Pure and Applied Geophysics

Buforn, Elisa; Mattesini, Maurizio

doi: 10.1007/s00024-020-02482-4pmid: N/A

Allen, Richard M.; Kong, Qingkai; Martin-Short, Robert

doi: 10.1007/s00024-019-02337-7pmid: N/A

The MyShake Platform is an operational framework to provide earthquake early warning (EEW) to people in earthquake-prone regions. It is unique among approaches to EEW as it is built on existing smartphone technology to both detect earthquakes and issue warnings. It therefore has the potential to provide EEW wherever there are smartphones, and there are now smartphones wherever there are people. The MyShake framework can also integrate other sources of alerts and deliver them to users, as well and delivering its alerts through other channels as needed. The MyShake Platform builds on experience from the first 3 years of MyShake operation. Over 300,000 people around the globe have downloaded the MyShake app and participated in this citizen science project to detect earthquakes and provide seismic waveforms for research. These operations have shown that earthquakes can be detected, located, and the magnitude estimated ~ 5 to 7 s after the origin time, and alerts can be delivered to smartphones in ~ 1 to 5 s. A human-centered design process produced key insights to the needs of users that have been incorporated into MyShake2.0 which is being release for Android and iOS devices in June 2019. MyShake2.0 will also deliver EEW alerts, initially in California and hopes to expand service to other regions.

Baptista, M. A.

doi: 10.1007/s00024-019-02344-8pmid: N/A

The Eurasia–Nubia plate boundary between the Azores and the Strait of Gibraltar has been the place of large tsunamigenic earthquakes. The tectonic regime is extensional in the Azores, transcurrent along the Gloria Fault, and compressional in the Strait of Gibraltar. Here, the plate boundary is not clearly defined. The knowledge of past events that occurred in the area constitutes an essential contribution to the evaluations of seismic and tsunami hazard in the North-East Atlantic. In this study, we present an overview of the six major events in the area and show the use of tsunami data to add some constraints on their source. The historical events occurred in the eighteenth-century between 1722 and 1761, while the twentieth-century events occurred between 1941 and 1975. We speculate that major tsunamigenic earthquakes that occur in the Iberia-Maghreb area take place at the boundaries of a lithospheric block approximately defined by the location the six events summarized here, which role and dynamics are not yet understood.

Catalán, M.; Martos, Y. M.; Martín-Davila, J.

doi: 10.1007/s00024-019-02380-4pmid: N/A

The Gulf of Cadiz has been affected by a long and complex geodynamic evolution. The lithospheric structure is poorly understood in this region, and it also shows a diffuse seismicity, spanning over a broad area. The Canary Archipelago has been extensively studied. Nevertheless, there are fundamental topics that are still under debate. No studies have addressed or suggested the possibility of a plausible geodynamic connection between both remarkable locations. In this study we integrate total tectonic subsidence (TTS), Curie point depth (CPD), Bouguer gravity anomaly, and seismic information. TTS shows the existence of a basement bulge in the area of the Canary Archipelago that extends to the north, and in the area of Madeira Island, which extends in a more subtle way to the north, too. Likewise, the CPD reaches the shallowest values in the same location at the Canary Archipelago. These two aspects suggest a cause-effect relationship between TTS and CPD at this specific area. Gravity data and CPD show a linear feature, which links the NW of the Canary Archipelago and the Gulf of Cadiz. The data we manage in this work show remarkable clues as: (a) the absence of a similar signal in the TTS, (b) the fact that CPD it is rather constant along this track, (c) CPD amplitude also almost doubles the values obtained on Canary Archipelago, and (d) the existence of a clear correlation between seismogenic depths and CPD, which points to the existence of a correlation between seismicity and the thermal architecture of the lithosphere. All of these evidences support the presence of a lithospheric thinning between Canary Islands and Gulf of Cádiz area, and in turn, the presence of an asthenospheric channel which feeds and alter locally the Eurasia–Africa Plate Boundary.

