Hazard Risk Mitigation Research Articles

The Multi-Parameter Wireless Sensing System (MPwise): Its Description and Application to Earthquake Risk Mitigation.

The Multi-Parameter Wireless Sensing (MPwise) system is an innovative instrumental design that allows different sensor types to be combined with relatively high-performance computing and communications components. These units, which incorporate off-the-shelf components, can undertake complex information integration and processing tasks at the individual unit or node level (when used in a network), allowing the establishment of networks that are linked by advanced, robust and rapid communications routing and network topologies. The system (and its predecessors) was originally designed for earthquake risk mitigation, including earthquake early warning (EEW), rapid response actions, structural health monitoring, and site-effect characterization. For EEW, MPwise units are capable of on-site, decentralized, independent analysis of the recorded ground motion and based on this, may issue an appropriate warning, either by the unit itself or transmitted throughout a network by dedicated alarming procedures. The multi-sensor capabilities of the system allow it to be instrumented with standard strong- and weak-motion sensors, broadband sensors, MEMS (namely accelerometers), cameras, temperature and humidity sensors, and GNSS receivers. In this work, the MPwise hardware, software and communications schema are described, as well as an overview of its possible applications. While focusing on earthquake risk mitigation actions, the aim in the future is to expand its capabilities towards a more multi-hazard and risk mitigation role. Overall, MPwise offers considerable flexibility and has great potential in contributing to natural hazard risk mitigation.

Formation of levees, troughs and elevated channels by avalanches on erodible slopes

Snow avalanches are typically initiated on marginally stable slopes with a surface layer of fresh snow that may easily be incorporated into them. The erosion of snow at the front is fundamental to the dynamics and growth of snow avalanches and they may rapidly bulk up, making them much more destructive than the initial release. Snow may also deposit at the rear, base and sides of the flow and the net balance of erosion and deposition determines whether an avalanche grows or decays. In this paper, small-scale analogue experiments are performed on a rough inclined plane with a static erodible layer of carborundum grains. The static layer is prepared by slowly closing down a flow from a hopper at the top of the slope. This leaves behind a uniform-depth layer of thickness $h_{stop}$ at a given slope inclination. Due to the hysteresis of the rough bed friction law, this layer can then be inclined to higher angles provided that the thickness does not exceed $h_{start}$, which is the maximum depth that can be held static on a rough bed. An avalanche is then initiated on top of the static layer by releasing a fixed volume of carborundum grains. Dependent on the slope inclination and the depth of the static layer three different behaviours are observed. For initial deposit depths above $h_{stop}$, the avalanche rapidly grows in size by progressively entraining more and more grains at the front and sides, and depositing relatively few particles at the base and tail. This leaves behind a trough eroded to a depth below the initial deposit surface and whose maximal areal extent has a triangular shape. Conversely, a release on a shallower slope, with a deposit of thickness $h_{stop}$, leads to net deposition. This time the avalanche leaves behind a levee-flanked channel, the floor of which lies above the level of the initial deposit and narrows downstream. It is also possible to generate avalanches that have a perfect balance between net erosion and deposition. These avalanches propagate perfectly steadily downslope, leaving a constant-width trail with levees flanking a shallow trough cut slightly lower than the initial deposit surface. The cross-section of the trail therefore represents an exact redistribution of the mass reworked from the initial static layer. Granular flow problems involving erosion and deposition are notoriously difficult, because there is no accepted method of modelling the phase transition between static and moving particles. Remarkably, it is shown in this paper that by combining Pouliquen & Forterre’s (J. Fluid Mech., vol. 453, 2002, pp. 133–151) extended friction law with the depth-averaged $\unicode[STIX]{x1D707}(I)$-rheology of Gray & Edwards (J. Fluid Mech., vol. 755, 2014, pp. 503–544) it is possible to develop a two-dimensional shallow-water-like avalanche model that qualitatively captures all of the experimentally observed behaviour. Furthermore, the computed wavespeed, wave peak height and stationary layer thickness, as well as the distance travelled by decaying avalanches, are all in good quantitative agreement with the experiments. This model is therefore likely to have important practical implications for modelling the initiation, growth and decay of snow avalanches for hazard assessment and risk mitigation.

