T Waves from the 1998 Papua New Guinea Earthquake and its Aftershocks: Timing the Tsunamigenic Slump

— T waves recorded at hydrophone and seismic stations following the Papua New Guinea earthquake of 17 July 1998 and its aftershocks show that a small event at 09:02 GMT featured source properties incompatible with an elastic dislocation of appropriate body-wave magnitude (m _b=4.4). These include an exceptional duration (47 s at the Wake Island hydrophone station WK31), a spectrum rich in high frequencies (7 to 12 Hz), and a generally low spectral amplitude. These characteristics can be explained by the model of an underwater slump, accelerating from a standstill and eventually slowing down. The relocation of the 09:02 event is compatible with its location within an amphitheater inside which shipboard cruises in 1998 and 1999 documented the presence of a 4 – km³ geologically fresh slump. We propose that the slump took place at 09:02 on 17 July 1998, i.e., 13 minutes after the mainshock, and that it generated the locally catastrophic tsunami, whose properties (amplitude and distribution of runup; timing) could not be explained by a dislocation model.     

Africa’s middle class: building houses and constructing identities in the small town of Buea,Cameroon

Across Africa households are commissioning new domestic buildings for their own use or to rent out rather than waiting for governments to supply urban shelter. We conceptualize this as two interwoven processes: building houses for a new middle class and the building of a new middle class identity. We draw particular attention to the impact of international migration on these processes. The paper uses quantitative data from building permits and qualitative data from 29 interviews and walk-round tours with homeowners and government officials in Buea, Cameroon. Despite some significant caveats (related to housing the urban poor, regulating building standards, and lost local government revenue), we argue that these individually initiated building projects do a reasonable job of meeting local demand for shelter. However, from the perspective of identity-building these buildings are less successful. Whilst homeowners take considerable pride in their building projects, the statements that these structures make about their identities are not always “read” by their neighbours in the way owners hope. New housing is the lens through which Buea’s residents interpret growing levels of inequality and its associated social tensions.     

Quantification of Hydrophone Records of the 2004 Sumatra Tsunami

The 2004 Sumatra-Andaman tsunami was recorded by hydrophones of the International Monitoring System at Site H08 near Diego Garcia, notably in frequency bands extending outside the range of the Shallow Water Approximation. Despite the severe high-pass filtering involved in this instrumentation, we show that the spectral amplitudes recovered around T = 87 s can be successfully explained by generation from the seismic source, in the framework of the normal mode theory of tsunami excitation. At the lower frequencies characteristic of more conventional tsunami waves (800 to 3200 s), the signal is probably present in the hydrophone records, but reliable deconvolution of its spectral amplitude is precluded by the fact that the instrumental filters lowered the tsunami signal to the level of resolution of the instrument digitizer. In the context of distant tsunami warning, hydrophone records could provide useful insight into high-frequency tsunami components, and even at lower, more conventional, frequencies, provided that an unfiltered channel could be recorded routinely.     

Observed very long period Rayleigh-wave phase velocities across the Canadian shield

summary . The recent Indonesian earthquake (1977 August 19; M_s =8.0) makes possible a direct determination of Rayleigh-wave phase velocities at very long periods (200 to 300 s) over a pure shield path (College, Alaska-State College, Pennsylvania) by the two-station method. Results are in good agreement with previous models derived by regionalization of great-circle phase velocities and discussed in a previous paper (Okal, Paper I). Great-circle phase velocities computed at College and State College are correctly predicted by the models derived in Paper I, and further substantiate one of its main conclusions: no substantial structural differences between oceans and continents at depths greater than 240 km, are necessary to account for the presently available seismic data.     