Udías, Agustín

doi: 10.1007/s00024-019-02323-zpmid: N/A

The occurrence of large earthquakes followed by tsunamis west off Saint Vincent Cape before the 1755 Lisbon earthquakes is the subject of a historical review. There are historical evidences of the occurrence of such earthquakes. Among the possible earthquakes with some historical records in this area, those with better information are those at years 241 B.C., 216 B.C., 881, 1356 and 1531. Although not a subduction zone very large earthquakes have happened in this area.

López Muga, Marina; Benayas, Irene; Tordesillas, Jose Manuel

doi: 10.1007/s00024-019-02340-ypmid: N/A

The Spanish Geophysical Data National Archive (Toledo Geophysical Observatory) is a centre created by Instituto Geográfico Nacional to collect all the geophysical documentation produced in all of the observatories IGN has had in operation throughout its history. All this information is transferred to the Geophysical Data National Archive where it is reviewed, classified and catalogued in a database, to be eventually filed in the records repository of the Archive, with the appropriate conditions for their future preservation. Its contents are digitized as backup and to meet the data requests received in this Archive. For the study of earthquakes throughout the twentieth century, this Archive has a large volume of information. The documents most consulted by researchers are the seismic records from the IGN Geophysical Observatories, but also the Archive houses an important collection of complementary information.

Cabieces, Roberto; Buforn, Elisa; Cesca, Simone; Pazos, Antonio

doi: 10.1007/s00024-020-02475-3pmid: N/A

Earthquakes with submarine foci are generally located with high uncertainties, and their focal mechanisms are poorly resolved, due to the confinement of the monitoring network onshore and the consequent poor azimuthal coverage. The use of amphibious networks, combining ocean-bottom seismometers (OBSs) and land stations, helps to reduce the epicentral distance and the azimuth gap, thus better constraining the hypocentral locations and decreasing the focal mechanism uncertainties. A second important factor in improving the location accuracy for offshore seismicity is the use of a suitable velocity model. The objective of this paper is to study how the combination of an amphibious network with 3D crustal models can improve offshore earthquake hypocenter locations and focal mechanisms. The study area is SW Iberia near Cape St. Vincent, which generated some of the most striking earthquakes and tsunamis in Europe in past centuries, such as the 1755 Lisbon earthquake (Imax = X). We deployed an array of six broadband OBSs 200 km offshore Cape St. Vincent to study the seismicity of the region for a period of 8 months. During this period, we detected 52 earthquakes, the largest with magnitude M (mbLg) ≈ 5. Thirty-eight earthquakes are relocated using land stations in Iberia and North Africa and the OBS array with different velocity models. Focal mechanisms and moment tensors are computed for a data set of seven earthquakes based on first-motion polarities, body-wave amplitude spectra and waveform cross-correlation. We show that if we include offshore OBS data and consider accurate 3D velocity models to locate earthquakes, the focal parameter uncertainties decrease substantially, thus improving the depth constraint. We also show that the locations and focal mechanisms obtained using the amphibious network agree with the regional stress pattern in the SW Iberia region.

Buforn, E.; López-Sánchez, C.; Lozano, L.; Martínez-Solares, J. M.; Cesca, S.; Oliveira, C. S.; Udías, A.

