Interplate seismicity at the CRISP drilling site: The 2002 Mw 6.4 Osa Earthquake at the southeastern end of the Middle America Trench
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Abstract We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica, a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ∼80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a nonlinear probabilistic approach within both, a 1‐D and a 3‐D P wave velocity models. The main shock occurred ∼25 km from the trench and probably along the plate interface at 5–10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ∼6 km depth. The location and depth coincide with the plate boundary imaged in prestack depth‐migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.Keywords:
Seafloor Spreading
Slab
Slab window
Transform fault
Seafloor Spreading
Pacific Plate
Convergent boundary
Seismic array
North American Plate
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Moment magnitude scale
Tsunami earthquake
Seismic moment
Thrust fault
Slow earthquake
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Based on the analyses of grouped activities of deep-focus (m b≥6.0) and shallow-focus (M S≥5.0) earthquakes in Northeast China, the spatial and temporal correlation between the deep-focus earthquake group and shallow-focus earthquake group was studied. The study was focused mainly on the characteristics of earthquake distribution along the collision zone between the west Pacific plate and Eurasian plate, as well as its relation to the form of the west Pacific subduction zone. Moreover, emphasis was also laid on the analyses of the effect of the west Pacific plate on the seismicity in Eurasian plate. The results show that in the region where the west Pacific plate subducts at low angle, the seismicity along the plate collision zone is very strong, and the effect of plate subduction on the seismicity in Eurasian continent is most significant. In this condition, the subduction zone is subjected to relatively strong compressive stress. However, in the region where the west Pacific plate subducts at high angle, the seismicity along the plate collision zone becomes weaker, and the effect of plate subduction to the seismic activity in Eurasian continent becomes less significant. In this condition, the tensile stress produced by the subduction zone at depth is enhanced. We propose, therefore, that the seismic activity of Northeast China will enter the active period of shallow earthquake group in forthcoming 10 years. In this period, 6 earthquakes of M S≥5.0 may occur in this region. It is emphasized that the monitoring and prediction of seismic activity in Northeast China should be enhanced.
Eurasian Plate
Pacific Plate
Focal mechanism
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Abstract The M w 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find ~3–5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within ~6 km of the deformation front. After the M w 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.
Seafloor Spreading
Tsunami earthquake
Accretionary wedge
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Remotely triggered earthquakes
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Slab
North American Plate
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Abstract The North China Seismic Region is not a typical intraplate seismic region. Not only does it show a pervasive neotectonic influence of global plate geodynamics, but it also displays an atypical seismic activity rate, especially for strong earthquakes, for a region far from plate boundaries. It is obvious that the seismicity is related to the active seismotectonics and geodynamics of the deforming interior of a continental plate. The features of the seismotectonic zones and the tectonic setting of magnitude 6.0 or greater earthquakes in the region are discussed here. We conclude that strong earthquakes occur mainly in places where NE-trending main and NW-trending auxiliary tectonic zones intersect. Three types of mechanisms are suggested which differ according to the characteristics of different seismotectonic zones and which relate to different shallow and deep structures. These observed types of strong earthquakes in North China may represent the main categories of continental intraplate seismicity.
geodynamics
Seismotectonics
Eurasian Plate
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Abstract The 2004 Sumatra-Andaman earthquake of M w 9.3 occurred in a region where a giant earthquake seemed unlikely from the point of view of tectonics. This clearly implies that our current understanding of strain accumulation processes of large earthquakes at subduction zones needs to be reexamined. The Ryukyu subduction zone is one such zone since no large earthquake has been anticipated there for reasons similar those pertaining to the Sumatra-Andaman arc. Based on our analysis of historical earthquakes, plate motion, back-arc spreading, and GPS observation along the Ryukyu trench, we highly recommend monitoring seafloor crustal deformation along this trench to clarify whether a large earthquake ( M w >8) could potentially occur there in the future.
Seafloor Spreading
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