Japan

Japan Mainshock Forecasts

Northern Honshu

Official research shows that a megaquake is likely to hit Northern Honshu in the near-term and Japanese authorities have already warned their people to be prepared (Megaquake advisory ended in Japan, but officials say disaster risk remains | NHK WORLD-JAPAN News).

From independent research, www.mainshockearthquakes.com, this author forecasts : a possible M 8.5 megaquake on around May 19 ±2 days, with a 95% probability that the magnitude will fall between M 8.4 and M 8.6.

This updated forecast (May 1 UTC) is based on ten estimates from the author’s various research analytical modules (Ecological Model Overview) for the magnitude and/or timing of this anticipated megaquake, with supporting graphs and information [see below].

Honshu’s previous M 9.1 Great Tohoku Earthquake [2011] is compared with current seismic data in the following way:

the first set of four graphs [Fig 1, below] examines:

(a) the magnitudes of these quakes, and
(b) their timing,

—all within a 750 km radius of the previous M 9.1 epicentre. The analysis is organized according to New Moon to New Moon (NM-NM) monthly intervals—this timeframe was selected because the 2011 M 9.1 Great Tohoku Earthquake occurred during a NM period, and the anticipated 2026 M 8.5 megaquake is also expected within this window.

Graph of largest monthly earthquakes in each New-Moon to New-Moon period before the Great Tohoku Quake, of 2011. Plots largest earthquakes in each NM-NM cycle from Jan 2010 to the M9.1 earthquake in 2011, and fits ecological-model triangle to data
Line graph showing the 750 NM-NM magnitudes of a Northern Honshu earthquake in 2026, with a blue line indicating daily magnitude fluctuations since January 1, 2025, and red annotations including a major magnitude of 8.6 on May 19, 2026.
Japan Northern Honshu 750km-radius search zone from New Moon to New Moon. Graph of largest monthly earthquake event timing from 2010 to the megaquake of 2011 Figure-3
Figure 4 Graph and fitted analytic geometry to Japan Northern Honshu largest monthly earthquakes 750km radius from M9.1 epicentre NM-NM timing for period 2025-2026 with latest Japan mainshock forecast

Analysis of Pre-Mainshock Seismic Intervals in Northern Honshu

Time Intervals Between Largest Monthly Earthquakes

Figures 3-4 provide a detailed visual representation of the time intervals between consecutive largest monthly earthquakes, specifically from one New Moon to the next, occurring within a 750-km radius of the epicentre of the Northern Honshu offshore M 9.1 mega-earthquake on 11/03/2011. These events are significant as they offer insight into the seismic patterns leading up to a major mainshock.

Theoretical vs Practical Oscillation Around the Lunar Period

In theory, the intervals between these significant monthly quakes are expected to oscillate around the lunar period, which is roughly 29.5 days. However, the actual data presented in Figure 3 indicates that the mean interval measured was 27.6 days, whereas Figure 4 shows that for current data this mean interval is 29.1 days. These slight deviations may suggest the influence of additional geological or tidal factors affecting the seismic cycle in this region. However, there may be lack of complete homogeneity between interacting earthquakes in this large zone with a radius of 750 km, as indicated by some irregularity in the curves connecting consecutive monthly points, particularly at the beginning of the sequences shown in these graphs.

Patterns Prior to the Mainshock

The classical mid or penultimate curve, as depicted in both figures, highlights the longest time interval between consecutive largest monthly earthquakes. This extended gap typically occurs before the onset of a major seismic event, known as the mainshock. The timing of these largest monthly earthquakes, in the period preceding the mainshock, tends to resolve in a nearly linear fashion towards the average interval or a common convergence point.

Convergence on the Mainshock Event

For a past mega-earthquake, Figure 3 demonstrates that the converging lines representing the above intervals all meet at 'Day 435', which corresponds to the day of the M 9.1 earthquake on Mar 11, 2011, and the subsequent devastating tsunami. This convergence visually emphasises the tendency for seismic activity to align towards a critical point in time just before a significant tectonic release.

For the currently anticipated mega-earthquake, Figure 4 shows curve amplitudes or time intervals, and parallel lines indicating levels of symmetry, converging towards the expected level which in this case is day 504 (May 19, 2026) representing 29 days since the previous large M 7.4 quake on Apr 20. This is typical of earthquake patterns preceding mainshocks wherein the main event resolves to ‘unity’ and occurs ‘one month’ after the previous month’s largest event. This pattern is supportive of the current forecast, but it in no way proves that it will happen around May 19. However, there is particularly strong supportive evidence from Figure 2 where the apex for the Triangular Module peaks at a M 8.6 on May 19.

Page updated May 2, 2026