The 14 August 2003 Lefkada Island (Greece) earthquake: Focal mechanisms of the mainshock and of the aftershock sequence

The central area of the Ionian Sea is dominated by the Cephalonia Transform Fault Zone (CTFZ) with a pronounced dextral strike-slip component of motion. The CTFZ has two main segments: the Lefkada Segment (LS) in the north and the Cephalonia Segment (CS) in the south. On 14 August 2003 an M_w 6.2 earthquake ruptured the Lefkada Segment and produced extensive damage, especially to the western coast of the island. Teleseismic waveform modelling revealed the multiple source character of the mainshock, which occurred as three sub-events along a ∼N12^∘E line. The first sub-event occurred at a depth of about 15 km, followed 2.5 s later by the second and largest sub-event at a depth of 11 km and the third sub-event 14 s after the second at a depth of 15 km. The total moment from the body waves of this sequence is about 22.3×10¹⁷ Nt m (M_w 6.2) with a source duration of ∼15 s. The rupture started at the northern part of the Lefkada fault Segment and propagated southwards. The second and third sub-events are located at 7 and 40 km to the south-east in respect to the first sub-event. The focal mechanisms of the two strongest sources indicate strike-slip faulting along the NE–SW trending Lefkada segment (sub-event 2: Strike = 12^∘, Dip = 81^∘, Rake = 174^∘; sub-event 3: Strike = 20^∘, Dip = 63^∘, Rake = −179^∘). Moment tensor inversion applied to regional broad band waveforms obtained from the Greek National Seismographic Network provided focal mechanisms for 23 aftershocks with magnitudes ranging from M_w 3.6 to 5.4. The aftershock sequence presented spatial and temporal variation. The aftershocks were concentrated in two clusters one at the northern part of the activated area and another at the southern part. Most of them were of strike-slip character, following the major tectonic lines of the area, although low-angle thrust and reverse faulting mechanisms were also observed. Thrust and reverse type mechanisms are mainly concentrated in the northern and mainland part of the Lefkada Island which probably indicates the segmented character of the fault and probable activation of adjacent structures.     

How deep was the Dudley (UK) earthquake of 22 September 2002?

On 22 September 2002, the largest UK earthquake (m_b4.3) of the last 10 years occurred near the town of Dudley in the West Midlands. Here we determine the earthquake focal mechanism and depth using data from stations at regional and teleseismic distances. Short-period teleseismic seismograms are interpreted in terms of P and surface reflections pP and sP. This analysis suggests that the source depth is deeper than the 9.7 km initially determined by the British Geological Survey (BGS). The relative amplitude method is applied to four teleseismic seismograms to support our interpretation of the surface reflections, and constrain the focal mechanism. Our preferred focal mechanism, a near vertical strike-slip with _s = 94°, = 88° and = –179°, is in reasonable agreement with a moment tensor determined by the Swiss Seismological Service. Synthetic regional surface wave seismograms match the observed seismograms for a model focal depth of 19.5 (±3.0) km and scalar moment, M₀, of 3.2 × 10¹⁵ N m. Our results emphasize that due to the well-known trade-off between depth and M₀ from inversions of long period (0.02–0.1 Hz) surface waves, it is preferable to combine long- and short-period data to constrain reliably the depth and hence estimate M₀. Our focal mechanism and depth are further validated by generating short-period synthetic seismograms that match the observations.     

Motion On the Sphere: Integrability and Families of Orbits

We study the problem of the motion of a unit mass on the unit sphere and examine the relation between integrability and certain monoparametric families of orbits. In particular we show that if the potential is compatible with a family of meridians, it is integrable with an integral linear in the velocities, while a family of parallels guarantees integrability with an integral quadratic in the velocities.     

