第62回統計地震学セミナー / The 62nd Statistical Seismology Seminar

Date&Time
2016年10月26日(水)
/ 26 October, 2016 (Wed) 15:00 – 17:00

Admission Free,No Booking Necessary

Place
統計数理研究所 セミナー室4 (D312B)
/ Seminar room 4 (D312B) @ The Institute of Statistical Mathematics
区切り線
Speaker1
Agnès Helmstetter 【15:00-16:00】
(Research fellow at CNRS)
Title
Repeating icequakes
Abstract

We have detected repeating icequakes on three different sites : an alpine glacier (Argentière, massif du Mont-Blanc, France), near the base of the western margin of the Greenland Ice Sheet, and on a rock-glacier (Gugla, Valais, Switzerland). Repeating icequakes are events with very similar waveforms, located at the base of a glacier, with quasi-periodic recurrence times of the order of minutes or hours, and progressive changes in magnitude. The activity of each cluster is intermittent.

Burst-like episodes can last for a few hours or months, and then disappear. In greenland, temporal changes of inter-event times and magnitudes are correlated with temperature, because surface meltwater yields an increase in basal water pressure and in glacier flow velocity.

But each cluster reacts differently to temperature changes, probably because the connectivity to the subglacial drainage system is different for each asperity. In contrast, we observed no correlation between temperature and repeating icequakes at Glacier d'Argentière and at Gugla rock Glacier. However, we observed bursts of repeating icequakes at Gugla triggered by snow falls. We suggest that the snow weight may have induced a transition between aseismic slip and unstable stick-slip. In addition to repeating basal icequakes, we also detected swarms of icequakes induced by crevasse opening, probably promoted by melt-water flow. These swarms of icequakes have very different statistical distributions in time, space and magnitude compared with repeating icequakes. Their recurrence times are power law distributed, their magnitudes obey the Gutenberg-Richter law, and the size of each cluster is several tens of meters. These different patterns may help to identify the triggering mechanisms of earthquake swarms, and to discriminate between fluid flow and aseismic slip.

区切り線
Speaker2
David Harte 【16:00-17:00】
(GNS Science New Zealand)
Title
Determining the Uncertainty in Earthquake Forecasts
Abstract

Forecasts based on a self-exciting model, like ETAS, are often produced by simulation. From these simulations, an empirical probability distribution can be derived for a forecast in a specified space-time- magnitude volume.

We will show that the forecast distribution can be characterised by probability generating functions. This shows how deeply complex the dependency structure is in such a model. While of theoretical interest, they remain intractable to me in a practical sense.

We then consider whether the forecast distribution can be approximated, using less computation than that required for simulation, by a "standard " multi-parameter probability distribution. The multiple parameters gives us the ability to at least fit a distribution with comparable mean and variance to that of the forecast distribution. One of the main questions is how to determine the forecast mean, and then given the mean,  the variance.