The Kefalonia Transform Zone (offshore Western Greece) with special emphasis to its prolongation towards the Ionian Abyssal Plain

Information concerning two seismic lines, the first located northwest of the Lefkada Island and the second from the deep Ionian basin to the gulf of Patras, is used to trace the Kefalonia Transform Zone (KTZ) and to explore its relation with the sedimentary sequences and the deeper geologic structures in the study area. In addition, sea bottom topography and fault plane solutions are combined in order to explore the prolongation of the KTZ into the Ionian Abyssal Plain (IoAP) and to describe its properties. The boundary between the subduction of the eastern Mediterranean oceanic crust under the overriding continental crust and the KTZ is well constrained by the seismic data in association with seismicity and regional stress field. The southern prolongation of the KTZ is located in the IoAP towards the direction between Kefalonia and Zakynthos Islands at depth greater than 15 km. The southern part of the KTZ exhibits a strike–slip motion with a thrust component according to fault plane solutions of moderate and strong earthquakes. The seismic section mostly confirms the existence of the thrust component and gives information about the tectonic status east and west of the KTZ.     

Satellite motion in a non-singular gravitational potential

We study the effects of a non-singular gravitational potential on satellite orbits by deriving the corresponding time rates of change of its orbital elements. This is achieved by expanding the non-singular potential into power series up to second order. This series contains three terms, the first been the Newtonian potential and the other two, here R ₁ (first order term) and R ₂ (second order term), express deviations of the singular potential from the Newtonian. These deviations from the Newtonian potential are taken as disturbing potential terms in the Lagrange planetary equations that provide the time rates of change of the orbital elements of a satellite in a non-singular gravitational field. We split these effects into secular, low and high frequency components and we evaluate them numerically using the low Earth orbiting mission Gravity Recovery and Climate Experiment (GRACE). We show that the secular effect of the second-order disturbing term R ₂ on the perigee and the mean anomaly are 4″.307×10⁻⁹/a, and −2″.533×10⁻¹⁵/a, respectively. These effects are far too small and most likely cannot easily be observed with today’s technology. Numerical evaluation of the low and high frequency effects of the disturbing term R ₂ on low Earth orbiters like GRACE are very small and undetectable by current observational means.     

Earthquake Triggering along the Xianshuihe Fault Zone of Western Sichuan,China

Western Sichuan is among the most seismically active regions in southwestern China and is characterized by frequent strong (M 6.5) earthquakes, mainly along the Xianshuihe fault zone. Historical and instrumental seismicity show a temporal pattern of active periods separated by inactive ones, while in space a remarkable epicenter migration has been observed. During the last active period starting in 1893, the sinistral strike–slip Xianshuihe fault of 350 km total length, was entirely broken with the epicenters of successive strong earthquakes migrating along its strike. This pattern is investigated by resolving changes of Coulomb failure function (CFF) since 1893 and hence the evolution of the stress field in the area during the last 110 years. Coulomb stress changes were calculated assuming that earthquakes can be modeled as static dislocations in an elastic halfspace, and taking into account both the coseismic slip in strong (M 6.5) earthquakes and the slow tectonic stress buildup associated with major fault segments. The stress change calculations were performed for faults of strike, dip, and rake appropriate to the strong events. We evaluate whether these stress changes brought a given strong earthquake closer to, or sent it farther from, failure. It was found that all strong earthquakes, and moreover, the majority of smaller events for which reliable fault plane solutions are available, have occurred on stress–enhanced fault segments providing a convincing case in which Coulomb stress modeling gives insight into the temporal and spatial manifestation of seismic activity. We extend the stress calculations to the year 2025 and provide an assessment for future seismic hazard by identifying the fault segments that are possible sites of future strong earthquakes.     

Analysis of a UV Event in a Polar Coronal Hole

We present observations of a UV event which occurred in a polar coronal hole. They were obtained by SUMER on SOHO in several chromospheric and transition region spectral lines. Its birth site was about 50 arc sec inside the limb and in a network lane showing a net outflow before its initiation. The event had an extension of about 5 arc sec along the slit, a duration of about 3 min and was characterized by a large increase of intensity together with a significant line broadening with, however, downflows of about 50 km s^–1 being dominant. Proper motions with a velocity of about 10 km s^–1 were also observed. The event appeared at middle transition (Ovi) temperatures and it simultaneously showed up in chromospheric (Oi, Ly ) and low transition region (Cii) temperatures. We discuss this event in view of different scenarios to account for it. Our event could be a part of the large family of quiet-Sun explosive events observed by Ryutova and Tarbell (2000) taking place in polar coronal holes that are triggered by magnetic reconnection in the low solar atmosphere.     

Least squares spectral analysis and its application to superconducting gravimeter data analysis

Detection of a periodic signal hidden in noise is the goal of Superconducting Gravimeter (SG) data analysis. Due to spikes, gaps, datum shrifts (offsets) and other disturbances, the traditional FFT method shows inherent limitations. Instead, the least squares spectral analysis (LSSA) has showed itself more suitable than Fourier analysis of gappy, unequally spaced and unequally weighted data series in a variety of applications in geodesy and geophysics. This paper reviews the principle of LSSA and gives a possible strategy for the analysis of time series obtained from the Canadian Superconducting Gravimeter Installation (CGSD), with gaps, offsets, unequal sampling decimation of the data and unequally weighted data points.     

