http://ajw.asahi.com/article/behind_news/social_affairs/AJ201505310018
A deeply focused earthquake with a magnitude of 8.5 (Japanese Meteorological Agency) struck the area west of the Ogasawara Islands at 8:24PM Saturday Japan time. The USGS pegged the quake as a M7.8. The quake measured an upper 5 on the Japanese seismic intensity scale of 0 to 7 (with 7 being the strongest) nearest the epicenter as well as in some parts of the main island of Honshu. The quake fortunately occurred at a very deep depth of 590km (367 miles) in the crust (thus no crustal deformation) which reduced the potential for a tsunami to be generated.
The epicenter was located some 1000km (621 miles) away from Tokyo. The quake was strong enough though where shaking could be felt all the way from Hokkaido to Okinawa (with varying degrees of seismic intensity ranging from 1-3). The strongest shaking however was felt on the main island of Honshu with parts of Kanagawa Prefecture like the town of Ninomiya facing Sagami Bay and Yokosuka in Yokohama, experiencing severe tremors that registered a lower-5 on the Japanese seismic intensity scale. The 23 wards of Tokyo experienced intensities of around 4 while some areas of Saitama Prefecture (just north of Tokyo) experienced a lower-5.
For comparison, the whole of Tokyo-to experienced shaking of 5 to upper-5 during the March 11th Great East Japan Earthquake (and with a much longer duration). Seismic waves from deeper earthquakes can travel much farther and have its energy focused in certain areas depending on the strata that it is traversing (thus why some areas farther away from the epicenter like Saitama experienced stronger tremors).
This quake for many started off as a mild shaking/rolling wobble for around 30 seconds which then subsided, and was then followed up by much stronger jolts/shaking for the next 30 seconds (again, location dependent). According to the experts, the deep focus caused extended lateral movements for up to a minute. While they also noted that a lot of aftershocks weren’t likely (due to the deep focus), at least one M6.3 quake occurred (but farther north from the original epicenter) around 6 hours later.
As expected, major rail lines in Tokyo came to a halt and inspections were carried out before service resumed around 30 minutes later. News reports also indicated some pockets of power outages as well as people being trapped in an elevator in a Tokyo skyscraper.
One common question is this a foreshock that could portend something larger? The March 11th megathrust quake was preceded by a M7.3 earthquake with an epicenter in the same general region two days earlier. Based on what I remember from my college geology courses (thus definitely not an expert though I’ve always had an interest in things like this) in this case though, the deeply focused aspect (think ancient oceanic rock that sheared under intense pressure) aspect of this quake makes this far less likely to be a foreshock since this quake was not a rupture/fracture along a fault line which usually results in a sudden release of strain energy; and that energy release can manifest itself in the form of both foreshocks and aftershocks.
