複合災害に立ち向かうために筑波研究学園都市の総力を挙げて取り組む研究プロジェクト

Research

Research

Special Seminars

Prof. Luis Rivera (Institut de Physique du Globe de Strasbourg, Strasbourg University, France)
Date/Time: November 12, 2012; 15:00 ~ 17:00;
Place: University of Tsukuba, Laboratory of Advanced Research Building B, Conference Room B112)
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Title: “Fast and Robust Source Determination of Large Earthquakes using W phase”

Abstract
Until recently, it was necessary to wait several hours before having a reliable source solution for large earthquakes. With the purpose of accelerating centroid moment tensor (CMT) determinations, we have recently developed a source inversion algorithm based on modelling of the W phase, a very long period phase (100–1000 s) arriving at the same time as the P wave. I have not not planned to talk too much about the algorithm itself but rather about the results we have been obtaining with it (of course I will be happy to come back to the methodological aspects either during the questions or at any other time).

I will show examples both at global and regional scales and both for offline and in real time applications. On the other hand, I would like to discuss the question of the uncertainties on the data and on the modeling assumptions. How to take them into account and how they affect the solution itself and the associated uncertainty.

Some of the material I will cover can be found in the following references:
1) http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.2008.03887.x/full
2) http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.2012.05419.x/full
3) http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.2012.05554.x/full

General description, examples, references, etc can also be found at: http://eost.u-strasbg.fr/wphase

Presentation download link: (PDF)
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・ Prof. Madariaga (Ecole Normale Superieure de Paris, France)
Date/Time: November 16, 2012; 15:00 ~ 17:00;
Place: University of Tsukuba, Laboratory of Advanced Research Building B, Conference Room B110)
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Title: “Kinematic and dynamic inversion and simple earthquake models: the heritage of Aki and Brune” by R. Madariaga and S. Ruiz

Abstract:
Almost 45 years ago Aki and Brune proposed that far field spectra had certain universal scaling properties that have been used to compute dynamic parameters like stress drop, energy moment ratios, etc. These properties can be modeled by the radiation from a sub-shear circular crack. This model does not apply to near field records because we need to include static and short-range effects. Here we propose a straightforward method to model near field records with simple and robust dynamic source models. For the moment, we have applied this approach to well recorded intermediate depth earthquakes in the magnitude range from 6.5 to 7.2. The data are strong motion records filtered in a frequency band from 0.02 to 1 Hz. Following Aki and Brune’s approach we describe the earthquake as a simple elliptical fault where the initial stress and rupture resistance are specified in a very simple fashion. As dynamic inversion is non-unique we chose the barrier model of Das and Aki to describe source properties. It is very simple to convert a barrier into an asperity model of the source.

The dynamic inversion problem is non-linear. In our inversions we use either the Neighbourhood Algorithm (NA) of Sambridge or a Genetic Algorithm developed by Cruz-Atienza. We use an L2 norm to compare the observed and synthetic seismograms. Once the best model has been found, a region in parameter space around the optimal solution is searched for well fitting solutions using a Monte Carlo approach. We present kinematic and dynamic inversion of the 24 July 2008 (Mw=6.8) Northern Iwate intermediate depth earthquake in Japan using strong motion records from the K-NET and Kik-Net networks. The rupture of this moderate magnitude earthquake is modeled with a single elliptical patch. Inversion converges to a source models with semi-minor axes of 4 km, maximum slip of about 4 m and large stress drops in the 30-45 MPa range. Rupture duration was less than 3 s because of very high sub Rayleigh rupture propagation speeds. Energy release rate for the best models was in the range 23-36 MJ/m2, a rather large value for events of this size. For both kinematic and dynamic inversions we found families of solutions that fit the strong motions data within a certain error, confirming the strong trade-off among inverted parameters.
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Presentation download link: (PDF)