Seismic Strain Rate and Flexure at the Hawaiian Islands Constrain the Frictional Coefficient

Abstract

Flexure occurs on intermediate geologic timescales (∼1 Myr) due to volcanic‐island building at the Island of Hawaii, and the deformational response of the lithosphere is simultaneously elastic, plastic, and ductile. At shallow depths and low temperatures, elastic deformation transitions to frictional failure on faults where stresses exceed a threshold value, and this complex rheology controls the rate of deformation manifested by earthquakes. In this study, we estimate the seismic strain rate based on earthquakes recorded between 1960 and 2019 at Hawaii, and the estimated strain rate with 10−18–10−15 s−1 in magnitude exhibits a local minimum or neutral bending plane at 15 km depth within the lithosphere. In comparison, flexure and internal deformation of the lithosphere are modeled in 3D viscoelastic loading models where deformation at shallow depths is accommodated by frictional sliding on faults and limited by the frictional coefficient (μf), and at larger depths by low‐temperature plasticity and high‐temperature creep. Observations of flexure and the seismic strain rate are best‐reproduced by models with μf = 0.3 ± 0.1 and modified laboratory‐derived low‐temperature plasticity. Results also suggest strong lateral variations in the frictional strength of faults beneath Hawaii. Our models predict a radial pattern of compressive stress axes relative to central Hawaii consistent with observations of earthquake pressure (P) axes. We demonstrate that the dip angle of this radial axis is essential to discerning a change in the curvature of flexure, and therefore has implications for constraining lateral variations in lithospheric strength.

Effects of a Weak Lower Crust on the Flexure of Continental Lithosphere

“…The solutions include 2D time‐dependent displacement, stress, strain rate, and surface flexure, and the grid is Lagrangian to best accommodate the viscoelastic deformation. The same finite element code, CitcomSVE, has been applied with 3D spherical geometry to post‐glacial rebound problems (Paulson et al., 2005; Zhong et al., 2003), and with 3D Cartesian geometry to model volcanic loading at ocean islands and seamounts (Bellas et al., 2020; Bellas & Zhong, 2021; Zhong & Watts, 2013).…”

The magmatic web beneath Hawai‘i

“…These injections would continuously modulate grain sizes in the PSC, prolonging conditions for seismic deformation in the host rock. This process could exploit lateral variations in strength ( 25 ) to produce the laterally compact seismogenic features that we observe.…”

Plate bending earthquakes and the strength distribution of the lithosphere

“…To aid discussion, we characterise the saturation moment of different rheological model as being 'weak', 'intermediate' and 'strong'. The proposed strength of the oceanic lithosphere in previous studies spans this range (Chapple & Forsyth, 1979;Bellas & Zhong, 2021). These regions are shown in Fig.…”

“…One of the confounding factors for seamount flexure is that thermal perturbation due to hotspot activity may reduce the apparent strength of the lithosphere (e.g., Pleus et al, 2020), although Bellas et al (2022) have argued that this is not capable of explaining seamount flexure generally. When thermal perturbation is ignored, strength models with a friction coefficient of 0.3 have been inferred by several studies Bellas & Zhong (2021); Pleus et al (2020). The latter study concludes that weaker members of published LTP laws are satisfactory (e.g., , which, along with a friction coefficient of 0.3, implies strength in the mid-intermediate range.…”