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Deep rock damage in the San Andreas Fault revealed by P- and S-type fault-zone-guided waves

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,
DOI: 10.1144/SP359.3

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Abstract

Damage to fault-zone rocks during fault slip results in the formation of a channel of low seismic-wave velocities. Within such channels guided seismic waves, denoted by Fg, can propagate. Here we show with core samples, well logs and Fg-waves that such a channel is crossed by the SAFOD (San Andreas Fault Observatory at Depth) borehole at a depth of 2.7 km near Parkfield, California, USA. This laterally extensive channel extends downwards to at least half way through the seismogenic crust, more than about 7 km. The channel supports not only the previously recognized Love-type- (FL) and Rayleigh-type- (FR) guided waves, but also a new fault-guided wave, which we name FF. As recorded 2.7 km underground, FF is normally dispersed, ends in an Airy phase, and arrives between the P- and S-waves. Modelling shows that FF travels as a leaky mode within the core of the fault zone. Combined with the drill core samples, well logs and the two other types of guided waves, FF at SAFOD reveals a zone of profound, deep, rock damage. Originating from damage accumulated over the recent history of fault movement, we suggest it is maintained either by fracturing near the slip surface of earthquakes, such as the 1857 Fort Tejon M 7.9, or is an unexplained part of the fault-creep process known to be active at this site.

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Additional Publication Details

Publication type:
Book chapter
Publication Subtype:
Book Chapter
Title:
Deep rock damage in the San Andreas Fault revealed by P- and S-type fault-zone-guided waves
ISBN:
9781862393370
DOI:
10.1144/SP359.3
Volume
359
Year Published:
2011
Language:
English
Publisher:
Geological Society of London
Contributing office(s):
Earthquake Science Center
Description:
15 p.
Larger Work Type:
Book
Larger Work Subtype:
Other Government Series
Larger Work Title:
Geology of the earthquake source: a volume in honour of Rick Sibson
First page:
39
Last page:
53
Number of Pages:
15
Country:
United States
State:
California
Other Geospatial:
San Andreas Fault