M 4.7 Earthquake along Lake Toba, Sumatera

There was a M=4.7 earthquake today in central Sumatra along the Sumatra fault system. Interesting that it is following the 6.1 further to the north just a few days ago. in the map below, the earthquakes associated with the M 6.1 are in yellow. today’s earthquake is in orange. Possibly triggered from the earlier earthquakes, though these are distanced by ~300 km (so seems less likely, depending upon the state of stress on the fault(s).
Today’s earthquake is aligned on the western edge of Lake Toba formed in the caldera left behind from the largest volcanic eruption on earth during the Quaternary period (~1.8 Ma until the present). This volcanic center is aligned with fracture zones located in the subducting India-Australia plate (eg the Investigator fracture zone).
here is the USGS web page for this earthquake. There is a write up about the tectonics of the region. They also have a map, view it with a grain of salt.
M=6.1 Sumatra

M 6.1 in northern Sumatra

nice strike slip earthquake here in northern sumatra. pretty big, hope not many people were hurt. but, much smaller than could be observed on this fault system.
the sumatra fault system is a right lateral strike slip fault system that is the result of oblique convergence on the subduction zone here. when plate motion on a thrust/reverse fault system is not perpendicular to the fault strike ( in other words, “oblique”), the the strain is partitioned to the thrust/reverse fault and a strike slip fault. the thrust/reverse fault will slip mostly with pure dip slip (i.e. a pure thrust earthquake). the amount of plate motion that is not perpendicular to the thrust fault is applied to the strike slip fault. the strike slip fault is called a forearc sliver fault. sumatra is an island partly formed by the volcanic arc related to the subduction zone (so the sumatra fault is more of an “arc” sliver, haha). there is another strike slip system (which may, in places, be a thrust/reverse system) in the forearc, called the mentawai fault [system].
another eg of a forearc sliver: the recent earthquake swarm in haida gwai (northwest of Vancouver island) was a thrust earthquake and the overlying queen charlotte fault is probably a forearc sliver in that location (but a strike slip fault further north?). by the time the queen charlotte becomes the fairweather fault, it is dominantly strike slip.
following the 2004 sumatra-andaman subduction zone earthquake, studies of coulomb stresses showed an increase of stress imparted onto the northern sumatra fault as the result of the subduction earthquake. this sort of stress increase is typically short lived (and really small compared to the amount of stress that is released by an earthquake). so, it would appear that the sumatra fault was not loaded sufficiently to be triggered by the subduction earthquake. so, this M=6.1 is probably not related to the subduction earthquake, but to the tectonic setting in general.
here is the USGS web page for this earthquake.
M=6.1 sumatra