Junior Research Group of Dr. Rebecca Harrington:: Physics of Earthquake Rupture  

Within one to two orders of magnitude, observable earthquake source parameters vary with earthquake size in a roughly predictable manner. However, many exceptions exist where the relationships deviate from the expected scaling. For example, the stress drop andthe amount of radiated energy associated with subduction zone earthquakes are less than with other types of earthquakes, and may vary with depth. Observed differences in source parameter scaling between earthquakes in different tectonic environments suggests that faulting style might have some influence on the amount of energy radiated in an earthquake.  Tectonic environment determines the faulting style, namely whether slip on the fault surface is vertical, oblique, or lateral, as well as the angle of faulting. While tectonic environment or faulting style may account for some of the observed differences between earthquakes, other physical differences may also influence scaling between source parameters. For example, it is not clear to what extent fault maturity influences radiated energy, stress drop, and earthquake duration. Fault maturity refers to the amount of cumulative displacement a fault surface has accumulated over its lifetime. The maturity causes variable fault surface roughness as modulated by cumulative displacement. Furthermore, the structure of coherent, intact rock vs. fractured, or damaged rock in a fault zone is a physical manifestation of fault displacement, and varies with distance to the fault surface depending on maturity. Depending on whether rupture occurs in intact rock on rough surface, or in damaged rock on a worn-down surface must also affect the dynamics of rupture.

Whether fault maturity or tectonicenvironment (and therefore faulting style) is more important is unclear. Relatively little attention has been paid to the effects of fault maturity and shallow fault zone structure on rupture dynamics. I would like to investigate and compare the relative influence between fault maturity and faulting environment on the rupture process by (1) examining the scaling relationships of earthquake characteristics over a wide range of Richter magnitudes and spatial scales for specific populations defined by fault maturity, and (2) by examining how they influence the radiation of high frequency energy during an earthquake. High frequency radiation is particularly important for structural building damage. Therefore, estimating the energy radiated in a particular frequency band during an earthquake is also relevant for assessing destructive potential. Through collaboration with  the University of California, Riverside, we will complete a deployment of seismic stations in southern California, in order to collect data tailored specifically to studying the maturity and structure of fault zones.  Current studies of earthquake source parameters examine source scaling only in the context of faulting environment, neglecting the effects of fault maturity on rupture dynamics. By considering how both factors might affect earthquake rupture in this study, we can use scaling relationships to more precisely estimate how the characteristics of earthquakes scale in a particular location, thereby enabling better hazard estimation.

The amount of recorded seismic waveform data with which earthquakes are studied has increased dramatically, especially over the last decade. However, extreme earthquakes happen infrequently, severely limiting our ability to study their characteristics in detail. It is therefore important to investigate what scaling relationships are appropriate, where scaling relationships break down between small and large earthquakes, and whether the physical causes for such a breakdown stem from differences in tectonic regime, faulting style, or fault maturity. In addition, we do not yet understand whether fault maturity or faulting style more strongly influences earthquake rupture. Once the scaling relationships are better understood, and the importance of physical differences in the faulting environment are studied, we can begin to make an accurate estimation of how damaging extreme earthquake might be in a given location.