From regional seismic hazard to “scenario earthquakes” for seismic microzoning: A new methodological tool for the Celano Project

We present the results of a probabilistic seismic hazard assessment and disaggregation analysis aimed to understand the dominant magnitudes and source-to-site distances of earthquakes that control the hazard at the Celano site in the Abruzzo region of central Italy. Firstly, we calculated a peak ground acceleration map for the central Apennines area, by using a model of seismogenic sources defined on geological-structural basis. The source model definition and the probabilistic seismic hazard evaluation at the regional scale (central Apennines) were obtained using three different seismicity models (Gutenberg–Richter model; characteristic earthquake model; hybrid model), consistent with the available seismological information. Moreover, a simplified time-dependent hypothesis has been introduced, computing the conditional probability of earthquakes occurrence by Brownian passage time distributions.Subsequently, we carried out the disaggregation analysis, with a modified version of the SEISRISK III code, in order to separate the contribution of each source to the total hazard.The results show the percentage contribution to the Celano hazard of the various seismogenic sources, for different expected peak ground acceleration classes. The analysis was differentiated for close (distance from Celano <20 km) and distant (distance from Celano >20 km) seismogenic sources. We propose three different “scenario earthquakes”, useful for the site condition studies and for the seismic microzoning study: (1) large (M=6.6) local (Celano-epicentre distance 16 km) earthquake, with mean recurrence time of 590 years; (2) moderate (M=5.5) local (Celano-epicentre distance 7.5 km) earthquake, with mean recurrence time of 500 years; and (3) large (M=6.6) distant (Celano-epicentre distance 24 km) earthquake, with mean recurrence time of 980 years.The probabilistic and time-dependent approach to the definition of the “scenario earthquakes” changes clearly the results in comparison to traditional deterministic analysis, with effects in terms of engineering design and seismic risk reduction.     

Mismatch between variations of solar indices, stratospheric ozone and UV-B observed at ground

In solar cycles 22–23, all solar indices showed maxima near 1990 and 2000 and minima in 1996. The maximum to minimum variation was only 1–2% in the UV range 240–350 nm. Dobson ozone intensities did not show any clear relationship with solar cycle and ozone variations were less than 10%. The UV-B (295–325 nm) observed at ground by Brewer spectrophotometers at some locations had variations of 50–100% for 295–300 nm, and 20–50% for 305–325 nm. The maxima were in different years at different locations (even with separations of only 300 km), did not match with the solar cycle, and were far too large to be explained on the basis of ozone changes (1% decrease of ozone is expected to cause 2% increase of UV-B). Thus, if the data are not bad, the UV-B changes do not match with solar activity or ozone changes and must be mostly due to other local effects (clouds, etc.?). When data are averaged over wide geographical regions, UV-B variation ranges are smaller (10–20%, probably because localised, highly varying cloud effects get filtered out), and are roughly as expected from ozone variations.     

Seasonal variability in mesopause region temperatures over Fort Collins, CO (41°N, 105°W) based on lidar observations, 1990 through 2007

Nearly 900 nocturnal temperature profiles (85–105 km) from the Colorado State University Na lidar at Fort Collins, CO (40.59N, 105.14W) from 1990 to 2007. After the removal of an episodic warming attributable to Mt. Pinatubo eruption, the time series is analyzed as the sum of the climatological mean, annual and semiannual oscillation, solar cycle effect and trends along with possible annual/semiannual modulation of the latter two. The direct seasonal variation is consistent with the concept of the two-level mesopause. The trends in summer and winter are comparable 90–96 km at −0.15±0.1 K/year. The summer trend turns positive above 96 km. The winter trend is negative with minimum of −0.3 K/year at 100 km but positive at 104 km. The negative trend values are a factor of five smaller than an earlier analysis of the early part of this data due to removal of an episodic event.     

