Two maps showing the location and main tectonic features of the 1987 Edgecumbe earthquake.
Location and main tectonic features of the 1987 Edgecumbe earthquake. (a) Map of North Island in New Zealand with active faults in gray. Stippled black lines show extent of the Taupō Volcanic Zone (TPZ) and North Island dextral fault system (NIDFS), where most active deformation is located. Dark blue square denotes location of Bay of Plenty. (b) Map with shaded topography of the Rangitāki Plains where the Edgecumbe earthquake occurred. Red lines denote active faults and stippled lined polygons regions where foreshock seismic activity concentrated before the 1987 earthquake. The black-white beach ball represents the focal mechanism of the Edgecumbe earthquake interpreted as extensional faulting along NE-oriented faults parallel to the TVZ. Credit: Delano et al. [2022], Figure 1
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Solid Earth

Since the 1990s, space geodesy has led to major breakthroughs in our scientific knowledge of seismic processes because it allows us to image surface deformation during recent earthquakes with high resolution. However, earthquake recurrence commonly spans several decades to millennia, and therefore imaging surface deformation during historical earthquakes is instrumental to gain further insight on the mechanics of, and hazards posed by continental faults. The use of modern computational tools to reconstruct surface deformation using historical aerial photographs has expanded the observational time window allowing to study older earthquakes and their relation with nearby faults and volcanic systems.

Using historical aerial photographs, Delano et al. [2022] reconstructed the surface topography of the Rangitāki Plains before and after the 1987 Edgecumbe earthquake in northern New Zealand and estimated the 3D deformation field caused by this Mw 6.5 seismic event. This earthquake is of particular interest because it generated widespread metric-scale displacements at the surface despite its moderate magnitude. The authors found surface displacements larger than those measured by field studies shortly after the earthquakes, as well as many previously-unrecognized small subtle faults capturing a wider aperture deformation field. This improved displacement data helped to better constrain subsurface fault geometries and generate a 3D model.

Using this model and the space-time evolution of the seismic foreshocks that preceded the 1987 earthquake, Delano et al. inferred that slip observed at the surface on the three main faults could have been triggered by inflation of a volcanic sill emplaced at about 8-9 kilometers depth below the Rangitāki plains. These results have implications for our understating of the relation between volcanism and earthquake triggering, and may help to evaluate future earthquake hazards.

Citation: Delano, J. E., Howell, A., Stahl, T. A., & Clark, K. (2022). 3D coseismic surface displacements from historical aerial photographs of the 1987 Edgecumbe earthquake, New Zealand. Journal of Geophysical Research: Solid Earth, 127, e2022JB024059.

—Daniel Melnick, Associate Editor, Journal of Geophysical Research: Solid Earth

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