35 research outputs found
Crack-Like Processes Governing the Onset of Frictional Slip
We perform real-time measurements of the net contact area between two blocks
of like material at the onset of frictional slip. We show that the process of
interface detachment, which immediately precedes the inception of frictional
sliding, is governed by three different types of detachment fronts. These
crack-like detachment fronts differ by both their propagation velocities and by
the amount of net contact surface reduction caused by their passage. The most
rapid fronts propagate at intersonic velocities but generate a negligible
reduction in contact area across the interface. Sub-Rayleigh fronts are
crack-like modes which propagate at velocities up to the Rayleigh wave speed,
VR, and give rise to an approximate 10% reduction in net contact area. The most
efficient contact area reduction (~20%) is precipitated by the passage of slow
detachment fronts. These fronts propagate at anomalously slow velocities, which
are over an order of magnitude lower than VR yet orders of magnitude higher
than other characteristic velocity scales such as either slip or loading
velocities. Slow fronts are generated, in conjunction with intersonic fronts,
by the sudden arrest of sub-Rayleigh fronts. No overall sliding of the
interface occurs until either of the slower two fronts traverses the entire
interface, and motion at the leading edge of the interface is initiated. Slip
at the trailing edge of the interface accompanies the motion of both the slow
and sub-Rayleigh fronts. We might expect these modes to be important in both
fault nucleation and earthquake dynamics.Comment: 19 page, 5 figures, to appear in International Journal of Fractur
Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence
The great Sumatra–Andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones [1,2,3,4]. In March 2005, the Sunda megathrust ruptured again, producing an event [5] of moment magnitude (Mw) 8.6 south of the 2004 rupture area, which was the site of a similar event in 1861 (ref. 6). Concern was then focused on the Mentawai area, where large earthquakes had occurred in 1797 (Mw = 8.8) and 1833 (Mw = 9.0) [6,7]. Two earthquakes, one of Mw = 8.4 and, twelve hours later, one of Mw = 7.9, indeed occurred there on 12 September 2007. Here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period. This indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture. This variability probably arises from the influence of non-permanent barriers, zones with locally lower pre-stress due to the past earthquakes. The stress state of the portion of the Sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007. The moment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833. The potential for a large megathrust event in the Mentawai area thus remains large
Learning by Heart: Cultural Patterns in the Faunal Processing Sequence during the Middle Pleistocene
Social learning, as an information acquisition process, enables intergenerational transmission and the stabilisation of cultural
forms, generating and sustaining behavioural traditions within human groups. Archaeologically, such social processes might
become observable by identifying repetitions in the record that result from the execution of standardised actions. From
a zooarchaeological perspective, the processing and consumption of carcasses may be used to identify these types of
phenomena at the sites. To investigate this idea, several faunal assemblages from Bolomor Cave (Valencia, Spain, MIS 9-5e)
and Gran Dolina TD10-1 (Burgos, Spain, MIS 9) were analysed. The data show that some butchery activities exhibit variability
as a result of multiple conditioning factors and, therefore, the identification of cultural patterns through the resulting cutmarks
presents additional difficulties. However, other activities, such as marrow removal by means of intentional breakage,
seem to reflect standardised actions unrelated to the physical characteristics of the bones. The statistical tests we applied
show no correlation between the less dense areas of the bones and the location of impacts. Comparison of our
experimental series with the archaeological samples indicates a counter-intuitive selection of the preferred locus of impact,
especially marked in the case of Bolomor IV. This fact supports the view that bone breakage was executed counterintuitively
and repetitively on specific sections because it may have been part of an acquired behavioural repertoire. These
reiterations differ between levels and sites, suggesting the possible existence of cultural identities or behavioural
predispositions dependant on groups. On this basis, the study of patterns could significantly contribute to the identification
of occupational strategies and organisation of the hominids in a territory. In this study, we use faunal data in identifying the
mechanics of intergenerational information transmission within Middle Pleistocene human communities and provide new
ideas for the investigation of occupational dynamics from a zooarchaeological approach