ALBERT

Description: The studies of the 2004 and 2005 Sumatra earthquakes showed the presence of the segmentation boundary limiting the rupture areas offshore Northern Sumatra. Recent geophysical studies provide new insight on the structure of this boundary and the changes in the subduction processes around Northern Sumatra. In this study we present new model obtained from refraction/reflection seismic modeling, MCS data, and relocated seismicity. The comparison with the crustal scale profile located in the rupture area of the December 2004 Sumatra earthquake reveals principal differences in the structure of the accretionary complex, as well as in the structure of the forearc crust. The segmentation boundary is linked to the differences in the sediment supply at the trench and the variations in the Sumatra block crustal thickness. In the southern segment frontal prism is well developed and manifested in the clear trust faulting, it is separated from the accretionary prism by a pronounced splay fault, which are not clearly observed in the northern segment. The width of the accretionary complex is much narrower in the southern segment, while having similar depth extend. The Vp velocity analysis suggests that in the southern segment the Mentawai fault is active, while north of Simeulue it is not or less active. The crustal thickness of the Sumatra basement is increasing towards south, which influences the backstop geometry and the dip angle of the downgoing plate; resulting in the steeper subduction in the south.

Description: The coupled plate interface of subduction zones—commonly called the seismogenic zone—has been recognized as the origin of fatal earthquakes. A subset of the aftershock series of the great Antofagasta thrust-type event (1995 July 30; Mw=8.0) has been used to study the extent of the seismogenic zone in northern Chile. To achieve reliable and precise hypocentre locations we applied the concept of the minimum 1-D model, which incorporates iterative simultaneous inversion of velocity and hypocentre parameters. The minimum 1-D model is complemented by station corrections which are influenced by near-surface velocity heterogeneity and by the individual station elevations. By relocating mine blasts, which were not included in the inversion, we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth. A study of the resolution parameters ALE and DSPR documents the importance of offshore stations on location accuracy for offshore events. Based on precisely determined hypocentres we calculate a depth of 46 km for the lower limit of the seismogenic zone, which is in good agreement with previous studies for this area. For the upper limit we found a depth of 20 km. Our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies; the results are also in agreement with thermal studies for the Antofagasta forearc region.

Description: Combined on- and offshore wide-angle seismic data were collected in southern Costa Rica along a profile that extends from the Middle America Trench and crosses the Pacific continental margin, the isthmus, and reaches to the Caribbean coast. Closely spaced marine airgun sources were used in the Pacific, coincident with the preexisting seismic reflection line P-1600 recorded by SHELL Int. Four large explosive charges were detonated in boreholdes onshore. The combined interpretation of the near-vertical and wide-angle data provides a detailed crustal model. The profile is located on the northern flank of Cocos Ridge, where we have determined that the subducting oceanic plate has a thickness of 11 to 12 km. It dips down with an average angle of 17° and can be imaged to a depth of at least 35 km with the available data. Erosion of the margin wedge is suggested by a thick low velocity zone overlying the downgoing slab. Like elsewhere along the Middle America Trench, the body of rock underlying forearc and slope sediment has high velocities (〉4.0 km/s). At deeper levels (〉5 km), the crustal rocks along the profile generally show high velocities (6.0 to 6.6 km/s). To the southwest, these high velocities can be followed to the mid-shelf area. The inverted Térraba Basin contains intermediate velocity sediment (3.0 km/s) with a total thickness of about 2 km. The Limón Basin has up to 6 km of Tertiary and Quaternary sediment. An intracrustal discontinuity is found at a depth of about 18 km, but cannot be determined in detail due to the limited offset of the available data.

Description: Within Europe there are more than 380 Ocean Bottom Seismometers (OBS) distributed across 10 instrument parks in 6 countries. At least 120 of these OBS are wideband or broadband, over 260 can be deployed for at least 6 months at a time and 140 for at least one year. New parks are planned in two other European countries, which should add over 70 OBSs to this “fleet”. However, these parks are under the control of individual countries or universities and hence to date this has made it difficult to organize large-scale experiments, especially for seismologists without marine experience. There has recently been an initiative to coordinate the use of these distributed instruments and their data products, to encourage large-scale experiments, possibly with onshore and offshore components, by seismologists who have not necessarily used OBSs before. The ongoing or planned developments include: Helping scientists with marine-specific formalities such as ship requests; clearer explanations of the noise floors of OBS instrumentation; improved clarity of instrument pricing and availability; standardized data output formats and data validation; and archiving in established seismological data centers. These efforts should allow improved experiment design in scientifically interesting regions with an offshore component and an easier, clearer way to organize large-scale, multi-country experiments. We will present details of this initiative to help organize large-scale experiments, the particularities of OBS sensors and marine deployments, the available instrumentation and new developments.

Description: We present results of a seismic refraction experiment which determines the crustal and uppermantle structure of an oceanic core complex (OCC) and its conjugate side located south of the 5◦S ridge–transform intersection at the Mid-Atlantic Ridge. The core complex with a corrugated surface has been split by a change in location of active seafloor spreading, resulting in two massifs on either side of the current spreading axis. We applied a joint tomographic inversion of wide-angle reflected and refracted phases for five intersecting seismic profiles. The obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness. An abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall. The observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface. The thickness of the high-velocity body is constrained by PmP reflections to vary along and across axis between〈3 and 5 km. The thickest crust is associated with the central phase of detachment faulting at the higher-elevated northern portion of the massif. Beneath the breakaway of the OCC the crust is 2.5 km thick and reveals significantly lower velocities. This implies that the fault initially exhumed low-velocity material overlying the gabbro plutons. In contrast, crust formed at the conjugate side during OCC formation is characterized by an up to 2-km-thick seismic layer 2 overlying a 1.7-km-thick seismic layer 3. Obtained upper-mantle velocities range from 7.3 to 7.9 kms−1 and seem to increase with distance from the median valley. However, velocities of 7.3–7.5 kms−1 beneath the older portions of the OCC may derive from deep fluid circulation and related hydrothermal alteration, which may likely be facilitated by the subsequent rifting. Our velocity models reveal a strongly asymmetric velocity structure across the ridge axis, associated with the accretion of gabbros into the footwall of the detachment fault and uppercrustal portions concentrated at the conjugate side. Our results do not support a substantial increase in the axial ridge’s melt supply related to the final phase of detachment faulting. Hence, the footwall rifting at 5◦S may be a generic mechanism of detachment termination under very low melt conditions, as predicted by recent numerical models of Tucholke et al.