Measuring Community Disaster Resilience in the Latvian Context: An Apply Case Using a Composite Indicator Approach

Despite a growing and considerable interest and implementation of disaster resilience framework methods in research, policy programs and engineering design, metrics and standards for a quantitative measuring of resilience are still an open issue.Recent literatures on hazard and disaster show that the resilience concept represents a guideline toward a valuable hazard risk management and mitigation. For the Latvian context this also represents a beneficial approach for the implementation of policy strategies based on (semi)quantitative framework aiming to enhance resilience within communities in order to properly and most efficient spend a limited availability of funds. In fact man-made and natural disasters are usually preceded by periods where communities develop toward increasing risk states until a loss occurred due to a specific disaster event. In regard to this aspect this study, principally based on the definition of the Community Disaster Resilience Framework (CDRF) developed by Mayunga, is aiming to evaluate the level of community resilience to disaster at the Latvian national level for a specific set of social, economic, human, physical, and environmental indicators. The method implements the concept of a composite-based, multi-criteria analysis aiming to measure baseline quantitative characteristics of the communities under investigation to potentially further enhance resilience within specific actions plans and/or policy mechanisms. The results are applied to the Latvian context and provide a tool to assess the variation in resilience in places giving a ranking from the most resilient region to the least.

Seismic site characterization at the western Cephalonia Island in the aftermath of the 2014 earthquake series

BackgroundThe site response during a strong earthquake event may be proven crucial for earthquake hazard assessment and risk mitigation. Two moderate magnitude earthquakes that occurred in early 2014 in Cephalonia produced the largest ground motion values ever recorded in Greece, highly exceeding the provisions of the effective seismic code implying for local effects. This motivated the investigation of site response in the epicentral area presented herein.MethodsWe applied the HVSR method on free-field ambient noise measurements obtained during an in situ survey. 68 measurements were adopted for site characterization after their validation using earthquakes and geotechnical data. The site response was approximated by the peak frequency and the amplification ratio of the HVSR curves.ResultsThe majority of measurements exhibit smooth lateral variations in the frequency range 0.7–17 Hz, at a factor up to 7 and they are clearly classified in two bands, a low (0.7–4 Hz) and a high one (5–17 Hz). Some discrepancies that are observed between microtremor measurements and earthquake recordings for peak frequencies <2 Hz and overall underestimated ambient noise HVSR amplification are likely explained by near-source, radiation pattern and/or nonlinear soil effects.ConclusionsHigh frequencies combined with low amplification correlate with damage in the hardest hit areas. Low frequencies are aligned in a NNE-SSW direction in the epicentral area, similar to the strike of the activated fault, indicating that the properties of rocks along the fault zone have possibly been affected by slippage and/or dynamic effects.

Probabilistic life-cycle cost-benefit analysis of portfolios of buildings under flood hazard

Nowadays, the number of residential buildings in flood prone zones is increasing significantly. Therefore, for these buildings, it is of vital importance to apply hazard risk mitigation strategies. In this paper, probabilistic cost-benefit analysis of flood risk mitigation strategies for portfolios of residential buildings is performed in a life-cycle context. Additionally, an efficient methodology is proposed to aid decision-makers on whether or not to retrofit portfolios of structures considering both expected and standard deviation of life-cycle cost. The vulnerability model of buildings under flood hazard is introduced considering serviceability and ultimate limit states. Probabilistic cost-benefit analysis is performed by comparing the effectiveness of different retrofit actions. Additionally, uncertainty and correlation effects are considered in the probabilistic loss and cost-benefit analysis procedures. The proposed approach is applied to a portfolio of residential buildings located in Florida to illustrate its capability and application. However, the proposed approach is general and can be applied to buildings located in other regions.

