Seismic velocity variations under island arcs: examples from the Philippines and Montserrat (Lesser Antilles)

Open Access
Author:
Sevilla, Winchelle Ian
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 29, 2010
Committee Members:
  • Charles James Ammon, Dissertation Advisor
  • Charles James Ammon, Committee Chair
  • Maureen Feineman, Committee Member
  • Barry Voight, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • subduction zones
  • Philippines
  • Montserrat
  • tomography
  • receiver functions
  • island arcs
Abstract:
Island arcs are geologically active and important structures. From a short-term perspective, they are a major source of seismic and volcanic hazards. From a longer-term perspective, arc processes are most likely a key component in the production of continental lithosphere. They are also the focus of numerous Geoscience investigations. In this thesis I investigate the seismic structure of island arcs at a regional (hundreds of kilometer) and a local (10's of km) scale. My goal in this work is to contribute to our efforts to understand the origin and evolution of these geologically important structures. I focus seismic imaging methods on two regions, the Philippine Island region and the northern Lesser Antilles island of Montserrat. The Philippine Island Arc (PIA) is commonly regarded as a complex structure in which subduction zones border its sides and the intra-arc, sinistral Philippine Fault System transects throughout its length. The arc is seismically active and volcanic activity spans almost the entire arc. While several studies provide a wealth of information on the tectonic and the geodynamic settings of PIA, few have looked carefully into the subsurface because they were limited by the availability of digital seismic data. For this reason, important data gaps exist, in particular the details of the subsurface seismic velocity structure. The recent deployments of relatively dense digital seismic stations offer an opportunity to conduct a detailed study on the arc's velocity structure. Data from this new seismic network are used to determine the three-dimensional (3-D) velocity structure of the PIA by applying the P–wave travel time tomography. A broad distribution of source depths and the arc-wide distribution of seismic stations allow tomographic imaging of structures down to 450 km depth with spatial resolution of about ~50 km resolution. The prominent features of the tomographic images include the low velocity zones correlating with the overlying volcanic structures and high velocity zones that more or less coincide with the Wadati-Benioff zones of the subduction zones. The slabs are imaged as 2–6% faster than the mantle velocity values of the IASP91 model. They commonly extend deeper than the seismicity suggesting that they penetrate aseismically to greater depths. Shallow low-velocity anomalies correlate with the fore-arc and intra-arc basins, in the mantle wedge at the top of the slabs, deep in the upper mantle, as well as the segments of the Philippine Fault Zones. Images of island arc structure are generally limited to the high, narrow frequency bands employed in active-source seismic experiments or smoothed over substantially in low-frequency global and regional tomography. A second focus of the work in this thesis is the imaging the regional– and local–scale structure of an active arc, the island of Montserrat, located in the northern Lesser Antilles arc. I use receiver functions and local-earthquake generated P-wave arrival times to estimate the first-order subsurface seismic structure in the vicinity of the island. The receiver functions provide information on the first-order shear velocity and Poisson's ratio of the crust. Although the precise sharpness of the crust-mantle transition is not resolved, this study can limit its thickness to less than ~4 km. The estimated mean crustal thickness of ~30 km and the observations suggest that the crust may be slightly thinner northwest of the island (~26–30 km) than it is to the south (~30–34 km). The high P–wave speed and Poisson's ratio indicate a generally mafic lower crust, with rocks of intermediate composition not precluded in the upper part of the lower crust. We did not find evidence for a thick high-speed lower crust (>7.4 km/s) as has been inferred in some other arcs. P-wave travel times are used to image the upper crust of the island, which includes the actively erupting Soufrière Hills Volcano. As with many studies of the island, imaging deeper than about 5–to–6 km beneath the volcano is very difficult because of limited ray coverage at depth. The earthquake-produced travel times primarily sample and illuminate the low-velocity upper regions of the island. The results are generally consistent with the results from recent active-source work imaging beneath the island.