Wilfredo Rojas

The Limón, Costa Rica earthquake of April 22, 1991: Back arc thrusting and collisional tectonics in a subduction environment

On April 22, 1991, a large earthquake (Mw = 7.7) occurred along the Caribbean coast of Costa Rica and western Panama. The rupture area of the fault mapped from the aftershocks is 45×85 km2;. The distribution of aftershocks and the local geological record suggest that faulting occurred on a blind thrust sheet that shallows toward the northeast. Uplift of the Caribbean coast ranging from 1.5 m near Puerto Limon and decreasing gradually toward the southeast was observed along the Caribbean. Northwest of Puerto Limon no significant coastal uplift was observed. This observation agrees with the aftershock data suggesting the rupture did not extend to the northwest of this location. The Limon earthquake also triggered aftershocks on secondary faults in the crust. These events are apparently associated with a family of imbricate thrust and strike-slip faults that lie in the eastern piedmont of the Talamanca Cordillera. The historical seismicity indicates that the Caribbean coast has been the site of several historical earthquakes with magnitudes greater than 7.0. On April 26, 1916, another earthquake (Ms = 6.9) took place in the same region. Summing the scalar seismic moment release along the Caribbean coast, the average rate of slip is approximately 0.8 cm/yr, compared with a value of 0.4 to 0.8 cm/yr along the Pacific subduction zone, depending on the estimated width of the seismogenic zone. Therefore a large fraction of the relative plate motion between the Cocos and Caribbean plates (9.8 cm/yr) appears to be taken up by crustal deformation in the back arc. The tectonic regime in the area appears to be dominated by the collision of the buoyant Cocos ridge with the subduction zone. The absence of a Wadati-Benioff zone where the Cocos ridge collides with the trench suggests the slab does not subduct beneath the Osa Peninsula; this is supported by the Pliocene gap of volcanism present in Costa Rica. Thus the predicted relative motion between the Cocos and Caribbean plates appears to be absorbed by a low rate of seismic moment release in the forearc and by a broad zone of active crustal shortening and underthrusting in the back arc. This type of tectonic deformation resembles more a collisional regime than a typical subduction zone environment.

A New Evaluation of Seismic Hazard for the Central America Region

A new evaluation of seismic hazard in the Central America region has been carried out, in the frame of the cooperation project RESIS II, financed by the Norway Cooperation Agency (NORAD). Different experts in seismic hazard from Costa Rica, Guatemala, Nicaragua , El Salvador, Norway and Spain participated in the study, which was aimed at obtaining results suitable for seismic design purposes. The analysis started with an exhaustive revision of the seismic catalogues of each country from which a global catalogue for CA has been configured and homogenised at moment magnitude, Mw. Seismotectonic models proposed for the region were revised and a regional zonation was proposed, taking into account seismotectonic data, seismicity, focal mechanisms, GPS observations and other evidences useful for defining seismic sources. In parallel, attenuation models for subduction and volcanic crustal zones were revised and the more suitable models were calibrated with strong motion data. Taking the previous inputs, the seismic hazard analysis was developed in terms of peak ground acceleration, PGA and spectral accelerations SA (T) for periods of 0.1, 0.5, 1 and 2 s, through the PSHA methodology (Probabilistic Seismic Hazard Assessment). As a result, different hazard maps were obtained for the quoted parameters, together with Uniform Hazard Spectra (UHS) in the main populations of Central America. This is the first study developed at regional scale after the last earthquakes that have occurred in the region and as a result the new generation of maps will be useful in the revision of seismic codes of the area.

El Sistema de Falla Navarro: desplazamientos izquierdos a lo largo del Cinturón Deformado del Centro de Costa Rica

In this study the Navarro fault system is defined based on geologic, geomorphologic, and seismological observations. This fault system is located between the northern slopes of the Talamanca cordillera and the southern and southeastern slopes of the Irazu and Turrialba volcanoes and is characterized by left-lateral displacements. The Navarro fault system trends between east-west and northeast-southwest and includes the following fault segments: Tarrazu, Navarro, Cachi, Urasca, Paraiso, and Maravilla. There are two transtensional structures along the fault system: the step over Estrella and the Ujarras tectonic depression. Earthquakes during 1973-2015 were relocated illuminating the fault segments of the southwest portion of the system. The focal mechanisms calculated mainly correspond to strike-slip faults in agreement with morphotectonic observations.

La falla Tobosi: Fuente del enjambre sísmico de Tobosi del 2011-2012 en el centro de Costa Rica

The Central Costa Rica Deformed Belt (CCRDB) is an ~100-km broad zone of deformation that marks the boundary between the Caribbean Plate and the Panama Microplate. From December 2011 to February 2012, an earthquake swarm took place in a portion of the CCRDB, near the town of Tobosi, 7 km southwest of the city of Cartago. In this study, data recorded by the National Seismological Network of Costa Rica (RSN: UCR-ICE) is used to relocate the earthquakes and calculate their focal mechanisms. Additionally, the tectonic geomorphology of the region is analyzed. The results show a transtension structure near the town of Tobosi, which comprises at least three faults, named: the Tobosi, Tablon, and Alumbre faults. It was found that the Tobosi fault is an active left-lateral strike-slip fault with a normal component and was the source of the Tobosi earthquake swarm.