doi: 10.1007/s00024-019-02336-8pmid: N/A

Seismic intensity for the February 28, 1969 (Mw = 7.8) earthquake have been re-evaluated using original documents in local archives, such as, contemporary newspapers, council minutes, monographic studies, among other sources for Spain, Portugal and Morocco and answers to macroseismic questionnaires for Morocco. This information is used to plot a new intensity map for the whole region affected by the earthquake: Portugal, Spain and Morocco. The intensity values vary from VIII to IX in the E-W coast of Algarve, southern Portugal, to II–III. Furthermore, we have relocated the hypocentres for main shock and 24 aftershocks using a new 3D crustal velocity model for the Gulf of Cadiz region and a non-linear probabilistic location methodology, most of them previously lacking a depth estimate. The new locations show an E-W distribution of epicenters, with focus located in the uppermost mantle, most of them with depths between 30 and 50 km. No earthquakes have been located at depths shallower than 30 km. A comparison between peak ground accelerations (PGAs) estimated from the observed intensities for the 1969 and the Lisbon 1755 earthquakes, and synthetic PGA values, generated assuming two different scenarios (using the 1969 and 2009 earthquakes) for the 1755 Lisbon event, shows that the observed damage produced by the 1755 earthquake may be better explained assuming a reverse dip-slip mechanism oriented in NE-SW direction, similar to that of the 2009 earthquake, rather than assuming focal mechanism similar to that of the 1969 earthquake.

Aparicio Florido, José Antonio

doi: 10.1007/s00024-019-02368-0pmid: N/A

In the interval between the El Asnam earthquakes of 1954 and 1980, both tsunamigenic and recorded on tide gauges of the western Mediterranean coasts, the earthquake and tsunami of 28 February 1969 took place. The epicentre was located in the Atlantic Ocean, southwest of Cape St. Vincent, and tsunami waves were recorded on tide gauges of the Atlantic coasts of Portugal, Spain and Morocco. This means there were three tsunami events over a 26-year period in the current NEAMTWS coverage area. There is no evidence of land sightings of tidal wave arrivals, except for two cases in Gijón (Spain) and near the mouth of the Bou Regreg River (Morocco), which will be discussed later. The behaviour of the sea in the coastal areas was apparently normal considering the presence of a storm that was moving from the southwest of the Iberian Peninsula, accompanied by very strong rainfall in Morocco, moderate to strong rainfall in Andalusia and southern Portugal, and weak rainfall or very cloudy skies in the rest of the area. However, on the high seas, both the earthquake and the tsunami effects were clearly felt by six ships of various tonnages at different distances and azimuths from the epicentre, even with the risk of shipwreck. The crews of these ships, which at the time of the earthquake were sailing across the Atlantic near the epicentral area, reported violent vibrations, very high waves and specific damages, which call into question the belief that seaquakes and tsunamis are not perceptible on deep waters, inviting us to analyse the risk posed by earthquakes with submarine epicentres for maritime navigation.

Pro, C.; Buforn, E.; Udías, A.; Borges, J.; Oliveira, C. S.

doi: 10.1007/s00024-019-02401-2pmid: N/A

The 28 February 1969 (Ms 8.0) Cape St. Vincent earthquake is the largest shock to have occurred in the region after the Lisbon earthquake of 1755. However, the study of the rupture process has been limited due to the characteristics of the available seismic data which were analogue records that were generally saturated at both regional and teleseismic distances. Indeed, these data consist of just one accelerograph record at the 25th April Bridge in Lisbon (Portugal) and the observed intensities in the Iberian Peninsula and northern part of Morocco. We have used these data to simulate the distribution of PGV (Peak Ground Velocity) for the 1969 event at regional distances (less than 600 km) by using a 3D velocity model. The PGV values are very important in seismic hazard studies. The velocity model and the methodological approach were tested by comparing synthetic and observed ground velocities at regional distances for two recent, well-studied earthquakes that occurred in this region, namely, the 2007 (Mw = 5.9) and the 2009 (Mw = 5.5) earthquakes. By comparing the synthetic and observed PGA (Peak Ground Acceleration) at Lisbon, the focal depth was estimated equal to 25 km and the seismic moment equal to 6.4 × 1020 N m (Mw = 7.8) for 1969 earthquake. With these parameters, PGV values were obtained for 159 sites located in the Iberian Peninsula and northern region of Morocco where we have felt intensity values. Using different empirical relations, the instrumental intensity values were calculated and compared with the felt intensities. As a result, the synthetic PGV values obtained in this study for the 1969 earthquake could be used as reference values, and the methodological approach would allow the PGV and intensity to be simulated for other events in the region.