Tsunami Simulations for Regional Sources in the South China and Adjoining Seas

We present 14 scenarios of potential tsunamis in the South China Sea and its adjoining basins, the Sulu and Sulawezi Seas. The sources consist of earthquake dislocations inspired by the the study of historical events, either recorded (since 1900) or described in historical documents going back to 1604. We consider worst-case scenarios, where the size of the earthquake is not limited by the largest known event, but merely by the dimension of the basin over which a coherent fault may propagate. While such scenarios are arguably improbable, they may not be impossible, and as such must be examined. For each scenario, we present a simulation of the tsunami??s propagation in the marine basin, exclusive of its interaction with the coastline. Our results show that the South China, Sulu and Sulawezi Seas make up three largely independent basins where tsunamis generated in one basin do not leak into another. Similarly, the Sunda arc provides an efficient barrier to tsunamis originating in the Indian Ocean. Furthermore, the shallow continental shelves in the Java Sea, the Gulf of Thailand and the western part of the South China Sea significantly dampen the amplitude of the waves. The eastern shores of the Malay Peninsula are threatened only by the greatest??and most improbable??of our sources, a mega-earthquake rupturing all of the Luzon Trench. We also consider two models of underwater landslides (which can be triggered by smaller events, even in an intraplate setting). These sources, for which there is both historical and geological evidence, could pose a significant threat to all shorelines in the region, including the Malay Peninsula.     

Deformation of monochromatic water wave trains propagating over a submerged obstacle in the presence of uniform currents

A nonlinear numerical model based on depth averaged equations and a relevant physical model have been investigated for the deformation of the water wave propagating over a submerged parabolic obstacle in the presence of uniform current. Physical and numerical modeling for wave with both following and opposing currents are done to explore the wave evolutions during passage over the submerged obstacle. A third-order Stokes dispersion relation is utilized in some cases in the computation. Separated flow zone is taken into consideration by two empirical equations obtained from the physical model testing done by the authors. Verification and validation of the numerical model by other published theoretical and experimental data are presented.     

One,two or no record of late neoproterozoic glaciation in South-East Cameroon?

Severe climate changes culminating in at least three major glacial events have been recognized in the Neoproterozoic sedimentary record from many parts of the world. Supportive to the global nature of these climatic shifts, a considerable amount of data have been acquired from deposits exposed in Pan-African orogenic belts in southwestern and western Africa. By comparison, published data from the Pan-African belts in Central Africa are scarce. We report here evidence of possibly two glacial events recorded in the Mintom Formation that is located on the margin of the Pan-African orogenic Yaoundé belt in South-East Cameroon.In the absence of reliable radiometric data, only maximum and minimum age limits of 640 and 580 Ma, respectively, can at present be applied to the Mintom Formation. The formation consists of two lithostratigraphic ensembles, each subdivided in two members (i.e., in ascending stratigraphic order the Kol, Métou, Momibolé, and Atog Adjap Members). The basal ensemble exhibits a typical glacial to post-glacial succession. It includes diamictites comprising cobbles and boulders in a massive argillaceous siltstone matrix, and laminated siltstones followed by, in sharp contact, a 2 m-thick massive dolostone that yielded negative δ¹³C values (<?3‰ V-PDB) similar to those reported for Marinoan cap carbonates elsewhere. However, uncertainty remains regarding the glacial influence on the siliciclastic facies because the diamictite is better explained as a mass-flow deposit, and diagnostic features such as dropstones have not been seen in the overlying siltstones. The Mintom Formation may thus provide an example of an unusual succession of non-glacial diamictite overlain by a truly glacial melt-related cap-carbonate.We also report the recent discovery of ice-striated pavements on the structural surface cut in the Mintom Formation, suggesting that glaciers developed after the latter had been deposited and deformed during the Pan-African orogeny. Striations, which consistently exhibit two principal orientations (N60 and N110), were identified in two different localities, in the west of the study area on siltstones of the Kol Member, and in the east on limestones of the Atog Adjap Member, respectively. N60-oriented striae indicate ice flow towards the WSW. Assigning an age to these features remains problematical because they were not found associated with glaciogenic deposits. Two hypotheses can equally be envisaged, i.e., either the striated surfaces are correlated: (1) to the Gaskiers (or Neoproterozoic post-Gaskiers) glaciation and represent the youngest Ediacaran glacial event documented in the southern Yaoundé belt; or (2) to the Late Ordovician Hirnantian (Saharan) glaciation, thereby providing new data about Hirnantian ice flows in Central Africa.     

Computation of oscillator strengths by a semi-empirical method for some elements of the iron-group and their solar photospheric abundance

Oscillator strengths are calculated by the STF method for selected transitions of Mn i. The results, applied to the analysis of solar spectra, support the ‘high’ solar photospheric abundance of manganese recently obtained on the basis of experimental gf-values.