Thomas Theunissen

University of Bergen | UiB · Department of Earth Science

The thick late syn‐ to early post‐rift shallow water evaporites in the most distal part of wide rifted margins is paradoxical with the deep depression at crustal breakup time predicted by isostatically compensated lithospheric thinning. Elevation of the distal margin and water depth during deposition of the late syn‐rift evaporites in the central S...

Continental rifted margins are often associated with widespread, thick evaporite (i.e., salt) deposits and pronounced salt tectonics. The majority of salt basins formed during the latest stages of rifting, prior to continental breakup. We use 2D thermo‐mechanical finite element modeling of lithospheric extension to investigate the interplay between...

Rifted margins are often associated with widespread and thick evaporite (salt) deposits and pronounced salt tectonics. The largest salt basins formed during the latest stages of rifting, immediately prior to continental breakup. Salt tectonics along these rifted margins commonly exhibit structural domains characterized by gravity‐driven updip exten...

The choice of crustal and mantle densities in numerical geodynamic models is usually based on convention. The isostatic component of the topography is not calibrated to fit observations resulting in not very well constrained elevations. The density distribution on Earth is not easy to constrain because it involves multiple variables (temperature, p...

The transition zone from continental crust to the mature mid-ocean ridge spreading center of the Iberia-Newfoundland magma-poor rifted margins is mostly composed of exhumed mantle characterized by highs and domes with varying elevation, spacing and shape. The mechanism controlling strain localization and fault migration explaining the geometry of t...

Tectonics and climate-driven surface processes govern the evolution of Earth’s surface topography. Topographic change in turn influences lithospheric deformation, but the elementary scale at which this feedback can be effective is unclear. Here we show that it operates in a single weather-driven erosion event. In 2009, typhoon Morakot delivered ~ 3...

Here we present high‐resolution 2‐D coupled tectonic‐surface processes modeling of extensional basin formation. We focus on understanding feedbacks between erosion and deposition and tectonics during rift and passive margin formation. We test the combined effects of crustal rheology and varying surface process efficiency on structural style of rift...

The largest compilation of seismological and gravimetry data is combined in a sequential inversion to shed light on unresolved crustal structures in Northern Taiwan and Southernmost Ryukyu area. Our density and seismic velocity models reveal a NW-SE trending Transfer Zone (TZ) between 23.25 and 24°N. This primary TZ delimits major changes in the Eu...

The largest compilation of seismological and gravimetry data are combined in a sequential inversion to shed light on unresolved crustal structures in Northern Taiwan and Southernmost Ryukyu area. Our density and seismic velocity models reveal a NW–SE trending Transfer Zone (TZ) between 23.25◦N and 24◦N. This primary TZ delimits major changes in the...

Local seismic networks are usually designed so that earthquakes are located inside them (primary azimuthal gap <<180°) and close to the seismic stations (0 - 100 km). With these local or near-regional networks (0 − 5°), many seismological observatories still routinely locate earthquakes using 1-D velocity models. Moving towards 3-D location algorit...

Subduction mega-thrust earthquakes in the SW Ryukyu trench pose a seismic and tsunami hazard. One of the objectives of this study is to estimate the downdip width of the seismogenic zone using numerical modeling to determine the temperature distribution along the plate interface. However, this approach depends strongly on the thermal parameters of...

We analyze in this study a new set of marine data including 3D local tomography, 1992–2008 relocated earthquakes and two recent multichannel seismic lines to characterize the deformation style in the collision area offshore east Taiwan. We have mapped in detail the Mohos of the converging plates as well as the subduction interface with a resolution...

The Pyrenean orogenic wedge is the consequence of the collision between the Iberian microplate and the southwesternmost part of the Eurasian plate from 55 to 25 Ma (Eocene to Oligocene). The shortening began since Late Cretaceous, about 100 My ago, leading to about 75 km from the west to 125 km to the east of continental crustal shortening. Before...

