P.A. Pirazzoli

Earthquake clustering in the eastern Mediterranean during historical times

Most coastal sectors which show evidence of Holocene coseismic uplift in Greece and the Eastern Mediterranean were raised during a short period called here the Early Byzantine tectonic paroxysm (EBTP) between the middle of the fourth and the middle of the sixth century A.D. The areas uplifted at that time include Cephalonia and Zante in the Ionian Islands, Lechaion and the Perachora Peninsula in the Gulf of Corinth, the Pelion coast of Thessaly, Antikythira and the whole of western Crete, a coastal sector near Alanya in southern Turkey, and the entire Levant coast from Hatay (Turkey) to Syria and the Lebanon. The amount of the EBTP uplift was generally between 0.5 m and 1.0 m but reached a maximum of about 9 m in southwestern Crete. In several areas (Zante, Pelion coast, Antikythira, western Crete, Alanya), the EBTP uplifted shoreline is the only evidence of Holocene emergence. In other areas, however, a similar uplift occurred earlier in the Holocene (Levant coast), or more recently (Cephalonia). Evidence of preseismic subsidence prior to the EBTP uplift has been reported from Thessaly, Antikythira, and Crete ; in both the latter islands, the EBTP uplift was preceded by a series of about 10 coseismic small subsidence movements, each measuring some tens of centimeters, which took place in the preceding 3000 years. No evidence was observed of postseismic vertical displacements.

Crustal block movements from holocene shorelines: Crete and antikythira (Greece)

Abstract Marks of several shorelines left by sea-level stands during the last 4000 yrs occur along the coasts of the islands of Crete and Antikythira. In this area, recent movements of the earths crust were identified by using a systematic survey based on multidisciplinary analyses, strengthened by 60 radiometric datings. Between 4000 and 1700 yr B.P., a block of lithosphere, A , about 150 km long (Fig. 10) which included Antikythira and the Levka Mountains in Crete, underwent a series of ten rapid subsidences (from 10 to 25 cm each time) without noticeable tilting. These movements ended about 1,530 ± 40 yr B.P. A further block of lithosphere, B , approximately 200 km long, was then uplifted about 10 m and inclined northeastwards in a single event. Block B includes block A and extends 50 km further eastwards, beyond the Ida Mountains, as far as the Iraklion sedimentary basin. The tectonic movements were preceded by vertically reversed displacements of short-duration on at least three occasions. If the A block subsidences can be ascribed to gravitational forces, then the B block uplift suggests a cause that originated at depth (crustal consumption and elastic rebound). To the east of the Ierapetra graben, the Sitian Peninsula appears to belong to a block of lithosphere, C , affected by a northeast tilting and separated by an active fault from the crustal block of Khrisi-Koufonisi.

Holocene sea-level changes in French Polynesia

Abstract New data on Holocene sea levels have been obtained in 28 atolls and 10 high islands in the Society, Tuamotu, Gambier and Austral Islands, in an area of the South Pacific as wide as western Europe. Sea-level indicators are often very accurate (±0.1 m) and include exposed corals, abandoned algal ridges and reef frameworks in growth position, emerged tidal notches, and skeletal reef conglomerates in which the position of the former low water level at the time of cementation has been determined by petrological analysis. Over 110 new samples have been dated by radiocarbon. The overall pattern is that of a MSL standstill at +0.8/+1.0 m between 5000 and 1250 yr B.P. A set of radiocarbon dated samples covering the period 4500-1250 yr B.P. almost continuously is evidence that lasting MSL drops could not ever have occurred below +0.7 m throughout this time. The highest sea-level peak (about +1.0 m) seems to have been reached between 2000 and 1500 yr B.P. Since 1500 yr B.P. MSL has been dropping gradually from about +1.0 m to the present position, which has been reached only recently. Some islands show however a slightly different pattern; the Holocene emergence is smaller than average in Moorea and Tahiti and larger in Rurutu, Makatea, Anaa, and possibly Hereheretue. Some data suggest also the possibility of a slight increase in the Holocene emergence towards the south. Possible geodynamic, isostatic, oceanic and climatic causes of the sea-level variations observed are discussed.

Extreme Sea Levels in the English Channel: Calibration of the Joint Probability Method

Abstract The probability of extreme sea levels along the coasts has always been statistically estimated from the series of local observations. The inference is never conclusive, and an attempt is given here to improve the methods already used with reference to the area of the English Channel. The joint probability method (JPM) is the starting point: In most cases it underestimates the return times (or overestimates extreme levels at a fixed time). The proposed extension is based on a more careful use of observed extremes by fitting a coefficient Cc deduced from the data set, which requires that the maximum record height be in agreement with the return period of the record length. This correction calibrates the whole series of extreme estimations to the observed maximum. Likewise an attempt to roughly explain this correction is given that explores the tide–surge interaction and seasonal dependence. The parameters are specifically computed for 15 tide-gauge stations, and the comparison is extended to other known methods, like the Gumbel one (in most cases overestimating the levels) and GEV simulations (which appear much better). Finally extreme levels with estimated return times of 10, 50, and 100 years, respectively, are proposed for each site, and a test for validity was performed by splitting certain long records into small samples, thus checking the spread of the results.

Episodic uplift deduced from Holocene shorelines in the Perachora Peninsula, Corinth area, Greece

Four raised shorelines, between +1.1 m and +3.5 m, have been identified at two localities on the Perachora Peninsula and were dated by AMS 14C to between 6400 and 1500 calibrated years ago. Uplift movements seem to have occurred in increments of 0.8 ± 0.3 m, with a return period of about 1600 years. The last uplift coincided with a regional tectonic paroxysm, which has already been documented in several areas of the eastern Mediterranean. The average uplift rate was probably faster during the Holocene than the average since the Last Interglacial period. No important vertical displacement occurred in this area at the time of (or after) the 1981 earthquake, but a new episode of coseismic uplift may be in preparation.

