Julián García-Mayordomo

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.

Incorporating Descriptive Metadata into Seismic Source Zone Models for Seismic-Hazard Assessment: A Case Study of the Azores-West Iberian Region

In probabilistic seismic-hazard analysis (PSHA), seismic source zone (SSZ) models are widely used to account for the contribution to the hazard from earth- quakes not directly correlated with geological structures. Notwithstanding the impact of SSZ models in PSHA, the theoretical framework underlying SSZ models and the criteria used to delineate the SSZs are seldom explicitly stated and suitably docu- mented. In this paper, we propose a methodological framework to develop and docu- ment SSZ models, which includes (1) an assessment of the appropriate scale and degree of stationarity, (2) an assessment of seismicity catalog completeness-related issues, and (3) an evaluation and credibility ranking of physical criteria used to delin- eate the boundaries of the SSZs. We also emphasize the need for SSZ models to be supported by a comprehensive set of metadata documenting both the unique character- istics of each SSZ and the criteria used to delineate its boundaries. This procedure ensures that the uncertainties in the model can be properly addressed in the PSHA and that the model can be easily updated whenever new data are available. The pro- posed methodology is illustrated using the SSZ model developed for the Azores-West Iberian region in the context of the Seismic Hazard Harmonization in Europe project (project SHARE) and some of the most relevant SSZs are discussed in detail. Online Material: Tables describing characteristics and boundaries of the seismic source zones.

Short Note: SeriesBuster: a Matlab program to extract spatio-temporal series from an earthquake database

SeriesBuster is a Matlab program made to extract seismic series attending to the temporal and spatial relations among records from earthquake catalogs. The program allows to compare earthquake records making use of a number of variables: maximum distance, maximum and minimum temporal difference, focal depth, or any other feature marked by a flag, by means of a set of filters and functions (Fig. 1). SeriesBuster works comparing all possible relations in all records from the catalog in order to ensure that no possible seismic series is missed and that the produced ones are complete. The user has the option of eliminating from the output file those series composed by less events than a minimum established. SeriesBuster shows an interactive histogram of the distribution of the series versus the number of events in order to facilitate the user’s decision. SeriesBuster can work with either geographic or cartesian coordinates. The former must be in decimal degrees format. The output file contains two added columns. The first one refers to the serial day from a reference

Probabilistic Seismic-Hazard Assessment of the Canary Islands

This article presents the first probabilistic seismic hazard assessment of the Canary Islands. The Canary Islands form a volcanic archipelago located on the passive margin of the African plate, 100 km off West Africa. Active volcanism has taken place on the islands in historical times, commonly together with the occurrence of volcanic-related seismic sequences, some of them felt as high as I msk = X. In 1989 a notorious seismic sequence ( m bLg 5.2) took place along a submarine fault located between the islands of Gran Canaria and Tenerife, clearly representing the occurrence of tectonic seismicity in the archipelago as well. In this article we review the geology and tectonics of the islands as well as recent paleoseismological findings on south Tenerife. We also revise, complete, and decluster the historical and instrumental seismic catalog of the islands. Seismic-hazard analysis is then performed following the standard Cornell (1968) method, defining three seismogenic sources and selecting an appropriate ground-motion attenuation relationship from Hawaiian data. Two hazard maps of the archipelago have been developed for return periods of 475 and 950 yr, as well as hazard curves for the capital cities. Calculated peak ground acceleration values at Santa Cruz de Tenerife and Las Palmas de Gran Canaria are 0.06 and 0.08 g , and 0.05 and 0.07 g , for the 475- and 950-yr return periods, respectively. Finally, we analyze the impact on hazard resulting from uncertainties associated with the seismogenic source model and the ground-motion attenuation relationship.

New insights on the seismogenic potential of the Eastern Betic Shear Zone (SE Iberia): Quaternary activity and paleoseismicity of the SW segment of the Carrascoy Fault Zone

The Carrascoy Fault (CAF) is one of the main active faults that form part of the Eastern Betic Shear Zone, a 450 km fault system that accommodates most of the convergence between the Eurasian (Iberia) and Nubian plates in the Betic Cordillera, south Spain. Although the CAF represents a major earthquake threat to the nearby City of Murcia, studies on its Quaternary tectonics and seismogenic potential are scarce to date. We present evidence that supports the division of the CAF into two overlapping segments with contrasting tectonic structure, Quaternary activity, and landform control: a SW segment, characterized by a broad fold-and-thrust zone similar to the forebergs defined in the Gobi-Altai region, and a NE segment, characterized by a sharp mountain front controlled by strike-slip tectonics. We attribute the differentiation into these two segments to the stresses associated with topography, which in turn is a consequence of the shortening component, at the middle Pleistocene, after circa 217.4 ka. For the SW segment we infer the occurrence of 9 to 11, Mw 6.7 paleoearthquakes in the last 30.2 kyr, and a slip rate of 0.37 ± 0.08 m/kyr. We date the occurrence of the last surface rupture event after 2750 B.P., and we estimate an average recurrence period of major events of 3.3 ± 0.7 kyr.

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.