Paul W. Burton

Greek tectonics and seismicity

Abstract The validity of existing tectonic models for the area of Greece is examined in the light of the new recalculated parameters for earthquakes of the region (Makropoulos and Burton, 1981). Relocated hypocentral positions are extracted from the catalogue to form radial and vertical distance-depth cross-sections centred on a reference point near the mid-point of the Aegean Volcanic arc, and these are used to form a three dimensional topography of the base of earthquake occurrence below 60 km. Isodepth maps are extracted from this topography as both three and two dimensional map presentations. These maps reveal several significant features of deep-seated tectonic processes in the region. Isodepths exceeding 150 km are seen in the northwest Aegean, and these are more closely linked to the Sporadhes and Gulf of Thermaicos, rather than the North Aegean trough. The 150 km isodepths are also seen in the northeast Aegean straddling the Dardanelles; in the northeastern part of the Peloponnesus, Gulf of Saronikos and eastern Gulf of Corinth in the southern Peloponnesus towards Crete; and extending from north of Kos to south of Rodos. The largest extent of deepest activity is seen south of Rodos and this continues towards southwest of Turkey. The subduction zone of the Hellenic arc is clear, but smooth Benioff zones are not the norm, and these data show that structural complexity is more readily observed. It is concluded that none of the proposed tectonic models completely explain the observed activity over the whole area, and rather than propose yet another model places where further work is still particularly necessary are identified.

Seismic hazard in Greece. II. Ground acceleration

Abstract In a previous paper (Makropoulos and Burton, 1985) the seismic hazard in Greece was examined in terms of magnitude recurrence using Gumbels third asymptotic distribution of extreme values and concepts of the physical process of strain energy release. The present study extends the seismic hazard methods beyond magnitude to the estimation of expectations of levels of peak ground acceleration exceedance thus allowing for a direct comparison between these two methodologies as well as establishing information relevant to design and planning criteria. The limited number of strong motion records do not permit regional study of attenuation of ground vibration in Greece. An average formula is derived from eight well known formulae which resulted from worldwide studies, this is: a = 2164 e0.70m (r+20)−1.80 cm s−2 where a is peak ground acceleration, m is earthquake magnitude and r is hypocentral distance in kilometres. This formula agrees with the observed values of peak ground acceleration values recorded in Greece. Acceleration seismic hazard is calculated at each of six chosen cities. Values of maximum acceleration with probability 70% of not been exceeded in the next 25, 50, 100, and 200 years are obtained along with corresponding values of velocity and displacement. The same detailed acceleration evaluation is then applied to the whole area of Greece by dividing it into cells of 0.5° lat × 0.5° long, and the results are illustrated through isoacceleration maps. Differences in magnitude and acceleration hazard maps reflect the fact that in acceleration hazard assessment the focal distance from a particular place in an important factor. The cities of Heraklion and Rodhos have the lowest acceleration hazard although the expected earthquakes may have large magnitude. Intermediate depth earthquakes characterise these two cities. Acceleration estimates, unlike magnitude hazard parameters, refer to a particular place and not to an area around it. Hence, even if two places have similar earthquake depth distributions, the hazards may differ significantly because of the different spatial distribution of the foci. This is observed in the case of Athens and Corinth. These cities have almost the same magnitude hazard, but the acceleration hazard is much lower for Athens where the hazard is mainly due to more distant earthquakes. The isoacceleration maps for Greece as a whole also define areas of high seismic hazard. These are the areas around Cephalonia and Leukas Islands in the Ionian Sea and the eastern Sporadhes, Lesbos Islands and Chalkidiki in the Northern Aegean Sea. At the 70% probability level the maximum acceleration is expected to be around 0.2g within the next 50 years. The areas where the maximum acceleration at the 70% probability level is expected to reach a value of 0.3g in the next 200 years are around Cephalonia and Leukas Islands and near the Dardanelles.

