"We've had similar swarms around Waimangu, Ngakuru and Waikite, very similar to these. It just happened you were right in the area of the epicentres.
"There are a number of faults that are all through the zone and it's not surprising we have activity right under the city.
"One of the main faultlines runs from Mount Edgecumbe to Edgecumbe and up through Waikite. That's a main lineament. There's another in Rotorua."
The first earthquake hit at 6.57pm. It was a 2.3 magnitude at a depth of 10km. There were a further 10, with the last the same depth and magnitude as the first, at 8.23pm.
But why do earthquakes swarm?
"Earthquakes can group into a swarm because stresses tend to concentrate at zones of weakness," said Dr Jolly.
"Think of slowly bending a stick. It eventually makes a cracking sound and if you keep bending the stick it tends to crack more at the weakest point. The earth does the same thing.
"There's nothing to be worried about. It's not something we would raise a flag of worry for - these things happen all the time.
"You can look at Matata, Rotoiti, Waimangu. Even in the past two to three years you can find clusters like these.
"These were under the city and that's why they were noticed more.
"Sometimes they can occur for a couple of hours, then you don't notice another one for months or years."
Within seconds of an earthquake happening the GNS Science website Geonet has already listed the quake and its statistics. The site is the go-to place for the internet-savvy to find out how strong the tremors were and where they were centred.
Dr Jolly said there were sensors around Lake Rotorua as well as in other locations in the Taupo Volcano Zone.
These didn't pick up every earthquake and GNS Science relied on members of the public to file "felt" reports" for the smaller quakes.
"Small but felt earthquakes won't get automatically on the system and if we see something that has been felt we get it online as quickly as possible."
The largest earthquake in the Bay of Plenty was the 6.5 Edgecumbe earthquake in 1987.
There was a swarm of earthquakes near Rotoiti in 2004 with the largest a magnitude 5.4.