I don't think it's gravity as that is well measured to the scale of nm/s^2 and any external localized field distortion would have been measured. If I have to g

I don't think it's gravity as that is well measured to the scale of nm/s^2 and any external localized field distortion would have been measured. If I have to guess, it would be that the below Indonesia (where the bulge is according to the article) there is more radiative cooling happening either due to a thinner crust or higher concentration of more thermally conductive elements in the inner and outer core/mantle/crust. These effects can be localized because the soup from which the Earths' insides were made was "liquid" to a point and so an even mix of elements is highly unlikely even after accounting for billions of years of gravitational differentiation. For example, the outer core is the only technically liquid part and it still has an incredibly high viscosity (1000s to billions of times the viscosity of water [1]) and definitely you wouldn't call that a liquid in your day-to-day life. [1] https://arxiv.org/abs/0709.3333 https://arxiv.org/abs/0709.3333