Enhancing sediment flux control and natural hazard risk mitigation through a structured conceptual planning approach

Sediment fluxes from mountain rivers contribute to shape the geomorphologic features of lowland rivers and to establish the physical basis for an optimal set of ecosystem functions and related services to people. Through significant public funding, the hydro-morphological regimes of mountain rivers in the European Alps have been progressively altered over the last century, with the aim to provide a safe dwelling space, to boost transport, mobility and to support economic growth. We claim that the underlying planning weaknesses contribute to determine these inefficient resource allocations, since flood risk is still high and the ecosystem services are far from being optimal. Hence, with the overall aim to enhance sediment flux control and hazard risk mitigation in such heavily modified alpine streams, we propose a structured design workflow which guides the planner through system analysis and synthesis. As a first step the proposed workflow sets the relevant planning goals and assesses the protection structure functionality. Then a methodology is proposed to achieve the goals. This methodology consists in characterising the hydrologic basin of interest and the sediment availability and determining the sediment connectivity to channels. The focus is set on the detailed analysis of existing river cross sections where the sediment continuity is interrupted (e.g. slit and check dams). By retaining relevant sediment volumes these structures prevent the reactivation of hydro-morphological and associated ecological functionalities. Since their actual performance can be unsatisfying with respect to flood risk mitigation (e.g. mainly old structures), we introduce specific efficiency indicators as a support for the conceptual design stage to quantify effects related to sediment flux control and risk management. The proposed planning approach is then applied to the Gadria system (stream, slit dam, retention basin and culvert), located in South Tyrol, Italy. This case study shows that design excellence is needed to re-establish the sediment continuity, while keeping flood risk below acceptable levels. Moreover, the detailed hydraulic analyses highlight that the slit dam is oversized and it could be redesigned to improve sediment continuity and to reduce maintenance costs.

Using boundary objects to stimulate transformational thinking: storm resilience for the Port of Providence, Rhode Island (USA)

Like many coastal ports around the world, Rhode Island’s Port of Providence in USA is at risk for climate-related natural hazards, such as catastrophic storm surges and significant sea level rise (0.5–2.0 m), over the next century. To combat such events, communities may eventually adopt so-called “transformational adaptation” strategies, like the construction of major new infrastructure, the reorganization of vulnerable systems, or changes in their locations. Such strategies can take decades or more to plan, design, find consensus around, fund, and ultimately implement. Before any meaningful decisions can be made, however, a shared understanding of risks, consequences, and options must be generated and allowed to percolate through the decision-making systems. This paper presents results from a pre-planning exercise that utilized “boundary objects” to engage the Port of Providence's stakeholders in an early dialogue about the transformational approaches to hazard–risk mitigation. The research team piloted the following three boundary objects as a means to initiate meaningful dialogue about long-term storm resilience challenges amongst key stakeholders of this exposed seaport system: (1) a storm scenario with local-scale visualizations, (2) three long-term transformational resilience concepts, and (3) a decision support tool called Wecision. The team tested these boundary objects in a workshop setting with 30 port business owners and policy makers, and found them to be an effective catalyst to generate a robust dialogue around a very challenging topic.

Aggravation factors for seismic response of sedimentary basins: A code-oriented parametric study

Although many numerical and instrumental studies over the last decades have focused on studying the effects of subsurface geometry on seismic ground motion, it has not yet been feasible to incorporate such effects into regular engineering practice, seismic building codes, earthquake hazard assessment and risk mitigation policies. In the present work, extensive numerical analyses of the linear viscoelastic response of trapezoidal sedimentary basins are performed to investigate the sensitivity of their 2D seismic response attributes to parameters related to the geometry of the basin (width, thickness and inclination angles of lateral boundaries) and the dynamic soil properties (shear and compressional wave velocities, soil density and attenuation). Basin effects are quantified and discussed through a period-dependent “aggravation factor”, defined as the ratio between 2D and 1D acceleration response spectra at the basin surface. This aggravation factor expresses the additional effect of the 2D response at different locations along the basin with respect to the corresponding 1D response that is normally accounted for in building codes. In order to identify the maximum amplification of ground motion that can be attributed to the 2D response of the basin, the main focus is put on the maximum values of aggravation factors, as well as their spatial distribution. It is shown that above the sloping edge of the basin, maximum values of aggravation factors less than one may occur, meaning that 2D response is deamplified with respect to the corresponding 1D response, particularly for steep slopes. At the nearly constant-depth part of the basin, maximum aggravation factors strongly depend on the shape ratio of the basin, defined as the ratio of the maximum thickness of the basin to its half-width. A general increase is observed with the shape ratio and the increase is more pronounced at the central part of the basin, where for high shape ratio, i.e. deep and narrow valleys, the computed median values are as high as 1.8 and 84th percentiles around 2.3. Maximum aggravation factors are higher and exhibit a larger scattering for basins with higher values of fundamental period at the center T0,c.

