Postearthquake relaxation evidence for laterally variable viscoelastic structure and water content in the Southern California mantle

Journal of Geophysical Research B: Solid Earth
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Abstract

I reexamine the lower crust and mantle relaxation following two large events in the Mojave Desert: the 1992 M7.3 Landers and 1999 M7.1 Hector Mine, California, earthquakes. Time series from continuous GPS sites out to 300 km from the ruptures are used to constrain models of postseismic relaxation. Crustal motions in the Mojave Desert region are elevated above background for several years following each event. To account for broadscale relaxation of the lower crust and mantle, the Burgers body model is employed, involving transient and steady state viscosities. Joint afterslip/postseismic relaxation modeling of the GPS time series up to one decade following the Hector Mine earthquake reveals a significant rheological contrast between a northwest trending “southwest domain” (that envelopes the San Andreas fault system and western Mojave Desert) and an adjacent “northeast domain” (that envelopes the Landers and Hector Mine rupture areas in the central Mojave Desert). The steady state viscosity of the northeast domain mantle asthenosphere is inferred to be ∼4 times greater than that of the southwest domain. This pattern is counter to that expected for regional heat flow, which is higher in the northeast domain, but it is explicable by means of a nonlinear rheology that includes dependence on both strain rate and water concentration. I infer that the southwest domain mantle has a relatively low steady state viscosity because of its high strain rate and water content. The relatively low mantle water content of the northeast domain is interpreted to result from the continual extraction of water through igneous and volcanic activity over the past ∼20 Myr.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Postearthquake relaxation evidence for laterally variable viscoelastic structure and water content in the Southern California mantle
Series title Journal of Geophysical Research B: Solid Earth
DOI 10.1002/2014JB011603
Volume 120
Issue 4
Year Published 2015
Language English
Publisher American Geophysical Union
Contributing office(s) Earthquake Science Center
Description 25 p.
First page 2672
Last page 2696
Country United States
State California