Shear-wave structure of the lithosphere above the Hawaiian hot spot from two-station Rayleigh wave phase velocity measurements

submitted to Geophysical Research Letters

Keith Priestley and Frederik Tilmann

Bullard Laboratories, Dept. of Earth Sciences, University of Cambridge, Cambridge, U.K.

( keith@esc.cam.ac.uk , tilmann@esc.cam.ac.uk )

Abstract

We have measured fundamental mode Rayleigh wave phase velocity dispersion from seismograms of five earthquakes recorded at stations on the islands of Hawaii and Oahu and inverted these data for the mantle velocity structure. The seismic lithosphere of the velocity model is km thick, which is similar to that of 80-90 Myr oceanic lithosphere. This indicates that no significant lithospheric thinning takes place above the mantle plume. Below the lithosphere the shear wave velocity decreases to 4.0 kms. The seismic model is consistent with the structure of the Hawaiian plume model of Watson and McKenzie [1991] which has a 72-km thick mechanical boundary layer with the shallowest melting occurring at 82 km depth. The difference between the dispersion measured on the Hawaii-Oahu path and that observed by Woods and Okal [1996] on the Oahu-Midway path can be explained by the cooling of the injected hot plume material.

Figures

(Click for full-size images)

Figure 1. Comparison of the phase velocity for the Hawaii-Oahu path with the phase velocity measured for the Oahu-Midway path [Woods and Okal, 1996] and pure-path phase velocities for four ages of Pacific crust [Nishimura and Forsyth, 1988].

     

Figure 2. Summary of the inversion results for the Hawaii--Oahu dispersion data. (a) The left most panel shows the starting (dotted line), an intermediate (dashed line) and the final (solid line) shear wave velocity models. The three panels to the right of the velocity models show the resolution kernels for: (i) the lid of the intermediate (dashed line) and final (solid line) models; (ii) the upper (dashed line) and lower (dot--dashed line) transition layers between the lid and low velocity layer of the intermediate model; and (iii) the low velocity layer of the intermediate (dashed line) and final (solid line) models. Panels (i) and (iii) have a scale of 1.0, (ii) has a scale of 0.5. (b) The match of the dispersion curve for the final velocity model (solid line) to the observed dispersion data. The dashed lines denote dispersion curves for a lithosphere with a thickness +/-7 km different from the thickness of the final velocity model.

Figure 3. Comparison of the final mantle velocity model from the Hawaii-Oahu dispersion data with the velocity model from the Oahu--Midway dispersion data [Woods and Okal 1996] and the velocity models from the pure-path dispersion data for the Pacific [Nishimura and Forsyth, 1988]. The four dotted curves going from slow to fast velocities are for <4, 4-20, 20-52, and 52-110 Myr old oceanic lithosphere.

Supplementary material

Figure S1. Map showing the southern end of the Hawaiian swell and the great circle paths of the events to the far station. The maximum deviation in the azimuth of the great circle path between the event and a station and the great circle path between the stations is 6o. Site KIP indicates two broadband stationns, which are maintained by IRIS and GEOSCOPE, respectively. Site HAW indicates a small aperture array of 6 Guralp CMG-3T broadband sensors. The stations are so close together that the signal does not significantly differ between them for the frequencies we are investigating. As no single station was up and showed data of the highest quality for all events, for each event the station with the lowest level of local noise was chosen.

Date Time Latitude Longitude Depth Mag BAZ in KIP Region
95/01/06 22:37:37.9 40.33 142.41 48 6.7 304.9 Near East Coast of Honshu
96/02/07 21:36:45.1 45.29 150.45 49 7.0 312.2 Kuril Islands
96/02/16 15:22:57.8 37.32 142.31 40 6.2 301.2 Off East Coast of Honshu
96/05/07 23:20:00.6 43.67 147.69 55 5.6 309.6 Kuril Islands
96/09/05 8:14:14.4 -22.32 -113.28 16 7.0 133.3 Easter Islands Region

Table S1. List of the events used in the inversion study

Table of dispersion measurements

Table S2. Fundamental mode Rayleigh dispersion measurements in tabular form.


Frederik Tilmann
Mon Sep 21 14:10:02 BST 1998