Pressure Ridges Research Articles

Sakhalin-1: Technology Development for Frontier Arctic Projects

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20208, "Sakhalin-1: Technology Development for Frontier Arctic Projects," by J.M. Hamilton, ExxonMobil Upstream Re - search, and J.E. Jones, ExxonMobil Development Company, originally prepared for the 2009 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. As the oil and gas industry pushes into frontier Arctic areas, new technologies and design criteria will be required to ensure safety and cost effectiveness of increasingly challenging developments. Sakhalin-1 provides a good example of a development in a frontier subarctic area where location-specific design criteria had to be developed and innovative-technology solutions were required to deal with demanding environmental conditions including sea ice, large breaking waves, and earthquakes. Introduction First production from the Exxon Neftegas Limited (ENL) Sakhalin-1 Phase-1 development of Chayvo field began in October 2005, marking the culmination of nearly 10 years of design-criteria and technology development. The development concept consists of an offshore drilling platform, an onshore drilling and treating facility at Chayvo, an oil-export pipeline that runs east/west across the island and Tatar Strait to mainland Russia, and a storage and tanker export terminal at DeKastri. The oil is exported by ice-strengthened tankers south through the Tatar Strait, which is partially covered by up to medium first-year ice (70 to 120 cm) in the winter. The offshore drilling platform, Orlan, is approximately 10 km offshore in approximately 15-m water depth. The platform annually is exposed to approximately 4000 km of drifting first-year pack ice up to 1.5-m (level ice) thickness, which originates in the Sea of Okhotsk north of the island and drifts south along the eastern coast. The controlling ice load features are pressure ridges, with keels extending to depths greater than 30 m (2000-year design value). The design ice load on the 80-m-wide, vertical-sided structure is approximately 385 MN. Design wave loads are generated by late-summer and fall storms that can produce breaking waves at the platform. Global wave loads are on the order of the ice loads, and wave run-up and locally high slamming pressures represent design challenges. Sakhalin Island seismicity is moderate to high, with the active Piltun fault system running north/south on the island as close as 10 km from onshore facilities and 20 km from the offshore platform. The 1995 Neftegorsk earthquake, which registered moment magnitude 7.1, had its epicenter approximately 50 km from the Sakhalin-1 facilities. All facilities must be designed for earthquakes, and the export pipeline, which crosses multiple faults, must be able to withstand several meters of fault displacement.

Erosional vs. accretionary shelf margins: the influence of margin type on deepwater sedimentation: an example from the Porcupine Basin, offshore western Ireland

