The Mw7.1 September 4, 2010 Darfield earthquake and the subsequent Mw6.3 February 22, 2011 Christchurch earthquake were the most damaging earthquakes in New Zealand since the 1931 Hawke?s Bay earthquake. The latter event caused enormous damage throughout Christchurch, New Zealand?s second largest city. These earthquakes occurred on previously unmapped faults in the eastern South Island. The behavior of this earthquake sequence has led to many questions about the fundamental characteristics of the fault on which they occurred and the local geology and tectonics. Five Victoria University of Wellington seismometers and nine PASSCAL seismometers were installed in the region surrounding the surface rupture of the Darfield earthquake within two weeks of its occurrence and were removed in mid January 2011. Using data recorded at these temporary stations and additional permanent regional stations, the researchers will study the aftershocks of the Darfield earthquake in order to investigate the seismically active fault structure, the temporal evolution of aftershocks and fault activity, and the regional seismic structure. A relocated aftershock catalog will be developed based on the GeoNet catalog for New Zealand. The investigators will examine how seismicity evolved throughout the fault system, with a focus on faults that had been identified beneath Christchurch prior to the February 2011 earthquake. The algorithm tomoDD will also be used to simultaneously solve for P- and S-wave velocities in the region surrounding the fault rupture. The benefit of this will be two-fold: the improved 3D velocity model will result in improved aftershock locations and it will constrain the local subsurface structure. For example, a velocity contrast across the fault could be used to constrain its age, which is currently a topic of controversy. Focal mechanisms will be calculated for aftershocks.In conjunction with aftershock locations, focal mechanisms will allow the researchers to constrain the directions of motion along the various fault segments that have been active in the aftershock sequence. By searching for repeating earthquakes and non-volcanic tremor, they will be able to explore the variety of ways in which the faults associated with the Darfield earthquake move. Repeating earthquakes may be caused by a section of fault that is experiencing aseismic creep, either before or after a large earthquake. By studying the first four months of the aftershock sequence of the 2010 Darfield earthquake, they will help establish the fault structure of previously unknown fault segments, understand the evolution of activity along them, and understand the factors that contributed to the more devastating 2011 Christchurch earthquake.
This project supported the analysis of aftershocks of the magnitude 7.1 earthquake near Christchurch, New Zealand, in September 2010, known as the Darfield earthquake. The data were collected with partial support from an NSF RAPID award in a four month deplyment of seismic instruments. The Darfield earthquake caused substantial damage in the Christchurch area, and appears to have triggered subsequent smaller earthquakes in 2011 directly underneath Christchurch that caused even greater damage. The faults on which these earthquakes occurred were unknown prior to this, so our work contributed to gaining first-order knowledge of the complex character of these faults. The main goals of the project were to investigate the complex geometry of the multiple faults that ruptured during and after the Darfield main shock, image the crustal structure in the region surrounding the earthquake sequence, assess the state of stress in the area, and search for possible precursory evidence for the 2011 earthquakes. The project supported the research of two senior investigators, a graduate student, and two undergraduate students. The results of the research were presented in three published research articles and eight conference presentations. We found that the aftershocks clearly defined all but one of the multiple faults involved in the earthquake sequence as defined by studies of the deformation of the Earth (geodesy), with the majority of fault slip on a segment named the Greendale fault. An additional set of faults on the eastern end of the sequence were also identified in our aftershock locations. The stress in the region was found to be quite favorable for causing slip on faults oriented east-west, as is the Greendale fault. We found evidence close to Christchurch of repeating small earthquakes, which are generally presumed to indicate the occurrence of non-seismic fault slip (fault creep). Aftershocks also migrated eastward towards Christchurch during the four months for which we had data. These observations, if they had been made in real time, potentially could have been signaled the possibility of continued earthquake activity beneath Christchurch.
