A microprocessor-based system for digitizing seismic events from magnetic-tape recordings

The task of converting analogue recordings of seismic events into digital form for computer input can be rendered faster and more reliable than hand-tracing on a digitizing table by the use of a simple microprocessor-based automatic system. We describe the circuitry and software of a system which takes 4 sec to digitize an event and 40 min to punch a card-deck containing 4096 data points; hand-tracing the same record on a digitizing table takes 8 hr. In addition, the automatic system removes the needs of correcting for duplicate points and axis alignment, and of interpolating to obtain equispaced data points. The system, as described, has been configured at minimal cost for input to an outdated mainframe facility; it demonstrates that microprocessor technology can be used to produce time- and labour-saving gains.</p

Gaps, tears and seismic anisotropy around the subducting slabs of the Antilles

Seismic anisotropy in and beneath the subducting slabs of the Antilles is investigated using observations of shear-wave splitting. We use a combination of teleseismic and local events recorded at three-component broadband seismic stations on every major island in the area to map anisotropy in the crust, the mantle wedge and the slab/sub-slab mantle. To date this is the most comprehensive study of anisotropy in this region, involving 52 stations from 8 seismic networks. Local event delay times (0.21 ± 0.12 s) do not increase with depth, indicating a crustal origin in anisotropy and an isotropic mantle wedge. Teleseismic delay times are much larger (1.34 ± 0.47 s), with fast shear-wave polarisations that are predominantly parallel to trend of the arc. These observations can be interpreted three ways: (1) the presence of pre-existing anisotropy in the subducting slab; (2) anisotropy due to sub-slab mantle flow around the eastern margin of the nearly stationary Caribbean plate; (3) some combination of both mechanisms. However, there are two notable variations in the trench-parallel pattern of anisotropy — trench-perpendicular alignment is observed in narrow regions east of Puerto Rico and south of Martinique. These observations support previously proposed ideas of eastward sublithospheric mantle flow through gaps in the slab. Furthermore, the pattern of anisotropy south of Martinique, near Saint Lucia is consistent with a previously proposed location for the boundary between the North and South American plates

Tectonic inversion in the Caribbean‐South American plate boundary: GPS geodesy, seismology, and tectonics of the Mw 6.7 22 April 1997 Tobago earthquake

On 22 April 1997 the largest earthquake recorded in the Trinidad‐Tobago segment of the Caribbean‐South American plate boundary zone (Mw 6.7) ruptured a shallow (~9 km), ENE striking (~250° azimuth), shallowly dipping (~28°) dextral‐normal fault ~10 km south of Tobago. In this study, we describe this earthquake and related foreshock and aftershock seismicity, derive coseismic offsets using GPS data, and model the fault plane and magnitude of slip for this earthquake. Coseismic slip estimated at our episodic GPS sites indicates movement of Tobago 135 ± 6 to 68 ± 6 mm NNE and subsidence of 7 ± 9 to 0 mm. This earthquake was anomalous and is of interest because (1) its large component of normal slip and ENE strike are unexpected given the active E‐W dextral shearing across the Caribbean‐South American plate boundary zone, (2) it ruptured a normal fault plane with a low (~28°) dip angle, and (3) it reactivated and inverted the preexisting Tobago terrrane‐South America ocean‐continent (thrust) boundary that formed during early Tertiary oblique plate convergence. Key Points The 22 April 1997 Mw 6.7 Tobago earthquake inverted a low-angle thrust fault Seismology, GPS, and modeling resolve coseismic slip, fault geometry, and moment Coseismic slip subsided Tobago and moved it NN

Water, oceanic fracture zones and the lubrication of subducting plate boundaries : insights from seismicity