The Ain Temouchent (Algeria) Earthquake of December 22<Superscript>nd</Superscript>, 1999

—On December 22^nd, 1999 an earthquake of Magnitude M_w : 5.7 occurred at Ain Temouchent (northwest Algeria). This moderate seismic event was located in a region characterized by a low seismic activity where few historical events have been observed. The earthquake, with a maximum intensity of VII (MSK scale), caused serious damages to the Ain Temouchent city and its surroundings. In the epicentral area, 25 people died and about 25,000 people were made homeless. Some minor breaks have been observed in several areas in the field. They were mainly related to minor collapses in the landscape or in volcanic cavities. The focal mechanism has been studied by using broadband data at regional and teleseismic distances, and different methods. The fault-plane solution has been estimated from first motions of P wave. Depth and source time function have been estimated from the modeling of body waveforms. Scalar seismic moment and source dimension have been obtained from spectral analysis. Results show thrust motion, with a horizontal pressure axis oriented in a NW-SE direction, a depth of 4 km and a simple source time function with time duration of 5 s. Scalar seismic moment estimated from waveform modeling is 4.7 × 10¹⁷ Nm, and spectral analysis gives a value of 1.7 × 10¹⁷ Nm and a source radius of 7.5 km.     

长江三峡及邻区构造应力场和震源错动类型的研究

在文献［３］提出的用计算机确定震源断层面解的基础上，进一步给出了解的置信度。置信度由两个因子组成（Ｃ_１，Ｃ_２）。其中Ｃ_１为初动符号的符合比，Ｃ_２为解的不可变动空间与全部空间的比。使用长江三峡及邻区（北纬２８°－３４°，东经１０８°－１１４°）１９７４年以来的初动观测资料，确定了Ｍｓ≥２．１的４０次地震的断层面解、该区的区域应力场和分区应力场及平均错动性质。此外还给出了黄陵背斜地区的应力场特征。结果表明，黄陵背斜的地震构造活动具有整体性，构造过程发生在１５～２０ｋｍ的深度，张应力轴的方向分布显示以背斜为中心的幅射状态。     

A source for the Reissner-Nordstrøm metric

An exact solution of the coupled Einstein-Maxwell equations representing the gravitational field in the interior of a sphere of charged incoherent matter in equilibrium is obtained which is a charged analogue of the static perfect fluid sphere solution with spheroidal 3-space obtained by Vaidya & Tikekar.     

地球信息的度量空间和Global GIS

论述了地球空间信息的度量空间，讨论了地球信息度量空间下若干基础技术问题；结合Global GIS的技术趋势，讨论了球模型技术方案的意义，指出了Global GIS椭球模型的重要意义，分析了Global GIS椭球模型中度量空间数据基础、可视化理论和技术、空间数据处理、分析理论和技术等问题。     

华东地区现代地壳应力场及地震断层错动性质

使用143组中、小地震单震震源机制解和17组小区域综合机制解资料,统计分析了华东地区的现代构造应力场特征。华东地区现今处在NEE向(80°左右)主压、NNW向(350°左右)主张应力场的控制下;主应力作用方式以水平和近水平为主。在应力场方向和作用方式基本一致的背景上,不同地震构造分区存在一些差异,这些差异可能与相应区域主要活动断裂的主体分布方向有关,可能表征了现存构造对地震错动特征的影响和控制作用。现代中、小地震震源机制解,历史中、强地震和现代有感地震最内等震线长轴方向等资料显示华东地区地震主要沿NE,NW2个方向破裂错动,兼有NNE,NEE,NWW或近EW方向。以走滑和近走滑方式为主,兼有少量斜向滑动。地震断层的错动方式存在某些分区差异。华东地区历史中、强地震以NE向破裂错动为主,而现代中强地震在陆域以NW-SE为主,海域NE,NW兼有     

Estimation of the stress levels in the focal region before and after the 2001 M =8.1 Western Kunlun Mountain Pass earthquake

A method estimating the stress level in the focal region of an earthquake is proposed here. Taking the 2001 M=8.1 Western Kunlun Mountain Pass earthquake as an example, we estimate its stress level in the focal region before and after it by this method. The results show that the stress level in the focal region just prior to the initiation of this event is approximately 6.3–8 MPa, and about 5–6.7 MPa remained in the focal region after its occurrence. The stress in the focal region decreased by roughly twenty percent after this event. Contribution No. 05FE3026, Institute of Geophysics, China Earthquake Administration.