ABSTRACTA 1000 km2 three‐dimensional (3D) seismic data survey that extends out from the western margin of the Porcupine Basin, offshore western Ireland reveals the internal geometry and depositional history of a large Palaeogene (Palaeocene–Early Eocene) shelf‐margin. Two wells intersect the margin thereby constraining the depositional environments. The 34/19‐1 well (landward end) intersects slope, shelf, marginal marine to coastal plain facies. The 35/21‐1 well (basinward end) intersects seismically imaged shelf‐margin clinoforms where base of slope back up to coastal plain deposits (source‐to‐sink) are represented. The basin‐fill stratal architecture of the Palaeogene succession reveals sediment deposition under two end member, basin physiographic styles: (1) an erosional margin style and (2) an accretionary or progradational margin style. Uplift of the western margin of the basin is suggested as the major cause of the initially oversteepened shelf‐slope erosional profile. Key characteristics of an erosional margin include sediment bypass of the shelf, canyon formation, and the development of significant onlapping submarine fan deposits on the lower slope. Failure on the slope is also revealed by several mass–transport complexes (MTCs) that carve out major erosive features across the slope. Three‐dimensional seismic analysis illustrates variations in size, geometry and depositional trend and transport mechanisms of the MTCs. Confined, thick chaotic seismic facies, erosional basal scours and syn‐depositional thrusting (pressure ridges) at terminus as opposed to thin, high‐amplitude discontinuous facies with an unconfined lobate terminus are interpreted to indicate slump‐ and slide‐dominated vs. debris flow‐dominated MTCs, respectively. The erosional margin was transformed into an accretionary margin when the gradient of the shelf‐slope to basin‐floor profile was sufficiently lowered through the infilling and healing of the topographic lows by the onlapping submarine‐fan deposits. This shallowing of the basin allowed nearshore systems to prograde across the deepwater systems. The accretionary margin was characterised by a thick sediment prism composed of clinoforms both at the shoreface/delta (tens of metres) and shelf‐margin (hundreds of metres) scales. Shelf‐margin clinoforms, the focus of this study, are the fundamental regressive to transgressive building blocks (duration 10–100 kyr) of the stratigraphic succession and can be observed on a larger scale (∼1 Myr) through the migration and trajectory patterns of the shelf‐edge. Trajectory pathways in the accretionary margin are accretionary in a descending or ascending manner. The descending style was characterised by a shelf‐slope break that migrated seawards and obliquely downwards as a result of a relative sea‐level fall. The descending trajectory geometry is lobate along strike suggestive of a point source progradation. Internally, the descending trajectory consists of downward stepping, steeply dipping shelf‐margin clinoforms that display extensive slumping and deposition of sediment on the lower slope indicative of rapid deposition. Furthermore, basin‐floor fans and associated ‘feeder’ channels extend basinwards beyond toe of slope. The ascending trajectory reflects a shelf‐slope break that is interpreted to have migrated seawards during steady or rising relative sea level. The ascending trajectory geometry is associated with significant lateral sediment dispersal along the shelf‐edge, reflecting distributary systems that were less ‘fixed’ or a greater reworking and longshore drift of sediment. Accretion involving the ascending shelf‐edge trajectory characteristically lacked significant basin‐floor deposits. Variable ascending trajectories are recognised in this study, as read from the angle at which the shelf‐slope break migrates. Horizontal to high angle ascending trajectories correspond to dominantly progradational and dominantly aggradational shelf‐edge trajectories, respectively. The sequence stratigraphic analysis of the Porcupine deltaic complex reveals a long‐term relative sea‐level rise.

On the estimation of ice thickness from scattering observations

This paper is inspired by the proposition that it may be possible to extract descriptive physical parameters – in particular the ice thickness, of a sea-ice field from ocean wave information. The motivation is that mathematical theory describing wave propagation in such media has reached a point where the inherent heterogeneity, expressed as pressure ridge keels and sails, leads, thickness variations and changes of material property and draught, can be fully assimilated exactly or through approximations whose limitations are understood. On the basis that leads have the major wave scattering effect for most sea-ice [Williams, T.D., Squire, V.A., 2004. Oblique scattering of plane flexural-gravity waves by heterogeneities in sea ice. Proc. R. Soc. Lon. Ser.-A 460 (2052), 3469–3497], a model two dimensional sea-ice sheet composed of a large number of such features, randomly dispersed, is constructed. The wide spacing approximation is used to predict how wave trains of different period will be affected, after first establishing that this produces results that are very close to the exact solution. Like Kohout and Meylan [Kohout, A.L., Meylan, M.H., 2008. An elastic plate model for wave attenuation and ice floe breaking in the marginal ice zone. J. Geophys. Res. 113, C09016, doi:10.1029/2007JC004434], we find that on average the magnitude of a wave transmitted by a field of leads decays exponentially with the number of leads. Then, by fitting a curve based on this assumption to the data, the thickness of the ice sheet is obtained. The attenuation coefficient can always be calculated numerically by ensemble averaging but in some cases more rapidly computed approximations work extremely well. Moreover, it is found that the underlying thickness can be determined to good accuracy by the method as long as Archimedean draught is correctly provided for, suggesting that waves can indeed be effective as a remote sensing agent to measure ice thickness in areas where pressure ridges are not sizeable, i.e. away from coastal regions of high deformation.

Arctic sea-ice ridges—Safe heavens for sea-ice fauna during periods of extreme ice melt?

