Lloyd Lynch

Remarkable cyclic ground deformation monitored in real‐time on Montserrat, and its use in eruption forecasting

Telemetered high-resolution tiltmeters were installed in Montserrat in summer of 1995, in December 1996, and in May 1997. The 1995 installations, several km from the Soufriere Hills vent, were too distant to yield useful data. However, the 1996 and 1997 installations on the crater rim revealed 6–14 h inflation cycles caused by magma pressurization at shallow depths (< 0.6 km below the base of dome). The tilt data correlated with seismicity, explosions, and pyroclastic flow activity, and were used to forecast times of increased volcanic hazard to protect scientific field workers and the general public.

Overview of the eruption of Soufriere Hills Volcano, Montserrat, 18 July 1995 to December 1997

The onset of phreatic volcanic activity at the Soufriere Hills volcano, Montserrat on 18 July 1995 followed a three-year period of heightened volcano-seismic activity beneath the island. Phreatic explosions gave way to continuous eruption of juvenile andesitic magma in the form of a lava dome on or around 15 November 1995. Magma production rate has varied, leading to changes in eruptive style. An explosive eruption on 17 September 1996 followed a period of enhanced dome growth and large-scale gravitational collapses from its eastern flank. Increasing dome volume led to stressing and overtopping of the confining crater walls to the southwest, north and west during early 1997. Sustained high magma production rate since June 1997 has led to three periods of major gravitational dome collapses followed by vulcanian explosive eruptions. Dome growth re-started immediately after the cessation of the latest of these explosive phases in October 1997 and continues as of December 1997.

Soufrière Hills Eruption, Montserrat, 1995–1997: Volcanic earthquake locations and fault plane solutions

A total of 9242 seismic events, recorded since the start of the eruption on Montserrat in July 1995, have been uniformly relocated with station travel-time corrections. Early seismicity was generally diffuse under southern Montserrat, and mostly restricted to depths less than 7 km. However, a NE-SW alignment of epicentres beneath the NE flank of the volcano emerged in one swarm of volcano-tectonic earthquakes (VTs) and later nests of VT hypocentres developed beneath the volcano and at a separated location, under St. Georges Hill. The overall spatial distribution of hypocentres suggests a minimum depth of about 5 km for any substantial magma body. Activity associated with the opening of a conduit to the surface became increasingly shallow, with foci concentrated below the crater and, after dome building started in Fall 1995, VTs diminished and repetitive swarms of ‘hybrid’ seismic events became predominant. By late-1996, as magma effusion rates escalated, most seismic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth - the diminution of shear failure earthquakes suggests the pathway for magma discharge had become effectively unconstricted. Individual and composite fault plane solutions have been determined for a few larger earthquakes. We postulate that localised extensional stress conditions near the linear VT activity, due to interaction with stresses in the overriding lithospheric plate, may encourage normal fault growth and promote sector weaknesses in the volcano.

Seismicity associated with dome growth and collapse at the Soufriere Hills Volcano, Montserrat

Varied seismicity has accompanied growth and collapse of the lava dome of the Soufriere Hills Volcano, Montserrat. Earthquakes have been classified as either volcano-tectonic, long-period or hybrid, and daily variations in the numbers of events have mapped changes in the style of eruption. Repetitive hybrid earthquakes were common during the first months of dome growth. In July 1996 the style of seismicity changed and regular, short-lived hybrid earthquake swarms became common. This change was probably caused by an increase in the magma flux. Earthquake swarms have preceded almost all major dome collapses, and have accompanied cyclical deformation, thought to be due to a build-up of pressure in the upper conduit which is later released by magma moving into the dome.

Observations of hybrid seismic events at Soufriere Hills Volcano, Montserrat: July 1995 to September 1996

Swarms of small repetitive events with similar waveforms and magnitudes are often observed during the emplacement of lava domes. Over 300,000 such events were recorded in association with the emplacement of the lava dome at Soufriere Hills Volcano, Montserrat, from August 1995 through August 1996. These events originated < 2–3 km deep. They exhibited energy ranging over ˜ 1.5–4.5 Hz and were broader band than typical long-period events. We term the events “hybrid” between long-period and volcano-tectonic. The events were more impulsive and broader band prior to, compared with during and after, periods of inferred increased magma flux rate. Individual swarms contained up to 10,000 events often exhibiting very similar magnitudes and waveforms throughout the swarm. Swarms lasted hours to weeks, during which inter-event intervals generally increased, then decreased, often several times. Long-duration swarms began about every two months starting in late September 1995. We speculate that the events were produced as the magma column degassed into adjacent cracks.

Eruption of Soufriere Hills Volcano in Montserrat continues

On July 18,1995, the Soufriere Hills volcano in Montserrat erupted for the first time in recorded history. The eruption began with intense fumarolic venting and phreatic explosions following 3 years of elevated seismicity. An andesite lava dome emerged on November 15, 1995, and continued to grow, with several periods of copious pyroclastic flow generation and an explosive eruption on September 17, 1996. The largest pyroclastic flows to date (on June 25, 1997) and a period of vulcanian explosions with fountain collapse (during early August 1997) indicate continued escalation of the eruption.

