Resumen: Actividad sísmica de magnitud moderada (5.0 ≤ mb ≤ 5.8) ha venido ocurriendo, al suroeste de Puerto Quepos en la región del Pacífico central de Costa Rica, historicamente. Esta actividad sísmica se presenta a manera de secuencias en las cuales generalmente ocurren al menos dos eventos principales con sus patrones de réplicas respectivos, usualmente espaciados por uno o más días. La característica más importantes de estas secuencias sísmicas, tal como lo propusieran Güendel y McNally (1981), son los períodos de quietud inter-sísmica, de entre 2 y 4 años, que anteceden y preceden a dicha actividad. Durante la revisión e impresión del presente artículo ocurrieron de manera similar, dos nuevos eventos en la misma región. Esta nueva secuencia sísmica inició con un evento de magnitud mb=5.1 el día 10 de agosto de 1999 y 10 días después, el 20 de agosto se presentó el evento de mayor magnitud mb=5.7, Ms=6.7, Mw=7.0. Estos dos nuevos eventos sísmicos ocurren tras tres años de “quietud sísmica” que precedieron a los eventos ocurridos en agosto y setiembre de 1996 y que sirvieron como motivación del presente estudio.

Resumen: La brecha sísmica de Nicoya es un segmento de la zona de subducción en el noroeste de Costa Rica. Las últimas tres rupturas produjeron grandes sismos en 1853, 1900 y 1950. Las evidencias que indican la existencia de un fuerte acople sísmico en el segmento de Nicoya son: a) un bajo nivel de sismicidad de fondo; b) el cese repentino de la propagación de las réplicas de los sismos de 1990 y de 1992 justo en los bordes de la brecha sísmica de Nicoya; c) el rápido movimiento de la Península de Nicoya hacia el NE aunado a subsidencia de su costa oeste y levantamiento de la costa este, compatibles con el patrón de deformación elástica resultante de un acoplamiento a lo largo de la interfase por debajo de esa península. Nuevos resultados de investigaciones científicas desde el 2001 han permitido afinar aun más el potencial de esta brecha sísmica. Mantenemos una red de monitoreo geodinámico en y alrededor de la península de Nicoya para el registro de las deformaciones pre-, co- y post-sísmicas.

Abstract: At the subduction zone in northwestern Costa Rica , the seismogenic zone lies directly beneath the Nicoya Peninsula , allowing for near source seismic studies of earthquake activity. We located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the Nicoya Peninsula . Using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth, 10 km in the south to 20 km in the north, that occurs where the subducting oceanic crust changes from warmer Cocos-Nazca Spreading center (CNS) origin to colder East Pacific Rise (EPR) origin. We argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath Nicoya , Costa Rica ; subducting colder oceanic crust deepens the seismogenic updip limit. I NDEX T ERMS : 3015 Marine Geology and Geophysics: Heat flow (benthic) and hydrothermal processes; 7209 Seismology: Earthquake dynamics and mechanics; 7220 Seismology: Oceanic crust; 7230 Seismology: Seismicity and seismotectonics. Citation: Newman, A. V., S. Y. Schwartz, V. Gonzalez, H. R. DeShon, J. M. Protti, and L. M. Dorman, Alongstrike variability in the seismogenic zone below Nicoya Peninsula , Costa Rica , Geophys. Res. Lett. , 29 (20), 1977, doi:10.1029/ 2002GL015409, 2002.

Abstract:We derive a P-wave 1D-velocity model for Costa Rica that may serve for routine high-precision earthquake location and as initial reference model for 3D seismic tomography. The velocity inversions are performed using 822 well-locatable events together with 14774 P-wave observations obtained by merging routine travel time data from 10 335 earthquakes in the period 1984 to 1997 collected by the Universidad Nacional de Costa Rica (OVSICORI), and 3510 earthquakes in the period 1992 to 1998 collected by the Red Sismológica Nacional (RSN) in Costa Rica. Special care is taken during the merging process to reduce the number of errors in the data and, in particular, to update, correct, and complete the station parameter list. Consistency and quality are given priority over completeness of the resulting data set. The final data set for the period 1984 to 1998 in Costa Rica consists of 11 848 local events with 13 2331 P-wave and 86 018 S-wave observations.

Abstract Small local earthquakes from two aftershock sequences in Porto dos Gaúchos, Amazoncraton—Brazil, were used to estimate the coda wave attenuation in the frequency band of 1 to 24 Hz. The time-domain coda-decay method of a single backscattering model is employed to estimate frequency dependence of the quality factor (Qc) of coda waves modeled using Qc = Q0 f η, where Q0 is the coda quality factor at frequency of 1 Hz and η is the frequency parameter.We also used the independent frequency model approach (Morozov, Geophys J Int, 175:239–252, 2008), based in the temporal attenuation coefficient, χ( f ) instead of Q( f ), given by the equation χ( f )=γ + π f Qe , for the calculation of the geometrical attenuation (γ ) and effective attenuation (Q−1e ). Qc values have been computed at central
frequencies (and band) of 1.5 (1–2), 3.0 (2–4), 6.0(4–8), 9.0 (6–12), 12 (8–16), and 18 (12–24) Hz for five different datasets selected according to the geotectonic environment as well as the ability to sample shallow or deeper structures, particularly the sediments of the Parecis basin and the crystalline basement of the Amazon craton. For the Parecis basin Qc = (98 ± 12) f (1.14±0.08), for the surrounding shield Qc = (167 ± 46) f (1.03±0.04), and for the whole region of Porto dos Gaúchos Qc = (99 ± 19) f (1.17±0.02). Using the independent
frequency model, we found: for the cratonic zone, γ = 0.014 s−1, Q−1 e = 0.0001, ν ≈ 1.12; for the basin zone with sediments of ∼500 m,
γ = 0.031 s−1, Q−1 e = 0.0003, ν ≈ 1.27; and for the Parecis basin with sediments of ∼1,000 m, γ = 0.047 s−1, Q−1e= 0.0005, ν ≈ 1.42. Analysis of the attenuation factor (Qc) for different values of the geometrical spreading parameter (ν) indicated that an increase of ν generally causes an increase in Qc, both in the basin as well as in the craton. But the differences in the attenuation between different geological environments are maintained for different models of geometrical spreading. It was shown that the energy of coda waves is attenuated more strongly in the sediments, Qc = (78 ± 23) f (1.17±0.14) (in the deepest part of the basin), than in the basement, Qc = (167 ± 46) f (1.03±0.04) (in the craton). Thus, the coda wave analysis can contribute to studies of geological structures in the upper crust, as the average coda quality factor is dependent on the thickness of sedimentary layer.