The seismicity of Tunis

Very little is known regarding tlie seisniicity of Tunisia and in particulartliat of the Tunis area. We know tliat Tunis was sliaken a fewtimes in the past but it is rather difficult to say witli certainty whateffect tliese earthquakes had on structures and what their intensitieswere. Information is partieularly lacking about tliose earthquakes beforethe 19tli century. It may, however, be signiflcant tliat liistorical record»for the area of Tunis indicate intense seismic activity during the 9th and18th centuries, altliough one must make some allowances for naturaiexaggerations in early statements.In order to disclose the seismic potentiality of suoli a poorly documentedarea it is, therefore, essential to resort to its geotectonic historyfor informatici!

Near-field horizontal and vertical earthquake ground motions

Strong-motion attenuation relationships are presented for peak ground acceleration, spectral acceleration, energy density, maximum absolute input energy for horizontal and vertical directions and for the ratio of vertical to horizontal of these ground motion parameters. These equations were derived using a worldwide dataset of 186 strong-motion records recorded with 15 km of the surface projection of earthquakes between Ms = 5:8 and 7.8. The effect of local site conditions and focal mechanism is included in some of these equations

Seismicity and associated strain of central Greece between 1890 and 1988

We examined the seismicity of central Greece between 1890 and 1988, using macroseismic and instrumental data, to ask two questions: (1) does the seismicity of this period reveal all the major tectonic structures that are known to be active?; and (2) what are the likely strains associated with the seismicity over this period? Many known active structures have been effectively aseismic for the last hundred years, and even the inclusion of all known large events earlier than 1890 reveals no activity associated with the NE coast of Evia, Gulf of Argos, or graben NE of Mt Parnassos. It is clear that even 100 years' data are inadequate for either a reasonable assessment of seismic risk or for a confident estimation of maximum magnitude. However, we are aware of no earthquakes in central Greece during the last 200 yr that were larger than Ms 7.0. It is probable that the maximum magnitude is restricted by the maximum length of fault segments, which appears to be around 15-20 km. The earthquakes of Ms ≥ 5.8 during 1890-1988 can account for a N-S displacement of around 45-70 cm (with maximum and minimum estimates a factor of two greater and smaller than this) across part of a 1890-1900 triangulation network in central Greece that was resurveyed in 1988. The contribution of smaller events may increase this displacement by about 50 per cent. This cumulative seismic displacement is similar to that estimated from the geodetic work (about 100 cm), but a detailed comparison of the two sets of observations will be reported elsewhere. A re-evaluation of all the important earthquakes of 1890-1988 in central Greece is presented in the Appendix, which summarizes information of use to both earth scientists and engineers

SHORT COMMUNICATION A DAMAGING SEAQUAKE

The term seaquake is used to mean shaking caused exclusively by an earthquake but felt on board a vessel at sea, excluding effects from tsunamis. Thousands of such occurrences are known which are occasionally listed in the casualty reports of Lloyd's List and in other sources, causing considerable concern to mariners but rarely serious structural damage to seaworthy vessels. Seaquakes, because the sensation they often create is one of the ship running aground, have been responsible for some of the fictitious reefs and shoals shown in early navigation charts. This research note brings to attention a little known case of a damaging seaquake which is of interest for the study of the vulnerability of offshore engineering structures and marine vessels, particularly submarines, as well as containers for the disposal of nuclear waste, and also provides information on the earthquake source responsible for the seaquake, that with the data available cannot be obtained by normal seismological methods. On the 28th February 1969 the motor tanker 'Ida Knudsen', a 32,000-tonne vessel built in 1958, was sailing in ballast from Lisbon to the Persian Gulf when it experienced a 'violent vertical shock'. This happened at about 02 h 45 min (GMT) when the ship was at a position 36-12"N-10.70"W in 2,700 fathoms of water. At the time the general state of the sea in this part of the Atlantic was 3 to 4 with moderate swell, and the windforce was 4 to 5. From the available log-extracts, maritime declaration and other survey documents it appears that as a result of the shock the vessel sustained very serious structural damage. In the wheelhouse, chartroom and radio station binnacles, compasses and permanent instruments were torn loose and collapsed. Doors and fixtures in the superstructure were torn loose and thrown about. The signal mast with the radarscanner was distorted and all its cross-bars were broken. Damage in the superstructure was more serious at midship than at the aft peak. From eyewitness accounts it appears that the vessel was lifted up bodily, the bow moving up faster than the bridge, and then the whole ship slammed back with violent vibrations, the whole event lasting about ten seconds. Serious damage was also caused both to the machinery and hull where piping was broken and leakage developed between tanks. After hours of drifting and with a misaligned propeller shaft the ship returned to Lisbon where it was drydocked and surveyed. ' The surveys proved that the hull, machinery and other equipment had sustained great damage and, on account of the permanent deformation and breaks, the ship had lost a substantial part of her longitudinal strength. The complete surface of the vessel's skin from cofferdam to cofferdam buckled, in places with permanent sets of 4cm and the hull was twisted to port by 18cm. Bulkheads, hull frames and girders were buckled or torn apart and all the wing tanks leaked. Moreover, the bottom parts of the side platings were torn away from the girders, in places by as much as 5cm, effects resembling those from an underwater mine explosion. The ship was condemned as a total loss. Later, she was rebuilt as 'Petros Hajikyriakos' (see Lloyd's Register of Ships). All this was apparently the result of an earthquake at 02 h 40mm 33 s (GMT) with an epicentre offshore of Gibraltar at 35.97"N-10.59"W (ISC normal depth determination), i.e. 20 km from where the ship was damaged. No other ships are known to have been in the near-field of this major earthquake (M, = 7.8, M , = 6.0 x loz7 dyn-cm).' A number of vessels further away

