Meaza Tsige

An Overview of the Damaging and Low Magnitude Mw 4.8 La Paca Earthquake on 29 January 2005: Context, Seismotectonics, and Seismic Risk Implications for Southeast Spain

This article presents an overview of the La Paca earthquake of magnitude mbLg 4.7, which occurred on 29 January 2005, with its epicenter located near the town of Avile´s in the Murcia region in southeast Spain. Despite its low magnitude, the earthquake caused important damage in two towns of the epicentral area, La Paca and Zarcilla de Ramos. These areas recorded intensities of VI–VII (European Macroseismic Scale, 1998) and sustained estimated economic losses amounting to 10 million €. Aftershocks continued for more than 2 weeks, producing considerable alarm in the population and mobilizing emergency services from the whole region. The La Paca seismic series is the third registered in the region in the past 8 years, being preceded by the Mula (1999) and southwest Bullas (2002) seismic series. These main events had also low magnitudes (mbLg 4.8) and caused damage levels similar to the 2005 earthquake. The case is an example of a moderate seismic zone where low-magnitude and frequent earthquakes have important implications on the seismic hazard and risk of the region. Although these are not the largest expected earthquakes, they have yielded important information for improving the knowledge of the seismic activity of the area. With this aim in mind, different topics have been analyzed from a multidisciplinary perspective, including seismicity, local tectonics and surface geology, focal mechanisms, macroseismic effects, and ground motion. Results indicate a local tectonic interpretation, consistent with a strike-slip focal mechanism, the confirmation of a triggering process between the 2002 and 2005 earthquakes, a geotechnical and ground-motion characterization for the damaged sites (supporting local amplification effects and estimated peak ground acceleration values of �0.1g), and an understanding of damage patterns in relation to local building trends. The results may be used as guidelines for future revisions of the Spanish Building Code (Norma de la Construccio´n Sismorresistente Espan˜ola [NCSE-02], 2002). The study results should contribute to risk mitigation in a region where strong-motion records from the maximum expected earthquakes are not available. This approach can be extended to other regions with similar seismic backgrounds and a lack of strong-motion records.

Lacustrine chalky carbonates: origin, physical properties and diagenesis (Palaeogene of the Madrid Basin, Spain)

The Palaeogene lacustrine chalky carbonates of the Madrid Basin are a peculiar type of very soft and friable carbonate facies with high porosity despite being covered by more than 800 m of sediment. Similar physical properties to those described in marine chalk reservoirs emphasize the interest in analysing and characterizing these carbonate facies within a lacustrine depositional system. Lithologically, they are calcitic and/or dolomitic poorly cemented carbonate muds with no significant amounts of skeletal debris. Clay minerals such as illite, smectite and palygorskite are present between the carbonate crystals. Palygorskite is the most common, covering the carbonate crystals and forming sheets between them. These lacustrine chalky carbonates were formed in the basinal areas of the lake as the result of inorganic carbonate precipitation and/or detrital sedimentation related to episodic reactivation of the adjacent fan systems. Their petrological, geochemical and physical properties indicate that few textural and compositional modifications occurred during diagenesis. Their main physical properties are a very low dry bulk and grain density (1.6–2.2 and 2.62 g/cm3, respectively) and medium to high porosity (10– 40%) due to micropores ( 2 Am, 30%). The convergence of lacustrine sedimentation dynamics (rapid sedimentation), the original mineralogy of these calcareous lacustrine muds (relatively stable low-magnesian calcite and dolomite), the early formation of the palygorskite cement of these muds, and the retention of Mg-enriched fluids in the pore system, were decisive in the partial inhibition of calcite cementation, compaction and recrystallization. The chalky carbonates are also intercalated between impermeable littoral carbonate facies that impeded fluid flow through their pore systems.

