Barry Parsons

The motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike-slip faults

Geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere. If the dominant forces acting on crustal blocks are tractions at their bases, then the long-term motion of each block will be given by the average velocity of the underlying lithosphere. Slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the South Island of New Zealand and Southern California are in good agreement with slip rates estimated geologically.

Crustal deformation during 1994–1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault

The positions of 115 ground marks in a 150 × 100 km area of oblique continental collision in the central Southern Alps, New Zealand, have been measured by Global Positioning System (GPS) two to four times between 1994 and 1998. Contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and Alpine fault. Across strike, more than 60% of the strain occurs within a band from 5 km NW to 20 km SE of the Alpine fault, but significant strain continues at least a further 60 km SE to near the edge of the Southern Alps foothills. Projections of the fault-parallel and fault-normal components of velocity onto an Alpine faultnormal profile show that about 85% of the NUVEL-1A model relative plate motion is observed within the GPS network. The surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50–70% of the relative plate rate in a region deeper than about 5–8 km on the down-dip extension of the SE dipping Alpine fault. Material shallower than this is behaving elastically and thus storing elastic strain in the region of the Alpine fault. The longer-wavelength displacements can be modeled either as distributed deformation beneath the Southern Alps, or by localization of elastic strain around the upper end of a discrete NW dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the SE boundary of the mountains if extrapolated to the surface. Strain determined from a small-scale survey network crossing the Alpine fault indicates no significant near-surface aseismic fault slip on the central Alpine fault over the past 25 years. Our results are consistent with independent geological evidence that the central section of the Alpine fault is capable of producing large to great earthquakes.

The 2009 L’Aquila earthquake (central Italy): A source mechanism and implications for seismic hazard

An edited version of this paper was published by AGU. Copyright (2009) American Geophysical Union.

A relation between the driving force and geoid anomaly associated with mid-ocean ridges

The driving force and geoid anomaly associated with the thermal structure of the oceanic plates are shown to be proportional to the first moment of the density structure with respect to depth and, hence, to each other. Both quantities exhibit the same functional dependence on age and this is given for two different thermal models. For the plate model the geoid anomaly and ridge driving force only increase slowly for ages greater than 40 m.y. in contrast to the half-space boundary layer model where a linear dependence on age holds for all ages. Isolation of the geoid anomaly related to the thermal structure of the plates would provide a direct measure of the magnitude of the ridge driving force.

Measurement of interseismic strain accumulation across the North Anatolian Fault by satellite radar interferometry

In recent years, interseismic crustal velocities and strains have been determined for a number of tectonically active areas through repeated measurements using the Global Positioning System. The terrain in such areas is often remote and difficult, and the density of GPS measurements relatively sparse. In principle, satellite radar interferometry can be used to make millimetric-precision measurements of surface displacement over large surface areas. In practice, the small crustal deformation signal is dominated over short time intervals by errors due to atmospheric, topographic and orbital effects. Here we show that these effects can be overcome by stacking multiple interferograms, after screening for atmospheric anomalies, effectively creating a new interferogram that covers a longer time interval. In this way, we have isolated a 70 km wide region of crustal deformation across the eastern end of the North Anatolian Fault, Turkey. The distribution of deformation is consistent with slip of 17-32 mm/yr below 5-33 km on the extension of the surface fault at depth. If the GPS determined slip rate of 24±1 mm/yr is accepted, the locking depth is constrained to 18±6 km.

The 2003 Bam (Iran) earthquake: Rupture of a blind strike-slip fault

A magnitude 6.5 earthquake devastated the town of Bam in southeast Iran on 26 December 2003. Surface displacements and decorrelation effects, mapped using Envisat radar data, reveal that over 2 m of slip occurred at depth on a fault that had not previously been identified. It is common for earthquakes to occur on blind faults which, despite their name, usually produce long-term surface effects by which their existence may be recognised. However, in this case there is a complete absence of morphological features associated with the seismogenic fault that destroyed Bam.

