Earthquake Seismology

Dynamic FE analysis of South Memnon Colossus including 3D soil-foundation-structure interaction

by Sara Casciati

Casciati S. and Borja R.I. (2004). “Dynamic FE analysis of South Memnon Colossus including 3D soil-foundation-structure interaction”. Computers & Structures, 82(20-21), 1719-1736. ISSN: 0045-7949.

DATE AND PLACE OF PUBLICATION: August 2004; Pergamon–Elsevier Science Ltd, Kidlington, Oxford OX5 1GB, Oxon, England.

ABSTRACT. A full three-dimensional dynamic soil–foundation structure interaction (SFSI) analysis of a famous landmark... more ABSTRACT. A full three-dimensional dynamic soil–foundation structure interaction (SFSI) analysis of a famous landmark in Luxor, Egypt, the South Memnon Colossus, is performed to investigate the response of this historical monument to seismic excitation. The analysis is carried out using the finite element (FE) method in time domain. The statue comprising the upper structure is modeled using 3D brick finite elements constructed from a photogrammetric representation that captures important details of the surface and allows the identification of probable zones of stress concentration. The modeling also takes into account the presence of a surface of discontinuity between the upper part of the statue and its fractured base. FE models of the foundation and the surrounding soil deposit are constructed and coupled with the statue model to analyze the seismic response of the entire system incorporating dynamic SFSI effects. These studies are useful for future conservation efforts of this historical landmark, and more specifically for designing possible retrofit measures for the fractured base to prevent potential collapse of the monument from overturning during an earthquake.

KEYWORDS: Soil–foundation–structure interaction; Multi-body deformable contact; Non-linear dynamic finite element analysis

The effectiveness of a distant accelerometer array to compute seismic source parameters: the April 2009 L'Aquila earthquake case history

by Nils Maercklin

N. Maercklin, A. Zollo, A. Orefice, G. Festa, A. Emolo, R. De Matteis, B. Delouis, A. Bobbio (2011). Bulletin of the Seismological Society of America, 101(1), 354-365, doi:10.1785/0120100124

The Mw 6.3 L'Aquila earthquake, central Italy, on April 6, 2009 has been recorded by the Irpinia Seismic Network... more The Mw 6.3 L'Aquila earthquake, central Italy, on April 6, 2009 has been recorded by the Irpinia Seismic Network (ISNet) about 250 km south-east of the epicenter. Up to 19 three-component accelerometer stations could be used to infer the main source parameters with different seismological methods. We obtained an approximate location of the event from arrival times and array-based backazimuth measurements and estimated the local magnitude (6.1) from an attenuation relation for southern Italy. Assuming an omega-square spectral model we inverted S-wave displacement spectra for moment magnitude (6.3), corner frequency (0.33 Hz), stress drop (2.5 MPa) and apparent stress (1.6 MPa). Waveform modeling using a point source and an extended source model provided consistent moment tensors with a centroid depth around 6 km and a prevalently normal fault plane solution with a dominant directivity toward south-east. The relatively high corner frequency and an overestimated moment magnitude of 6.4 from moment tensor inversions are attributed to the rupture directivity effect. To image the rupture geometry we implemented a beamforming technique that back-projects the recorded direct P-wave amplitudes into the earthquake source region. A NW-SE striking rupture of 17 km length is imaged, propagating with an average velocity up to 3 km/s. This value is significantly higher than our estimate of 2.2 km/s from S-wave spectra. Our case study demonstrates that the use of array techniques and a dense accelerometer network can provide quick and robust estimates of source parameters of moderate-size earthquakes located outside the network.

The Transition of Dynamic Rupture Styles in Elastic Media Under Velocity-Weakening Friction

