Discrepancy between GPS (5 yrs) and archaeoseismic (3 kyr) slip rate across the Ateret site (Dead Sea fault): Secular variations versus distributed slip
Abstract
The well known arctan profile for interseismic strike slip displacement (Savage & Burford, 1973) provides a surprisingly good fit to a Crusader castle wall offset during two consecutive co-seismic ruptures (~1.5 m: M~7.5 20.5.1202 AD; ~0.5 m: M~6.6 30.10.1759 AD). This theoretical profile serves us to (1) evaluate individual slip events in the preceding millennia and (2) assess distributed deformation for the discrepancy with GPS results. The perfect fit can be understood as plastic relaxation of elastic deformation acquired during slip to ~1 m depth. This is the scale for the thickness of debris above the basaltic bedrock. A similar depth fits three preceding slip events; the inferred trace of the buried fault is shifted laterally ~2 m. The arctan formula enables resolution of the tangled early slip events. Slip rate calculated from the archaeologic catalog corroborated by paleoseismic trenching, amounts to 3 mm/yr, compared with GPS: 3.5-6.5 mm/yr. In order to assess the possible role of secular variations in the rate of seismic release we analyze the seismite archive of Lake Lisan (late Pleistocene) - Dead Sea (Holocene). The last two millennia are associated with the largest rate of earthquakes during the last 50 kyr, so secular variations are not likely to be blamed. Distributed deformation is independently evident in rotation of the paleomagnetic vectors of ~ 1Myr old lava flows (Heimann & Ron, 1993). To assess the possible role of distributed deformation we use the locking depth model for a secondary parallel fault indicated by the tectonics. We find that the discrepancy between GPS and archaeologic slip rate of 0.5-3.0 mm/yr can readily be accommodated with the secondary fault buried at a few hundred m depth, causing the twisting of the overlying basalt. Mechanical stratification seems to control the depth of locking on either of the spatial and temporal scales in this study: on a km scale, basalt overlying soft sediments (inter-seismic twisting of basalt); on a meter scale, archaeological debris over basalt (co-seismic twisting of debris layer). LIDAR scan map (2m x7 m) of the Crusader wall and the arctan solution for locking-depth of 1 m.
Short (GPS) versus long (archaeo-paleoseism) term estimates of slip rate- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T33A2206A
- Keywords:
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- 1209 GEODESY AND GRAVITY / Tectonic deformation;
- 1242 GEODESY AND GRAVITY / Seismic cycle related deformations;
- 7250 SEISMOLOGY / Transform faults;
- 8106 TECTONOPHYSICS / Continental margins: transform