Seismic hazard evaluation for specific seismic regions of the world

Tsapanos, T.M. and Burton, P.W., 1991. Seismic hazard evaluation for specific seismic regions of the world. Tectonophysics, 194: 153-169. Seismic hazard is estimated for fifty of the most seismically active countries of the world using the technique of Gumbel’s third asymptotic distribution of extreme values and hazard maps are produced. For this purpose a catalogue of 9700 earthquakes with magnitudes M 2 5.5 has been compiled and earthquakes in the time period 1898-1985 are examined here. An equal area grid point mesh, of about 5 o latitude X 5 o longitude, is chosen to represent the local seismicity. This cell size corresponds approximately to the area of Romania which is the smallest country examined. The results estimate the most probable maximum magnitude for a time period of 85 years, which exceeds magnitude 7 for all fifty of the cellular seismic regions, and the contour maps and tables show its spatial variation and rank the countries in terms of overall relative seismic hazard. The maps effectively produce a brief seismic hazard atlas.

Seismic hazard in Greece. I. Magnitude recurrence

Two different methods are applied to the earthquake catalogue for Greece (Makropoulos and Burton, 1981), MB catalogue, to evaluate Greek seismic hazard in terms of magnitude: earthquake strain energy release and Gumbels third asymptotic distribution of extreme values. It is found that there is a close relationship between results from the two methods. In places where the cumulative strain energy release graphs include at least one well defined cycle of periodicity of strain release, then the parameters of the third type asymptote are well defined with small uncertainties. In almost all cases the magnitude distribution shows a remarkably good third type asymptotic behaviour. The results are presented in the form of graphs and contour maps of annual and 80-year modes, and magnitudes with 70% probability of not being exceeded in the next 50 and 100 years. For six of the most heavily industrial and highly populated centres of Greece magnitude hazard parameters are also derived and examined in more detail, thereby illustrating the direct applicability of the methods in terms of zoning. The close agreement between observed and predicted extreme magnitudes shows that the sample period considered (1900–1978), is long enough to obtain statistically stable estimates. For Athens the upper bound magnitude is found to be 6.7 ± 0.3 (within 100 km) and 6.8 ± 0.4 (100 km) from the two methods respectively, whereas for Corinth an earthquake of magnitude 6.5 has a mean return period of 43 years. Greece as a whole has an upper bound magnitude 8.7 ± 0.6 and earthquakes of a size similar to the 1903 Kithira event (M ≈ 8.0) have a mean return period of about 200 years. The significantly different maps contouring magnitudes of the annual and 80-year modes result from the fact that each place has its own distribution curvature for magnitude occurrence, and thus they are not a linear extrapolation of each other. However, as longer return periods are considered, these differences become small because the expected magnitudes approach the regional upper bound. A feature common to all these maps is the existence of three well defined aseismic blocks: 1. (a) the Attikocycladic block 2. (b) the Ptolemais basin and 3. (c) the block formed by the northeastern part of Greece. Well defined areas of high seismic hazard which correlate with the most tectonically active areas are: 1. (a) along the Hellenic arc: the Greek-Albania border, Leukas-Cephalonia Islands and the southeastern end of the arc 2. (b) the western end of the North Anatolian fault and 3. (c) the Chalkidiki peninsula and the northern Sporadhes Islands.

An earthquake study in the Lake Baringo basin of the central Kenya Rift

A 100 × 80 km2 earthquake recording network was operated for three months (January–March 1990) in the Lake Baringo region of the Kenya Rift Valley. Twenty-nine seismic sites were occupied by short-period stations over a region including the Elgeyo escarpment, the Kerio Valley, the Tugen Hills and Lake Baringo itself. Eighty local events of ML < 2.0 have been located within 50 km of the network. These events are situated within the central part of the rift, showing some association with the main rift faults but mainly clustering beneath Lake Baringo at a depth of about 5 km and occurring as swarm activity. Ninety percent of the events occurred at depths shallower than 12 km. The brittle-ductile transition zone in this area is determined at a depth of 12–16 km, similar to that in the Lake Bogoria area immediately to the south. A preliminary study of focal mechanisms of suitable events indicates WNW-ESE extension across the Lake Baringo basin and suggests the presence of sub-vertical rupture surfaces beneath the lake. These may be caused by the insertion of basic dykes into the upper crust, and the rupture process assisted by high temperature and pressure geothermal fluids. The inversion of local P-wave arrival time data for the upper crustal velocity structure beneath Lake Baringo identifies a low-velocity zone beneath surface geothermal activity, a relationship previously demonstrated for the Lake Bogoria region.

