Seismotectonic analysis of active detachment faulting at the TAG segment of the Mid- Atlantic Ridge, 26°N
Abstract
Detachment faulting appears to accommodate a high percentage of extension on slow-spreading ridges. Detachments exhume lower crustal and mantle rocks to the seafloor, and are capable of providing long-term, high-permeability pathways for high-temperature hydrothermal circulation. Recent results have elucidated the morphological characteristics of detachment faults, but the nature of extension on these fault systems is not well understood. Here we present a detailed analysis of the space-time behavior of 20,730 well-constrained microearthquakes observed during a 245-day period at the TAG segment of the Mid-Atlantic Ridge (26°N). We observe a high, continuous rate of seismicity throughout the deployment that lacks the mainshock-aftershock sequences characteristic of most active faults. Frequency-magnitude analysis yields a b-value of ~1.5 long the detachment, which is intermediate between standard values for tectonic (~0.8-1.2) and volcanic (~1.8-2.0) environments. Clustering analyses using waveform cross- correlation techniques to quantify similarity suggest that seismicity is characterized by repeated slip on small fault patches. The recurrence interval between earthquakes within each patch ranges from 1 to 3 days. The moment release rate for detachment fault earthquakes observed during our study is roughly 5×1021 dyn-cm/yr, which is approximately an order of magnitude lower than the rate observed during a previous 3-week microearthquake survey from the same area in 1985, indicating that seismic slip rates are highly variable on decadal time scales. Both of these moment rates are orders of magnitude less than the nominal rate expected based on a simple model of extension (~1024 dyn-cm/yr), indicating that either slip along the detachment is predominately aseismic, or extension rates during both observation periods were significantly less than the long-term geological average. The peculiar nature of seismicity on the detachment fault (high-rates of small events without mainshock-aftershock sequences) may result from the composition (e.g., altered mafic/ultra-mafic rocks) or the stress state (e.g., high pore pressure) of the fault surface.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T41D..06D
- Keywords:
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- 3025 Marine seismics (0935;
- 7294);
- 3035 Midocean ridge processes;
- 3075 Submarine tectonics and volcanism;
- 7245 Mid-ocean ridges;
- 8004 Dynamics and mechanics of faulting (8118)