**1. Introduction**

166 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

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Hindu Kush region, *Phys. Earth and Planet. Inter*. 78, 69–95.

Grafenberg data, *Bull. Seism. Soc. Am.* 74, 1645–1659.

region, *J. Geophys. Res*. 102, 22799–22813.

The Azores Archipelago is a region of intense geodynamical activity, with particular morphology and tectonics, which results in an intense seismic and volcanic activity (cf. Fig. 1). The seismic activity recorded in the region is very high, predominating low magnitude events organized in space and time in seismic crisis, sometimes triggered by stronger

Fig. 1. Epicentral distribution of the aftershocks recorded between 9 and 31 of July 1998, located with MAC model (Senos et al., 1980). The Harvard-CMT focal mechanism is showed, pointing to the main shock location. Location of SIVISA permanent seismic stations and the temporary portable stations of IM and IGIDL/CGUL as triangles. Isobaths interval of 300m (Lourenço et al., 1998). Inset: Azores Triple Junction between North-America, Eurasia and Nubia plates.

earthquakes such as the 1980 (Ml~7.2) Terceira earthquake or the 1998 (Ml~5.8) Faial earthquake. Besides the interest of such a natural laboratory for Geosciences, the study and understanding of such phenomena is also critical in terms of civil protection and associated risk assessment, because such seismic activity has often a significant impact in economic and human terms. However, the available knowledge on the mechanisms responsible for the observed seismicity pattern and its relationship with tectonic and volcanic features of the region is still unsatisfactory.

The occurrence of the July 9th 1998 earthquake near the island of Faial, together with the fact that the instrumental coverage of the seismic sequence generated by this earthquake applied for the first time digital seismic stations, opened a window of opportunity for a comprehensive analysis on the mechanisms involved in the generation of such seismicity. The main event triggered a seismic crisis of thousands of aftershocks, strongly monitored in the weeks following the event, whose analysis allowed defining the characteristics of such seismicity and its relationship with volcanic and tectonic phenomena associated with the Tectonic Triple Junction environment.

The analysis of the aftershocks aimed at one hand the spatiotemporal characterization, by reviewing and refining the hypocentral locations, the determination of the seismic generation mechanisms and identification of the main active structures (Dias, 2005; Matias et al., 2007); on the other hand, to determine the crustal structure, with the refinement of the existing 1-D models, determining the local three-dimensional structure, analysis of seismic anisotropy and correlation with the state of crustal stress (Dias, 2005; Dias et al., 2007).

#### **2. Temporary seismic network and data selection**

At the time of the July 9th 1998 earthquake, the existing permanent seismic network of SIVISA (Seismological Surveillance System of Azores) was composed by 17 analog single-component (vertical) stations and 8 three-component analog stations, all with A/D converters, plus 4 digital stations. All stations were short-period, the analog stations - equipped with MARK L4C seismometers of 0.5 s (2 Hz) and the digitals with Lennartz LE-3D seismometers, 1 Hz. The only exception was the broad-band station of CMLA in São Miguel island, belonging to the worldwide network IRIS/IDA, equipped with a very broad-band triaxial Streckeisen STS-2 seismometer. This network was installed in 8 islands of the Azores Archipelago.

In the days following the main earthquake, the Meteorology Institute (IM), the Infante D. Luíz Geophysical Institute (IGIDL) and the Geophysical Centre of the University of Lisbon (CGUL)1, in order to improve the instrumental coverage of the aftershocks by the SIVISA permanent network, deployed several portable digital seismic stations: IM placed 5 stations on Faial and Pico islands, while 2 additional stations were placed by IGIDL-CGUL on S. Jorge island. The seismometers used on all temporary stations were Lennartz LE-3D sensors, analogue to those used in the permanent digital stations. This increased the seismic monitoring network to a total of 14 stations located on the three islands surrounding the epicentral area (cf. Fig. 1). The stations sampling rates were 100 Hz for the IGIDL-CGUL stations, 125 Hz for IM stations; due to limited storage capacity, the data recorded by the 5 IM temporary stations were latter decimated to half, being stored with 62.5 Hz.

