Source process and tectonic implications of the great 1975 North Atlantic earthquake

Abstract

Summary. The Atlantic segment of the Africa–Europe plate boundary has usually been interpreted as a transform boundary on the basis of the bathymetric expression of the Gloria fault and dextral strike‐slip first‐motion mechanisms aligned along the Azores–Gibraltar line of seismicity. The 1975 May 26 earthquake (Ms=7.9) was assumed to fit into this framework because it occurred in the general area of this line and has a similar first‐motion focal mechanism (strike=288°, dip=72°, slip angle=184°). However, several anomalies cast doubt on this picture: the event is abnormally large for an oceanic transform event; a sizeable tsunami was excited; the aftershock area is unusually small for such a large event; and most significantly, the epicentre is 200 km south of the presumed plate boundary. The Rayleigh wave radiation pattern indicates a change in focal mechanism to one with a significant dip‐slip component. The short duration of the source time history (20 s, as deconvolved from long‐period P‐waves), the lack of directivity in the Rayleigh waves, and the small one‐day aftershock area suggest a fault length less than 80 km. One nodal plane of the earthquake is approximately aligned with the trace of an ancient fracture zone. We have compared the Pasadena 1‐90 record of the 1975 earthquake to that of the 1941 North Atlantic strike‐slip earthquake (200 km to the NNW) and confirmed the large size of the 1941 event (M=8.2). The non‐colinear relationship of the 1975 and 1941 events suggests that there is no well‐defined plate boundary between the Azores and Gibraltar. This interpretation is supported by the intraplate nature of both the 1975 event and the large 1969 thrust event 650 km to the east. This study also implies that the largest oceanic strike‐slip earthquakes occur in old lithosphere in a transitional tectonic regime.

Revision of the Portuguese catalog of tsunamis

“…These events not only follow the Gloria Fault itself but also a second source zone also striking almost EW, ca. 200 km south (Lynnes and Ruff, 1985). This can correspond to either the development of a tectonic block or the re-activation of a previous transform domain (Kaabouben et al, 2008).…”

Fault parameters and tsunami excitation of the May 23, 1989, MacQuarie Ridge Earthquake

“…Both observation and theory showed that this is not the case. The North Atlantic earthquake of 1975, a strike-slip event, generated observable tsunamis [Lynnes and Ruff, 1985]. Tsunamis were also recorded from the 1906 San Francisco earthquake.…”

New study on the 1941 Gloria Fault earthquake and tsunami

“…Antunes (1944) presented the first epicentre location, using only data from the Portuguese seismic network, at −18.9 • E, 38.7 • N. Gutenberg and Richter (1949) relocated the epicentre at −18.5 • E, 37.50 • N and computed the earthquake magnitude as M s = 8.2. Later, Lynnes and Ruff (1985), using a master-event technique relative to a better characterized event (26 May 1975), relocated the epicentre at −19.1 • E, 37.6 • N. Udias et al (1976) computed the earthquake magnitude as 8.3.…”

“…The predominant focal mechanism is strike-slip (Buforn et al, 1998(Buforn et al, , 2004Argus et al, 1989). Among these events are those that occurred on 25 November 1941 (Udias et al, 1976;Lynnes and Ruff, 1985), 9 June 1969 (Argus et al, 1989), 26 May 1975(Buforn et al, 1998Argus et al, 1989;Kaabouben et al, 2008) and 17 October 1983 (Argus et al, 1989). Some of these earthquakes generated tsunamis, described in historical documents and recorded by tide stations, namely on 31 March 1761, 25 November 1941and 26 May 1975(Baptista et al, 2006Kaabouben et al, 2008;Baptista and Miranda, 2009).…”