Geodetic strain of Greece in the interval 1892–1992

A first-order triangulation of Greece was carried out in the 1890s. Reoccupation, using Global Positioning System receivers, of 46 of the 93 original markers yielded estimates of the deformation of the region over the intervening interval. Broad regions have similar geodetic strain over the 100-year time span. Strain north of the Gulf of Korinthos is predominantly north-south extension, though with a significant east-west component. The central Peloponnisos is relatively stable, whereas the gulfs of the southern Peloponnisos are all characterized by uniaxial east-west extension. The seismic expression of strain for the entire region, calculated from the seismic moment tensors of earthquakes of M S ≥ 5.8 during the past 100 years, accounts for only 20-50% of the geodetically determined strain. At a scale of 50-100 km, the fraction of the strain that is expressed seismically varies much more than this range. In particular, whereas seismic strain in the eastern Gulf of Korinthos over the past 100 years is commensurate with the geodetic strain, there is rapid extension across the western Gulf of Korinthos (∼0.3 μstrain yr -1 ), with negligible seismic strain for the 100 year period prior to 1992. The Egion earthquake of June 1995 in the western Gulf of Korinthos released only a small proportion (≤20%) of the elastic strain that had accumulated in that region. The observed distribution of displacements can be explained by the relative rotation of two plates with a broad accommodation zone between them, but it is equally consistent with the deformation that would be expected of a sheet of fluid moving toward a low-pressure boundary at the Hellenic Trench. A simple calculation implies that if the region does behave as a fluid, then its effective viscosity is ∼10 22 ∼10 23 Pa s. Such viscosities are consistent with the deformation of a lithosphere obeying a rheological law similar to that obtained for olivine in the laboratory.

Triggered slip: Observations of the 17 August 1999 Izmit (Turkey) Earthquake using radar interferometry

We use Synthetic Aperture Radar interferometry (InSAR) to map the displacement field of the 17 August 1999 Izmit earthquake, which largely conforms to that predicted for an elastic upper crust. We determine the earthquake source parameters and show that slip continues farther west than the mapped fault ruptures. We also show that additional sub-surface displacements occurred on parallel strands of the North Anatolian Fault Zone. We argue that this was caused by changes in static stress accompanying the mainshock, or by the dynamic release of regional stresses.

Venus Tectonics: Initial Analysis from Magellan

Radar imaging and altimetry data from the Magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the Venus surface. The plains record a superposition of different episodes of deformation and volcanism; strain is both areally distributed and concentrated into zones of extension and shortening. The common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes. Ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening; the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression. Venus displays two geometrical patterns of concentrated lithospheric extension: quasi-circular coronae and broad rises with linear rift zones; both are sites of significant volcanism. No long, large-offset strike-slip faults have been observed, although limited local horizontal shear is accommodated across many zones of crustal shortening. In general, tectonic features on Venus are unlike those in Earths oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide, and separated by stronger and less deformed blocks hundreds of kilometers in width, as in actively deforming continental regions on Earth.

Geodetic estimate of seismic hazard in the Gulf of Korinthos

The recent 15 June 1995, M0 = 6.0 × 1018 N m, Aigion earthquake in the western Gulf of Korinthos has focussed attention on the seismic hazard of the region. Although there have been few large earthquakes in the region during this century, the historical record suggests that there may have been many large earthquakes there in the interval 1750–1900. We present geodetic data that give estimates of the rate of extension of the Gulf of Korinthos during this century and which suggest that less than half of the elastic strain in the central and western Gulf of Korinthos has been released by earthquakes during this century. In contrast, the seismic and geodetic strains in the eastern Gulf of Korinthos are in agreement with each other. If the discrepancy between seismic and geodetic strains in the western Gulf of Korinthos that has accumulated during this century is removed in earthquakes, the moment release will be equivalent to several Ms > 6.5 earthquakes.