High Resolution Studies of the Earthquake Source

Trustees Of Boston University, Boston

Investigators

Abstract

The aim of this work is to obtain higher resolution, more precise source parameters of small earthquakes than are currently available. To accomplish this, the highest quality existing data are being analyzed using techniques that compare closely located earthquakes to improve the precision of both relative and absolute source parameters. Small earthquakes do not in themselves present a major natural hazard, but they provide much of our understanding of the source processes of larger damaging earthquakes. Many models of earthquake nucleation and rupture dynamics, and earthquake triggering require assumptions about small-scale behavior, for example, whether there is a minimum sized earthquake or nucleation patch. Observations to constrain these parameters are therefore critical. To date, studies using shallow and deep borehole recordings containing high frequency signals have shown that the static stress drop is constant down to the smallest earthquakes we can measure, but the scatter and uncertainty in the individual measurements are large. Measurements of seismic energy, which are fundamental to understanding the rupture dynamics have even larger uncertainties. These uncertainties severely limit the use of such observations to constrain models of the earthquake source process. In this study, new techniques are used that take advantage of closely-located earthquakes to obtain significantly more precise source parameters of small earthquakes than in earlier studies. The data analyzed first are from the deep borehole recording at Cajon Pass, California where local earthquakes were recorded with seismometers between 1 and 3 km depth over a total of four years. These seismograms remain the cleanest, least attenuated recordings of small earthquakes available. In previous work source parameters were estimated for some of the earthquakes recorded at 2.5 km depth, simply to address long standing scaling controversies. Since then, the importance of more precise measurements of energy and stress drops, as well as quantification of the onset of the seismograms has become clear for solving fundamental questions about earthquake rupture processes. Earthquakes recorded by the borehole network at Parkfield, California, (where there is considerable controversy about small earthquake stress drops), and a high frequency network at Acu Dam, northeastern Brazil, installed to monitor reservoir-induced seismicity, are also being analyzed for comparison. At each location, the catalogues are searched for similar earthquakes, including foreshocks and aftershocks. The waveforms and spectra of the clustered earthquakes, which can be assumed to have identical path effects, are compared to obtain high-resolution relative source parameters. Detailed study of the onsets of the waveforms is also performed to investigate the earthquake nucleation process and determine, for example, whether it differs with magnitude. These measurements are then used to improve the constraints on models of the nucleation and dynamics of earthquake rupture. This work will improve our understanding of the earthquake source process, which is fundamental to any improvements in hazard minimization. It funds a graduate student and also undergraduate assistants to enhance education.