Stuart Henrys さんと Laura Wallace さんのセミナー
日時：9月12日（木）14時〜16時
場所：新館セミナー室

Laura Wallace, Univ. of Texas, Austin:

Title:Slow slip and interseismic coupling at the Hikurangi subduction
margin, New Zealand: Implications for physical controls on megathrust
behavior

Abstract:
Campaign and continuous GPS measurements in the North Island of New
Zealand show marked along-strike variations in slip behavior of the
Hikurangi subduction thrust.  The southern Hikurangi interface
undergoes deep interseismic coupling (down to 30 km depth), while most
of the interface at the northern and central Hikurangi margin is
dominated by aseismic creep and episodic slow slip events (SSEs). The
character of SSEs at Hikurangi also undergoes strong along-strike
variations.  At southern Hikurangi, deep (30-50 km), long-duration (1
year), infrequent (5 year recurrence), large (Mw 7.0) SSEs occur along
the down-dip transition from interseismic coupling to aseismic creep.
Plate boundary slip on the shallow interface (<15 km depth) at
northern Hikurangi is dominated by frequent (1-2 year recurrence),
short (1-3 weeks), moderate to large (Mw 6.3-6.8) SSEs, with steady,
aseismic creep below 15 km depth.  In contrast, at central Hikurangi,
the majority of the megathrust between <10-50 km depth undergoes SSE
slip, indicating that the physical conditions conducive to SSE slip
may be inherently broad.  Along a single transect at the central
Hikurangi margin, we observe a full spectrum of SSE durations,
magnitudes, depths, and recurrence characteristics, including short,
shallow (<15 km) SSEs beneath Hawke Bay, long-duration, deep (30-50
km) large SSEs in the Manawatu region, and newly observed moderate
duration (~3 months) SSEs at 30-40 km depth, directly down-dip of the
shallow Hawke Bay SSEs.
Along-strike changes in megathrust behavior at Hikurangi are also
accompanied by along-strike variations in convergence rate, sediment
thickness on the incoming plate, degree of accretion vs. subduction
erosion, upper plate stress regime (i.e., a shift from back-arc
extension to transpression in the upper plate), geochemical signature
of fluids emerging within the forearc, seismic attributes of the plate
interface and upper plate, among other characteristics. These features
make the Hikurangi margin an ideal location to evaluate the controls
on megathrust behavior.  I will discuss some potential mechanisms to
explain the along-strike variations, including the effects of seamount
subduction, and the link between upper plate stress, permeability,
fluid flow, and fluid pressure along the interface.

Stuart Henrys, GNS Science, NZ

Title: SAHKE geophysical transect reveals crustal and subduction zone 
structure at the southern Hikurangi margin, New Zealand 

Abstract:
The Seismic Array HiKurangi Experiment (SAHKE) investigated the 
structure of the forearc and subduction plate boundary beneath the 
southern North Island along a 350 km transect. Tomographic inversion 
of first-arrival travel times was used to derive a 15-20 km deep 
P-wave image of the crust. The refracted phases and migrated 
reflection events image subducting slab geometry and crustal 
structure. In the west, Australian Plate Moho depth decreases 
westward across the Taranaki Fault system from 35 to ~28-30 km. In 
the east, subducted Pacific Plate oceanic crust is recognised to have 
a  positive velocity gradient, but becomes less distinct beneath the 
Tararua Ranges, where the interface increases in dip at about 15 km 
depth from <5° to >15°. This bend in the subducted plate is 
associated with vertical clusters in seismicity, splay fault 
branching, and low-velocity high-attenuation material that we 
interpret to be an underplated subduction sedimentary channel. We 
infer that a step down in the decollement transfers slip on the 
plate interface at the top of a subduction channel to the oceanic 
crust and drives local uplift of the Tararua Ranges. Reflections from 
the Wairarapa Fault show that it is listric and soles into the top of 
underplated sediments, which in turn abut the Moho of the over-riding 
plate at ~32 km depth, near the downdip end of the strongly locked 
zone. The change in dip of the Hikurangi subduction interface is 
spatially correlated with the transition from geodetically determined 
locked to unlocked areas of the plate interface.