Tracking high-frequency seismic source evolution: 2004 Mw 8.1 Macquarie event

The 2004 Mw 8.1 event on 23 December 2004 near the Macquarie Ridge is a very large intraplate event that has been overshadowed by the M w 9.3 Sumatra-Andaman event only 3 days later. We are able to track the progress of source evolution by estimating th

Three-dimensional visualization of a near-vertical slab tear beneath the southern Mariana arc

The use of a three-dimensional ray-tracing inversion algorithm has greatly enhanced the resolution of gradients and strong variations in wave speeds to create improved P wave tomographic images of the Mariana arc region. The images obtained from the Mariana arc region show relatively low amplitudes of heterogeneity due to the limited number of seismic stations in the area. Despite these limitations, detailed interpretations of the three-dimensional geometry and morphology of the Pacific Plate subducting beneath the Philippine Sea Plate have provided a three-dimensional model of the steep dip of the Pacific plate and the curvature of the slab beneath the Mariana arc in unprecedented detail. The new P wave tomography and seismicity depict a previously unidentified E-W trending near-vertical tear in the subducting plate at the southern end of the Mariana arc that divides the arc into two distinct segments: a steeply dipping curved slab penetrating the lower mantle and a short (∼250 km depth) slab along the Challenger Deep segment of the arc. The slab tear is likely to be the result of the need to accommodate the reduced volume the slab must occupy as it is subducted beneath the Philippine Sea plate along a curved arc

Tracking earthquake source evolution in 3-D

Starting from the hypocentre, the point of initiation of seismic energy, we seek to estimate the subsequent trajectory of the points of emission of high-frequency energy in 3-D, which we term the 'evocentres'. We track these evocentres as a function of time by energy stacking for putative points on a 3-D grid around the hypocentre that is expanded as time progresses, selecting the location of maximum energy release as a function of time. The spatial resolution in the neighbourhood of a target point can be simply estimated by spatial mapping using the properties of isochrons from the stations. The mapping of a seismogram segment to space is by inverse slowness, and thus more distant stations have a broader spatial contribution. As in hypocentral estimation, the inclusion of a wide azimuthal distribution of stations significantly enhances 3-D capability. We illustrate this approach to tracking source evolution in 3-D by considering two major earthquakes, the 2007 Mw 8.1 Solomons islands event that ruptured across a plate boundary and the 2013 Mw 8.3 event 610 km beneath the Sea of Okhotsk. In each case we are able to provide estimates of the evolution of high-frequency energy that tally well with alternative schemes, but also to provide information on the 3-D characteristics that is not available from backprojection from distant networks. We are able to demonstrate that the major characteristics of event rupture can be captured using just a few azimuthally distributed stations, which opens the opportunity for the approach to be used in a rapid mode immediately after a major event to provide guidance for, for example tsunami warning for megathrust events

Crustal properties from seismic station autocorrelograms

Stations on the Australian continent receive a rich mixture of continuous ground motion withambient seismic noise from the surrounding oceans, and numerous small earthquakes in the earthquake belts to the north in Indonesia, and east in Tonga-Kermadec, a

Structural controls on the Mw 9.0 2011 Offshore-Tohoku earthquake

Joint seismic tomography exploiting P and S wave arrivals conducted before the 2011 Offshore Tohoku earthquake reveals an area comparable to the faulting surface for the 2011 March 11 event with different properties from other areas along the shallow part of the subduction zone. The differences are revealed by using a measure R of the relative variations in shear wavespeed and bulk-sound speed. Within the faulting area there are patches on the subduction zone with slightly reduced S wavespeed, and thus negative R, that appear to separate portions of the rupture with very different character. On the down-dip side there is strong short-period radiation, whilst the largest slip occurs up-dip with most energy release at longer periods. Segmentation of the slip process can be imaged by back projection of seismograms from the US Array; the areas of greatest energy release at short periods lie down-dip from the negative R anomalies. The main seismic moment release from broad-band seismograms lies on the updip side of the same anomalies. The structural variations on the subduction zone thus separate two regions with fundamental differences in the rupture process, stronger long-period radiation up-dip and stronger short-period radiation down-dip. These variations are likely to reflect features brought into the subduction zone, which may have acted as asperities that allowed this event to build up 30-40. m of strain in the near trench zone, making it much bigger than expected. Thus minor changes in the character of the subducted plate can have a significant influence on the behaviour of a great earthquake

Spiral-arm seismic arrays

Seismic arrays havemany uses for signal enhancement, from surface-wave characterization of the near surface to teleseismic detection in the context of monitoring nuclear tests. Many variants of the geometrical configuration of stations have been used with the objective of maximizing potential resolution of the incoming wavefronts direction of arrival. Aversatile class of array configurations, with good resolution properties, can be constructed with multiple spiral arms. The array response is comparablewith the same number of full circles, but with far fewer stations and is robust to minor position changes in emplacement. The desirable properties of the spiral-arm arrays are illustrated for a permanent array in the Precambrian Pilbara craton in northwestern Australia and for a temporary array on ancient sediments in southern Queensland, Australia. In each case, the practical array response is very good and matches the theoretical expectations. The spiral-arm configuration allows the deployment of relatively large aperture arrays with a limited number of stations, which is advantageous in a broad range of seismic applications, including near-surface characterization