1900年代初頭、東京帝国大学（現在の東京大学）の教授である今村明恒は、仁和地震や宝永地震などの過去の地震の歴史記録について議論しました。今村は、これらの五畿七道大地震がすべて南海道沖を震源とする巨大な地震であり、津波を伴っていたと提案しました。また、今村は1928年に、南海道地震研究所（現在の東京大学地震研究所、和歌山地震観測所）を私財で設立しました。澤村武雄（1951年）は、昭和南海地震後に行われた水路部による測量の結果から、四国南部での南東上りの傾動が明らかになり、室戸岬の隆起と高知平野の沈降など、歴史的な地震で知られている地殻変動とほぼ一致していることを示しました。また、白鳳から昭和にかけての南海道沖地震の震源が、大規模な北傾斜の断層線上に並ぶことから、この衝上断層を「南海スラスト」と名付けました。その後、1960年代にプレートテクトニクスが発展し、金森博雄（1972年）は昭和東南海・南海地震の震源断層モデルを求め、これらの地震が南海トラフのプレート境界で発生していることを明らかにしました。
2003年時点での「東南海、南海地震等に関する専門調査会」による検討では、今後発生が予測される南海トラフの地震の中で最大のものはマグニチュード8.7で、破壊領域は長さ約600kmの3連動である東海・東南海・南海地震とされていました。しかし、2011年の東北地方太平洋沖地震の発生後、この仮定は見直されました。これらの3つの地震が同時にまたは短い間隔で発生した場合、太平洋ベルト全体にわたる地震動による被害が拡大し、地域間の支援が実質的に不可能となると見られています。そのため、地方自治体は地震の連鎖発生を考慮した災害対策を急務としています。2010年には、初めてこれらの3つの地震が同時に発生することを想定した訓練が実施されました。
津波に関しては、東海地震、東南海地震、南海地震が同時に発生するか、数分から数十分の時間差で発生する場合、波の高さが重なり合って土佐湾西部と東海沿岸の一部の狭い範囲で最大で約10メートルに達する可能性があるとシミュレーションされています。特に浜岡原発に近い御前崎付近では、同時発生の場合と比較して海上の波の高さが2倍以上になり、11メートルに達することがあります。さらに、南海トラフから琉球海溝まで全長1,000キロに及ぶ断層が一斉に破壊されることで、広範な地域でM9クラスの超巨大地震や二連動のM9クラス超巨大地震が発生する可能性が近年指摘されています。
In the early 1900s, Professor Akitsune Imamura, who was a professor at Tokyo Imperial University (now the University of Tokyo), discussed the historical records of major earthquakes such as the Ninna Earthquake and Hoei Earthquake. He proposed that these Five Provinces Seven Circuits Great Earthquakes, including those in the Kinai region, were all massive earthquakes accompanied by tsunamis, originating off the coast of Nankaido. Imamura further established the Nankai Earthquake Research Institute (now the University of Tokyo Earthquake Research Institute, Wakayama Earthquake Observatory) in 1928 using his personal funds. Takeshi Sawamura (1951) revealed southeastward tilting in the southern part of Shikoku based on measurements conducted by the Ministry of Public Works after the Showa Nankai Earthquake. This tilting was found to be consistent with crustal movements associated with the uplifting of Muroto Cape and subsidence in the Kochi Plain, both known from historical earthquakes. Moreover, the earthquake sources of successive Nankai Trough earthquakes from the Baien to the Showa periods, sharing common characteristics, were found to align along a large north-dipping fault line extending from Off Choshi to Off Ashizuri, prompting the naming of this thrust fault as the "Nankai Thrust" [41][145][146]. Subsequently, in the 1960s, as plate tectonics developed, Hiroo Kanamori (1972) sought a fault model for the Showa Tokai and Nankai Earthquakes, revealing that these earthquakes occur at the plate boundary of the Nankai Trough [147].
As of 2003, the "Special Investigation Committee on Tokai, Tonankai, and Nankai Earthquakes" considered the largest anticipated earthquake on the Nankai Trough, among those predicted to occur in the future, to have a magnitude of 8.7 and a destructive range of approximately 600 km in three consecutive movements, namely Tokai, Tonankai, and Nankai Earthquakes [148]. However, this assumption was reassessed after the occurrence of the 2011 Tohoku Earthquake. If these three earthquakes were to occur simultaneously or in rapid succession, like the Ansei Earthquakes, seismic damage due to ground shaking would extend across the entire Pacific Belt, making regional mutual support and relief efforts practically impossible. Consequently, local governments are urged to implement disaster measures considering the occurrence of a cascade of earthquakes. In 2010, for the first time, a training exercise assuming the simultaneous occurrence of these three earthquakes was conducted [149].
Regarding tsunamis, simulations suggest that if the Tokai, Tonankai, and Nankai Earthquakes were to occur either simultaneously or with a time lag of several minutes to several tens of minutes, the overlapping of wave heights could lead to tsunami heights reaching nearly 10 meters in some narrow areas along Tosa Bay and the eastern coast of Tokai. Particularly, near the Hamaoka Nuclear Power Plant, the offshore wave height could be more than twice as high as during a simultaneous occurrence, reaching 11 meters [150]. Furthermore, the occurrence of a cascade of earthquakes every few hundred years involving the entire fault from the Nankai Trough to the Ryukyu Trench, leading to a large-scale M9-class cascade earthquake or two consecutive M9-class mega-earthquakes, has been suggested in recent years [152][153]. The total length of the fault in this case would rival that of the 2004 Sumatra Earthquake, with an average recurrence interval of approximately 1,700 years. This hypothesis is supported by the discovery of extensive uplifting features at three locations, Muroto Cape (Shizuoka Prefecture), Cape Ashizuri (Kochi Prefecture), and Kikaijima (Kagoshima Prefecture), compared to the predicted uplift from conventional Nankai Trough-related earthquakes. However, some researchers argue that these features are associated with inland crustal earthquakes rather than massive plate boundary earthquakes [161].