Decade-Scale Correlation Between Crustal Deformation, Geophysical Variation And Length Of Day

Decade fluctuation in length of day (lod) is one typical features of the Earth's rotation variation, which is generally attributed to exchanges of angular momentum between the core and the mantle. Three possible core-mantle coupling mechanisms, topographic coupling, electromagnetic coupling and gravitational coupling, have been proposed, but no one was thoroughly, verified or ruled out. Similar to decade fluctuation of lod, crustal deformation and geophysical field also fluctuate on time scales of several years to decade years, the mechanisms are not well understood yetIn order to explore the possible correlation between fluctuations of lod and variations of crustal deformation as well as geophysical observations, we extensively collected observation data obtained at earthquake-precursor observatories (sites) in Chinese mainland, which including fault deformation monitoring data of nearly 200 fault sites, ground tilt observations of over 30 gauges, stress/strain observations of over 20 gauges, gravity change observations of 7 stations, shallow soil temperature observations of 8 stations, the time spans of the collected observation data are generally 10 to 30 years. Apart from the observation data of earthquake-precursor observatories, we also collected the repeated leveling data of Hexi-Qilian mountain area with time span of 30 years, and repeated gravity measurement data of Beijing area with time span of 20 years. By Internet resources and e-mail communication, we also obtained the deformation data of San Andreas Fault, California, as well as the tilt/strain observations of Friuli area, NE Italy. The annual values of geomagnetic filed at 200 observatories all over the world were also downloaded from the World Data Center.Time series comparisons demonstrate that, quite part of fluctuations in crustal deformation and geophysical observations are dynamically correlated to decade variation of lod. The decade correlation between lod and crustal deformation as well as geophysical observations usually appears as globally quasi-synchronous change, location-dependent and direction-dependent. The decade correlation not only exists in original observations, but also appears in the detrended observations in many cases.As observed, the lod-correlated variation of fault deformation and tilt observations can reach respectively several centimeters and 4 to 5 arc seconds at maximum, the maximum variation of strain, stress and gravity correlated to lod fluctuation, then, can reach 10^-6 order, several kPa and several decades Gal respectively, regional vertical deformation change correlated to lod can also reach the maximum of several centimeters (for example, the Hexi corridor-Qilian mountain area).Theoretically, decade variation of the Earth's rotation rate may induce decadal fluctuations of surface displacement, tilt, strain and gravity by centrifugal potential or force, the induced theoretical variations, however, are generally 4⁵ order less than the observed fluctuations, so rotational rate change of the earth is not the major mechanism for decadal correlation between crustal deformations, geophysical variations and length of day.Jointly referring to research results on decade variation of lod and secular variation of geomagnetic field, the most possible mechanism for decade correlation between crustal deformations and length of day is: flow disturbance of the outer core induces decadal fluctuation of the mantle's rotational rate on one hand by angular momentum exchange (rigidity behavior of the mantle and the crust), and causesdecadal fluctuations of crustal deformation on the other hand by core-mantle boundary (CMB) forces (elastic behavior of the mantle and the crust), both decadal variations of lod and crustal deformations arise from core-mantle coupling process.Decadal variation of main geomagnetic filed directly arises from electromagnetic induction induced by flow disturbance of the outer core. Decade fluctuations of surface gravity observations are closely related to core-mantle coupling induced crus...