Seismic risk of circum-pacific earthquakes I. Strain energy release

Commonly used earthquake “whole process” frequency - magnitude and strain energy - magnitude laws are merged to obtain an analytic expression for an upper bound magnitude to regional earthquake occurrenceM3, which is expressed primarily in terms of the annual maximum magnitudeM1 and the magnitude equivalent of the annual average total strain energy releaseM2. Values ofM3 are also estimated graphically from cumulative strain energy release diagrams. Both methods are illustrated by application to the high seismicity of the circum-Pacific belt, using Duda’s (1965) data and regionalisation. Values ofM3 obtained analytically, with their uncertainties, are in agreement with those obtained graphically. Empirical relations are then obtained betweenM1,M2, andM3, which could be of general assistance in regional seismic risk considerations if they are found to be of a universal nature. For instance.M3 andM2 differ by one magnitude unit in subregions of the circum-Pacific.

Seismic risk of circum-Pacific earthquakes: II. Extreme values using Gumbel's third distribution and the relationship with strain energy release

In a previous paper (Makropoulos andBurton, 1983) the seismic risk of the circum-Pacific belt was examined using a ‘whole process’ technique reduced to three representative parameters related to the physical release of strain energy, these are:M1, the annual modal magnitude determined using the Gutenberg-Richter relationship;M2, the magnitude equivalent to the total strain energy release rate per annum, andM3, the upper bound magnitude equivalent to the maximum strain energy release in a region.The risk analysis is extended here using the ‘part process’ statistical model of Gumbels IIIrd asymptotic distribution of extreme values. The circum-Pacific is chosen being a complete earthquake data set, and the stability postulate on which asymptotic distributions of extremes are deduced to give similar results to those obtained from ‘whole process’ or exact distributions of extremes is successfully checked. Additionally, when Gumbel III asymptotic distribution curve fitting is compared with Gumbel I using reduced chi-squared it is seen to be preferable in all cases and it also allows extensions to an upper-bounded range of magnitude occurrences. Examining the regional seismicity generates several seismic risk results, for example, the annual mode for all regions is greater thanm(1)=7.0, with the maximum being in the Japan, Kurile, Kamchatka region atm(1)=7.6. Overall, the most hazardous areas are situated in this northwestern region and also diagonally opposite in the southeastern circum-Pacific. Relationships are established between the Gumbel III parameters and quantitiesm1(1),X2 and ω, quantities notionally similar toM1,M2 andM3 although ω is shown to be systematically larger thanM; thereby giving a physical link through strain energy release to seismic risk statistics. Inall regions of the circum-Pacific similar results are obtained forM1,M2 andM3 and the notionally corresponding statistical quantitiesm1(1),X2 and ω, demonstrating that the relationships obtained are valid over a wide range of seismotectonic enviroments.

The «London» earthquake of 1580, April 6

Abstract A large number of accounts of the effects of the 1580, April 6, “London” earthquake have been collected and examined. At least four aftershocks have been identified, the largest of which occurred on May 1 or 2. Intensities have been assigned for a number of localities on the evidence of contemporary reports and other data, including the nature of the constructions employed in damaged structures. An assessment of the suitability of six variants of widely used intensity scales is made by assigning intensities independently on each scale, for the complete data set. Marine effects, the regions affected by the aftershocks and the spatial distribution of intensities show that the epicentre probably lay offshore in the Straits of Dover. The maximum intensity was assigned as an inferred 9 using both the MSK scale and the Modified Mercalli Scale (Brazee version), the two scales found to be preferable. This earthquake affected all of northern France, Britain possibly as far north as Edinburgh, and the Low Countries and Germany beyond Cologne and Duisburg. Employing suitable relationships between intensity, magnitude, and attenuation, yields a Richter magnitude in the range 6.2 to 6.9. It is suggested that this earthquake may have been caused by movement along a fault of Variscan trend at a depth of about 33 km; that is, at the base of the crust.

