Challenges in Characterizing Low-Slip Rate Faults: Paleoseismic Case Study of the Late Quaternary Pajarito Fault System in the Rio Grande Rift, Los Alamos, New Mexico

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems
Type: Oral
Date: 4/28/2020
Time: 04:30 PM
Room: 240
Description: 

The Pajarito fault system (PFS) is a low slip rate (~0.1 mm/yr), 50-km-long seismogenic fault system that bounds the western margin of the Rio Grande Rift. The PFS crosses the edge of Los Alamos National Laboratory (LANL) in Los Alamos, NM and is the largest contributing seismic source to LANL. Thus, understanding its paleoseismic record is key to characterizing seismic hazard at the Lab. The PFS consists of four distinct faults from south to north: Pajarito (PF), Guaje Mountain (GMF), Rendija Canyon (RCF) and Santa Clara Canyon (SCF) faults. The PF is separated from faults to the north by a 4.5-km gap to the SCF and a 1 to 3-km-wide step to the RCF, characterized by horse-tail splay fault terminations and a change in dip polarity. Despite multiple paleoseismic studies on the PF, RCF and GMF over the past three decades it is still unclear if these faults have ruptured together or independently in the late Quaternary.

We performed comprehensive mapping, LiDAR data analyses, geochronology and paleoseismic trenching at multiple sites along the PF to refine past earthquake timing. Our findings suggest the most recent event on the PF occurred in the latest Holocene after ~1.6 to 2.3 ka B.P. One site on the PF records as many as four events in the past 74 ka, suggesting an average recurrence >10 ka. Complicating factors in evaluating both segmentation and earthquake recurrence of the PFS include geometric fault complexity, periods of erosion, low rates of deposition and a lack of datable material in pre-Holocene deposits. Paleoseismic trench observations indicate fault zone complexities resulted in distributed, small Holocene surface displacements that could not be differentiated by mapping alone. These factors suggest multiple investigative techniques are required to characterize complex low-slip faults and it is critical that seismic hazard evaluations consider these types of uncertainties when evaluating concepts of segmentation and earthquake recurrence.

Presenting Author: Robert Givler

Authors