The large tsunami of 26 December 2004: Field observations and eyewitnesses accounts from Sri Lanka, Maldives Is. and Thailand

Abstract Post-event field surveys were conducted and measurements were taken in Sri Lanka and Maldives about two weeks after the catastrophic Indian Ocean tsunami of 26 December 2004. The measurements taken were cross-checked after interviewing with local people. In the southwest, south and east coastal zones of Sri Lanka maximum water levels ranging from h = 3 m to h = 11 m a.m.s.l. were estimated. The highest values observed were in the south of the island: Galle h ∼ 10 m, Hambantota h ∼ 11m. Maximum inundation of d ∼ 2 km was observed in Hambantota. The heavy destruction and thousands of victims caused in coastal communities, buildings and infrastructure, like railways and bridges, is attributed not only to physical parameters, like the strength of the tsunami hydrodynamic flow, coastal geomorphology and the wave erosional action in soil, but also to anthropogenic factors including the increased vulnerability of the non-RC buildings and the high population density. Local people usually described the tsunami as a series of three main waves. The leading wave phase was only a silent sea level rise of h ≤ 1.5 m and d ≤ 150 m, while the second wave was the strongest one. The first two waves occurred between 09:00 and 09:30 local time, depending on the locality. It is well documented that near Galle, southern part, the strong wave arrived at 09:25:30. In the west coast the third wave was a late arrival which possibly represents reflection phases. In Maldives, three waves were also reported to arrive between 09:00 and 09:30 local time. Maximum water level was only h ∼ 3 m in Laamu Atoll, which is interpreted by the wave amplitude damping by the coral reef to the east of the island complex as well as to that the tsunami did not arrived at high tide time. Damage was observed in several islands of Maldives but this was minimal as compared to the heavy destruction observed in Sri Lanka. About 25 Greek eyewitnesses, who happened to experience the tsunami attack in Padong and Blue Lagoon Port of Phuket island as well as in Maya Bay, Phi-Phi islands, Thailand, were interviewed on the basis of a standard questionnaire. The first sea motion was a retreat of at least 100 m. Then, two main waves arrived, the first being the strong one occurring at about 09:55–10:05 local time, with h ∼ 6m in Padong causing significant destruction and human victims. The collected information clearly indicates that the tsunami propagated as the leading crest wave to the west side, e.g. in Sri Lanka and Maldives, and as the leading trough wave to the east, e.g. in Thailand

An assessment of the tsunami risk in Muscat and Salalah, Oman, based on estimations of probable maximum loss

We present a method for determining an initial assessment of tsunami risk, with application for two coastal areas of Oman. Using open source GIS and seismic databases we carry out a tsunami risk assessment using a deterministic and probabilistic approach based on worst-case scenarios. A quick and effective method for estimating tsunami run-up without the use of complex modelling software is an important step in disaster risk reduction efforts as many government and emergency response organisations do not possess the expertise to carry out or interpret tsunami inundation numerical models. Estimates of probable maximum loss were calculated using a simple method of building identification and a revised building damage assessment technique. A series of tsunami risk maps were created for the coastal settlements of Muscat and Salalah, with the aim of improving tsunami response. We find Muscat to be at far greater risk of tsunami damage than Salalah; this is due in part to Muscat's proximity to potential tsunamigenic sources and the cities current level of urban infrastructure. Whilst much of the infrastructure in Salalah is currently at low risk from tsunami, development pressures could lead to increased risk within the region. It is hoped that the assessment of risk may go some way to a government led disaster risk reduction strategy being implemented in coastal Oman. The methods detailed provide a cheap and efficient means to quantify tsunami risk in many coastal Middle Eastern countries, of which several have poor disaster risk reduction strategies

Εφαρμογή των λανθανόντων μοντέλων Markov και σχετικών μοντέλων σε μελέτες σεισμών

