Distributed Acoustic Sensing (DAS) is a promising technique to improve the rapid detection and characterization of earthquakes. Previous DAS studies mainly focus on the phase information but less on the amplitude information. In this study, we compile earthquake data from two DAS arrays in California, USA, and one submarine array in Sanriku, Japan. We develop a data-driven method to obtain the first scaling relation between DAS amplitude and earthquake magnitude. Our results reveal that the earthquake amplitudes recorded by DAS in different regions follow a similar scaling relation. The scaling relation can provide a rapid earthquake magnitude estimation and effectively avoid uncertainties caused by the conversion to ground motions. Our results show that the scaling relation appears transferable to new regions with calibrations. The scaling relation highlights the great potential of DAS in earthquake source characterization and early warning.

Detecting crustal deformation during transient deformation events at offshore subduction zones remains challenging. The spatiotemporal evolution of slow slip events (SSEs) on the offshore Hikurangi subduction zone, New Zealand, during February–July 2019, is revealed through a time-dependent inversion of onshore and offshore geodetic data that also account for spatially varying elastic crustal properties. Our model is constrained by seafloor pressure time series (as a proxy for vertical seafloor deformation), onshore continuous Global Navigation Satellite System (GNSS) data, and Interferometric Synthetic Aperture Radar (InSAR) displacements. Large GNSS displacements onshore and uplift of the seafloor (10-33 mm) require peak slip during the event of 150 to >200 mm at 6-12 km depth offshore Hawkes Bay and Gisborne, comparable to maximum slip observed during previous seafloor pressure deployments at north Hikurangi. The onshore and offshore data reveal a complex evolution of the SSE, over a period of months. Seafloor pressure data indicates the slow slip may have persisted longer near the trench than suggested by onshore GNSS stations in both the Gisborne and Hawkes Bay regions. Seafloor pressure data also reveal up-dip migration of SSE slip beneath Hawke Bay occurred over a period of a few weeks. The SSE source region appears to coincide with locations of the March 1947 Mw 7.0–7.1 tsunami earthquake offshore Gisborne and estimated Great earthquake rupture sources from paleoseismic investigations offshore Hawkes Bay, suggesting that the shallow megathrust at north and central Hikurangi is capable of both seismic and aseismic rupture.

We derive the 3-D S-wave velocity structures of sediments and upper crust in the region off Ibaraki by applying ambient noise tomography to a dense array of short-period ocean bottom seismometers (OBSs). The cross-spectra were calculated using 27- or 142-day continuous seismic data, and the phase velocities of the fundamental and the first-higher Rayleigh wave modes are obtained in the frequency ranges of 0.1–0.25 Hz and 0.17–0.3 Hz, respectively. Our 1-D S-wave velocity inversion based on the trans-dimensional Markov chain Monte Carlo method revealed multiple sedimentary layers above the acoustic basement and the upper crustal structure. The 1-D structure was then used as a reference model to conduct ambient noise tomography and non-linear inversion of the 3-D S-wave velocity structure by collecting data of the local 1-D S-wave velocity structure. Our 3-D S-wave velocity structure revealed three main points: (1) The acoustic basement is situated at a depth of ~4 km depth; (2) the crustal structure is more complex than the that of the sedimentary layers; and (3) the southern region has a complex crustal structure in which subducting seamounts were identified by previous P-wave velocity tomographies.

The off-Ibaraki region is a convergent margin at which a seamount subducts. An intensive event location was performed around the subducting seamount to reveal the regional seismotectonics of this region. By applying a migration-based event location to an Ocean Bottom Seismic network record of both P- and S-waves, over 20,000 events were determined in the off-Ibaraki region below ~M4. The seismicity showed clear spatiotemporal patterns enough to identify the seismicity changes and geometry of the interface. At the updip side, the shallow tectonic tremors and earthquakes are shown to be spatially complementary bounded by an updip limit of the seismogenic zone. At the downdip side, a semicircular low-seismicity zone was identified, which is possibly a rupture area of the Mw7.9 event. The event depth profile exhibited a gently sloped planar downdip interface subparallel to the subducting slab. This plane appears to be stably active from 2008 to 2011. Comparison with the active source seismic survey profiles exhibits that this planar downdip interface is several kilometers deeper than the top of the oceanic crust. After the Mw7.9 event, a high-angle downdip seismic interface was activated above the planar interface. Further, below the planar downdip interface, broadly scattered events occurred with a swarm manner. We successfully illuminated the complicated subsurface structures around the subducting seamount. It is suggested that most of the event occur along or below the plate interface as the top of the oceanic crust.