S U M M A R YThe Pampean flat slab of central Chile and Argentina (30 • -32 • S) has strongly influenced Cenozoic tectonics in western Argentina, which contains both the thick-skinned, basementcored uplifts of the Sierras Pampeanas and the thin-skinned Andean Precordillera fold and thrust belt. In this region of South America, the Nazca Plate is subducting nearly horizontally beneath the South American Plate at ∼100 km depth. To gain a better understanding of the deeper structure of this region, including the transition from flat to 'normal' subduction to the south, three IRIS-PASSCAL arrays of broad-band seismic stations have been deployed in central Argentina. Using the dense SIEMBRA array, combined with the broader CHARGE and ESP arrays, the flat slab is imaged for the first time in 3-D detail using receiver function (RF) analysis. A distinct pair of RF arrivals consisting of a negative pulse that marks the top of the oceanic crust, followed by a positive pulse, which indicates the base of the oceanic crust, can be used to map the slab's structure. Depths to Moho and oceanic crustal thicknesses estimated from RF results provide new, more detailed regional maps. An improved depth to continental Moho map shows depths of more than 70 km in the main Cordillera and ∼50 km in the western Sierras Pampeanas, that shallow to ∼35 km in the eastern Sierras Pampeanas. Depth to Moho contours roughly follow terrane boundaries. Offshore, the hotspot seamount chain of the Juan Fernández Ridge (JFR) is thought to create overthickened oceanic crust, providing a mechanism for flat slab subduction. By comparing synthetic RFs, based on various structures, to the observed RF signal we determine that the thickness of the oceanic crust at the top of the slab averages at least ∼13-19 km, supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat. The overthickened region is broader than the area directly aligned with the path of the JFR, however, and indicates, along with the slab earthquake locations, that the flat slab area is wider than the JFR volcanic chain observed in the offshore bathymetry. Further, RFs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench-parallel faults. One explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted. Alternatively, it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region.

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[1] The Central American subduction zone in northern Costa Rica shows along-strike variations in both the incoming and overriding plates. By analyzing the subducting oceanic Moho (M1) and the upper plate Moho (M2) with receiver functions, we investigate the variability in the hydration state of the subducting Cocos Plate and the nature of crustal terranes within the overriding Caribbean Plate. We calculate high-quality P and PP wave receiver functions using broadband data of the Global Seismology Network; Geoscope Project; and the CRSEIZE, Pocosol, and Corisubmod experiments. In addition, we estimate the depth (H) and vertically averaged Vp/Vs (k) to Moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the H and k estimates. Our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the Nicoya Peninsula, as compared to a serpentinized oceanic mantle subducting beneath the northern part. In the Caribbean Plate, we describe the nature of the Mesquito, Nicoya, and Chorotega terranes by integrating new and published Vp/Vs estimates. Both the Nicoya and Chorotega terranes display high Vp/Vs (1.80-1.92) consistent with their oceanic character. In contrast, the oceanic Mesquito Terrane mostly displays lower Vp/Vs (1.62-1.80) more compatible with continental crust, which may indicate that subduction zone magmatism is modifying the crust to display continental character. Our estimates show that the deepest M2 ($42 km) is observed in the volcanic arc region whereas the shallowest M2 ($27-33 km) is observed in parts of the fore-arc and back-arc regions.

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ABSTRACT:The first relationship between Modified Mercalli Intensity (MMI) scale and the horizontal component of Peak Ground Acceleration (PGA) was developed for the Costa Rican region using regression analysis of 108 earthquakes (2.8 < Mw < 7.7) that occurred between 1983 and 2004. For each PGA instrumentally recorded, a single MMI value was assigned based on the geographic proximity to the PGA observation. A total of 330 values of both the largest PGA of the two horizontal components (PGA max ) and the average of the two horizontal components (PGA ave ) were associated to MMI values. The correlations obtained are: MMI=2.30log(PGA max )+0.92 and MMI=2.33log(PGA ave )+0.76 for MMI II-V and MMI=3.82log(PGA max )-1.78 and MMI=4.60log(PGA ave )-3.38 for MMI V-VII. Predicted PGA intervals for each intensity unit were proposed based on these equations. There is a fairly remarkable agreement between the PGA max vs. MMI found in this study and the correlation calculated by Wald et al. (1999a) for California. Keywords: Modified Mercalli Intensity, Peak Ground Acceleration, Regression Analysis, Macroseismic Information, Costa Rica. RESUMEN:La primera correlación entre la intensidad Mercalli Modificada (MMI) y la componente horizontal de la aceleración pico del suelo (PGA) fue desarrollada para Costa Rica a partir de la regresión de 108 sismos (2,8 < Mw < 7,7) que ocurrieron entre 1983 y 2004. Para cada PGA registrado instrumentalmente, un único valor de MMI fue asignado basado en la proximidad geográfica del PGA observado. Un total de 330 valores de la aceleración más alta entre las dos componentes horizontales (PGA max ) y del promedio de las dos componentes horizontales (PGA ave ) fue asociado con MMI. Las correlaciones obtenidas son: MMI=2,30log(PGA max )+0,92 y MMI=2,33log(PGA ave )+0,76 para MMI II-V y MMI=3,82log(PGA max )-1,78 y MMI=4,60log(PGA ave )-3,38 para MMI V-VII. Con base en estas ecuaciones fueron propuestos intervalos de PGA para cada unidad de intensidad. La correlación entre PGA max y MMI encontrada en este estudio es notablemente similar a la correlación calculada por Wald et al. (1999a) para California.