[S-SS14] Active faults and paleoseismology

[SSS14-21] Trench survey excavated across the surface ruptures along the Huaytapallana Fault associated with the 1969 Earthquake

*Yoshiki Shirahama¹, Lorena Nicole Rosell Guevara², Anderson Rafael Palomino Tacuri², Carlos Benavente Escobar², Hisao Kondo³ (1.The University of Tokyo, 2.Geological, Mining and Metallurgical Institute, 3.National Institute of Advanced Industrial Science and Technology)

Keywords:Trench survey, Peru, Huaytapallana Fault, Surface rupture

Peru is a seismically active region due to the subduction of the Nazca Plate into the South American Plate. While studies have been conducted on large earthquakes in subduction zones, inland earthquakes have not been well studied. Inland earthquakes are essential for seismic hazard assessment because they occur near urban areas and can cause strong shaking and damage due to fault displacement. The Huaytapallana fault extends west of Huancayo City, about 200 km east of Lima City. This fault is located on the western flank of the Eastern Cordillera of the Central Andes of Peru. It is the epicenter of two earthquakes recorded in 1969: the Ms 5.7 earthquake on July 24th and the Ms 6.2 earthquake on October 1st. The earthquakes have produced surface ruptures, but their distribution is unknown because they are located at high elevation areas (>4000 m above sea level), making field investigations difficult. We reinterpreted the surface ruptures from high-resolution AW3D satellite images and reproduced 1-m grid DEMs. The distribution of the surface ruptures, about 21 km long with a 2.5-km gap, was revealed. The surface ruptures were reported to have displaced about 2 m vertically and without strike-slip in the northern part, about 1.8 m vertically, and with 0.7 m left-lateral displacement in the southern part. Although few tectonic landforms are along the surface ruptures, left-lateral displacement of lacustrine terrace scarps along a glacial lake was observed in the northern part.
We conducted a trenching survey to investigate paleo-earthquakes before the 1969 earthquake and to determine the seismic activity of the Huaytapallana fault. The trench, approximately 4 m wide, 20 m long, and 3 m deep, was excavated across the rupture at approximately 70 m south of the glacial lake, where left-lateral displacement was observed. Glacial outwash deposits and overlying lake sediments were observed on the trench wall, and several northeast uplift faults that cut and deformed the glacial outwash deposits were recognized. The most northeastern fault was nearly vertical, and a graben depression of the strata occurred near the fault. Two reverse thrust faults were found on the southwest side of the vertical fault. The combined vertical displacement of the fault zone is approximately 2 m, suggesting that the 1969 earthquake caused most of the deformation observed on the trench wall. On the uplift side, the outwash deposits were generally inclined to the northwest and unconformably overlain by lacustrine deposits, suggesting faulting activity before the deposition of the lake sediments. In this presentation, we discuss the results of trench observations and the activity of the Huaytapallana fault.