Seismic structure of the Nicaragua convergent margin in the area of the 1992 tsunamigenic slow earthquake from wide-angle (WAS) and multichannel seismic (MCS) data.

Prada, Manel, Melendez, Adria, Sallares, Valenti, Ranero, Cesar, McIntosh, Kirk D. and Grevemeyer, Ingo (2010) Seismic structure of the Nicaragua convergent margin in the area of the 1992 tsunamigenic slow earthquake from wide-angle (WAS) and multichannel seismic (MCS) data. [Talk] In: AGU Fall Meeting 2010. , 13.-17.12.2010, San Francisco, USA .

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We present two 2D models of the Nicaragua convergent margin obtained by travel-time inversion of WAS data recorded along perpendicularly intersected profiles. Profile NIC120 (NW-SE) is located on the continental slope parallel to the trench and it was acquired during the cruise EW-0005 (2000). It is 190 km long and includes 12 Ocean Bottom Hydrophones (OBH). The other profile (NE-SW), recorded in 28 OBHs and 7 land stations, is ~280 km long and perpendicular to the trench. This profile is the sum of two profiles acquired during the cruise EW-0005 and the cruise SO173-1 (2003). For the travel-time inversion we followed a top-to-bottom layer stripping strategy aimed to determine the position of a single reflector and the velocity distribution above it. This process allows us to keep velocity contrasts across sharp geological interfaces. Subsequently, a Monte Carlo-based mean deviation analysis was performed to estimate the model parameter uncertainty. The velocity model for the NE-SW profile shows three different areas in the overriding plate: the sedimentary cover on top of the basement which in turn is divided into western and eastern bodies. Upper continental mantle velocities are found at ~10 km below a >5 km thick sedimentary basin, which indicates the presence of very thin (5-6 km) crust that thickens to ~20 km toward the trench. The mantle wedge shows relatively low seismic velocity (7.0-7.5 km/s), indicating a high degree of serpentinization. Under the lower slope, there is a low velocity zone which might be an indication of fluids escaping from subducted sediments and a higher degree of fracturing. The interplate reflector fits well with that of coincident MCS data, except for a segment under the low velocity zone. The misfit could be explained by anisotropy due to fluids and fractures. In the model of the NE-SW profile shows the sedimentary cover is ~4 km thick with velocities between 2 km/s and 3.5 km/s and its bottom boundary fits well with coincident MCS data. Below, the basement is ~12 km thick with velocities between 4 km/s and 6.5 km/s and has a strong lateral variation coindicing with the chaotic facies of MCS profile indicating the presence of fractures and fluids. The depth of the interplate reflector increases from SE to NW and matches well that of coincident MCS data, again with a certain misfit. Various matching features are observed at the intersection of the two models. Firstly, the depth of the interplate is ~17 km. Secondly, the thickness of the sedimentary layer is ~4 km with very similar velocity distributions. Finally, the low velocity zone appears in both models with again similar velocity values. Eventually, the velocity models help to understand the geological structures involved in the 1992 earthquake. The hypocenter is located at the base of the overriding plate and its aftershocks occured all along the interplate. The shallow rupture explains the huge tsunami, while the aftershocks in the serpentinized mantle indicate that nucleation could be possible in such conditions.

Document Type: Conference or Workshop Item (Talk)
Keywords: Nicaragua convergent margin, seismic structure, subduction zone processes
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Date Deposited: 02 Sep 2016 07:23
Last Modified: 02 Sep 2016 07:23

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