Ground motions and source mechanisms of earthquakes in multiscales : microseismicity to macroseismicity

Abstract

This thesis focuses on the source mechanisms of Earthquakes of different scales, from the micro-seismicity in oil/gas fields in Oman, to moderate earthquakes in Kuwait, and finally to pico-seismicity, i.e. acoustic emission, in the laboratory. To investigate the source mechanisms as well as their uncertainties, a waveform-based Bayesian moment tensor inversion approach was developed and validated by synthetic tests. The Bayesian approach estimates the source parameters and the uncertainties by generating a posterior probability density function of the source parameters. The effects of location, velocity model, and error model of the data on the posterior prediction of the source parameters are discussed. This Bayesian moment tensor inversion method was first applied to the well-documented induced seismicity data in an oil/gas field in Oman. On Chapter 3, we move on to another mideast country - Kuwait. We conducted ground motion calculations in Kuwait due to regional large earthquakes and to local seismicity. We found that the regional earthquakes with low-frequency and long-duration surface waves were most likely to affect tall buildings, while the local smaller earthquakes are most likely to affect small and old structures constructed before the adoption of building codes. Using the Bayesian moment tensor inversion method, we studied the source mechanisms of local earthquakes in Kuwait. Historically, Kuwait has low local seismicity; however, in recent years the KNSN has monitored more and more local earthquakes. In 2015, two local earthquakes - Mw4.5 on 03/21/2015 and Mw4.1 on 08/18/2015 - have been recorded by both the Incorporated Research Institutions for Seismology (IRIS) and KNSN, and were widely felt by people in Kuwait. Most earthquakes in Kuwait occurred close to oil/gas fields. The earthquakes are generally small (Mw < 5) and are shallow with focal depths of about 2 to 8 km. We determined the location and source mechanism of these local earthquakes, with the uncertainties, using a Bayesian inversion method. Our results show that most likely these local earthquakes occurred on pre-existing faults and may have been triggered by oil field activities. In Kuwait, where oil fields are close to populated areas, these induced earthquakes could produce ground accelerations high enough to cause damage to local structures. Chapter 4 is devoted to the study of acoustic emissions during the rock fracturing experiments in the laboratory. The laboratory fracturing can produce pico-seismicity with magnitude as small as -7. Three event-detection and three location algorithms have been implemented to the acoustic emission (AE) data from the fracturing experiment of a cylindrical Berea sandstone sample (Diameter: 36.43 mm; Height: 76.7 mm). The first P-amplitude and waveform-based Bayesian moment tensor inversion algorithms have been applied to the AE data to study the source mechanisms of this fracturing related pico-seismicity. The location, and sensor calibration are discussed in the thesis. The main contribution from this thesis are: 1) Developing a waveform-based Bayesian moment tensor approach; 2) Understanding the source mechanisms of local earthquakes in Kuwait, and simulating the ground motion due to regional and local earthquakes in Kuwait; 3) Characterizing the laboratory-scale fractures using the fracturing related acoustic emission data.