by Alice - Agnes Gabriel

Co-authored with J.-P. Ampuero, L. A. Dalguer, and P. M. Mai

Although kinematic earthquake source inversions show dominantly pulse-like rupture behavior, seismological... more Although kinematic earthquake source inversions show dominantly pulse-like rupture behavior, seismological observations, laboratory experiments and theoretical models indicate that earthquakes can operate with two dierent rupture styles, either as pulses or as cracks, and can propagate at subshear or supershear speeds. The determination of rupture style and speed has important implications for ground motions and may inform about the state of stress and strength of active fault zones. Here we conduct 2D in-plane dynamic rupture simulations with a spectral element method to investigate the diversity of rupture styles on faults governed by velocity-and-state-dependent friction with dramatic velocity-weakening at high slip rate. Our rupture models assume uniform initial stresses and are articially initiated by prescribing time-weakening over a time-dependent nucleation area. We identify the conditions that lead to dierent rupture styles. In particular we investigate the transitions between decaying, steady-state and growing pulses,
cracks, sub-shear and super-shear ruptures as a function of background stress, nucleation duration and characteristic velocity at the onset of severe weakening. Our models show
that small changes of background stress or nucleation energy can lead to dramatic changes of rupture style. Each rupture style transition features a characteristic value of dynamic
stress at the point of transition. We characterize the asymptotic properties of steady state and self-similar pulses as a function of background stress, in particular their slip prole, peak slip rate, rupture speed and rise time. We show that an earthquake may not be restricted to a single rupture style, but that complex rupture patterns can emerge that consist of multiple rupture fronts, possibly involving dierent styles and back-propagating
fronts. For instance, growing pulses lead to re-activation of slip due to gradual stress build up near the hypocenter. We also demonstrate the possibility of a super-shear transition
for pulse-like ruptures. Identifying this diversity of rupture patterns in real earthquakes poses an interesting observational challenge.

I terremoti in Campania in età romana e medioevale. Sismologia e sismografia storica

by Fernando La Greca

in "Annali Storici di Principato Citra", V, 1, 2007, pp. 5-34.

Investigation of the frequency content of ground motions recorded during strong Vrancea earthquakes, based on deterministic and stochastic indices

by Iolanda Craifaleanu

Proceedings of the Eighth International Conference on Structural Dynamics, EURODYN 2011, Leuven, Belgium, July 4-6, paper (on CD-ROM), ISBN 978-90-760-1931-4

The paper presents results from a recent study, involving a thorough analysis, based on several deterministic and... more The paper presents results from a recent study, involving a thorough analysis, based on several deterministic and stochastic indices, of the frequency content of ground motions recorded during strong Vrancea seismic events. The study aims to better reveal the spectral characteristics of the analyzed ground motions. Observations are correlated with available information on local site conditions, geology, wave propagation patterns etc., in order to investigate the influence of different factors. Over 300 accelerograms, recorded during the strong Vrancea seismic events mentioned above, were used in the study. New improved digitization of the analyzed ground motions is utilized. Various analytical estimators of the predominant spectral period, such as those based on velocity and input energy spectra, as well as the smoothed spectral predominant period, the average spectral period, the central period etc. are calculated and compared. Peaks of power spectral density are also analyzed. Spectral bandwidth measures, as the Vanmarcke and the Longuet Higgins indices, are evaluated. The variability of the analyzed parameters from an event to another is pointed out, as well as, when relevant, their regional variability, mapped by using GIS techniques. The capacity of the analyzed estimators of the predominant period to describe the frequency contents of the ground motions is assessed comparatively, and relevant parameters are identified. The results are correlated and validated by using information provided by spectral bandwidth measures. A comprehensive image of the spectral contents of Vrancea ground motions, during 4 significant seismic events, is developed. Conclusions are analyzed with respect to observations made by seismological and geological methods.

P-wave attenuation tomography of Mount St. Helens: preliminary results from Coda-Normalised Spectra.

by Stephen Hicks

de Siena, L.; Hicks, S.; Waite, G. P.; Moran, S. C.
American Geophysical Union, Fall Meeting 2010, abstract #S31A-2044

The P-wave velocity structure of Mount St. Helens has been recently imaged with local earthquake tomography, using the... more The P-wave velocity structure of Mount St. Helens has been recently imaged with local earthquake tomography, using the data recorded since its eruption in 1980. Part of this dataset has been processed to obtain a preliminary frequency dependent Qp image of the crust below the volcanic cone. We extended the so-called coda-normalization method, usually applied to S-waves, to the measurement of path-dependent P-wave attenuation. A scattering model has been developed in order to select the best time-window to measure the P-wave spectral amplitude on each trace. The objective is to average the effect of the source radiation pattern with the properties of early coda. The weighting matrix in the final inversion is dependent on the source radiation pattern and the spectral amplitude of noise. The Discrete Picard Condition and the Discrepancy Principle have been applied to investigate the maximum resolution available in each part of the medium. Truncated Singular Value Decomposition as well as Zeroth-, first- and second-order Tikhonov regularization techniques have been investigated by using the multi-resolution inversion code (MuReATA). The interpretation of the preliminary results is carried out by using cluster analysis on velocity and attenuation measurements.