Abstract The abundances and distribution of metazoan within-ice meiofauna (13 stations) and under-ice fauna (12 stations) were investigated in level sea ice and sea-ice ridges in the Chukchi/Beaufort Seas and Canada Basin in June/July 2005 using a combination of ice coring and SCUBA diving. Ice meiofauna abundance was estimated based on live counts in the bottom 30 cm of level sea ice based on triplicate ice core sampling at each location, and in individual ice chunks from ridges at four locations. Under-ice amphipods were counted in situ in replicate (N=24–65 per station) 0.25 m2 quadrats using SCUBA to a maximum water depth of 12 m. In level sea ice, the most abundant ice meiofauna groups were Turbellaria (46%), Nematoda (35%), and Harpacticoida (19%), with overall low abundances per station that ranged from 0.0 to 10.9 ind l−1 (median 0.8 ind l−1). In level ice, low ice algal pigment concentrations ( 3 m where abundances were up to 42-fold higher compared with level ice. We propose that the summer ice melt impacted meiofauna and under-ice amphipod abundance and distribution through (a) flushing, and (b) enhanced salinity stress at thinner level sea ice (less than 3 m thickness). We further suggest that pressure ridges, which extend into deeper, high-salinity water, become accumulation regions for ice meiofauna and under-ice amphipods in summer. Pressure ridges thus might be crucial for faunal survival during periods of enhanced summer ice melt. Previous estimates of Arctic sea ice meiofauna and under-ice amphipods on regional and pan-Arctic scales likely underestimate abundances at least in summer because they typically do not include pressure ridges.

Tectonic Geomorphology of the Southernmost Sagaing Fault and Surface Rupture Associated with the May 1930 Pegu (Bago) Earthquake, Myanmar

Abstract The Sagaing fault is a continental transform fault between the India and Sunda plates that connects spreading centers in the Andaman Sea and the continental convergence zone along the Himalayan front. Several M >7 earthquakes occurred along the fault in the last century, and Global Positioning System campaigns revealed a right-lateral slip rate of 18 mm/yr, about half of the total India–Sunda displacement rate of 35 mm/yr. However, there are few fundamental geologic data on the Sagaing fault, such as detailed fault trace locations, information on late Quaternary slip rate, or spatial extent of surface ruptures during historical earthquakes. We conducted geologic field investigations along the southernmost 120 km long stretch of the fault zone that ruptured during the M 7.3 1930 Pegu (Bago) earthquake. We found well-defined tectonic geomorphic features across a deltaic lowland, including fault scarps, tectonic depressions, stream offsets, and pressure ridges. The sense of displacement is predominantly right-lateral strike-slip with vertical motion less than 1/5 of horizontal motion. Based on right-lateral offsets of stream channels, terrace risers, and property boundaries, we estimate coseismic displacement during the 1930 earthquake as ≥3.0 m. This in turn gives us a preliminary recurrence interval of surface-rupturing earthquakes on the Sagaing fault east of Yangon as ≥160 yr. Coulomb stress change induced by the May 1930 Pegu (Bago) earthquake may have triggered the December 1930 Pyu earthquake immediately to the north. This study demonstrates that detailed geomorphic investigations are key for a better assessment of seismic hazard for the Sagaing fault.

On the Role of Tropospheric Circulation in Recent Climate Changes in the Antarctic Peninsula Region

Abstract The state of tropospheric circulation in the West Antarctic sector (WAS) is considered during 1990s, the warmest decade in the Antarctic Peninsula (AP) region. Regional warming has progressed almost coherently with the second phase of global warming and is related to oceanic variability, specifically PDO-ENSO conditions. Atmospheric circulation in 1990s comprises a prevailing cyclogenesis west of the Antarctic Peninsula sector along with frequent weather modifications (the winter season in particular is examined) and the ridge of high pressure to the east. Predominant atmospheric circulation types for the recent decade are found to be stable in time causing smaller air temperature oscillations on different time scales. The circulation background responsible for the stabilization of air temperature growth in the AP region immediately after the turn of the millennium is shown.

A TORNADO WITH ANTICYCLONIC ROTATION AT CLOONCAN, CO. MAYO

On 31 July 2004 a tornado that was unusual in several ways occurred in the west of Ireland. It had an anticyclonic, rather than the more normal cyclonic, rotation, and it occurred under relatively unpromising synoptic conditions, where there was a slack surface-pressure gradient close to the centre of a ridge of high pressure over north-west Europe. However, regional low-level wind shear and strong convergence are shown to have created a favourable environment over Co. Mayo, aided by a limited amount of convective available potential energy (CAPE) that accumulated beneath an inversion at 816hPa and by a strong vertical moisture profile. The role of the contrasting thermal properties between Lough Carra and the adjacent land surface in producing an anticyclonically rotating tornado from a horizontal vortex that developed within the sheared environment is considered. All of these features are found to be strongly localised. Comparisons are made with the non-mesocyclonic tornado environments documented in US research, and a number of contrasts are noted. The tornado track was only partially traceable due to the terrain, and it reached its widest point of 18m where it entered bogland after 0.75km.