Where giant earthquakes may come

AbstractGiant earthquakes (Mw≥8.5) usually occur on the boundary between subducting and overriding plates of converging margins, but it is not yet clear which (if any) subduction zones are more prone to produce such a kind of events. Here we show that subduction zones may have different capabilities to produce giant earthquakes. We analyze the frequency‐magnitude distribution of the interplate earthquakes at subduction zones that occurred during 1976–2007 and calculate the propensity (defined as the average annual rate) of giant events for about half of the subduction zones. We find that the b value of interplate earthquakes is significantly different among the subduction zones, and out‐of‐sample giant earthquakes (before 1976 and after 2007) have occurred preferentially in high‐propensity areas. Besides the importance for seismic hazard assessment and risk mitigation, our results seem to indicate that a higher seismicity rate does not necessarily imply a higher likelihood to generate giant earthquakes, and the way in which the stress is released at a subduction interface does not change significantly after the occurrence of such events.

Improving Decision Making about Natural Disaster Mitigation Funding in Australia—A Framework

Economic losses from natural disasters pose significant challenges to communities and to the insurance industry. Natural disaster mitigation aims to reduce the threat to people and assets from natural perils. Good decisions relating to hazard risk mitigation require judgments both about the scientific and financial issues involved, i.e., the efficacy of some intervention, and the ethical or value principles to adopt in allocating resources. A framework for selecting a set of mitigation options within a limited budget is developed. Project selection about natural disaster mitigation options needs to trade off benefits offered by alternative investments (e.g., fatalities and injuries avoided, potential property and infrastructure losses prevented, safety concerns of citizens, etc.) against the costs of investment. Such costs include capital and on-going operational costs, as well as intangible costs, such as the impact of the project on the visual landscape or the loss of societal cohesion in the event of the relocation of part of a community. Furthermore, dollar costs of any potential project will need to be defined within some prescribed budget and time frame. Taking all of these factors into account, this paper develops a framework for good natural hazard mitigation decision making and selection.

Toward IAVCEI guidelines on the roles and responsibilities of scientists involved in volcanic hazard evaluation, risk mitigation, and crisis response

Toward IAVCEI guidelines on the roles and responsibilities of scientists involved in volcanic hazard evaluation, risk mitigation, and crisis response

A survey instrument to isolate effectiveness of a novel risk communication intervention

Risk communications can be used to inform, persuade and facilitate public support for hazard risk mitigation and preparedness. Some common risk communications practices, such as the use of fear-based appeals, may inadvertently trigger maladaptive behavior. A brochure, film or advertisement may be just one part of a larger campaign to promote hazard readiness, and these items can and should be tested separately for efficacy and effectiveness prior to broad deployment. The purpose of this article is to show the development of a survey instrument, the 25-item earthquake-resistant construction knowledge and opinions (EKO) scale. This scale was created to test a non-emergency communications intervention designed to encourage earthquake-resistant school construction. It was utilized in a randomized controlled trial measuring change in outcomes after viewing a 20-minute film (trial results are reported elsewhere). The intervention film featured community members in Nepal who had worked toward making their school buildings earthquake safe. Study participants were 761 adult community members at schools with buildings in need of seismic work. Factor analysis yielded four questionnaire subscales labeled knowledge, feasibility, would-help-school and would-recommend-to-others, accounting for 55.7 % of the variance. Internal consistency was acceptable, with Cronbach’s α of 0.78–0.93 for all but the knowledge factor (α = 0.48). The EKO scale may be adapted to measure risk communications in other settings. Research on further development and application of the EKO scale and other similar questionnaires may facilitate the evaluation of risk communication interventions and help improve understanding of factors that contribute to intervention effectiveness.

