Source characteristics of seismic and aseismic events across spatial and temporal regimes

• Main Author: Nanjundiah, Priyamvada
• Other Authors: Wei Shengji
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Science::Physics::Geophysics and geomagnetism
• Online Access: https://hdl.handle.net/10356/138081
• Institution: Nanyang Technological University

The source characteristics of slow and fast earthquakes can help us interpret the evolution of the mechanics that govern the seismic cycle. The variability of source characteristics can be observed along different tectonic regimes such as strike-slip faults vs megathrusts, across different time scales like earthquakes and slow-slip events. However, on a regional scale, the local tectonic setting can have a significant impact on these parameters as observed in Central Pamir region. In this thesis, I aim to study source parameters of seismic and aseismic events and the impact of the aforementioned spatial and temporal scales on these source parameters. I catalogued 109 slip distributions from 52 earthquakes and 17 slow-slip events on thrust and strike-slip faults in various tectonic settings around the world with slip distributions constrained by geodesy in combination with other data. The width, potency, and potency density of slow and fast earthquakes were determined based on static slip distributions. I define and derive potency density as a ratio of the average slip to the rupture radius, to use as a measure of anelastic deformation unbiased by assumptions regarding the elastic properties of the surrounding medium. From the analysis of the catalogued events, it is observed that strike-slip earthquakes had the highest potency densities amongst all the events while slow slip events(SSE) had the lowest potency densities. Among megathrust events, continental thrust earthquakes and Tsunami earthquakes showed much higher potency densities as well as centroid depths compared to their deeper subduction counterparts with SSEs being the deepest events. The tendency of rupture characteristics to vary with fault type and centroid depth provides a prognosis for future slow and fast earthquakes. To better understand the effect of regional tectonics on specific earthquakes, I focus on the 2015 Mw 7.2 Lake Sarez earthquake sequence that occurred in Tajikistan. The mainshock was the largest crustal event to occur in this region in the past century. The event showed three significant segments along strike that were constrained by pixel-offset tracking of Sentinel-1A SAR imagery. Joint inversion of geodetic and teleseismic waveform data indicated a unilateral rupture towards the northeast. With a rupture speed of ∼3 km/s, a significant portion of the seismic moment distributed at depths < 10 km. To better understand the regional stresses, I estimated the focal mechanisms of 59 aftershocks using regional broadband data. Approximately two-thirds of the aftershocks were strike-slip earthquakes with depths ranging from 10 to 22 km, consistent with the mainshock. The remaining aftershocks exhibited oblique-normal or thrust components, highlight- ing the role of smaller faults in accommodating both the east-west extensional and the north-south compressional tectonic loading. This event helped shed light on a previously unknown fault cutting through the Pamir Plateau. Along with an analysis of the recent seismicity in the Pamir region, I attempt to understand the large-scale tectonics of the Pamir region from a geomorphic and geologic perspective. In chapter 4, I present a detailed analysis of fault traces in and around the Pamir Plateau mapped on high-resolution satellite imagery. I focus on the major fault systems such as the Karakoram, the Main Pamir Thrust, the Darvaz fault, and the Chaman Fault systems and on how these fault systems interact with the Sarez Karakul fault. We also map river and moraine offsets which help constrain the location as well as the slip rate on the fault. Preliminary discrete element analysis of the region was able to reproduce the major fault systems along with the new fault mapped by us, thus highlighting the importance of the new Sarez-Karakul fault in accommodating the overall tectonic stresses of the region.