The 7.8 magnitude quake that hit New Zealand's South Island in November 2016, is responsible for a series of large slow slip events, according to a new study led by scientists at the University of Texas, Austin.

Slow slip events are similar to earthquakes, as they involve more rapid than normal movement between two pieces of the Earth's crust along a fault. However, unlike earthquakes, movement in these so-called "silent earthquakes" can take weeks to months to occur, instead of seconds. Usually, slow slip events are the ones to trigger damaging earthquakes, but scientists from the University of Texas, in an effort to explore the linkage between the two types of seismic activity, have for the first time documented such a clear example of the opposite.

 A GPS network operated by GeoNet, a partnership between GNS Science and the New Zealand Earthquake Commission, detected slow slip events beneath the North Island, along the shallow part of the Hikurangi subduction zone that runs along and across New Zealand, in the months following the Kaikoura earthquake. Some of the events occurred as far as 300 miles away from the epicenter of the M7.8 quake.

The slow slip occurred at a depth less than 9 miles below the surface and spanned an area of more than 6,000 square miles offshore from the Hawke's Bay and Gisborne regions. A deeper slow slip event was also triggered on the subduction zone at 15-24 miles beneath the Kapiti Coast region and is still ongoing today.

"This is probably the clearest example worldwide of long-distance, large-scale slow slip triggering," said the lead author of the study, Laura Wallace, a research scientist at the University of Texas Institute for Geophysics (UTIG).

An offshore "sedimentary wedge" — a mass of sedimentary rock piled up at the edge of the subduction zone boundary offshore from the North Island' east coast – probably helped towards triggering the events. Such layers of compliant rock are particularly susceptible to trapping seismic energy, which promotes slip between the plates at the base of the sedimentary wedge where the slow slip events occurred.

"Our study also suggests that the northward traveling rupture during the Kaikoura quake directed strong pulses of seismic energy towards the North Island, which also influenced the long-distance triggering of the slow slip events beneath the North Island," said Yoshihiro Kaneko, a seismologist at GNS Science.

Source: TEXAS Geosciences

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Engineering Seismology, Instrumentation