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Like the general seismicity increase in the month following the SSE, −38˚
repeating families of interplate events (Fig. 1) also cluster in time at -40
the end of the SSE. They are spatially concentrated within the slow
slip patch and associated with a well-imaged subducted seamount -30
(Bell et al., 2010). Seamount
Tectonic tremor was also identified toward the end and continuing -20
after the slow slip event. Like the interplate earthquake families,
tremor is also co-located with slow slip and localized in the vicinity GISBORNE Seamount
-15
of subducted seamounts (Fig. 2). The subsequent, rather than 200
synchronous occurrence of tremor and interplate earthquakes and -12 100
slow slip suggests that seamount subduction plays the dominant 50
role in the stress state of the shallow megathrust. While northern -39° -9
Hikurangi seamounts appear to primarily subduct aseismically, their
subduction may generate elevated pore-fluid pressures in accumulated 50 km
underplated sediment packages and a complex, interconnected 178° 179°
fracture network such that tremor and microseismicity occur as SSE Duration
seismic components of seamount subduction during shallow slow
slip. This study indicates that the location of subducted seamounts May 1, July 1 Sept. 1 Nov. 1 Jan. 1, March 1 May 1 July 1
is strongly correlated with the distribution of SSE-associated tectonic 2014 2015
tremor and repeating earthquakes. The seamounts appear to be
responsible for slow slip, tremor, and microseismicity rupturing −38˚00' 177°30’ 178°00’ 178°30’ 179°00’ 179°30’
adjacent regions in a range of slip processes. −40 PUZ
Ongoing work more fully utilizes the rich data set of local earthquakes −30 TWGZ −20 −15 −12 −9
and includes analysis of seismic attenuation using the body wave MWZ −6
spectra of local earthquakes, local earthquake seismic velocity TKGZ
tomography, and earthquake source parameter analysis including −38˚30' RAGZ CNGZ S2
OBB
focal mechanisms and seismic moment. Knowing the physical state of LOBS1 LOBS2
the subducting plate interface is important for the slow slip modeling, RIGZ EOBS3 EOBS4
E
EO
EO
O O
EOBS1 11
OB
and our attenuation and velocity tomography models will be key to SNGZ 50 50 100 100 200 200 200 LOBSBS EOBS2 2 S1
LOBS8
infer the physical properties and structure in the area where slow slip PRGZ
occurs. For example, recent work revealed large differences between −39˚00' KNZ EOBS5 LOBS6
OB
SSE slip inversions that assume homogeneous elastic properties
versus those that utilize a more realistic elastic structure (Williams MHGZ PUKE
and Wallace, 2015). ■
Figure 1. (top) Repeating earthquake families identified near the −39˚30'
plate interface color coded by time of occurrence of first earthquake
in the family. Note the concentration of events where the seamount
is subducting and their timing just after the 2014 slow slip event SSE Duration (Wallace et al., 2016)
244 254 264 274 284 294 304
Figure 2. (bottom) Tectonic tremor associated with the 2014 Gisborne (Sep 1) (Oct 1) (Oct 31)
SSE color-coded by time. Tremor is primarily located on and adjacent Julian Day 2014
to subducted seamounts Puke and S1 and mostly occurs at the end
and after slow slip has ended. Triangles indicate the location of the
land-based stations of the New Zealand National Seismic Network
and HOBITSS OBS used to locate tremor.
NSF GeoPRISMS Awards # 1551683 ,
1551922, 1551758
References

Bell, R., R. Sutherland, D.H.N. Barker, S. Henrys, S. Bannister, L.M. Schwartz, A.F. Sheehan, (2016), Slow slip near the trench at the
Wallace, J. Beavan, (2010), Seismic reflection character of the Hikurangi subduction zone, New Zealand. Science, 352(6286), 701–
Hikurangi subduction interface, New Zealand, in the region of 704. doi.org/10.1126/science.aaf2349
repeated Gisborne slow slip events. Geophys. J. Int., 180(1), 34–48. Williams, C.A., L.M. Wallace, (2015), Effects of material property variations
doi.org/10.1111/j.1365-246X.2009.04401.x on slip estimates for subduction interface slow slip events, Geophys.
Wallace, L.M., S.C. Webb, Y. Ito, K. Mochizuki, R. Hino, S. Henrys, S., Res. Lett., 42(2), 1113-1121. doi.org/10.1016/j.epsl.2018.01.002

Spring 2018 Issue No. 40 GeoPRISMS Newsletter • 21