Strong ground acceleration seismic hazard in Greece and neighboring regions

Abstract In an early paper [Tectonophysics 117 (1985) 259] seismic hazard in Greece was analyzed using a relatively homogeneous earthquake catalogue spanning 1900–1978 and a strong motion attenuation relationship adapted to use in Greece. Improved seismic hazard analyses are obtained here using Gumbels asymptotic extreme value distribution applied to peak horizontal ground acceleration occurrence, but now taking into account the increased length and quality of earthquake catalogue data spanning 1900–1999 and the burgeoning information on earthquake strong motion data and attenuation relationships appropriate for Europe and, explicitly, Greece. Seismic acceleration hazard results tabulated for six cities reveal (e.g. using arbitrarily the 50-year p.g.a. with 90% probability of not being exceeded) changes of about 10% in the new calculated values: two cities show an increase and four a decrease. These are relatively small and reassuring adjustments. Inspection of the available attenuation relationships leads to a preference for the models of Theodulidis and Papazachos, particularly with the model modification to produce a ‘stiff soil’ site relationship, as these relationships explicitly exploit the Greek strong motion database. Isoacceleration maps are produced for Greece as a whole from each attenuation relationship inspected. The final set of maps based on the Theodulidis and Papazachos models provide a foundation for comparison with the Seismic Hazard Zones adopted in the New Greek Seismic Code where scope can be found to modify zone shape and the level at which p.g.a.s are set. It should be noted that the generation of the present isoacceleration maps is based on a seismogenic zone-free methodology, independent of any Euclidean zoning assumptions.

K -means cluster analysis and seismicity partitioning for Pakistan

Pakistan and the western Himalaya is a region of high seismic activity located at the triple junction between the Arabian, Eurasian and Indian plates. Four devastating earthquakes have resulted in significant numbers of fatalities in Pakistan and the surrounding region in the past century (Quetta, 1935; Makran, 1945; Pattan, 1974 and the recent 2005 Kashmir earthquake). It is therefore necessary to develop an understanding of the spatial distribution of seismicity and the potential seismogenic sources across the region. This forms an important basis for the calculation of seismic hazard; a crucial input in seismic design codes needed to begin to effectively mitigate the high earthquake risk in Pakistan. The development of seismogenic source zones for seismic hazard analysis is driven by both geological and seismotectonic inputs. Despite the many developments in seismic hazard in recent decades, the manner in which seismotectonic information feeds the definition of the seismic source can, in many parts of the world including Pakistan and the surrounding regions, remain a subjective process driven primarily by expert judgment. Whilst much research is ongoing to map and characterise active faults in Pakistan, knowledge of the seismogenic properties of the active faults is still incomplete in much of the region. Consequently, seismicity, both historical and instrumental, remains a primary guide to the seismogenic sources of Pakistan. This study utilises a cluster analysis approach for the purposes of identifying spatial differences in seismicity, which can be utilised to form a basis for delineating seismogenic source regions. An effort is made to examine seismicity partitioning for Pakistan with respect to earthquake database, seismic cluster analysis and seismic partitions in a seismic hazard context. A magnitude homogenous earthquake catalogue has been compiled using various available earthquake data. The earthquake catalogue covers a time span from 1930 to 2007 and an area from 23.00° to 39.00°N and 59.00° to 80.00°E. A threshold magnitude of 5.2 is considered for K-means cluster analysis. The current study uses the traditional metrics of cluster quality, in addition to a seismic hazard contextual metric to attempt to constrain the preferred number of clusters found in the data. The spatial distribution of earthquakes from the catalogue was used to define the seismic clusters for Pakistan, which can be used further in the process of defining seismogenic sources and corresponding earthquake recurrence models for estimates of seismic hazard and risk in Pakistan. Consideration of the different approaches to cluster validation in a seismic hazard context suggests that Pakistan may be divided into K = 19 seismic clusters, including some portions of the neighbouring countries of Afghanistan, Tajikistan and India.