Synoptic-Scale and Mesoscale Environments Conducive to Forest Fires during the October 2003 Extreme Fire Event in Southern California

Abstract This study has employed both observational data and numerical simulation results to diagnose the synoptic-scale and mesoscale environments conducive to forest fires during the October 2003 extreme fire event in southern California. A three-stage process is proposed to illustrate the coupling of the synoptic-scale forcing that is evident from the observations, specifically the high pressure ridge and the upper-level jet streak, which leads to meso-α-scale subsidence in its exit region, and the mesoscale forcing that is simulated by the numerical model, specifically the wave breaking and turbulence as well as the wave-induced critical level, which leads to severe downslope (Santa Ana) winds. Two surges of dry air were found to reach the surface in southern California as revealed in the numerical simulation. The first dry air surge arrived as a result of moisture divergence and isallobaric adjustments behind a surface cold front. The second dry air surge reached southern California as the meso-α- to meso-β-scale subsidence and the wave-induced critical level over the coastal ranges phased to transport the dry air from the upper-level jet streak exit region toward the surface and mix the dry air down to the planetary boundary layer on the lee side of the coastal ranges in southern California. The wave-breaking region on the lee side acted as an internal boundary to reflect the mountain wave energy back to the ground and created severe downslope winds through partial resonance with the upward-propagating mountain waves.

A mechanism for African monsoon breaks: Mediterranean cold air surges

Surges of cold air from the Mediterranean into northern Africa during the boreal summer are documented, and their influence on monsoon breaks is analyzed using Tropical Rainfall Measuring Mission rainfall estimates and reanalysis products. Between 1998 and 2006, 6–10 cold air surges occurred each summer, with low‐level temperature anomalies ranging from less than −1 K to over −6 K. Composite analysis indicates that cold air surges over northern Africa persist for 2–10 days and travel equatorward at approximately 5.5 m s–1, which is 0.5–1.5 m s–1 faster than the observed climatological low‐level meridional flow. Northern African cold surges have characteristics similar to surges observed elsewhere in the world, including a hydrostatically induced ridge of surface pressure and an amplified upper tropospheric ridge/trough pattern. The African cold surge is preceded by the passage of a shortwave trough and an intensification of the upper tropospheric subtropical westerly jet streak over the Mediterranean Sea. These events are associated with increased confluence in the jet entrance region over the central Mediterranean, an enhanced direct secondary circulation, subsidence, and low‐level ageostrophic northerly flow over northeastern Africa. Composite analysis shows that the passage of a cold surge is associated with an enhancement in convective activity over southern Algeria, western Niger, northern Mali, and Mauritania 2 to 5 days before the surge reaches the eastern Sahel (∼17.5°N), when northeasterly flow channeled between the Atlas and Ahaggar Mountains strengthens and transports relatively moist air from the western Mediterranean and eastern North Atlantic over the region and increases moisture convergence over western Africa north of 20°N. Over the eastern Sahel of Sudan and eastern Chad, the composite results reveal a break in convective activity and decrease in low‐level convergence when the surge arrives that persists for about 6 days. These results offer great promise for improving the short‐range prediction of rainfall over northern Africa.

STEREO observations of shock formation in the solar wind

During solar minimum, the majority of shocks observed at 1 AU are associated with interactions between fast and slow streams, appearing as the pressure ridge between the streams narrows, and as the fast mode speed becomes smaller than the velocity jump between streams. Shocks are believed to strengthen when following shock waves catch up with the leading shock. STEREO observations confirm this behavior for very weak shocks. We can also put limits on the location of the shock‐forming region by examining the periods of alignment between Venus and STEREO. In a period in which we see nine shocks at 1 AU, we see no shocks at 0.72 AU. Hence, stream interactions from 0.72 AU to 1 AU are responsible for the development of most of the shocks observed at 1 AU during solar minimum.