Equations for the estimation of strong ground motions from shallow crustal earthquakes using data from Europe and the Middle East : horizontal peak ground acceleration and spectral acceleration

This article presents equations for the estimation of horizontal strong ground motions caused by shallow crustal earthquakes with magnitudes Mw ≥ 5 and distance to the surface projection of the fault less than 100km. These equations were derived by weighted regression analysis, used to remove observed magnitude-dependent variance, on a set of 595 strong-motion records recorded in Europe and the Middle East. Coefficients are included to model the effect of local site effects and faulting mechanism on the observed ground motions. The equations include coefficients to model the observed magnitude-dependent decay rate. The main findings of this study are that: short-period ground motions from small and moderate magnitude earthquakes decay faster than the commonly assumed 1/r, the average effect of differing faulting mechanisms is not large and corresponds to factors between 0.8 (normal and odd) and 1.3 (thrust) with respect to strike-slip motions and that the average long-period amplification caused by soft soil deposits is about 2.6 over those on rock sites. Disappointingly the standard deviations associated with the derived equations are not significantly lower than those found in previous studies

Equations for the estimation of strong ground motions from shallow crustal earthquakes using data from Europe and the Middle East : vertical peak ground acceleration and spectral acceleration

This article presents equations for the estimation of vertical strong ground motions caused by shallow crustal earthquakes with magnitudes M w 5 and distance to the surface projection of the fault less than 100km. These equations were derived by weighted regression analysis, used to remove observed magnitude-dependent variance, on a set of 595 strong-motion records recorded in Europe and the Middle East. Coefficients are included to model the effect of local site effects and faulting mechanism on the observed ground motions. The equations include coefficients to model the observed magnitude-dependent decay rate. The main findings of this study are that: short-period ground motions from small and moderate magnitude earthquakes decay faster than the commonly assumed 1/r, the average effect of differing faulting mechanisms is similar to that observed for horizontal motions and is not large and corresponds to factors between 0.7 (normal and odd) and 1.4 (thrust) with respect to strike-slip motions and that the average long-period amplification caused by soft soil deposits is about 2.1 over those on rock sites

Statistical modeling of ground motion relations for seismic hazard analysis

We introduce a new approach for ground motion relations (GMR) in the probabilistic seismic hazard analysis (PSHA), being influenced by the extreme value theory of mathematical statistics. Therein, we understand a GMR as a random function. We derive mathematically the principle of area-equivalence; wherein two alternative GMRs have an equivalent influence on the hazard if these GMRs have equivalent area functions. This includes local biases. An interpretation of the difference between these GMRs (an actual and a modeled one) as a random component leads to a general overestimation of residual variance and hazard. Beside this, we discuss important aspects of classical approaches and discover discrepancies with the state of the art of stochastics and statistics (model selection and significance, test of distribution assumptions, extreme value statistics). We criticize especially the assumption of logarithmic normally distributed residuals of maxima like the peak ground acceleration (PGA). The natural distribution of its individual random component (equivalent to exp(epsilon_0) of Joyner and Boore 1993) is the generalized extreme value. We show by numerical researches that the actual distribution can be hidden and a wrong distribution assumption can influence the PSHA negatively as the negligence of area equivalence does. Finally, we suggest an estimation concept for GMRs of PSHA with a regression-free variance estimation of the individual random component. We demonstrate the advantages of event-specific GMRs by analyzing data sets from the PEER strong motion database and estimate event-specific GMRs. Therein, the majority of the best models base on an anisotropic point source approach. The residual variance of logarithmized PGA is significantly smaller than in previous models. We validate the estimations for the event with the largest sample by empirical area functions. etc