FPLUME-1.0: An integral volcanic plume model accounting for ash aggregation

Abstract. Eruption source parameters (ESP) characterizing volcanic eruption plumes are crucial inputs for atmospheric tephra dispersal models, used for hazard assessment and risk mitigation. We present FPLUME-1.0, a steady-state 1-D (one-dimensional) cross-section-averaged eruption column model based on the buoyant plume theory (BPT). The model accounts for plume bending by wind, entrainment of ambient moisture, effects of water phase changes, particle fallout and re-entrainment, a new parameterization for the air entrainment coefficients and a model for wet aggregation of ash particles in the presence of liquid water or ice. In the occurrence of wet aggregation, the model predicts an effective grain size distribution depleted in fines with respect to that erupted at the vent. Given a wind profile, the model can be used to determine the column height from the eruption mass flow rate or vice versa. The ultimate goal is to improve ash cloud dispersal forecasts by better constraining the ESP (column height, eruption rate and vertical distribution of mass) and the effective particle grain size distribution resulting from eventual wet aggregation within the plume. As test cases we apply the model to the eruptive phase-B of the 4 April 1982 El Chichón volcano eruption (México) and the 6 May 2010 Eyjafjallajökull eruption phase (Iceland). The modular structure of the code facilitates the implementation in the future code versions of more quantitative ash aggregation parameterization as further observations and experiment data will be available for better constraining ash aggregation processes.

Near-source Doppler radar monitoring of tephra plumes at Etna

Over the last twenty years Mount Etna has produced more than one hundred explosive events ranging from light ash emissions to violent sub-plinian eruptions. Significant hazards arise from tephra plumes which directly threaten air traffic, and generate fallout affecting surrounding towns and infrastructures. We describe the first radar system, named VOLDORAD 2B, fully integrated into a volcano instrumental network dedicated to the continuous near-source monitoring of tephra emissions from Etna's summit craters. This 23.5cm wavelength pulsed Doppler radar is operated in collaboration between the Observatoire de Physique du Globe de Clermont-Ferrand (OPGC) and the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo (INGV-OE) since 2009. Probed volumes inside the fixed, northward-pointing conical beam total about 1.5km in length, covering the summit craters which produced all recent tephra plumes. The backscattered power, related to the amount of particles crossing the beam, and particle along-beam velocities are recorded every 0.23s, providing a proxy for the tephra mass eruption rate. Radar raw data are transmitted in real-time to the volcano monitoring center of INGV-OE in Catania and are used to automatically release alerts at onset and end of eruptive events. Processed radar parameters are also made available from the VOLDORAD database online (http://voldorad.opgc.fr/). In addition to eruptive crater discrimination by range gating, relative variations of eruption intensity can be tracked, including through overcast weather when other optical or infrared methods may fail to provide information. Short-lived dense ash emissions can be detected as illustrated for weak ash plumes from the Bocca Nuova and New South East craters in 2010. The comparison with thermal images suggests that the front mushroom of individual ash plumes holds the largest particles (coarse ash and small lapilli) and concentrations at least within the first hundred meters. For these short-lived ash plumes, the highest particle mass flux seems to occur typically within the first 10s. We also analyze data from the first lava fountain generating an ash and lapilli plume on 12 January 2011 that initiated a series of 25 paroxysmal episodes of the New South East Crater until April 2012. We illustrate the pulsating behavior of the lava fountain and show that vertical velocities reached 250ms−1 (with brief peaks exceeding 300ms−1), leading to mean and maximum tephra fluxes (DRE) of 185 and 318m3s−1 (with peaks exceeding 380m3s−1) respectively, and a total volume of pyroclasts emitted during the lava fountain phase of 1.3×106m3. Finally, we discuss capacities and limits of the instrument, along with future work aimed at providing source term inputs to tephra dispersal models in order to improve hazard assessment and risk mitigation at Etna.

Identifying cryptotephra units using correlated rapid, nondestructive methods: VSWIR spectroscopy, X‐ray fluorescence, and magnetic susceptibility

AbstractUnderstanding the frequency, magnitude, and nature of explosive volcanic eruptions is essential for hazard planning and risk mitigation. Terrestrial stratigraphic tephra records can be patchy and incomplete due to subsequent erosion and burial processes. In contrast, the marine sedimentary record commonly preserves a more complete historical record of volcanic activity as individual events are archived within continually accumulating background sediments. While larger tephra layers are often identifiable by changes in sediment color and/or texture, smaller fallout layers may also be present that are not visible to the naked eye. These cryptotephra are commonly more difficult to identify and often require time‐consuming and destructive point counting, petrography, and microscopy work. Here we present several rapid, nondestructive, and quantitative core scanning methodologies (magnetic susceptibility, visible to shortwave infrared spectroscopy, and XRF core scanning) which, when combined, can be used to identify the presence of increased volcaniclastic components (interpreted to be cryptotephra) in the sedimentary record. We develop a new spectral parameter (BDI1000VIS) that exploits the absorption of the 1 µm near‐infrared band in tephra. Using predetermined mixtures, BDI1000VIS can accurately identify tephra layers in concentrations &gt;15–20%. When applied to the upper ∼270 kyr record of IODP core U1396C from the Caribbean Sea, and verified by traditional point counting, 29 potential cryptotephra layers were identified as originating from eruptions of the Lesser Antilles Volcanic Arc. Application of these methods in future coring endeavors can be used to minimize the need for physical disaggregation of valuable drill core material and allow for near‐real‐time recognition of tephra units, both visible and cryptotephra.

