Paul A. Johnson

Effects of acoustic waves on stick–slip in granular media and implications for earthquakes

It remains unknown how the small strains induced by seismic waves can trigger earthquakes at large distances, in some cases thousands of kilometres from the triggering earthquake, with failure often occurring long after the waves have passed. Earthquake nucleation is usually observed to take place at depths of 10–20 km, and so static overburden should be large enough to inhibit triggering by seismic-wave stress perturbations. To understand the physics of dynamic triggering better, as well as the influence of dynamic stressing on earthquake recurrence, we have conducted laboratory studies of stick–slip in granular media with and without applied acoustic vibration. Glass beads were used to simulate granular fault zone material, sheared under constant normal stress, and subject to transient or continuous perturbation by acoustic waves. Here we show that small-magnitude failure events, corresponding to triggered aftershocks, occur when applied sound-wave amplitudes exceed several microstrain. These events are frequently delayed or occur as part of a cascade of small events. Vibrations also cause large slip events to be disrupted in time relative to those without wave perturbation. The effects are observed for many large-event cycles after vibrations cease, indicating a strain memory in the granular material. Dynamic stressing of tectonic faults may play a similar role in determining the complexity of earthquake recurrence.

Using time-reversal to locate non-volcanic tremor and to fulfill the monitoring objectives of the nuclear-test ban treaty

In this paper are presented the latest results of our group effort to apply Time Reversal (TR) to different seismology problems. The first problem considered is source location of non-volcanic tremor (NVT). NVT episodes involve quasi-continuous emissions of seismic energy making the identification of distinct events and phase arrival times very difficult. We locate 2 NVT episodes that were recorded near Hemet: one triggered by the 2002 Denali earthquake and one by the 2009 Mexicali earthquake. Locations indicate sources slightly off the known scarp of tectonic faults. In both cases, determination of the source mechanism is impossible. The second problem is determination of the depth of seismic events. There is currently no robust method to estimate the depth of small events (below Mw5.0), when depth determination is crucial in discriminating small earthquakes from man-made blast, since the blast will have to be within about 2 km of the surface where few earthquakes occur. We propose to use stacked autocor...

Chapter 12. Field Observations

Impact of biomass burning on surface water quality in Southeast Asia through atmospheric deposition: field observations P. Sundarambal, R. Balasubramanian, P. Tkalich, and J. He Tropical Marine Science Institute 14 Kent Ridge Road, National University of Singapore, 119223, Singapore Department of Chemical and Biomolecular Engineering, National University of Singapore, Engineering Drive 1, 117576, Singapore Division of Environmental Science and Engineering, National University of Singapore, Engineering Drive 1, 117576, Singapore Singapore Delft Water Alliance, National University of Singapore, Engineering Drive 1, 117576, Singapore Received: 27 February 2010 – Accepted: 12 March 2010 – Published: 25 March 2010 Correspondence to: R. Balasubramanian (eserbala@nus.edu.sg) Published by Copernicus Publications on behalf of the European Geosciences Union.