An Approach for Rapid Assessment of Seismic Hazards in Turkey by Continuous GPS Data

The Earth is being monitored every day by all kinds of sensors. This leads an overflow of data in all branches of science nowadays, especially in Earth Sciences. Data storage and data processing are the problems to be solved by current technologies, as well as by those accessing and analyzing these large data sources. Once solutions have been created for collecting, storing and accessing data, then the challenge becomes how to effectively share data, applications and processing resources across many locations. The Global Positioning System (GPS) sensors are being used as geodetic instruments to precisely detect crustal motion in the Earth's surface. Rapid access to data provided by GPS sensors is becoming increasingly important for deformation monitoring and rapid hazard assessments. Today, reliable and fast collection and distribution of data is a challenge and advances in Internet technologies have made it easier to provide the needed data. This study describes a system which will be able to generate strain maps using data from continuous GPS stations for seismic hazard analysis. Strain rates are a key factor in seismic hazard analyses. Turkey is a country prone to earthquakes with a long history of seismic hazards and disasters. This situation has resulted in the studies by Earth scientists that focus on Turkey in order to improve their understanding of the Earth's crust structure and seismic hazards. Nevertheless, the construction of models, data access and analysis are often not fast as expected, but the combination of Internet technologies with continuous GPS sensors can be a solution to overcome this problem. This system would have the potential to answer many important questions to assess seismic hazards such as how much stretching, squashing and shearing is taking place in different parts of Turkey, and how do velocities change from place to place? Seismic hazard estimation is the most effective way to reduce earthquake losses. It is clear that reliability of data and on-line services will support the preparation of strategies for disaster management and planning to cope with hazards

Insights into the 1968–1997 Dasht-e-Bayaz and Zirkuh earthquake sequences, eastern Iran, from calibrated relocations, InSAR and high-resolution satellite imagery

The sequence of seismicity in the Dasht-e-Bayaz and Zirkuh region of northeastern Iran, which includes 11 destructive earthquakes within a period of only 30 years, forms one of the most outstanding examples of clustered large and intermediate-magnitude seismic activity in the world.We perform a multiple-event relocation analysis, with procedures to remove systematic location bias, of 169 earthquakes, most of which occurred in the period 1968–2008, to better image the distribution of seismicity within this highly active part of Iran. The geographic locations of the clustered earthquakes were calibrated by the inclusion of phase arrivals from seismic stations at short epicentral distances, and also by matching the relative locations of the three largest events in the study to their mapped surface ruptures. The two independent calibration methods provide similar results that increase our confidence in the accuracy of the distribution of relocated epicentres. These calibrated epicentres, combined with the mapping of faults from high-resolution satellite imagery, and from an InSAR-derived constraint on fault location in one case, allow us to associate individual events with specific faults, and even with specific segments of faults, to better understand the nature of the active tectonics in this region during the past four decades. Several previous assumptions about the seismicity in this region are confirmed: (1) that the 1968 August 30 Mw 7.1 Dasht-e-Bayaz earthquake nucleated at a prominent segment boundary and left-step in the fault trace, (2) that the 1968 September 11 Mw 5.6 aftershock occurred on the Dasht-e-Bayaz fault at the eastern end of the 1968 rupture and (3) that the 1976 November 7 Mw 6.0 Qayen earthquake probably occurred on the E–W left-lateral Avash Fault. We show, in addition, that several significant events, including the 1968 September 1 and 4 (Mw 6.3 and 5.5) Ferdows earthquakes, the 1979 January 16 (Mw 6.5) and 1997 June 25 (Mw 5.9) Boznabad events and the 1979 December 7 (Mw 5.9) Kalat-e-Shur earthquake are likely to have ruptured previously unknown faults. Our improved description of the faulting involved in the 1968–1997 earthquake sequence highlights the importance of rupturing of conjugate left- and right-lateral faults in closely spaced events, or potentially even within a single earthquake, as was likely the case at the eastern end of the 1979 November 27 (Mw 7.1) Khuli-Buniabad main shock. The high level of clustered seismic activity probably results from the simultaneous activity on left- and right-lateral faults, an inherently unstable arrangement that must evolve rapidly. The combination of high-resolution satellite imagery and calibrated earthquake locations is a useful tool for investigating active tectonics, even in the absence of detailed field observations