The effect of carbon dioxide cooling on trends in the F2-layer ionosphere

The effect of carbon dioxide (CO₂) cooling on trends of h_mF₂ and N_mF₂ are investigated using a coupled thermosphere and ionosphere general circulation model. Model simulations indicate that CO₂ cooling not only causes contraction of the upper atmosphere and changes of neutral and ion composition but also changes dynamics and electrodynamics in the thermosphere/ionosphere. These changes determine the altitude dependence of ionospheric trends and complex latitudinal, longitudinal, diurnal, seasonal, and solar cycle variations of trends of h_mF₂ and N_mF₂. Under the CO₂ cooling effect, trends of N_mF₂ are negative with magnitude from 0% to −40% for doubled CO₂, depending on location, local time, season, and solar activity. The corresponding trends of h_mF₂ are mostly negative with a magnitude from 0 to −40 km, but can be positive with a magnitude from 0 to 10 km at night, with maximum positive trends occurring after midnight under solar minimum conditions.     

Seasonal oscillations in mesospheric temperatures at low-latitudes

Using 3 years of high-quality temperature measurements (2002–2004) recorded from Maui, HI (20.8°N), we have investigated the characteristics of mesospheric seasonal oscillations at low-latitudes. Measurements of the near-infrared OH (6,2) and O₂ (0,1) nightglow emission layers (centered at 87 and 94 km) independently reveal a distinct semi-annual oscillation (SAO) and annual oscillation (AO) with amplitudes of 3.8 and 2.0 K, respectively. An observed asymmetry in the seasonal variation of the nocturnal mean, previously reported by Taylor et al. [2005. Characterization of the semi-annual-oscillation in mesospheric temperatures at low-latitudes. Advances in Space Research 35, doi:10.1016/j.asr.2005.05.111] from this site is shown to be due to a superposed AO of amplitude 50% of the SAO signature. Detailed investigations of the local-time variation of the SAO amplitude and phase combined with TIME-GCM simulations of the seasonal variation of the diurnal tide strongly suggest a large local-time dependence of the amplitude (but not phase) of the observed SAO. These data indicate that the true mean temperature SAO amplitude could be as high as 7 K at this latitude.     

Neogene volcanic activity of western Syria and its relationship with Arabian plate kinematics

The Cenozoic (mostly Neogene) volcanic activity in Syria is part of the extensive magmatism that took place in the Mashrek Region, Middle East, from upper Eocene to Holocene (40–0.0005 Ma). Samples in western Syria are mostly high TiO₂ (TiO₂ 1.8–3.7 wt.%) alkaline mafic rocks (basanites, hawaiites and alkali basalts) plus rare transitional/tholeiitic basalts and basaltic andesites) with within-plate-like trace element signature.On the basis of incompatible trace element content, the volcanic activity in Syria has been divided into two stages: the first lasting from 25 to 5 Ma and the second from 5 to recent times. Indeed, the Syrian lavas show incompatible trace element content increasing with decreasing age from 25 to 5 Ma, followed by an abrupt decrease to low values roughly at the Miocene–Pliocene boundary. This temporal shift in composition is related to major tectonic re-organization occurred during upper Miocene.The proposed petrogenetic model invokes three steps: (a) passive upwelling of the shallow asthenosphere during the development of the Dead Sea transform fault system. Different degrees of partial melting were followed by variable extents of fractional crystallization and limited upper crustal contamination; (b) the Miocene–Pliocene boundary tectonic change enhanced passive decompression of the same sources and a consequent increase in degree of partial melting resulting in low incompatible trace element content of the relatively high-volume liquids; (c) after this phase, the incompatible trace element content in the basaltic magmas increased as consequence of fractional crystallization processes.Major and trace element content similarities with the rest of the circum-Mediterranean igneous rocks are consistent with a common relatively shallow origin for the Cenozoic anorogenic magmatism of the entire circum-Mediterranean area (the so-called Common Magmatic Reservoir). Because much of the igneous activity in the studied area is concentrated near the Dead Sea fault, the origin of Cenozoic magmatism in Syria (and in the rest of the circum-Mediterranean area) reflects a strong lithospheric control on the loci of partial melting. Mantle plumes from lower mantle and/or north-westward channelling of the Afar mantle plume is not needed to explain volcanic activity in Syria and the Mashrek area.     

