Great Earthquakes in Slow-Subduction, Low-Taper Margins

Abstract

The seismic hazard presented by slow subduction zones is not well known. While there is a widely accepted apparent relation between “fast-young plate” subduction and great earthquake generation (e.g., Chile, 1960), the seismic record indicates that slow subduction zones are also capable of generating mega-thrust earthquakes (M > 8.2). Available data on the recurrence interval for slow subduction margins, suggests that repeat times are longer than for more rapid convergence mar- gins (on the order of several hundred to a few thousand years). For several of these margins, however, no shallow dipping thrust earthquake focal mechanisms are observed and no mega-thrust earthquakes known either. Slow subduction zones (v ≤ 4 cm/year) are typically characterized by thick sedi- mentary sections on the incoming plate (2–6 km) and by a broad accretionary wedge (100–250 km). The “taper” of these accretionary wedges is mechanically related to the basal and internal friction and ranges from about 2° to 12°. Some wedges have extremely shallow mean surface and basal slopes (about 1–2° each, taper <4°) indicat- ing a very weak decollement layer. These include: Barbados Ridge, Makran, Hikurangi, Mediterranean Ridge, Calabria, Gibraltar/Cadiz, and Cascadia/Washington. Nankai, Sumatra and E. Alaska have slightly higher tapers of about 5–7°. Most of these low-taper wedges have very slow to slow convergence rates (0.5–4 cm/year). The presence of these wide accretionary wedges strongly affects the type and amount of deformation above the “up-dip limit” of the seismogenic zone. The ther- mally insulating effect of a wide and thick wedge of sediment produces a wide, shal- low transition zone (between the 100°C and 150°C isotherms) as well as a substantial (up to 80 km wide) region between this and the front of the wedge, where the amount and timing of deformation is poorly understood. Indeed, recent seismological data from Nankai indicate “very-low-frequency” shallow-thrust earthquakes beneath the accretionary wedge, long considered to be “aseismic,” underscoring the unusual mechanical behavior in the transition zone. As the rigidity of the high-porosity wedge sediments is low, for an earthquake of a given seismic moment, more co- seismic slip will occur and for a longer duration, than for a deeper earthquake in more consolidated units. Thus shallow earthquakes in the wedge are more effi cient at generating a strong tsunami. Many of the margins with very broad accretionary wedges have produced extremely strong earthquakes (M9) in the past, as well as giant tsunamis.