Transit Systems and Ridership under Extreme Weather and Climate Change Stress: An Urban Transportation Agenda for Hazards Geography

AbstractThis review examines studies that detail the effects of weather and climate on urban transit systems and ridership. The aim is to help revitalize a hazards geography research agenda to support stakeholder decision‐making on hazard risk and disaster vulnerability for US transit systems and ridership. Largely representative of the literature are field perspectives outside of hazards geography whose predispositions to positivism neglect equity concerns of vital importance to hazard risk mitigation, vulnerability reduction, adaptation, and resilience research and practitioner communities. The article argues for hazards geography's perspectival insight to fill this void. Themes discussed in the article include hazards geography value, transit system vulnerability to meteorological and non‐meteorological factors, place‐based climate change vulnerability assessments, policymaker and managerial perceptions of climate change, and transit ridership choice. The article concludes with a call to hazards geographers to conduct critical assessments of structural and non‐structural adaptation measures, investigate mobility decision‐making and coping strategies, and develop theoretical understandings on adaptation policymaking and practice in relation to hazard risk and disaster vulnerability.

Growing City and Rapid Land Use Transition: Assessing Multiple Hazards and Risks in the Pokhara Valley, Nepal

Pokhara is one of the most naturally beautiful cities in the world with a unique geological setting. This important tourist city is under intense pressure from rapid urbanization and population growth. Multiple hazards and risks are rapidly increasing in Pokhara due to unsustainable land use practices, particularly the increase in built-up areas. This study examines the relationship among urbanization, land use/land cover dynamics and multiple hazard and risk analysis of the Pokhara valley from 1990 to 2013. We investigate some of the active hazards, such as floods, landslides, fire, sinkholes, land subsidence and earthquakes, and prepare an integrated multiple hazard risk map indicating the highly vulnerable zones. Land use and land cover maps from 1990 and 2013 from Landsat images (30 m resolution) have been prepared and analyzed for the spatial dynamics of urbanization and the transition of land use and land cover. In the 23-year period, the built-up area more than doubled from 24.03 km² to 54.20 km². Although the landscape in the urban, peri-urban and rural areas appears to be fragmented, different drivers play pivotal roles in landscape change in these areas. The results provide substantial information for establishing innovative action plans for disaster risk management in the valley. Recommendations are made for the most suitable places for future urban expansion in the valley. This study is important for raising awareness among policy makers and other public officials to include multiple hazard risk mitigation in land use policies and plans. Establishing connections between urban expansions, escalating population growth and multiple hazards and risk assessment will also improve in modelling the latent impact of future catastrophes and emergency preparedness.

Seismicity and Seismotectonics of Jeddah-Makkah Region, West-Central Saudi Arabia