Long-term variability in mesopause region temperatures over Fort Collins, Colorado (41°N, 105°W) based on lidar observations from 1990 through 2007

During 1990–2007, there were 894 lidar observations of nocturnal mesopause region temperatures over Fort Collins, Colorado. In an earlier analysis with data to April 1997, an unexpected episodic warming, peaking in 1993 with a maximum value over 10 K, was reported and attributed to the Mount Pinatubo eruption in June 1991. With all data, long-term temperature trends from a 7-parameter linear regression analysis including solar cycle effect and long-term trends leads to a cooling of as much as 6.8 K/decade at 100 km, consistent with some reported observations but larger than model predictions. Including the observed episodic warming response in an 11-parameter nonlinear regression analysis reduces the maximum long-term cooling trends to 1.5 K/decade at 91 km, with magnitude and altitude dependences consistent with the prediction of two models, Spectral Mesosphere/Lower Thermosphere Model (SMLTM) and Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA). In addition, the mid-latitude middle-atmospheric response to solar flux variability in Thermosphere–Ionosphere-Energetics and Dynamics (TIMED)/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperatures is presented.     

Collision of the Ganges–Brahmaputra Delta with the Burma Arc: Implications for earthquake hazard

We take a fresh look at the topography, structure and seismicity of the Ganges–Brahmaputra Delta (GBD)–Burma Arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential. The GBD, the world's largest delta, has been built from sediments eroded from the Himalayan collision. These sediments prograded the continental margin of the Indian subcontinent by  400 km, forming a huge sediment pile that is now entering the Burma Arc subduction zone. Subduction of oceanic lithosphere with > 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land. The prism starts at an apex south of the GBD shelf edge at  18°N and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit. The front of the prism is blind, buried by the GBD sediments. Thus, the deformation front extends 100 km west of the surface fold belt beneath the Comilla Tract, which is uplifted by 3–4 m relative to the delta. This accretionary prism has the lowest surface slope of any active subduction zone. The gradient of the prism is only  0.1°, rising to  0.5° in the forearc region to the east. This low slope is consistent with the high level of overpressure found in the subsurface, and indicates a very weak detachment. Since its onset, the collision of the GBD and Burma Arc has expanded westward at  2 cm/yr, and propagated southwards at  5 cm/yr. Seismic hazard in the GBD is largely unknown. Intermediate-size earthquakes are associated with surface ruptures and fold growth in the external part of the prism. However, the possibility of large subduction ruptures has not been accounted for, and may be higher than generally believed. Although sediment-clogged systems are thought to not be able to sustain the stresses and strain-weakening behavior required for great earthquakes, some of the largest known earthquakes have occurred in heavily-sedimented subduction zones. A large earthquake in 1762 ruptured  250 km of the southern part of the GBD, suggesting large earthquakes are possible there. A large, but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism, and is the only known candidate for a rupture of the plate boundary along the subaerial part of the GBD–Burma Arc collision zone.     

Temperature variability in central Mexico and its possible association to solar activity

Minimum extreme temperature variability from five meteorological stations in the central part of Mexico covering a period from 1920 to 1990 is examined. We found a correlation coefficient (r=0.65) between these temperature records and geomagnetic activity. Furthermore, by performing spectral analysis peaks were obtained with similar periodicities to those found in the sunspot number, the magnetic solar cycle, cosmic ray fluxes and geomagnetic activity; all of these phenomena are modulated by solar activity. Signals with periodicities comparable to those observed in El Niño and the Quasi-Biennial Oscillation were also identified. We conclude that the solar signal is probably present in the minimum extreme temperature record of the central part of Mexico.     