A TORNADO WITH ANTICYCLONIC ROTATION AT CLOONCAN, CO. MAYO

On 31 July 2004 a tornado that was unusual in several ways occurred in the west of Ireland. It had an anticyclonic, rather than the more normal cyclonic, rotation, and it occurred under relatively unpromising synoptic conditions, where there was a slack surface-pressure gradient close to the centre of a ridge of high pressure over north-west Europe. However, regional low-level wind shear and strong convergence are shown to have created a favourable environment over Co. Mayo, aided by a limited amount of convective available potential energy (CAPE) that accumulated beneath an inversion at 816hPa and by a strong vertical moisture profile. The role of the contrasting thermal properties between Lough Carra and the adjacent land surface in producing an anticyclonically rotating tornado from a horizontal vortex that developed within the sheared environment is considered. All of these features are found to be strongly localised. Comparisons are made with the non-mesocyclonic tornado environments documented in US research, and a number of contrasts are noted. The tornado track was only partially traceable due to the terrain, and it reached its widest point of 18m where it entered bogland after 0.75km. John Tyrrell (email: j.tyrrell@ucc.ie), Department of Geography, UCC, Cork.

External Influences on Nocturnal Thermally Driven Flows in a Deep Valley

AbstractThe dynamics that govern the evolution of nighttime flows in a deep valley, California’s Owens Valley, are analyzed. Measurements from the Terrain-Induced Rotor Experiment (T-REX) reveal a pronounced valley-wind system with often nonclassical flow evolution. Two cases with a weak high pressure ridge over the study area but very different valley flow evolution are presented. The first event is characterized by the appearance of a layer of southerly flow after midnight local time, sandwiched between a thermally driven low-level downvalley (northerly) flow and a synoptic northwesterly flow aloft. The second event is characterized by an unusually strong and deep downvalley jet, exceeding 15 m s−1. The analysis is based on the T-REX measurement data and the output of high-resolution large-eddy simulations using the Advanced Regional Prediction System (ARPS). Using horizontal grid spacings of 1 km and 350 m, ARPS reproduces the observed flow features for these two cases very well. It is found that the low-level along-valley forcing of the valley wind is the result of a superposition of the local thermal forcing and a midlevel (2–2.5 km MSL) along-valley pressure forcing. The analysis shows that the large difference in valley flow evolution derives primarily from differences in the midlevel pressure forcing, and that the Owens Valley is particularly susceptible to these midlevel external influences because of its specific geometry. The results demonstrate the delicate interplay of forces that can combine to determine the valley flow structure on any given night.

Explaining the patterns formed by ice floe interactions

We provide a quantitative explanation of the patterns commonly observed during the collision of two floating ice sheets (ice floes): simple rafting, finger rafting, or the formation of a pressure ridge. In particular, we analyze the equilibrium configurations of the ice in each of the two types of rafting and show that these are only possible if the ice thickness is below a threshold value that we determine in terms of the strength and elastic modulus of the ice. We construct a regime diagram to characterize the regions of thickness‐strength parameter space for which each type of deformation can occur. Using typical values for the strength of sea ice, we find that finger rafting can only occur in ice with thickness ≲8 cm, and simple rafting can typically only occur in ice with thickness ≲21 cm. These estimates are quantitatively consistent with most field observations reported in the literature. Finally, by quantifying two common complications that occur in reality, we are also able to account for some of the discrepancies between theory and observation that remain. We show that the presence of rubble may allow ice with a thickness of up to 1 m to perform simple rafting, but multiple rafting of thin sheets does not increase the maximum rafting thickness.

Ocean wave scattering by natural sea ice transects

Ocean waves propagate in continuous sea ice as flexural‐gravity waves, so named because their dispersion is influenced by bending of the ice plate and by buoyancy. Such waves are scattered by topographic heterogeneity in the ice, e.g., features like pressure ridges, yet current models fail to account for the complexity that is symptomatic of sea ice. Here a two‐dimensional wave scattering model is described that allows real data from natural sea ice to be assimilated rather than synthetic ice terrain. On the basis of linear wave theory and the elastic thin plate equation, the model assumes an unbroken compliant sea ice cover with no free edges and uses Green's Functions to obtain an integral equation that is solved numerically. While scattering in physical stretches of sea ice can vary greatly, all transects behave as low‐pass filters and can display resonances that relate to how features are separated or to transect length, the latter being an undesirable trait of the model. In the paper an association is made between the statistical properties of the sea ice terrain and the nature of the scattering that results. Not unexpectedly, sea ice with greater relative draft variation is more reflective. It is proposed that this observation can be used to identify the draft variation for an unknown transect if its scattering response is known. Finally, the progression of median energy density for ensembles of similarly sampled transects is examined at wave periods of less than 10 s and is found to decay exponentially with distance traveled, in accord with the few observations available.