The temporary stations were operational until the end of July 1998, allowing a significant increase in the network registering capability, lowering the magnitude detection threshold

<sup>1</sup> IGIDL and CGUL were latter merged into Institute D. Luiz (IDL).

earthquakes such as the 1980 (Ml~7.2) Terceira earthquake or the 1998 (Ml~5.8) Faial earthquake. Besides the interest of such a natural laboratory for Geosciences, the study and understanding of such phenomena is also critical in terms of civil protection and associated risk assessment, because such seismic activity has often a significant impact in economic and human terms. However, the available knowledge on the mechanisms responsible for the observed seismicity pattern and its relationship with tectonic and volcanic features of the

The occurrence of the July 9th 1998 earthquake near the island of Faial, together with the fact that the instrumental coverage of the seismic sequence generated by this earthquake applied for the first time digital seismic stations, opened a window of opportunity for a comprehensive analysis on the mechanisms involved in the generation of such seismicity. The main event triggered a seismic crisis of thousands of aftershocks, strongly monitored in the weeks following the event, whose analysis allowed defining the characteristics of such seismicity and its relationship with volcanic and tectonic phenomena associated with the

The analysis of the aftershocks aimed at one hand the spatiotemporal characterization, by reviewing and refining the hypocentral locations, the determination of the seismic generation mechanisms and identification of the main active structures (Dias, 2005; Matias et al., 2007); on the other hand, to determine the crustal structure, with the refinement of the existing 1-D models, determining the local three-dimensional structure, analysis of seismic anisotropy and correlation with the state of crustal stress (Dias, 2005; Dias et al., 2007).

At the time of the July 9th 1998 earthquake, the existing permanent seismic network of SIVISA (Seismological Surveillance System of Azores) was composed by 17 analog single-component (vertical) stations and 8 three-component analog stations, all with A/D converters, plus 4 digital stations. All stations were short-period, the analog stations - equipped with MARK L4C seismometers of 0.5 s (2 Hz) and the digitals with Lennartz LE-3D seismometers, 1 Hz. The only exception was the broad-band station of CMLA in São Miguel island, belonging to the worldwide network IRIS/IDA, equipped with a very broad-band triaxial Streckeisen STS-2

In the days following the main earthquake, the Meteorology Institute (IM), the Infante D. Luíz Geophysical Institute (IGIDL) and the Geophysical Centre of the University of Lisbon (CGUL)1, in order to improve the instrumental coverage of the aftershocks by the SIVISA permanent network, deployed several portable digital seismic stations: IM placed 5 stations on Faial and Pico islands, while 2 additional stations were placed by IGIDL-CGUL on S. Jorge island. The seismometers used on all temporary stations were Lennartz LE-3D sensors, analogue to those used in the permanent digital stations. This increased the seismic monitoring network to a total of 14 stations located on the three islands surrounding the epicentral area (cf. Fig. 1). The stations sampling rates were 100 Hz for the IGIDL-CGUL stations, 125 Hz for IM stations; due to limited storage capacity, the data recorded by the 5

The temporary stations were operational until the end of July 1998, allowing a significant increase in the network registering capability, lowering the magnitude detection threshold

seismometer. This network was installed in 8 islands of the Azores Archipelago.

IM temporary stations were latter decimated to half, being stored with 62.5 Hz.

1 IGIDL and CGUL were latter merged into Institute D. Luiz (IDL).

region is still unsatisfactory.

Tectonic Triple Junction environment.

**2. Temporary seismic network and data selection** 

to events between 0.9 and 1.1 ML (Dias, 2005). During this period thousands of events were detected, with 4627 located aftershocks (Matias et al., 2007).

In order to maximize the quality and reliability of the results, for the one-dimensional modelling a set of 692 events was selected with the following criteria (Dias, 2005; Matias et al., 2007): minimum number of 6 stations recording an event, with one station at least located on S. Jorge island and azimuthal gap (GAP) smaller than 180º. For the 3-D modelling and seismic anisotropy analysis the criteria were more severe, with the used data set reduced to 688 and 438 events, respectively.
