Very strong mag. 6.6 earthquake - 309 km southeast of Lissabon, Lisboa, Portugal, on Sunday, March 15, 1964 at 22:30 GMT

Date & time: Mar 15, 1964 22:30:28 UTC
Magnitude: 6.6
Depth: 25.0 km
Epicenter latitude / longitude: 36.221°N / 7.59°W (North Atlantic Ocean, Portugal)
Antipode: 36.221°S / 172.41°E
Nearby towns and cities:
93 km (58 mi) SSE of Olhao (Olhão) (pop: 29,200) --> See nearby quakes!
94 km (58 mi) SSE of Faro (pop: 41,400) --> See nearby quakes!
96 km (60 mi) SSE of Laranjeiro (Moncarapacho) (pop: 20,500) --> See nearby quakes!
101 km (63 mi) S of Tavira (pop: 15,100) --> See nearby quakes!
105 km (65 mi) SSE of Quarteira (pop: 18,800) --> See nearby quakes!
109 km (68 mi) SSE of Loule (Loulé) (pop: 23,100) --> See nearby quakes!
113 km (70 mi) SSE of Albufeira (pop: 15,900) --> See nearby quakes!
133 km (82 mi) SE of Portimao (pop: 38,500) --> See nearby quakes!
138 km (86 mi) SE of Lagos (pop: 18,800) --> See nearby quakes!
309 km (192 mi) SSE of Lissabon (pop: 517,800) --> See nearby quakes!
Primary data source: USGS (United States Geological Survey)
Estimated released energy: 5 x 10¹⁴ joules (139 gigawatt hours, equivalent to 1.2 x 10⁵ tons of TNT or 7.5 atomic bombs!) | about seismic energy

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Based on its magnitude, the fault that was active during the quake ruptured along a surface of approx. 398 km² (=154 sqare miles) as a first-order estimate. The length of the rupture zone thus was probably around 35 km (21 mi).
Aftershocks typically occur during the weeks and months following the quake at or near the same fault, at distances of up to approx. two times the length of the rupture zone.
The often broadly linear arrangement of aftershock epicenters encompasses the rupture zone of the main shock (check on the map below to verify).

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