Jeddah-Makkah regionis have been suffering from earthquake crisis where some moderate to destructive earthquakes have been recorded. These earthquake activities are oriented along major faults or clustered in certain spots. Moreover, these earthquake events have annual recurrence periods, so the identification of these seismogenic source zones is of utmost importance for mapping the most hazardous localities which should be avoided in the future urban planning. Historical and instrumental earthquakes have been collected from national and international data centers and unified in catalogue. The existence of microearthquakes inland suggests that there is a significant level of tectonic activity at away from the axial trough of the Red Sea. Then, seismogenic source zones have been defined depending on the major tectonic trends; distribution of earthquake epicenters, seismicity rate (a & b-values) and fault plane solution of major earthquakes. It is concluded that Jeddah-Makkah region is affected by the outlined five seismogenic source zones; three of these zones aligned of the main Red Sea axial trough (southwestern Jeddah, western Jeddah, and northwestern Jeddah zones), while the other two zones are located in the land area of the region (Thewal-Rabegh and Jeddah-Makkah zones). These inland zones correlated well with the main trends of major tectonics which refleet the reactivation of tectonic movements along these fault trends. The Red Sea zones are in agreement with the main path of the axial trough. The range of b-value in these identified zones is 0.65 to 1.03 through these identified zones. The area characterized by higher b-values could be indicative of a relative low stress regime which was a result of resulting from the stress release by the earthquakes. Whereas, the areas of lower b-values can be considered as an evidence of a relatively higher stress regime associated with a dominantly extensional stresses. Based on aforementioned, the region is suffering from different stress level accumulations which, in turn, cause earthquakes with different magnitudes. Accordingly, deployment of local seismograph network through Jeddah-Makkah region is highly recommended. These results will support, to a great extent, seismic hazard assessment and risk mitigation of the region.

Site response assessment of an urban extension site using microtremor measurements, Ahud Rufeidah, Abha District, Southwest Saudi Arabia

Microtremor horizontal-to-vertical spectral ratio (HVSR) method has been conducted at 33 sites in Ahud Rufeidah urban expansion zone in order to assess the fundamental frequency and the corresponding amplitude of the sediments. Clear HVSR peaks have been identified in the majority of the surveyed area. The eastern and northern parts of the area are characterized by two well-separated peaks which indicate distinct shallow and deep impedance contrasts. The frequency map of sediments shows a distribution in the range of 0.86–3.13 Hz. The observed frequencies can be related to the total thickness of Quaternary alluvial sediments (sand, gravel, and shale) deposited over the gneiss bedrock. Lower resonance frequencies are attained at sites in the northern part, while the higher values are attained at sites in the southern part. The amplitudes of HVSR peaks are in the range 3–15. In general, the higher peak amplitudes are identified at lower frequencies. Since low fundamental frequencies are related to bedrock, this can be an indication of high-impedance contrast between alluvial sediments and gneiss bedrock. The results of this study represent one step for seismic hazard assessment and risk mitigation of this urban area where great damage can be attained in case of strong earthquakes. Hence, these results should be taken into consideration before establishing the new urban constructions in the area of study.

Global volcanic unrest in the 21st century: An analysis of the first decade

We define volcanic unrest as the deviation from the background or baseline behaviour of a volcano towards a behaviour which is a cause for concern in the short-term because it might prelude an eruption. When unrest is preceded by periods of quiescence over centuries or millennia it is particularly difficult to foresee how a volcano might behave in the short-term. As a consequence, one of the most important problems is to assess whether unrest will culminate in an eruption or not. Here, we review and evaluate global unrest reports of the Smithsonian Institution Global Volcanism Program (GVP) between January 2000 and July 2011. The aim of the evaluation is to establish the nature and length of unrest activity to test whether there are common temporal patterns in unrest indicators and whether there is a link between the length of inter-eruptive periods and unrest duration across different volcano types. A database is created from the reported information on unrest at 228 volcanoes. The data is categorised into pre-eruptive or non-eruptive unrest indicators at four different subaerial volcano types and submarine volcanoes as defined by the GVP. Unrest timelines demonstrate how unrest evolved over time and highlight different classes of unrest including reawakening, pulsatory, prolonged, sporadic and intra-eruptive unrest. Statistical tests indicate that pre-eruptive unrest duration was different across different volcano types. 50% of stratovolcanoes erupted after about one month of reported unrest. At large calderas this median average duration of pre-eruptive unrest was about twice as long. At almost five months, shield volcanoes had a significantly longer unrest period before the onset of eruption, compared to both large calderas and stratovolcanoes. At complex volcanoes, eruptive unrest was short lived with only a median average duration of two days. We find that there is only a poor correlation between the length of the inter-eruptive period and unrest duration in the data; statistical significance was only detected for the pair-wise comparison of non-eruptive unrest at large calderas and stratovolcanoes. Results indicate that volcanoes with long periods of quiescence between eruptions will not necessarily undergo prolonged periods of unrest before their next eruption.Our findings may have implications for hazard assessment, risk mitigation and scenario planning during future unrest crises.