Discrete valued hidden Markov Models (HMMs) are used to model time series of event counts in several scientific fields like genetics, engineering, seismology and finance. In its general form the model consists of two parts: the observation sequence and an unobserved sequence of hidden states that underlies the data and consist a Markov chain. Each state is characterized by a specific distribution and the progress of the hidden process from state to state is controlled by a transition probability matrix. We extend the theory of HMMs to the multivariate case and apply them to seismological data from different seismotectonic environments. This extension is not straightforward and it is achieved gradually by assuming different multivariate distributions to describe each state of the model. We consider different multivariate distributions with Poisson marginals, starting from the known multivariate Poisson distribution (MPHMMs) and then extending to copula based distributions to allow for full flexibility. Different copula families have been selected, with different number of parameters and different properties. An EM type algorithm has been constructed to estimate the models' parameters and some simulation experiments have been conducted for the parameter estimation method to be evaluated. Real data applications with seismological data are conducted to illustrate the usefulness of the proposed models. All the proposed models constitute a class of time series models for multivariate counts which is less developed and has itself particular interest. The results obtained from the applications of the proposed models to geophysical data are promising. Of course the application field is not restricted to geophysical data only but it is wide enough to include data from many different scientific disciplines. Extensions of the models are feasible with various ways.Εφαρμογές των διακριτών Λανθανόντων Μοντέλων Markov έχουν γίνει σε χρονολογικές σειρές συχνοτήτων γεγονότων σε διάφορα επιστημονικά πεδία όπως αναγνώριση φωνής, γενετική, σεισμολογία και οικονομικά. Στη γενική τους μορφή τα μοντέλα αποτελούνται από δύο μέρη: το πρώτο μέρος είναι μία παρατηρούμενη ακολουθία από συχνότητες γεγονότων και το δεύτερο μέρος είναι μία μη παρατηρούμενη ακολουθία καταστάσεων που αποτελούν μία Μαρκοβιανή αλυσίδα. Κάθε κατάσταση χαρακτηρίζεται από μία συγκεκριμένη κατανομή και η μετάβαση της Λανθάνουσας ακολουθίας από μία κατάσταση σε άλλη καθορίζεται από τον πίνακα πιθανοτήτων μεταπήδησης της Μαρκοβιανής αλυσίδας. Το αντικείμενο μελέτης αυτής της έρευνας είναι η επέκταση αυτών των μοντέλων στην πολυμεταβλητή περίπτωση και η εφαρμογή τους σε σεισμολογικά δεδομένα από διαφορετικά σεισμοτεκτονικά περιβάλλοντα. Αυτή η επέκταση δεν είναι αυτονόητη και γίνεται σταδιακά υποθέτοντας διαφορετικές πολυμεταβλητές κατανομές να περιγράφουν τις καταστάσεις του μοντέλου.Αρχικά η επέκταση γίνεται αντιστοιχίζοντας μία πολυμεταβλητή κατανομή Poisson με κάθε κατάσταση του μοντέλου και μετά θεωρούμε μία πολυμεταβλητή κατανομή που βασίζεται στα copula για μεγαλύτερη ευελιξία. Επιλέχθησαν διαφορετικές οικογένειες copula, με διαφορετικό αριθμό παραμέτρων και ιδιότητες. Εκτίμηση των παραμέτρων του μοντέλου γίνεται με τη χρήση του αλγόριθμου ΕΜ και αξιολόγηση της επίδοσης του με προσομοίωση από διαφορετικά σετ πραγματικών τιμών. Εφαρμογή των Λανθανόντων μοντέλων Μάρκοβ με σεισμολογικά δεδομένα παρουσιάζονται. Όλα τα προτεινόμενα μοντέλα αποτελούν μία κατηγορία μοντέλων για χρονολογικές σειρές που δεν είναι πολύ αναπτυγμένη και παρουσιάζει ιδιαίτερο ενδιαφέρον. Τα αποτελέσματα που προκύπτουν από εφαρμογές των μοντέλων σε σεισμολογικά δεδομένα είναι ενθαρρυντικά. Φυσικά τα μοντέλα αυτά δεν περιορίζονται μόνο σε εφαρμογές με γεωφυσικά δεδομένα αλλά βρίσκουν εφαρμογή σε ένα ευρύ φάσμα επιστημονικών πεδίων. Επιπλέον επεκτάσεις των μοντέλων είναι εφικτές με ποικίλους τρόπους

The large tsunami of 26 December 2004: Field observations and eyewitnesses accounts from Sri Lanka, Maldives Is. and Thailand.