To improve earthquake location, we create a 3-D a priori P-wave velocity model (3-DVM) that approximates the large velocity variations of the Ecuadorian subduction system. The 3-DVM is constructed from the integration of geophysical and geological data that depend on the structural geometry and velocity properties of the crust and the upper mantle....

An active seismic experiment has been conducted across the southern Ryukyu margin east of Taiwan over the whole trench-arc-backarc system in May 2009. Twenty-four ocean bottom seismometers (OBS) were deployed from the Ryukyu trench to the southern Okinawa trough over the Ryukyu arc and forearc. Wide angle seismic data were recorded by the OBS array...

We present an improvement of the maximum intersection (MAXI) method in which absolute earthquake location is defined by the maximum intersection number of hyperbolic equal differential time (EDT) volumes (one EDT volume being described as all grid nodes satisfying the arrival-time differences between two stations, a tolerance value known as TERR)....

The southernmost part of the Ryukyu subduction, where the Philippine Sea Plate is subducting under the Eurasian Plate, is known to be a very seismically active region of transition from a north-dipping subduction along the Ryukyu subduction to an ~ SE–NW collision along the Taiwanese orogenic wedge. In this paper, we will focus on the Ryukyu forear...

The southern termination of the Ryukyu arc-trench system underwent a complex polyphased and extremely rapid tectonic evolution during the last 5 to 8 My. At first, the relative motion of the Philippine Sea plate (PSP) has changed about 5 My ago from a northward to a northwestward motion relative to Eurasia. Secondly, the Ryukyu trench has propagate...

The purpose of this work is to specify the deformation offshore East of Taiwan by characterizing major active faults in particular at the level of the Ryukyu forearc. The major problem associated with the study of this region is that earthquakes are located outside the permanent networks. Consequently, this led to some unprecise earthquake location...

The Ryukyu subduction is known to generate very few earthquakes in its central segment contrarily to its two extremities. We focus in this study on the southernmost part of the Ryukyu subduction zone offshore east Taiwan. Our first task was to build a homogeneous earthquake catalogue for the period 1900–2007. The new catalogue provides homogenized...

In Ecuador, the Nazca plate is subducting beneath the North Andean Block. This subduction triggered, during the last century, 4 major earthquakes of magnitude greater than 7.7. Between 1994 and 2007, the Geophysical Institute (Escuela National Politecnica, Quito) recorded about 40 000 events in whole Ecuador ranging from Mb 1.5 to 6.9. Unfortunatel...

Taiwan is one of only a few sites of arc-continent collision and as a result experiences a great number of earthquakes every year, including large and destructive earthquakes with a high recurrence rate. One of the main aims of the TAIGER project are determination of the nature of the earthquakes around Taiwan and the assessment of earthquake hazar...

The occurrence of a M9 earthquake off Sumatra in 2004 in a region considered as sustaining moderate stress in the upper plate has deeply « shaken » the geodynamics community. The Ryukyu arc-trench system that extends from SW Japan to Taiwan exhibits similarities with the Sumatra-Andaman arc-trench system. During the last century, the Ryukyu subduct...

In the framework of the "Active tectonics and seismic hazard in Taiwan" (ACTS ANR) project, we perform a three-dimensional delay-time passive tomography obtained from a large seismological dataset in order to image the crustal structures and active deformation zones that may accommodate the relative convergence in the Ryukyu forearc area. The catal...

Quality of absolute earthquake location depends on the available velocity model (i.e. the ray path estimate), the quality of arrival time pickings, the seismic array geometry and the location algorithm. Although the deployment of Ocean Bottom Seismometers allows to improve the azimuthal coverage in subduction zones, the well resolved earthquake loc...

The Ryukyu Subduction is known to be weakly coupled in its central segment contrarily to its two extremities, i.e. off SW Japan in the North and close to Taiwan in the South. On Eastern Taiwan, the relative convergence between the Philippine Sea Plate (PSP) and the Eurasian Plate is mainly accommodated onshore along the Longitudinal Valley Fault sy...