Additional Report for Azores-Gibraltar Fracture Zone

Azores-Gibraltar Fracture Zone

Azores, N Atlantic Ocean


Index of Monthly Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

10/1988 (SEAN 13:10) 850 events; tremor near submarine volcano

11/1988 (SEAN 13:11) New swarm on another section of fracture zone

12/1988 (SEAN 13:12) Continued earthquakes and tremor

03/1989 (SEAN 14:03) Seismicity since May 1988 summarized


Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

All times are local (= UTC - 1 hour)

10/1988 (SEAN 13:10) 850 events; tremor near submarine volcano

A seismic swarm began 16 October about 30 km S of Sao Miguel Island. [Several older houses were damaged in Vila Franca do Campo and Povoacao on the S coast of Sao Miguel Island. Most of the epicenters were located along a branch of the Azores-Gibraltar fracture zone. [The strongest shock recorded by the WWSSN was an mb 5.1 event at 0615 on 16 October (37.58°N, 25.37°W, 10 km depth)]. In mid-November, the swarm was ending after 850 events had been recorded by the Azores Univ seismic array.

Since early November, tremor has been registered at the Sao Miguel and Terceira Islands stations of the Univ seismic network [but see 14:3]. This tremor appeared to originate from Don Joao de Castro Bank (38.23°N, 26.63°W, 14 m depth), a shallow submarine volcano that erupted in December 1720. Several earthquakes centered near the volcano reached MM V at Terceira, 80 km away.

Information Contacts: V. Forjaz, Univ of the Azores; NEIC; Lisbon International Service.

11/1988 (SEAN 13:11) New swarm on another section of fracture zone

The seismic swarm along a branch of the Azores-Gibraltar Fracture Zone was ending in mid-November. No surface evidence of submarine volcanism has been reported.

On 21 November at 1556, a second swarm started with an [Ms 5.6] shock centered 25 km NW of Sao Miguel along the Sete Cidades Fault, a branch of the same fracture zone. The hypocenter was 53 km SE of Don Joao de Castro Bank [preliminary location by WWSSN at 37.97°N, 26.11°W, 10 km depth]. By the next day, the number of seismic events/hour had decreased from 24 to 4; several of the shocks were migrating along the main portion of the Sao Miguel Fault. The swarm caused 30 houses to collapse and damaged hundreds of other buildings. Modified Mercalli intensities were VII-VIII in the NW part of the island and V at the capital.

Information Contacts: V. Forjaz, Univ of the Azores; NEIC; Lisbon International Service.

12/1988 (SEAN 13:12) Continued earthquakes and tremor

As of 27 December, a total of 1,300 events had been registered since the activity began. Several patterns of epicentral migration had been noted along the Azores-Gibraltar Fracture Zone and the Congro regional fault. Volcanic tremor remained frequent. Fumarole temperatures were about 10°C higher than normal at Furnas Caldera on Sao Miguel Island. Its most recent eruption, in 1630, deposited tephra over most of the island and killed 191 people.

Information Contacts: V. Forjaz, Univ of the Azores; NEIC; Lisbon International Service.

03/1989 (SEAN 14:03) Seismicity since May 1988 summarized

The following is a summary of significant seismic activity in the E Azores (figure 1) since May 1988 [through January 1989]. "A swarm of microearthquakes, accompanied by weak harmonic tremor, occurred beneath the NE flank of Agua de Pau Volcano on Sao Miguel (epicentral zone 1 on figure 2) 23-24 May 1988. Seismometers recorded 383 earthquakes; the largest had intensities of MM IV-V in villages along the N coast. A similar but smaller swarm occurred in the same area in 1983 and was recorded by USGS seismographs. Numerous, small, normal faults (including some that show scissor-type movement) displace basaltic, tristanitic, and trachytic vent deposits and flows in this area. However, the area has had no eruptions for about 3,000 years and is the least active of the five volcanic zones on Sao Miguel that have erupted during Holocene time.

Figure 1. Sketch map showing islands in the eastern Azores. Approximate epicenters of the 3-5 October 1988 swarm at the submarine volcano Don Joao de Castro Bank (4) and the 21 November 1988 and 21 January 1989 earthquakes in the Hirondelle Basin (6 and 7) are shown. Courtesy of R. Moore.
Figure 2. Approximate epicenters for earthquakes on and near the island of Sao Miguel, May-October, 1988. Courtesy of R. Moore.

"A small swarm of microearthquakes occurred 24-26 June on the S flank of Agua de Pau Volcano (zone 2). Sixty-four earthquakes were recorded; the largest had intensities of MM III-IV in nearby villages. No tremor accompanied this episode.

"A small swarm of microearthquakes occurred 6 July near the S coast of Sao Miguel (zone 3). Fifty-one earthquakes were recorded; the largest was felt and had an intensity of MM III-IV. No tremor accompanied this episode.

"A small swarm of earthquakes occurred within the volcano Don Joao de Castro Bank (zone 4) 3-5 October. Sixty-two earthquakes were recorded; no harmonic tremor was detected. A felt earthquake occurred in the same area 23 October; it had intensities of MM IV-V on Terceira and MM III-IV on the N coast of Sao Miguel.

"A swarm of tectonic earthquakes, many of which were felt, occurred beneath the ocean floor about 30 km S of the extinct Povoaçao Volcano (zone 5) 16-20 October 1988. Seismometers recorded 409 earthquakes during this period; the largest had an intensity of MM VI in towns on the S coast of Sao Miguel. Felt aftershocks occurred as recently as late February 1989. No harmonic tremor accompanied this activity. Epicenters were aligned along a N-NW trend; hypocenters generally ranged from 10 km to as shallow as 400 m. Furnas Volcano, which adjoins Povoaçao on its W side and most recently erupted in A.D. 1630, has numerous boiling drowned hot springs and derivative 'fumaroles' that emit only water vapor. Temperatures of the hot springs depend on elevation and range from 98.5° to 100°C. The associated 'fumaroles' are much cooler and are subject to further cooling during periods of heavy rain.

"The largest Azores earthquake of the last year, M 5.8, occurred within the S Hirondelle Basin (zone 6), SE of Don Joao de Castro Bank, on 21 November. The earthquake was widely felt in the E Azores and caused minor damage on the W part of Sao Miguel, where intensities were MM VI-VII. The quake was apparently tectonic in origin as no harmonic tremor occurred during its normal aftershock sequence. An earthquake of MM V (measured near the NW coast of Sao Miguel) occurred 21 January 1989 within the S Hirondelle Basin. The earthquake was probably an aftershock of the 21 November event. No tremor was recorded."

Information Contacts: A. Rodrigues da Silva, Consorcio Geotermico de S. Miguel; R. Moore, USGS; National Institute of Meteorology and Geophysics, Portugal.