Understanding the surface hydrology of the Lake Eyre Basin: Part 1—Rainfall

This is the first of two papers that describe the surface hydrology of the Lake Eyre Basin (LEB) (1,140,000 km 2) in central Australia and compares some key characteristics with those observed from arid regions globally. This paper concentrates on annual rainfall, whereas the second paper is devoted to streamflow. The first part describes the LEB's climate (arid to semi-arid), which is dominated by a subtropical high pressure ridge stretching latitudinally across central Australia. Then follow major analyses that include the characteristics of rainfall, wet and dry spell lengths and cumulative surpluses and deficiencies, rainfall trends and intra- and inter-decadal fluctuations, and the relationship between rainfall and El Niño-Southern Oscillation (ENSO). The paper concludes with six conclusions, the key ones being: the variability of the annual rainfall (based on the coefficient of variation) in the LEB is approximately 60% greater than that found for stations located in arid regions in the rest of the world; there is a bias towards longer lengths of dry years than observed in the rest of Australia; and, there is a significant lag correlation between rainfall and ENSO, particularly in the east and in the latter part of a year.

Emplacement and inflation of natrocarbonatitic lava flows during the March–April 2006 eruption of Oldoinyo Lengai, Tanzania

The most voluminous eruption of natrocarbonatite lava hitherto recorded on Earth occurred at Oldoinyo Lengai in March–April 2006. The lava flows produced in this eruption range from blocky 'a'a type to smooth-surfaced inflated pahoehoe. We measured lava inflation features (i.e. one tumulus and three pressure ridges) that formed in the various pahoehoe flows emplaced in this event. The inflation features within the main crater of Oldoinyo Lengai are relatively small-scale, measuring 1-5 m in width, 2.5–24.4 m in length and with inflation clefts less than 0.4 m deep. Their small sizes are in contrast to a tumulus that formed on the northwestern slope of the volcano (situated ~1140 m below the crater floor). The tumulus is roughly circular, measures 17.5 × 16.0 m, and is cut by a 4.4 m deep axial inflation cleft exposing two separate flow units. We measured the elastic properties (i.e. shear- and bulk moduli) of natrocarbonatitic crust and find that these are similar to those reported for basaltic crust, and that there is no direct correlation between magmastatic head and pressure required to form tumuli. All inflated flows in the 2006 event were confined by lateral barriers (main crater, erosional channel or erosional gully) suggesting that the two most important factors for endogenous growth in natrocarbonatitic lava flows are (1) lateral barriers that prevent widening of the flow, and (2) influx of new material beneath the viscoelastic and brittle crust.

Sea ice as an evaluation target for HEM modelling and inversion

We investigate helicopter electromagnetic (HEM) inversion schemes applied to synthetic and measured HEM sea ice profiling data. Direct HEM data-to-ice-thickness inversion is compared to three different formal, least squares layered earth inversion algorithms. By making several approximations, it is possible to directly invert a single channel measurement (i.e., the in-phase or quadrature component of a single frequency measurement) to obtain an estimate of sea ice thickness. Measurements from multiple input channels, however, can be used in a layered earth inversion to simultaneously recover several model parameters such as sea ice thickness, sea ice conductivity and sub-ice bathymetry. Synthetic data sets for a particular two-frequency HEM system showed that simple least squares inversion algorithms produce reliable estimates of sea ice thickness in cases where the ice is thicker than 3 m. These methods could also recover acceptable estimates of sea ice thickness when a thin, conductive, partially melted sea ice layer was present, and could determine shallow, sub-ice bathymetry in brackish water. As expected, 1D transformations and inversions of synthetic data for a three-dimensional pressure ridge keel structure contained artifacts, notably broadening of the apparent width of the keel. Prior to inverting a field data set acquired over rather thin (~ 0.5 m) Antarctic sea ice, we found it necessary to recalibrate the phase angle of the measurements using a phasor diagram-based method. Direct transformation of a single channel from the recalibrated data set produced more accurate estimates of sea-ice thickness than formal inversion of multi-channel data. We suggest that the least squares inversion methods are inferior in this situation because of the particular characteristics of the two-frequency HEM system used in this evaluation; the extreme differences in sensitivity of high and low frequency data components, the overall low sensitivity to sea ice conductivity (especially for thin ice), and the partially low signal-to noise ratios of the measurements. The data sets used in this study will be made available to the public to allow alternate inversion approaches to be applied and evaluated. It is suggested that inclusion of parameter bounds and other forms of regularization could help to improve the inversion results.

