Using deep slow slip in New Zealand to constrain slip partitioning

Abstract:

Underneath New Zealand’s North Island, the Pacific plate subducts obliquely beneath the Australian plate. Just to the south, subduction ceases and the plate boundary transitions to the mainly strike-slip, steeply dipping Alpine fault that runs along the South Island. In the region of the southern North Island, the relative plate motion has significant components of both convergence and along strike motion, and slip is partitioned between the main Hikurangi subduction interface and a series of shallower strike-slip faults running thurough the North Island (Wallace and Beavan, GRL, 2010). This region also hosts deep (~50 km), long duration (~1 year) slow slip events (SSEs). From early 2013 to 2015, continuous GPS stations maintained by GeoNet in this region recorded two such deep SSEs on the Hikurangi megathrust. The first SSE occurred on the Kapiti slow slip patch, just southwest of the North Island coast. This patch previously hosted slow slip events (SSEs) in 2003 and 2008 (Wallace and Beavan, JGR, 2010). The 2014 Kapiti SSE is unique because it was rapidly decelerated following increased normal stress (clamping) caused by a nearby M 6.3 earthquake (Wallace et al., GRL, 2014). However, GPS data indicates that slip did not stop entirely, and soon after the Manawatu slow slip patch, just to the northeast of the Kapiti patch, ruptured in another SSE. This patch previously had large SSEs in 2004/2005 and 2010/2011. Given the previous repeat interval of ~5.5 years, the 2014/2015 Manawatu SSE is early; however with only 3 SSEs it is difficult to tell how regular SSEs on this patch are usually.

Here we show Network Inversion Filter derived models of slow slip for the various phases of the Kapiti and Manawatu SSEs, which indicate a possible continuous migration of slip from the Kapiti SSE patch to the Manawatu SSE patch, and we quantify the shear stress increase on the Manawatu patch after the Kapiti SSE. Additionally, we explore allowing the Network Inversion Filter to vary the direction of slip on the plate interface to better fit the data. Using this method we can estimate how much of the strike-slip and dip-slip components of the relative plate motion are being accommodated by the main thrust interface, and infer how much slip is being accommodated by the strike-slip faults and forearc rotation.