A Non-Extensive Statistical Physics View in the Spatiotemporal Properties of the 2003 (Mw6.2) Lefkada,Ionian Island Greece,Aftershock Sequence

Investigation of the spatiotemporal properties of the 2003 Lefkada seismic sequence is performed through non-extensive statistical physics. Information on highly accurate aftershock source parameters became feasible from the recordings of a portable digital seismological network that was installed and operated in the study area, during the evolution of the seismic sequence. Thus, the spatiotemporal distribution of aftershocks onto the main and neighboring fault segments was investigated in detail, enabling the recognition of four distinctive seismicity clusters separated by less active patches. The aftershock spatiotemporal properties are studied here, using the ideas of non-extensive statistical physics (NESP). The cumulative distribution functions of the inter-event times and the inter-event distances are presented using the data set in each seismicity cluster, and the analysis results in values for the statistical thermodynamic q _T and q _D parameters for each cluster, where q _T varies from 1.16 to 1.47 and q _D from 0.42 to 0.77 for the inter-event times and distances distributions, respectively. These values confirm the complexity and non-additivity of the spatiotemporal evolution of seismicity, and the applicability of the NESP approach in investigating aftershocks sequence. The temporal pattern is discussed using the closely connected to NESP approach of superstatistics, which is based on a superposition of ordinary local equilibrium statistical mechanics. The result indicates that the temporal evolution of the Lefkada aftershock sequence in clusters A, B and C is governed by very low number of degrees of freedom, while D is a less organized seismicity structure with a much higher number of degrees of freedom.     

Correlation of Static Stress Changes and Earthquake Occurrence in the North Aegean Region

A systematic analysis is made of static Coulomb stress changes and earthquake occurrence in the area of the North Aegean Sea, Greece, in order to assess the prospect of using static stress changes to construct a regional earthquake likelihood model. The earthquake data set comprises all events of magnitude M ≥ 5.2 which have occurred since 1964. This is compared to the evolving stress field due to constant tectonic loading and perturbations due to coseismic slip associated with major earthquakes (M ≥ 6.4) over the same period. The stress was resolved for sixteen fault orientation classes, covering the observed focal mechanisms of all earthquakes in the region. Analysis using error diagrams shows that earthquake occurrence is better correlated with the constant tectonic loading component of the stress field than with the total stress field changes since 1964, and that little, if any, information on earthquake occurrence is lost if only the maximum of the tectonic loading over the fault orientation classes is considered. Moreover, the information on earthquake occurrence is actually increased by taking the maximum of the evolving stress field since 1964, and of its coseismic-slip component, over the fault orientation classes. The maximum, over fault orientation classes, of linear combinations of the tectonic loading and the evolving stress field is insignificantly better correlated with earthquake occurrence than the maximum of the tectonic loading by itself. A composite stress-change variable is constructed from ordering of the maximum tectonic loading component and the maximum coseismic-slip component, in order to optimize the correlation with earthquake occurrence. The results indicate that it would be difficult to construct a time-varying earthquake likelihood model from the evolving stress field that is more informative than a time-invariant model based on the constant tectonic loading.     

断层相互作用对中国川滇地区地震危险评估的意义

研究了1904年以来发生在中国四川、云南地区的强烈地震所伴生的库仑应力变化. 研究区涉及了中国大陆最活动的地震带. 由于欧亚板块与印度板块沿喜马拉雅会聚带的碰撞, 菱形区域的构造运动受到西藏高原向东扩展的影响,在东——西方向上受到主要板内走滑断裂运动的调节. 在研究区,断层顺时针逐步旋转,转动角达90deg;, 造成了应力场的复杂性. 计算了由强地震(MSge;6.5) 的同震滑动和连续构造加载在主要断裂上产生的库仑破裂函数变化(Delta;CFF). 在应力评估模型的每一步, 对是否能触发下一次地震的可能性都进行了判断. 最后,模型给出了下一次地震中断层易于破裂的证据,对未来危险性进行了评估.      

Crustal and upper mantle structure of the Kozani-Grevena area obtained by non-linear inversion of P and S travel times

The present study focuses on the P and S crustal and uppermost mantle velocity structure in the broader Kozani-Grevena area. The velocity structure is derived from the inversion of travel times of local events. The main data source is the travel times from the aftershock sequence of the large event of 13 of May 1995 (M_w = 6.6) which occurred in the study area. An appropriate preconditioning of the final linearized system is used to reduce ray density effects on the results. An attempt is made to interpret the features and details of the crustal structure in terms of the geotectonic setting of the area. The observed features of the deeper crustal and uppermost mantle structure are in very good agreement with previous results. Specifically, a crustal thickening is observed along a ENE-WSW direction, perpendicular to the well-known Dinaric trend (NNW-SSE) of the geological formations of the area, in accordance with the theoretical expectation of a thicker crust under the accretion prism which starts at the SW edge of the study area.     

3-D Crustal Velocity Structure in Northwestern Greece

— The three-dimensional crustal velocity structure in the area of the northwestern Greek mainland was determined by P-wave travel time inversion, applying a two-step tomography procedure. The data set consists of the travel-time residuals of 584 well located earthquakes. In order to improve the initial (reference) velocity model, before the inversion of travel times, the minimum 1-D model was determined. Several tests were conducted to estimate model stability and hypocenter uncertainties. The velocity distribution in the shallow layers (4 and 7 km) is strongly affected by the crustal thickness variation and the complex tectonics. A first, well-defined velocity discontinuity appears at a depth of 3–6 km, along the Hellenides Mountain chain. A second low velocity anomaly is detected at a depth of 9–12 km and may be connected with the Alpidic orogenesis. Another interesting feature appears beneath the Amvrakikos Gulf (horstgraben structure), where relatively low velocities (<6.0 km^-1) appear to a depth of 20 km. Finally, a well-pronounced velocity boundary is found at a depth of 16 km. In general, low velocities are predominant along the Dinarides-Hellenides Mountain chain, rather typical for the upper crust.Acknowledgement. The authors thank the referees for their useful comments. Moreover, we would like to thank the General Secretariat for Research and Technology of Greece, for the partial support of this study.