Slow Slip, Tremor, and Earthquakes in New Zealand

Wallace, L. M., N. Bartlow, I. Hamling, and B. Fry (2014), Quake clamps down on slow slip, Geophys. Res. Lett., 41, 8840–8846, doi:10.1002/2014GL062367.

Bartlow, N. M., L. M. Wallace, R. J. Beavan, S. Bannister, and P. Segall (2014), Time-dependent modeling of slow slip events and associated seismicity and tremor at the Hikurangi subduction zone, New Zealand, J. Geophys. Res. Solid Earth, 119, 734–753, doi:10.1002/2013JB010609.

Cumulative Hikurangi slip Figure 1: Cumulative slow slip in the Hikurangi subduction zone of New Zealand, beginning 2010 - mid 2012. The east side of the plate interface is at 10 km depth, and the west side is at 70 km depth. Named slow slip patches are labeled in (a). (b) is the same as (a), with the addition of earthquakes (seismicity) within 10 km of the plate interface. Larger circles represent bigger earthquakes.

Manawatu SSE Figure 2: Culumative slip during the long (almost 1 year) 2010 - 2011 Manawatu slow slip event. Green dots indicate the estimated locations of tremor sources, from Idehara, K., Yabe, S., and Ide, S. Regional and global variations in the temporal clustering of tectonic tremor activity, Earth, Planets and Space, 2014, 66, 66, doi:10.1186/1880-5981-66-66, 2014.

New Zealand is home to the Hikurangi subduction zone, where the Pacific tectonic plate collides with and sinks below the Australian plate. Subduction zones host the largest earthquakes in the world, and it's estimated that the Hikurangi subduction zone has the potential for an earthquake of magnitude close to 9.

In beween large earthquakes, the Hikruangi subduction zone is not dormant. In fact, this is a very active area with many small and medium sized earthquakes every year, some of which are damaging. Hikurangi is also home to slow slip events, tecotnic tremor, and slow-slip triggered earthquakes. In a slow slip event, the deeper extent of the interface between the two tectonic plates slips, as in an earthquake, but does so more slowly. While earthquake slip occurs over the course of seconds, slow slip occurs over the course of days to weeks. The vast majority of this slip is "silent", meaning it cannot be detected with seismic instruments. But, slow slip events move the surface of the earth up to a few cm, and this movement can be detected using GPS and other instruments (see the page on Cascadia for more). A few of the slow slip events in Hikurangi include tectonic tremor signals, which are very small seismic signals, while others are completely silent. While slow slip and tremor are not dangerous to humans or the environment, they provide a window into the physics of subduction zones. Additionally, by characterizing and monitoring slow slip events, scientists may be able to detect changes leading up to a future large earthquake.

Figure 1 shows the total amount of slow slip (colors) over a 2.5 year period from the beginning of 2010 through mid-2012. Slow slip occurs mainly in shallow regions in the north, and deeper regions in the south. The second panel of Fig. 1 shows seismicity within 10 km of the plate interface. Most seismicity is located between the slow slip regions, with very little seismicity overlapping the slow slip regions. This implies that the slow slip regions have frictional properties that do not allow for sudden, seismic slip (as in earthquakes) and the region in between has frictional properties that allow for earthquakes, but not slow slip. In this way, slow slip events help scientists forcast the location and potential rupture areas of future earthquakes.

Dr. Bartlow has carried out GPS-data based studies of multiple Slow Slip events in Hikurangi using the Network Inversion Filter (NIF) software package (see the software page). She has built the first self-consistent catalog of these slow slip events, including magnitudes, durations, and slip-rates. In Hikurangi, slow slip events occur at a variety of depths. Shallow slow slip (around 10 km depth) occurs over the course of about a week, while deep slow slip events (around 40 km depth) can take a year to occur. Dr. Bartlow has found that the relationship between slow slip events and tremor is quite different in Hikurangi compared to the more well-studied case of Cascadia. In Hikruangi, tremor located near the Manawatu slow slip event is located deeper than the slip, and not co-located with the actively slipping region as in Cascadia (Fig. 2). Dr. Bartlow has also studied in detail an earthquake swarm that was thought to be triggered by the Cape Turnagain slow slip event in 2011. Dr. Bartlow's models show that these earthquakes were most likely triggered by stress changes caused by slow slip (Fig. 3). A movie of the 2011 Cape Turnagain slow slip event and earthquake swarm can be seen below as well.

Cape Turnagain SSE Animation of the Cape Turnagain slow slip event. Estimated slip rate is shown in the colors, and earthquakes appear as circles scaled by their magnitude.
Cape Turnagain SSE Figure 3: (a) A possible model of cumulative slow slip during the 2011 Cape Turnagain slow slip event. This model fits the avaialble GPS data within the uncertainty, with the requirement that shear stress increase in the region outlined in black, where the associated earthquake swarm occurs. (b) Shear stress on the plate interface calculated from the slip in (a). This shear stress pattern can explain the seismic swarm, assuming a stress triggering hypothesis.

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