Post-event field surveys were conducted and measurements were taken in Sri Lanka and Maldives about two weeks after the catastrophic Indian Ocean tsunami of 26 December 2004. The measurements taken were cross-checked after interviewing with local people. In the southwest, south and east coastal zones of Sri Lanka maximum water levels ranging from h = 3 m to h = 11 m a.m.s.l. were estimated. The highest values observed were in the south of the island: Galle h ∼ 10 m, Hambantota h ∼ 11 m. Maximum inundation of d ∼ 2 km was observed in Hambantota. The heavy destruction and thousands of victims caused in coastal communities, buildings and infrastructure, like railways and bridges, is attributed not only to physical parameters, like the strength of the tsunami hydrodynamic flow, coastal geomorphology and the wave erosional action in soil, but also to anthropogenic factors including the increased vulnerability of the non-RC buildings and the high population density. Local people usually described the tsunami as a series of three main waves. The leading wave phase was only a silent sea level rise of h ≤ 1.5 m and d ≤ 150 m, while the second wave was the strongest one. The first two waves occurred between 09:00 and 09:30 local time, depending on the locality. It is well documented that near Galle, southern part, the strong wave arrived at 09:25:30. In the west coast the third wave was a late arrival which possibly represents reflection phases. In Maldives, three waves were also reported to arrive between 09:00 and 09:30 local time. Maximum water level was only h ∼ 3 m in Laamu Atoll, which is interpreted by the wave amplitude damping by the coral reef to the east of the island complex as well as to that the tsunami did not arrived at high tide time. Damage was observed in several islands of Maldives but this was minimal as compared to the heavy destruction observed in Sri Lanka. About 25 Greek eyewitnesses, who happened to experience the tsunami attack in Padong and Blue Lagoon Port of Phuket island as well as in Maya Bay, Phi-Phi islands, Thailand, were interviewed on the basis of a standard questionnaire. The first sea motion was a retreat of at least 100 m. Then, two main waves arrived, the first being the strong one occurring at about 09:55–10:05 local time, with h ∼ 6 m in Padong causing significant destruction and human victims. The collected information clearly indicates that the tsunami propagated as the leading crest wave to the west side, e.g. in Sri Lanka and Maldives, and as the leading trough wave to the east, e.g. in Thailand

The East Aegean Sea strong earthquake sequence of October?November 2005: lessons learned for earthquake prediction from foreshocks

International audienceThe seismic sequence of October?November 2005 in the Samos area, East Aegean Sea, was studied with the aim to show how it is possible to establish criteria for (a) the rapid recognition of both the ongoing foreshock activity and the mainshock, and (b) the rapid discrimination between the foreshock and aftershock phases of activity. It has been shown that before the mainshock of 20 October 2005, foreshock activity is not recognizable in the standard earthquake catalogue. However, a detailed examination of the records in the SMG station, which is the closest to the activated area, revealed that hundreds of small shocks not listed in the standard catalogue were recorded in the time interval from 12 October 2005 up to 21 November 2005. The production of reliable relations between seismic signal duration and duration magnitude for earthquakes included in the standard catalogue, made it possible to use signal durations in SMG records and to determine duration magnitudes for 2054 small shocks not included in the standard catalogue. In this way a new catalogue with magnitude determination for 3027 events was obtained while the standard catalogue contains 1025 events. At least 55 of them occurred from 12 October 2005 up to the occurrence of the two strong foreshocks of 17 October 2005. This implies that foreshock activity developed a few days before the strong shocks of 17 October 2005 but it escaped recognition by the routine procedure of seismic analysis. The onset of the foreshock phase of activity is recognizable by the significant increase of the mean seismicity rate which increased exponentially with time. According to the least-squares approach the b-value of the magnitude-frequency relation dropped significantly during the foreshock activity with respect to the b-value prevailing in the declustered background seismicity. However, the maximum likelihood approach does not indicate such a drop of b. The b-value found for the aftershocks that followed the strong shock of 20 October 2005 is significantly higher than in foreshocks. The significant aftershock-foreshock difference in b-value is valid not only if the entire aftershock sequence is considered but also if only the segment of aftershocks that occurred within the first 24-h or the first 48-h after the mainshock of 20 October 2005 are taken into account. This difference in b-value should be examined further in other foreshock-aftershock sequences because it could be used as a diagnostic of the mainshock occurrence within a few hours after its generation