Evolution of Pajonales Landslide (Tirajana Depression, Gran Canaria): A Case of Advancing Landslide

We studied the evolution of different stages of Pajonales landslide (Tirajana Depression, Gran Canaria), based on the geotechnical investigations of both in situ and mobilized volcanic materials. The deposit extends over 560 ha and it has undergone successive reactivations, some during the 20th century. The landslide comprises four large bodies that have successively broken away from a single initial rock mass. The main scarp affects the lava flows with intercalations of pyroclastic materials of the Roque Nublo Group and later volcanic activity (5.5 Ma to present-day). The basal surface of the landslide developed in old rocks of the Mogan Group (14.0–13.3 Ma), which are rhyolitic and trachytic ignimbrites with hydrothermal alteration related to the infilling of the Tejeda caldera. This alteration caused silty-clay layers presenting a low friction angle, high plasticity and expansive behavior. Representative samples of pyroclasts and soils from landslides were collected and laboratory tests were performed to identify them, determine unit weight, grain-size, plasticity, and shear strength. The morphology of slope prior to sliding was reconstructed considering the location of the failure surfaces and scarps of first and second generation. Failure surfaces for each stage of sliding were identified using software of limit equilibrium analysis. These surfaces have been developed through clayey-silt levels that result from the alteration of pyroclastic materials, showing the most unfavorable geotechnical parameters (minimal or residual values). Moreover, the presence of water is a triggering factor, since total or partial saturation of the materials is required. Finally, the landslide is an advancing type: as successive reactivations occur, the sliding masses are broken down into smaller ones moving towards the Tirajana ravine.

Effectiveness of Deep Drainage Wells as a Slope Stabilization Measure: The Reactivation of the Diezma Landslide (Southern Spain)

The effectiveness of the stabilization measures, particularly deep drainage wells, have been analysed in a complex landslide affecting a major motorway in Southern Spain. A complete failure analysis of this instability, named as the Diezma landslide, has been performed considering different steps: (1) The slope before and after the A-92 motorway construction; (2) The slope during the 2001 Diezma landslide; (3) The slope after the stabilisation measures; (4) The slope at the 2010 reactivation; (5) Possible future reactivation. The good performance of the drainage wells was verified successfully by means of electrical resistivity tomography cross-sections. The future stability of the Diezma landslide depends on the correct performance of the drainage systems after periods of heavy rain. In addition, the reactivation of the Diezma landslide is expected in the case that a low magnitude earthquake (Mw 4.0–5.0), relatively common in the study area, occurs close to the landslide location.

Geological and Structural Control of Earthquake-Induced Landslides in El Salvador

El Salvador is located at one of the most seismically active areas in Central America. It has suffered several destructive earthquakes during the past 100 year, which have caused severe damage and great loss of life, mainly due to the earthquake-triggered landslides. The triggered landslides are of different types, ranging from relatively shallow falls and slides in rock and debris landslides to large deep-seated landslides, being the latter the most damaging. A detailed analysis of historical and recent coseismic-landslides (failure mechanism, distribution and concentration), together with geological and geotechnical aspects of materials and location of main geological structures, has been carried out. The study shows that there is a great concentration of landslides in the central part of the country, especially near to the El Salvador Fault Zone (ESFZ) independent to the distance and origin of the earthquake. Despite the possible lithological and topographical amplification which are associated mainly to the unconsolidated pyroclastic deposits and the volcanic cones with steep slopes, here it seems that there has been a great ground motion independently to the origin of the earthquake and focal distance, due to local seismic wave deviation or guiding and seismic energy liberation along the fault zone.

Multiphase carbonate cementation related tofractures in the Upper Jurassic limestones, Maestrat Basin (Iberian Range, Spain)

Abstract In the western part of the Penyagolosa subbasin (Maestrat Basin, Spain), carbonate cementation occludes fractures and infills stylolites in Tithonian-Berriasian limestones. Field relationships, petrography, cathodoluminesence and geochemical analyses (microprobe, fluid inclusions, oxygen, carbon and strontium isotopes) of the carbonate cements show that the paragenetic sequence includes (A) calcite cements in echelon tension gashes (– 11.37 δ 18 O VPDB). (B) Scarce isolated rhombic dolomite replacement cement