Seismica

Locating the Nordstream explosions using polarization analysis

2022-10-08T10:04:20-04:00

The seismic events that preceded the leaks in the Nordstream natural gas pipelines in the Baltic Sea have been interpreted as explosions on the seabed. We use a polarization-based location method initially developed for marsquakes to locate the source region without the need for a subsurface velocity model. We show that the 2 largest seismic events can be unambiguously attributed to the methane plumes observed on the sea surface. The two largest events can be located with this method, using 4 and 5 stations located around the source, with the uncertainties in elliptical bounds of 30 x 30 km and 10 x 60 km, respectively. We can further show that both events emitted seismic energy for at least ten minutes after the initial explosion, indicative of resonances in the water column or the depressurizing pipeline.

Source Model and Characteristics of the 27 July 2022 MW 7.0 Northwestern Luzon Earthquake, Philippines

2022-08-30T05:20:10-04:00

The geometry and kinematics of the causative fault of the 27 July 2022 moment magnitude (M_w) 7.0 earthquake, which is one of the strongest to hit northern and central Luzon in the past 30 years, were estimated through inverse modeling of line-of-sight interferometric synthetic aperture radar deformation. We modeled rupture along multiple candidate faults based on fit with the pattern of line-of-sight deformation, consistency with focal mechanisms, and compatibility with the known kinematics of the mapped active faults in the region. Our preferred fault model, located west of and parallel to the Abra River Fault (ARF), exhibits localized reverse-slip (average 67° rake) at 15-35 km down-dip. Peak slip occurs at 13-16 km depth, with 95 cm of pure reverse-slip. The existence of a reverse-slip dominated ARF-parallel fault rupture is consistent with a complex shear partitioning model, wherein the NW-SE oblique plate convergence is accommodated not only by the sinistral strike-slip Philippine Fault Zone and the major subduction zones, but also by minor faults in intervening crustal blocks.

The root to the Galápagos mantle plume on the core-mantle boundary

2022-09-10T20:05:59-04:00

Ultra-low velocity zones (ULVZs) are thin anomalous patches on the boundary between the Earth's core and mantle, revealed by their effects on the seismic waves that propagate through them. Here we map a broad ULVZ near the Galápagos hotspot using shear-diffracted waves. Forward modelling assuming a cylindrical shape shows the patch is ~600 km wide, ~20 km high, and its shear velocities are ~25% reduced. The ULVZ is comparable to other broad ULVZs mapped on the core-mantle boundary near Hawaii, Iceland, and Samoa. Strikingly, all four hotspots where the mantle plume appears rooted by these ‘mega-ULVZs’, show similar anomalous isotopic signatures in He, Ne, and W in their ocean island basalts. This correlation suggests mega-ULVZs might be primordial or caused by interaction with the core, and some material from ULVZs is entrained within the plume. For the Galápagos, the connection implies the plume is offset to the west towards the base of the mantle.

Seismic interferometry in the presence of an isolated noise source

2022-09-14T10:58:36-04:00

Seismic interferometry gives rise to a correlation wavefield that is closely related to the Green’s function under the condition of uniformly distributed noise sources. In the presence of an additional isolated noise source, a second contribution to this wavefield is introduced that emerges from the isolated source location at negative lapse time. These two contributions interfere, which may bias surface wave dispersion measurements significantly. To avoid bias, the causal and acausal parts of correlation functions need to be treated separately. We illustrate this by applying seismic interferometry to field data from a large-N array where a wind farm is present within the array

Tilt Corrections for Normal Mode Observations on Ocean Bottom Seismic Data, an example from the PI-LAB experiment

2022-07-08T16:04:38-04:00

Earth's normal modes are fundamental observations used in global seismic tomography to understand Earth structure. Land seismic station coverage is sufficient to constrain the broadest scale Earth structures. However, 70% of Earth's surface is covered by the oceans, hampering our ability to observe variations in local mode frequencies that contribute to imaging small-scale structures. Broadband ocean bottom seismometers can record spheroidal modes to fill in gaps in global data coverage. Ocean bottom recordings are contaminated by signals from complex interactions between ocean and solid Earth dynamics at normal mode frequencies. We present a method for correcting tilt on broadband ocean bottom seismometers by rotation. The correction improves the ability of some instruments to observe spheroidal modes down to ₀S₄. We demonstrate this method using 15 broadband ocean bottom seismometers from the PI-LAB array. We measure normal mode peak frequency shifts and compare with 1-D reference mode frequencies and predictions from 3-D global models. Our measurements agree with the 3-D models for modes between ₀S₁₄ - ₀S₃₇ with small but significant differences. These differences likely reflect real Earth structure. This suggests incorporating ocean bottom normal mode measurements into global inversions will improve models of global seismic velocity structure.

The launch of Seismica: a seismic shift in publishing

2022-10-20T11:08:32-04:00

Seismica, a community-run Diamond Open Access (OA) journal for seismology and earthquake science, opened for submissions in July 2022. We created Seismica to support a shift to OA publishing while pushing back against the extreme rise in the cost of OA author processing charges, and the inequities this is compounding. Seismica is run by an all-volunteer Board of 47 researchers who fulfil traditional editorial roles as well as forming functional teams to address the needs for technical design and support, copy editing, media and branding that would otherwise be covered by paid staff at a for-profit journal. We are supported by the McGill University Library (Québec, Canada), who host our website and provide several other services, so that Seismica does not have any income or financial expenditures. We report the process of developing the journal and explain how and why we made some of the major policy choices. We describe the organizational structure of the journal, and discuss future plans and challenges for the continued success and longevity of Seismica.