Probabilistic Seismic Hazard Assessment at the Eastern Caribbean Islands

A probabilistic seismic hazard analysis has been performed to compute probabilistic seismic hazard maps for the eastern Caribbean region (10° N-19° N, 59° W-64° W), which includes in the north the Leeward Islands (from Anguilla to Dominica) and in the south the Windward Islands (from Martinique to Grenada), Barbados, Trinidad, and Tobago. The analysis has been conducted using a standard logic-tree approach that allowed systematically taking into account the model-based (i.e., epistemic) uncertainty and its influence on the computed ground motion param- eters. Hazard computations have been performed using a grid of sites with a space resolution of 0.025 degrees covering the territory of the considered islands. Two different computation methodologies have been adopted: the standard Cornell-McGuire approach (Cornell, 1968; McGuire, 1976) based on the definition of appropriate seismogenic zones (SZ), and the zone-free approach developed by Woo (1996), which overcomes the ambiguities related with the definition of seis- mic sources. The interplay and complexities between shallow crustal, intraplate, and interface subduction seismicity of the Caribbean region have been thoroughly investigated. By merging all available databases, a comprehensive and updated earthquake catalog for the region has been compiled. Also, a thorough investigation has been undertaken to identify the most suitable ground motion prediction equa- tions to be used in the analyses. Uniform hazard spectra have been calculated for the horizontal component of ground motion (rock and level site conditions), 4 return periods (RP) (95-, 475-, 975-, and 2475-yr), and 22 spectral accelerations (SA) with structural periods ranging from 0 to 3 s. SAs at 0.2 and 1.0 s for 2475-yr RP have been calculated to allow the definition of seismic hazard in the region of study according to the International Building Code (IBC, International Code Council (ICC), 2009).

Attenuation of seismic waves in the Trinidad and Tobago area

Abstract The attenuation of seismic waves from earthquakes located within the area bounded by 9–12°N and 60–63°W was estimated from short-period seismograms. Coda- Q , Q c , determinations were made for each of the six seismograph stations within the area, while spectral Q values from P-phases, Q α , were estimated for station TRN. The S-S single-scattering model was assumed for coda generation, and the ω −2 source model was assumed for the spectral Q determinations. The Q c values show a strong frequency dependence in the frequency range 1.5–12 Hz. The value of Q at 1 Hz, Q o , was found to lie within the range 107–132, while the rate of frequency dependence, n , extends from 0.80 to 1.06 for shallow events. For intermediate-depth events, Q o varies from 101 to 173 and n from 0.80 to 1.02. The Q α values obtained show a spatial variation within the region, the highest attenuation was obtained on land Trinidad.

Quantifying 10 Years of Improved Earthquake‐Monitoring Performance in the Caribbean Region

ABSTRACT Over 75 tsunamis have been documented in the Caribbean and adjacent regions during the past 500 years. Since 1500, at least 4484 people are reported to have perished in these killer waves. Hundreds of thousands are currently threatened along the Caribbean coastlines. Were a great tsunamigenic earthquake to occur in the Caribbean region today, the effects would potentially be catastrophic due to an increasingly vulnerable region that has seen significant population increases in the past 40–50 years and currently hosts an estimated 500,000 daily beach visitors from North America and Europe, a majority of whom are not likely aware of tsunami and earthquake hazards. Following the magnitude 9.1 Sumatra–Andaman Islands earthquake of 26 December 2004, the United Nations Educational, Scientific and Cultural Organization (UNESCO) Intergovernmental Coordination Group (ICG) for the Tsunami and other Coastal Hazards Early Warning System for the Caribbean and Adjacent Regions (CARIBE‐EWS) was established and developed minimum performance standards for the detection and analysis of earthquakes. In this study, we model earthquake‐magnitude detection threshold and P ‐wave detection time and demonstrate that the requirements established by the UNESCO ICG CARIBE‐EWS are met with 100% of the network operating. We demonstrate that earthquake‐monitoring performance in the Caribbean Sea region has improved significantly in the past decade as the number of real‐time seismic stations available to the National Oceanic and Atmospheric Administration tsunami warning centers have increased. We also identify weaknesses in the current international network and provide guidance for selecting the optimal distribution of seismic stations contributed from existing real‐time broadband national networks in the region.

The earthquake sequence of June 1992 near Saba, West Indies

Abstract During June 1992, a minor earthquake swarm occurred close to the island of Saba, a small composite volcano which is the northernmost member of the inner volcanically active Lesser Antilles arc. The initial phase of the seismic activity was recorded at some permanent eastern Caribbean seismic stations, the nearest of which was located 55 km away. On 14 June 1992, a seismograph was installed on Saba to improve the detection and location capacities of the permanent network with regard to this swarm. Sixty events were detected of which only fifteen could be located. The largest earthquake of the sequence occurred on 11 June 1992, had a duration magnitude of M D 4.5 and was felt at Modified Mercalli intensities IV–V in Saba and III in St. Kitts. The epicentres exhibit a SW-NE trend and focal depths are mainly clustered in the range 10–16 km. A t -value of 0.73 was obtained for the sequence. Spectral analysis yielded various source parameters: moments in the range 0.38 × 10 13 −23.66 × 10 13 Nm, source radii from 87 to 126 m and stress drops from 1.3 to 79.9 bar. Moment increases with increasing magnitude while stress drop appears to decrease with decreasing seismic moment, similar to many other studies. Although the evidence as to the nature of the origin of this swarm is not conclusive, it seems to suggest a tectonic rather than volcanic origin.