The Holocene Paleoseismic History of a Complex Normal Fault System: The Pajarito Fault System Near Los Alamos, New Mexico and Its Implications for Seismic Hazard

Session: Exploring Rupture Dynamics and Seismic Wave Propagation Along Complex Fault Systems [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The Pajarito fault system (PFS) is a 50-km-long seismogenic fault system forming the active western margin of the Rio Grande rift in the vicinity of Los Alamos in north-central New Mexico. The PFS has a complex expression and includes four distinct faults: the main Pajarito (PF), Rendija Canyon (RCF), Guaje Mountain (GMF) and Sawyer Canyon (SCF) faults. LiDAR analyses and geologic mapping show the surficial trace of the PF is variably expressed as a faulted monocline and is separated from the RCF, GMF and SCF by a 1 to 4-km-wide gap. Within this gap, the PF is thought to terminate in a series of horse-tail fault splays and changes in dip polarity to strands to the north.

Despite numerous paleoseismic investigations on segments of the PFS over the past three decades, analyses of available evidence from studies performed between 1990-2003 have yielded non-definitive conclusions of its late Quaternary rupture history. Summarizing a selection of paleoseismic studies, McCalpin (2005) suggests evidence on the main PF for a single Holocene event approximately 1.4 - 2.2 ka. Alternatively, Lewis et al. (2009) examined previous paleoseismic studies on the PF, RCF and GMF and suggests up to three separate Holocene events across all PFS segments (PF: 1.4-2.2 ka; RCF: ~4-~7 ka; GMF; >8.1 ka). However, no single trench showed evidence for multiple Holocene events. We present data from modern paleoseismic studies across the PF conducted between 2016-2018 that suggest the MRE on the PF occurred between 1.6 – 2.3 ka. These studies do not provide evidence for events on the main PF that temporally overlap with interpreted events on the RCF and GMF. We explore the possibility that the PFS is more complex than previously understood and could behave in different ways than those considered by past studies. Since most slip events observed on the PFS are relatively small (5-25 cm), we explore whether slip could be accommodated via mechanisms other than large-magnitude seismic events.

Presenting Author: Emily Schultz-Fellenz

Authors