Critical phenomena in thermal conductivity: Implications for lower mantle dynamics

Microscopic mechanisms for heat transport in dense minerals (phonon scattering and photon attenuation) exhibit aspects of threshold behavior, discussed qualitatively here. For all minerals examined so-far using laser-flash analysis, the lattice component of the thermal conductivity of the mantle asymptotes to a constant above a critical temperature of 1500 K. Radiative transfer calculated from absorption spectra has thresholds in both grain-size and Fe content, and a rather complex dependence on temperature. These critical phenomena impact convection of the lower mantle, because the lattice contribution tends to destabilize the cold boundary layers, whereas radiative transfer mostly promotes stability in the lower mantle, unless the grains are large and Fe-rich, which makes convection chaotic and time-dependent. The specific behavior suggests that flow in the lower mantle is sluggish, whereas flow in the upper mantle-transition zone is time-dependent. The decrease in k_rad as Fe/(Fe + Mg) increases beyond 0.1 may be connected with formation of lower mantle, thermo-chemical plumes through positive feedback.     

Are fiddler crabs potentially useful ecosystem engineers in mangrove wastewater wetlands?

The effect of different organic-rich sewage concentration (0%, 20% and 60% diluted in seawater) and absence or presence of mangrove trees on the survival, bioturbation activities and burrow morphology of fiddler crabs species was assessed. After 6 months, males of both species always showed higher survival (80%) when compared to females (20%). Crabs inhabiting pristine conditions achieved higher survival (67–87%) than those living in sewage-exposed mesocosms (40–71%). At 60% sewage loading, fiddler crabs processed less sediment (34–46%) during feeding and excavated slightly more sediment (45–80%) than at pristine conditions. While percent volume of the burrow chambers increased (13–66%) at contaminated mesocosms for both vegetation conditions, burrows were shallower (33%) in bare cells loaded with sewage. The results show that fiddler crabs presented moderate mortality levels in these artificial mangrove wetlands, but mainly in sewage impacted cells. However, they still function as ecosystem engineers through bioturbation activities and burrow construction.     

Studies on turbulent diffusion processes and evaluation of diffusivity values from hydrodynamic observations in Corpus Christi Bay

The physical process of dispersion which can be attributed to turbulence (turbulent diffusion) or shear (shear-augmented diffusion) within the flow field is very important as it ultimately governs the distribution of constituents of interest within the environment. A series of diffusion experiments were conducted in Corpus Christi Bay, TX with the purpose of characterizing turbulent diffusion through dispersion coefficients or turbulent diffusivity, K_i (i=x, y, z) dependent on the degree of randomness or turbulence intensity, I.Measured with a boat-mounted acoustic doppler current profiler (ADCP), the Eulerian velocity time-series of fluid particles in random motion, u_i was used in the evaluation of the Eulerian time-scale of turbulence, T^E based on the velocity correlation function, R^E with T^E being related to the Lagrangian time-scale T^L through a scaling parameter, β(=T^L/T^E). Surface currents were obtained with high frequency (HF) Radar equipment deployed over the study area from which the horizontal velocity gradients were determined.Within the spatial scale of the experiment (1000 m), the observed low horizontal gradients (10⁻⁴ s⁻¹) allowed for the generation of velocity time-series from an ADCP mounted on a moving platform. A numerical scheme for evaluating turbulent diffusivity values was developed on the basis of Eulerian current measurements and calibrated through the statistics of an evolving dye patch for the scaling parameter β which in this scheme was found to be in the range 1–3.     

A multidisciplinary effort to assign realistic source parameters to models of volcanic ash-cloud transport and dispersion during eruptions