The Rachaya‐Serghaya fault system (Lebanon): Evidence of coseismic ruptures, and the AD 1759 earthquake sequence

The Rachaya and Serghaya faults are the easternmost fault branches of the Dead Sea Transform Fault within the Lebanese restraining bend. They lie east of the Yammouneh fault (the main strand of the Dead Sea Transform Fault within the restraining bend), extend along the western and eastern flanks of the Anti‐Lebanon range, respectively, and show left‐lateral strike‐slip movement manifested as offset drainage. We studied both faults through combined field investigations in geomorphology and paleoseismology. Young fault scarps, mole tracks, pressure ridges and offset streams detected along the faults' traces attest to recent coseismic ruptures. Two paleoseismic investigations highlight their seismogenic potential and indicate earthquake recurrence along them: the Rachaya and Serghaya faults are active and the sources of recent historical earthquakes, the last of which might be the 30 October–25 November 1759 (Ms 6.6 and 7.4) earthquake sequence that caused severe damage in the eastern Mediterranean region. Such a possible correlation suggests that the two faults are probably structurally interconnected, as movement on one fault may stimulate movement on the other fault. In addition, both faults may define together an active seismogenic fault system that accommodates some of the regional displacement that takes place within the Lebanese restraining bend. Our results highlight that the seismogenic potential of the Rachaya and Serghaya faults must be included in any seismic hazard assessment of the region.

Comparison of Kemi-I and Confederation Bridge cone ice load measurement results

A Joint Finnish Industry Project in 1983–87 measured cone ice loads by installing an instrumented cone around the Kemi-I lighthouse in the Gulf of Bothnia. A Canadian University/Joint Industry Project has been conducting cone ice load measurements since 1997 on two instrumented piers of the Confederation Bridge, in the Southern Gulf of St Lawrence. The diameters of the cones at the waterline are 10 and 14 m respectively. Ice conditions are more severe at Kemi-I but ice movement is faster and ridge encounters more frequent at Confederation Bridge. However, all types of first year ice features have been encountered at both locations. The cone ice load measurements and observations indicate that ice failure patterns and broken floe clearing mechanisms are, in general, similar. Comparisons with reference to ice thickness, temperature, and velocity, are made on: rubble formation, rubble surcharge height, rubble jamming, pressure ridge failure modes, dynamic effects, and measured and predicted ice loads. The results pave the way for a better understanding of ice failure against cones and cone ice load design.

The 17 February 2006 Guinsaugon rock slide-debris avalanche, Southern Leyte, Philippines: deposit characteristics and failure mechanism

The Guinsaugon rock slide-debris avalanche was the most catastrophic single landslide event in Philippine history, with 14–18 M m3 of debris instantly burying an entire village. Hummocky topography, pressure ridges and other internal structures suggest that the landslide deposit was emplaced as a debris avalanche and debris flow. Susceptibility to planar and wedge failures as well as to toppling due to rock discontinuities were demonstrated using kinematic analysis and SMR. Limit equilibrium analysis on planar failures yielded factors of safety ranging from 0.8 to 1.5. The study showed that pore pressure on discontinuities had a more significant effect on the slope stability than seismicity. For wedge failures, there is a sudden drop in the factor of safety at pore water pressures of 258–306 kPa. At the site, the pore water pressure may have been as high as 490 kPa. The possibility of such a landslide event in the future is discussed.