Slow slip source characterized by lithological and geometric heterogeneity

Area of Science:

• Geophysics
• Tectonics
• Seismology

Background:

• Slow slip events (SSEs) accommodate significant tectonic plate motion at subduction zones.
• The specific faults hosting SSEs, particularly at shallow depths, remain poorly understood.
• The Hikurangi subduction zone provides a unique setting to study shallow SSEs (<2 km depth).

Purpose of the Study:

• To investigate the physical properties of fault rocks involved in shallow slow slip events.
• To link geophysical imaging with direct sampling of subduction zone materials.
• To understand the role of material heterogeneity in promoting SSEs.

Main Methods:

• International Ocean Discovery Program (IODP) expeditions for direct sampling of subduction zone material.
• Analysis of drilled rock samples to determine lithological and physical properties.
• Integration of drilling data with regional seismic reflection imaging.

Main Results:

• Heterogeneous lithologies and variable physical properties were identified in the material entering the SSE source region.
• These heterogeneous materials are located at shallow depths (<2 km) offshore New Zealand.
• Observations suggest a link between fault zone heterogeneity and SSE occurrence.

Conclusions:

• Lithological, mechanical, and frictional heterogeneity within the fault zone promote SSEs and slow earthquake phenomena.
• Geometric complexity, such as the subduction of rough crust, further enhances SSEs.
• Shallow SSEs at Hikurangi offer insights into the fundamental processes governing plate motion accommodation.