During volcanic eruptions, volcanic ash transport and dispersion models (VATDs) are used to forecast the location and movement of ash clouds over hours to days in order to define hazards to aircraft and to communities downwind. Those models use input parameters, called “eruption source parameters”, such as plume height H, mass eruption rate Ṁ, duration D, and the mass fraction m₆₃ of erupted debris finer than about 4 or 63 μm, which can remain in the cloud for many hours or days. Observational constraints on the value of such parameters are frequently unavailable in the first minutes or hours after an eruption is detected. Moreover, observed plume height may change during an eruption, requiring rapid assignment of new parameters. This paper reports on a group effort to improve the accuracy of source parameters used by VATDs in the early hours of an eruption. We do so by first compiling a list of eruptions for which these parameters are well constrained, and then using these data to review and update previously studied parameter relationships. We find that the existing scatter in plots of H versus Ṁ yields an uncertainty within the 50% confidence interval of plus or minus a factor of four in eruption rate for a given plume height. This scatter is not clearly attributable to biases in measurement techniques or to well-recognized processes such as elutriation from pyroclastic flows. Sparse data on total grain-size distribution suggest that the mass fraction of fine debris m₆₃ could vary by nearly two orders of magnitude between small basaltic eruptions ( 0.01) and large silicic ones (> 0.5). We classify eleven eruption types; four types each for different sizes of silicic and mafic eruptions; submarine eruptions; “brief” or Vulcanian eruptions; and eruptions that generate co-ignimbrite or co-pyroclastic flow plumes. For each eruption type we assign source parameters. We then assign a characteristic eruption type to each of the world's  1500 Holocene volcanoes. These eruption types and associated parameters can be used for ash-cloud modeling in the event of an eruption, when no observational constraints on these parameters are available.     

A 14-year monthly climatology and trend in the 35–65 km altitude range from Rayleigh Lidar temperature measurements at a low latitude station

A sodium resonance lidar at 589 nm has been operated in São José dos Campos, Brazil (23°S, 46°W) since 1972 mainly for studies related to the origin, chemistry and dynamics of the mesospheric sodium layer. Beginning in 1993, the improved laser capability has also enabled the processing of the Rayleigh signal from which the temperatures from 35 to 65 km are retrieved on a nightly mean basis. We used these nightly profiles to determine the monthly temperature profiles from 1993 to 2006. The mean temperature characteristics for each year and for the whole period are obtained. Seasonal thermal amplitude is small (6 K peak to peak at 40 and 60 km). Compared with the MSISE-90 model, a large difference is noted, with temperature lower than the model below the stratopause and higher above. Also the seasonal variation has a large difference with better agreement occurring around local winter, but with temperatures higher by 8–10 K at the equinoxes. The semiannual component is dominant over the annual at all altitudes. Linear trends with decreasing temperature of 1.09, 2.29 and 1.42 K/decade are observed at 40, 50 and 60 km, respectively.     

Quasi-biennial oscillation in foF2 at the south crest of the equatorial anomaly

The aim of this paper is to report some periodicities observed in the ionospheric parameter foF2 measured at Tucuman (26.9°S; 65.4°W), station placed near the southern crest of the equatorial anomaly. For that, monthly medians of foF2 at several hours of LT for the period 1958–1987 are used. The data are run with Fast Fourier Transform (FFT). Data gaps (4–5 months) are filled by means of linear interpolation. Several periodicities are present. Besides the solar cycle dominant dependence (11 years), semi-annual, annual, five years and quasi-biennial periodicities are also observed. A marked quasi-biennial periodicity is observed at daytime and nighttime hours being their greater amplitude at local noon and midnight. Different mechanisms or combined effects possibly cause them. It is suggested that the solar activity by means of extreme ultraviolet radiation (EUV), which present a quasi-biennial oscillation (QBO) and it is responsible for the ionization, could be the dominant mechanism for the diurnal quasi-biennial periodicity of foF2. At night, since the photoionization by extreme ultraviolet radiation is not significant and the F2 layer is lower than during daytime (100 km) other mechanism may be operative for the quasi-biennial periodicity observed. Possibly the stratospheric QBO contributes to the modulation of the observed behaviour in foF2 at night. This result is preliminary because it needs to be extended to other stations so as to extract definite conclusions. Moreover, we cannot dismiss the possibility of a combined effect of both these mechanisms mainly at daytime and/or QBO influence of geomagnetic parameters.     

Hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S

We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an  18 km² area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a  24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at  3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.     

OSL dating of paleoshorelines at Lagkor Tso, western Tibet

Lagkor Tso, a saline lake located south of Gertse in western Tibet exhibits spectacular flights of paleoshorelines. Optically stimulated luminescence (OSL) dating on quartz using the single-aliquot regenerative-dose (SAR) protocol from five paleoshoreline deposits shows that the lake level was 130 m higher than the present lake surface 5.2 ka ago. The lake level dropped rapidly by 25 m between 5.2 ka and 3.7 ka ago. Lake shrinkage further accelerated between 3.7 ka and 3.2 ka ago, when the lake level was just 74 m above the present lake surface. Luminescence characteristics and problematic samples are discussed.     

