Lithospheric flexure under the Hawaiian volcanic load: Internal stresses and a broken plate revealed by earthquakes

Abstract

Several lines of earthquake evidence indicate that the lithospheric plate is broken under the load of the island of Hawai`i, where the geometry of the lithosphere is circular with a central depression. The plate bends concave downward surrounding a stress‐free hole, rather than bending concave upward as with past assumptions. Earthquake focal mechanisms show that the center of load stress and the weak hole is between the summits of Mauna Loa and Mauna Kea where the load is greatest. The earthquake gap at 21 km depth coincides with the predicted neutral plane of flexure where horizontal stress changes sign. Focal mechanism P axes below the neutral plane display a striking radial pattern pointing to the stress center. Earthquakes above the neutral plane in the north part of the island have opposite stress patterns; T axes tend to be radial. The M6.2 Honomu and M6.7 Kiholo main shocks (both at 39 km depth) are below the neutral plane and show radial compression, and the M6.0 Kiholo aftershock above the neutral plane has tangential compression. Earthquakes deeper than 20 km define a donut of seismicity around the stress center where flexural bending is a maximum. The hole is interpreted as the soft center where the lithospheric plate is broken. Kilauea's deep conduit is seismically active because it is in the ring of maximum bending. A simplified two‐dimensional stress model for a bending slab with a load at one end yields stress orientations that agree with earthquake stress axes and radial P axes below the neutral plane. A previous inversion of deep Hawaiian focal mechanisms found a circular solution around the stress center that agrees with the model. For horizontal faults, the shear stress within the bending slab matches the slip in the deep Kilauea seismic zone and enhances outward slip of active flanks.

Why Is Crustal Underplating Beneath Many Hot Spot Islands Anisotropic?

“…(a) The standard model in which Hawaii Island is supported by an elastic Pacific Plate predicts poorly the depth variation of inferred earthquake stresses. (b) The model suggested by Klein () in which Hawaii Island is supported by edge‐bending of a broken plate fits the patterns of seismicity better. (c) Metasomatic underplating prescribes volume increases within the upper portion of the mantle lithosphere, generating an extensional strain in the overlying crust that resembles the strain associated with the broken‐plate model.…”

“…Owing to the restricted aperture of the land‐based seismic network on Hawaii Island, only axes of greatest pressure were estimated for earthquakes during 1970–2007, with Richter magnitude ≥2.5. Klein () found marked differences in focal parameters for seismic events above and below a horizontal plane at 21‐km depth that displayed almost no seismicity. Below 21‐km depth, most earthquakes displayed P axes that aligned with radial spokes emanating from a “stress center” at the center of the island, near the Mauna Loa summit and a cluster of long‐period earthquakes at 40–60‐km depth that Klein () identifies with the principal magma conduit for Mauna Loa.…”

“…Klein () found marked differences in focal parameters for seismic events above and below a horizontal plane at 21‐km depth that displayed almost no seismicity. Below 21‐km depth, most earthquakes displayed P axes that aligned with radial spokes emanating from a “stress center” at the center of the island, near the Mauna Loa summit and a cluster of long‐period earthquakes at 40–60‐km depth that Klein () identifies with the principal magma conduit for Mauna Loa. Above 21‐km depth, the P axes of lithospheric quakes are more scattered, but tend to align their P axes with circles about this stress center, perpendicular to the radial‐spoke alignment of the deeper quakes.…”

“…Klein () notes that Hawaii's earthquake mechanisms are inconsistent with stresses expected if plate flexure supports the island; that is, the lower half of the lithosphere should be in tension. The T axes of deeper earthquakes should be aligned with radial spokes from the center of the load (Figure a), and the upper half of the lithosphere should be in compression, with radially oriented P axes.…”

“…The plate flexure proposed by Klein () poses obstacles for the magmatic underplating model. Compression in the lower lithosphere would impede the diffuse porous‐flow or fracture‐guided melt migration processes described by Richards et al () and Olugboji and Park ().…”