltic cover and cause an actual eruption dur ing the crisis in 2009.

Evidence of magma activation beneath the Harrat Lunayyir basaltic field (Saudi Arabia) from attenuation tomography

Abstract. We present a seismic attenuation model for the crust beneath the Cenozoic basaltic field of Harrat Lunayyir (western Saudi Arabia), where a strong seismic swarm occurred in 2009. The tomography inversion uses the envelope shape of the S wave seismograms from over 300 strong events (M &lt; 3.5). The resulting attenuation structures appear to be consistent with the distribution of seismic velocities. The obtained 3-D attenuation model distinguishes the low-attenuation zones down to 5 km depth corresponding to the rigid basaltic cover. At greater depths, we detect a high-attenuation anomaly coinciding with the main seismicity cluster. We propose that this zone corresponds to the upper part of the conduit area ascending from deeper magma sources. According to the distributions of local events, fluids and melts from this conduit appear to reach a depth of ∼2 km, but were not able to reach the surface and cause the eruption in 2009.

Investigating the P wave velocity structure beneath Harrat Lunayyir, northwestern Saudi Arabia, using double‐difference tomography and earthquakes from the 2009 seismic swarm

AbstractIn 2009, a swarm of more than 30,000 earthquakes occurred beneath the Harrat Lunayyir lava field in northwest Saudi Arabia. This event was just one of several seismic swarms to occur in this region over the past decade. Surface deformation associated with the seismicity, modeled in previous studies using Interferometric Synthetic Aperture Radar (InSAR) data, is best attributed to the intrusion of a 10 km long dyke. However, little is known about the velocity structure beneath Harrat Lunayyir, making assessment of future seismic and volcanic hazards difficult. In this study, we use local double‐difference tomography to generate a P wave velocity model beneath Harrat Lunayyir and to more precisely locate earthquakes from the 2009 seismic swarm. A pronounced fast velocity anomaly, centered at ~15 km depth with a shallower extension to the N‐NW, is interpreted as an area of repeated magmatic intrusion. The crust surrounding the fast intrusion is slower than that suggested by broader‐scale models for the Arabian Shield. The largest magnitude events occurred early in the swarm, concentrated at shallow depths (~2–8 km) beneath northern Harrat Lunayyir, and these events are associated with the dyke intrusion. Later, deep earthquakes (~15 km) beneath the southern end of the study region as well as a group of intermediate‐depth events connecting the shallow and deep regions of seismicity occurred. These later events likely represent responses to the local stress conditions following the intrusion. Our results are unique since harrat magma systems are rarely imaged, and our observations, coupled with the seismic history in this region, suggest that future volcanic intrusions beneath Harrat Lunayyir are likely.