A report on long-term trends and variabilities in middle atmospheric temperature over Gadanki (13.5°N, 79.2°E)

The present study reports long-term variabilities and trends in the middle atmospheric temperature (March 1998–2008) derived from Rayleigh backscattered signals received by the Nd:YAG lidar system at Gadanki (13.5°N, 79.2°E). The monthly mean temperature compositely averaged for the years 1998–2008 shows maximum temperature of 270 K in the months of March–April and September at altitudes between 45 and 55 km. The altitude profile of trend coefficients estimated from the 10 years of temperature observations using regression analysis shows that there exists cooling at the rate with 1σ uncertainty of 0.12±0.1 K/year in the lower stratospheric altitudes (35–42 km) and 0.2±0.08 K/year at altitudes near 55–60 km. The trend is nearly zero (no significant cooling or warming) at altitudes 40–55 km. The regression analysis reveals the significant ENSO response in the lower stratosphere (1 K/SOI) and also in mesosphere (0.6 K/SOI). The solar cycle response shows negative maxima of 1.5 K/100F10.7 units at altitudes 36 km, 41 km and 1 K/100F10.7 units at 57 km. The response is positive at mesospheric altitude near 67 km (1.3 K/100F10.7 units). The amplitudes and phases of semiannual, annual and quasi-biennial oscillations are estimated using least squares method. The semiannual oscillation shows larger amplitudes at altitudes near 35, 45, 62 and 74 km whereas the annual oscillation peaks at 70 km. The quasi-biennial oscillations show larger amplitudes below 35 km and above 70 km. The phase profiles of semiannual and annual oscillations show downward propagation.     

Nature of sediment dispersal off the Sepik River, Papua New Guinea: preliminary sediment budget and implications for margin processes

A sediment budget is constructed for the slope and narrow continental shelf off the Sepik River in order to estimate the relative importance of turbid plumes versus bottom gravity transport through a near-shore submarine canyon in the dispersal of sediment across this collision margin. ²¹⁰Pb geochronology and inventories of Kasten cores are consistent with the northwestward dispersal of sediment from the river mouth via hypopycnal and possible isopycnal plumes. Sediment accumulation rates are 5 cm yr⁻¹ on the upper slope just off of the Sepik mouth, decreasing gradually to 1 cm yr⁻¹ toward the northwest, and decreasing abruptly offshore (<0.2 cm yr⁻¹ at 1200 m water depth). A sediment budget indicates that only about 7–15% of the Sepik River sediment discharge accumulates on the adjacent open shelf and slope. The remainder presumably escapes offshore via gravity flows through a submarine canyon, the head of which extends into the river mouth. The divergent sediment pathways observed off the Sepik River (i.e., surface and subsurface plumes versus sediment gravity flows through a canyon) may be common along high-yield collision margins of the Indo–Pacific archipelago, and perhaps are analogous to most margins during Late Quaternary low sea-level conditions.     

The lunar tides in the Antarctic mesosphere and lower thermosphere

Horizontal winds in the mesosphere and lower thermosphere over the Antarctic have been measured by a meteor radar at Rothera (67.5°S, 68.0°W) and MF radar at Davis (68.6°S, 78.0°E). Data from Rothera recorded over a 20-month interval in 2005–2006 and data from Davis recorded over the 13-year interval 1994–2006 are examined to investigate the monthly mean behaviour of the lunar semidiurnal tide. Both data sets show a clear signal of the 12.42-h lunar semidiurnal (M₂) tide. The amplitude reaches values as large as 8 m s⁻¹. The vertical wavelengths of the tide vary seasonally from 10 to 65 km. Comparisons of the phase of the tide measured over the two sites reveals that it does not purely consist of a migrating wavenumber 2 mode. This suggests that other, non-migrating, modes are likely to be present.