A one million-year-long sequence of marine terraces on Sumba Island, Indonesia

Abstract An exceptional sequence of raised coral reef terraces is visible at Cape Laundi, on the north coast of the island of Sumba, in the Savu Sea, in which at least eleven terraces wider than 100 m, six of them even being over half a kilometer wide, are geomorphologically distinguishable between an ancient patch reef now at +475 m and the present sea level. ESR date estimations of almost unrecrystallized corals, most of them in situ, make possible a tentative identification of the terraces corresponding to isotope Stages 15 (ca. 600 ka ago) and 9 (ca. 330 ka ago). When the uplift trend deduced from the present altitude of dated terraces (0.49 ± 0.01 mm/yr) is extrapolated to the whole raised section, most geomorphological features appear to correspond to interglacial stages of the astronomically calibrated oxygen-isotope record, up to stage 27 (ca. 0.99 Ma), beyond the Matuyama/Jaramillo geomagnetic boundary. The upper terraces would in this case be twice as old as previously estimated (ca. 0.5 Ma). Main terraces seem to correspond to high sea levels at certain interglacial stages and substages, but in some cases the action of low sea levels at glacial stages cannot be ruled out. For terraces between 30 and 70 m in altitude, Th/U and ESR dates reveal that aragonitic corals at similar elevations may differ in age as much as 100 ka; this can be explained by taking into account a polycyclic origin for several terraces, reconstructions available of changes in the eustatic sea level during the last 250 kyr, and the fact that coral reef development may have been scattered at some periods only, without totally covering subjacent, older formations. The Sumba sequence of marine terraces at Cape Laundi is among the longest and most complete for the past million years reported in the literature. Its relatively good state of preservation will probably give impetus to a number of new studies on the chemical, physical, isotopic, morphological, palaeoclimatic, pedological, ecological and diagenetic changes which have occurred during the past one million years.

Crustal block movements from Holocene shorelines: Rhodes Island (Greece) *

Abstract Signs of up to eight stepped Holocene shorelines, which have been reported in a previous study on the east coast of Rhodes Island, have been reinvestigated in detail on the basis of a new geomorphological survey, identification and petrological analysis of many new samples of exposed marine organic crusts and over 30 radiocarbon datings. Confirmation was obtained of the view that the island can be charactered by considering small crustal blocks up to a dozen kilometres long with each block being affected by a specific tectonic history, and a general trend towards an uplift increasing from south to north. Two successive phases of submergence-emergence have been revealed by petrogenetic sequences in many samples, which give evidence of the occurrence of positive and negative vertical movements. This implies possible rejuvenation effects of the later submergence phase on the apparent age of pre-existing algal crusts, and those have been taken into account and estimated. The most likely sequence of vertical crustal movements to have occurred has been specified for each block and a time range has been ascribed to each former shoreline. Independent up and down movements, increasing in amplitude from south to north, appear in most blocks. There is a recurrent periodicity varying from a few hundred to one or at most two thousand years. The largest vertical displacement observed during the late Holocene—a sudden uplift movement of about 3.8 m in the northernmost part of the island—is likely to be linked with the earthquake which destroyed the Colossus in 222 B.C.

Late Holocene sea level indicators from twelve atolls in the central and eastern Tuamotus (Pacific Ocean)

New sea-level data for the late Holocene period are reported from twelve atolls of the Tuamotu archipelago: Faaite, Hikueru, Marokau, Hao, Amanu, Tatakoto, Pukarua, Nukutavake, Vairaatea, Tureia, Nukutipipi, and Hereheretue. The data come from coral conglomerate outcrops, coral colonies in growth position, in situ reef framework and marine notches; they give consistent results, and their ages are controlled by 29 radiocarbon datings. The Holocene MSL remained 0.8±0.2 m higher than at present, from before 4000 years BP until at least 1500 years BP, then dropped gradually to the present level. This pattern is very similar to that reported from the northwest Tuamotus, suggesting the absence of measurable differential vertical movements over a distance greater than 1300 km during the late Holocene. Effects of the 1983 cyclones on reef morphology are reported from some atolls and radiocarbon dates of some storm-generated reef blocks are given.

Historical coseismic uplift on Euboea Island, Greece

In the Aegean region it is usually accepted that uplift is confined to the southern areas, near the outer island arc, whereas subsidence prevails in the inner basin areas. However, we give evidence of elevated notches andLithophaga shells, dated by AMS14C, showing that Holocene coseismic uplift of the order of about a metre occurred in inner Aegean areas too and affected parts of the coasts of north and central Euboea Island between 510 and 380 B.C. and also possibly between 1050 and 900 B.C.

Surges, atmospheric pressure and wind change and flooding probability on the Atlantic coast of France

A statistical study of hourly tide-gauge records at Saint-Gildas, Saint-Nazaire, Port Tudy, Brest, Le Conquet and Roscoff, as well of three-hourly wind and air pressure records at Belle Ile and Ouessant over the last 50 years, has made possible the definition of the main meteorological factors which are associated with the development of positive or negative surges. Recent changes in these meteorological factors indicate that atmospheric depressions and strong surge winds are becoming less frequent and that the ongoing trends of climate variability show a decrease in the frequency and hence the gravity of coastal flooding. However the highest sea surges of the last decades did not occur at the time of spring high tide. There is a significant statistical probability that coastal flooding reaching higher elevations than in the recent past may occur in the near future, in spite of the improvement of climatic trends.