Abstract
The down-dip limit of the seismogenic zone and up-dip and down-dip limits of the deep low-frequency tremors in southwest Japan are clearly imaged by the hypocentre distribution. Previous studies using smooth constraints in inversion analyses estimated that long-term slow slip events (L-SSEs) beneath the Bungo Channel are distributed smoothly from the down-dip part of the seismogenic zone to the up-dip part of the tremors. Here, we use fused regularisation, a type of sparse modelling suitable for detecting discontinuous changes in the model parameters to estimate the slip distribution of L-SSEs. The largest slip abruptly becomes zero at the down-dip limit of the seismogenic zone, is immediately reduced to half at the up-dip limit of the tremors, and becomes zero near its down-dip limit. Such correspondences imply that some thresholds exist in the generation processes for both tremors and SSEs. Hence, geodetic data inversion with sparse modelling can detect such high resolution in the slip distribution.
Highlights
In many subduction zones along the Pacific Rim, deeper extents of the source areas for megathrust earthquakes have recorded slow earthquakes in the past two decades[1]
We focused only on the spatial distribution to estimate the detailed distribution of the total slips during each Long-term slow slip events (L-SSEs) event
The L-SSE areas are clearly separated into two parts [the major slip area at the up-dip side] with two sharp boundaries (Figs 3 and 4). (i) The shallow boundary between the seismogenic zone and the major L-SSE area corresponds to the isotherm at 350 °C9. (ii) The deep boundary between the major L-SSE area and the minor L-SSE area corresponds to the up-dip limit of the tremor area
Summary
In many subduction zones along the Pacific Rim, deeper extents of the source areas for megathrust earthquakes have recorded slow earthquakes in the past two decades[1]. Among the various types of slow earthquakes, well-studied events include deep low-frequency tremors[5] with dominant frequency of 2–8 Hz, especially in the Nankai and Cascadia subduction zones. There is a spatial gap between megathrust earthquakes and deep ETSs. Long-term slow slip events (L-SSEs) with durations of 0.5–5 years[3] and recurrence interval of >5 years occur within this gap of the Nankai subduction zone. The upper (up-dip) boundary of the L-SSEs area was located close to the sources of two M > 7 interplate earthquakes (the 1946 M 8.0 Nankai earthquake and the 1968 M 7.5 Hyuga-nada earthquake) This implies partitioning between seismogenic and aseismic slip areas as highlighted from slow slip events[13]. Temporal correspondence has been reported between the occurrence of L-SSEs and the acceleration of deep tremors beneath the Bungo Channel[14]
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