We investigate the relationship between subduction processes and related seismicity for the Lesser Antilles Arc using the Gutenberg–Richter law. This power law describes the earthquake-magnitude distribution, with the gradient of the cumulative magnitude distribution being commonly known as the b-value. The Lesser Antilles Arc was chosen because of its along-strike variability in sediment subduction and the transition from subduction to strike-slip movement towards its northern and southern ends. The data are derived from the seismicity catalogues from the Seismic Research Centre of The University of the West Indies and the Observatoires Volcanologiques et Sismologiques of the Institut de Physique du Globe de Paris and consist of subcrustal events primarily from the slab interface. The b-value is found using a Kolmogorov–Smirnov test for a maximum-likelihood straight line-fitting routine. We investigate spatial variations in b-values using a grid-search with circular cells as well as an along-arc projection. Tests with different algorithms and the two independent earthquake cataloges provide confidence in the robustness of our results. We observe a strong spatial variability of the b-value that cannot be explained by the uncertainties. Rather than obtaining a simple north–south b-value distribution suggestive of the dominant control on earthquake triggering being water released from the sedimentary cover on the incoming American Plates, or a b-value distribution that correlates with on the obliquity of subduction, we obtain a series of discrete, high b-value ‘bull's-eyes’ along strike. These bull's-eyes, which indicate stress release through a higher fraction of small earthquakes, coincide with the locations of known incoming oceanic fracture zones on the American Plates. We interpret the results in terms of water being delivered to the Lesser Antilles subduction zone in the vicinity of fracture zones providing lubrication and thus changing the character of the related seismicity. Our results suggest serpentinization around mid-ocean ridge transform faults, which go on to become fracture zones on the incoming plate, plays a significant role in the delivery of water into the mantle at subduction zones

Analysis and forward modeling of seismic anisotropy during the ongoing eruption of the Soufriere Hills Volcano, Montserrat, 1996-2007

Volcanic stress field analysis has been lauded as a potentially powerful tool for midterm to long-term eruption forecasting. However, because tectonic processes can also produce localized stress field reorientations, evidence for a direct causal link between observed stress field reorientations and magmatic activity is of critical importance. In this study, we show that local stress field reorientations preceding changes in volcanic activity at the Soufrière Hills Volcano, Montserrat, are observable using two independent measures of crustal stress (local (volcano-tectonic) earthquake fault plane solutions and measurements of shear wave splitting in regional earthquakes). We further demonstrate that the local stress field orientation during a 6 month period preceding the onset of eruptive activity at Soufrière Hills in 1999 is highly localized and spatiotemporally variable and that the spatial pattern of precursory local stress orientations is consistent with numerically modeled patterns of stress resulting from pressurization of a vertical dike. These observations provide compelling evidence for a direct causal link between pressurization of midlevel volcanic conduit systems by ascending magma and precursory local stress field reorientations and demonstrate that seismological analysis can be used to detect subtle local changes in stress that herald eruptive activity

Fig. S6: Monitoring volcano deformation at La Soufrière, St Vincent during the 2020–21 eruption with insights into its magma plumbing system architecture

Unwrapped interferogram using ALOS-2 images obtained between 07/04/2021 and 21/04/2021 (ascending track, path 36 and frame 250). This exemplifies the inability of ALOS-2 radar to detect deformation related to the explosive phase (9–22 April 2021) due to the low signal-to-noise ratio and widespread lack of coherence

Fig. S1: Monitoring volcano deformation at La Soufrière, St Vincent during the 2020–21 eruption with insights into its magma plumbing system architecture

Surface deformation in the 2018–21 time-interval from Sentinel-1 small-baseline analysis. Top: mean line-of-sight velocity from Sentinel-1 descending track 156. Positive velocity corresponds to motion towards the satellite. The image is in radar geometry, and has been flipped and rotated so as to align approximately with a geographical orientation. X- and Y-coordinates are the number of pixels in range and azimuth, respectively. Bottom: time-series of displacement for five selected pixels. Locations of the pixels are displayed with the same symbols on the upper panel. Orange dashed line shows the onset of the effusive phase (27 December 2020)

Fig. S4: Monitoring volcano deformation at La Soufrière, St Vincent during the 2020–21 eruption with insights into its magma plumbing system architecture

Distribution of best-fitting model solutions for I2 pre-eruption (effusive) deformation using (a) point-, (b) dyke- and (c) penny-shaped crack sources. Modelled dyke using Sentinel-1 data for I3 phase from Joseph et al. (2022) shown as black open rectangle