Tectonic plates are enormous slabs of solid rock. Their sizes vary greatly, from just a few hundred to thousands of kilometres across. Thickness also varies, from less than 50 km to over 250 km. 

Tectonic plates move slowly but surely. They also deform—bending under compression and stretching under extension. Tectonic earthquakes occur when the forces between two plates are so high that they break faults in the lithospheric rock under our feet.  

Nowadays, scientists use Global Positioning System (GPS) and similar satellites to track how specific points on the earth’s surface move. All plates have boundaries that they share with their neighbouring plates. The four main types of boundaries are convergent boundaries, where plates move one against the other; divergent boundaries, where plates move apart; transform plate boundaries, where plates move sideways; and diffuse plate boundaries, where the motion between plates is distributed throughout complex fault networks. 

Mainland Portugal is located on the Eurasian plate (EU). This plate has a divergent boundary that moves Eurasia away from the North American plate, and a convergent region along which Eurasia is pushed toward the African (Nubian) plate. 

Tectonic plates are made of the earth’s lithosphere, which comprises the crust (the earth’s outermost solid layer) and the underlying uppermost mantle. The composition of the plates varies according to whether they are continental or oceanic. The continental crust is made of lightweight materials in comparison to the dense and heavy oceanic crust. The continental crust and lithosphere are much thicker than their oceanic equivalents. The difference in thickness and density of the oceanic and continental materials explains the difference in their surface elevations. Like icebergs, continents have deep roots inside the earth’s mantle.

Tectonic plates are made of Earth’s lithosphere, which comprises the crust (Earth’s outermost solid layer) and the underlying uppermost mantle. The composition of the plates varies according to whether they are continental or oceanic. The continental crust is made of lightweight materials in comparison to the dense and heavy oceanic crust. The continental crust and lithosphere are much thicker than their oceanic equivalents. The difference in thickness and density of the oceanic and continental materials explains the difference in their surface elevations. Like icebergs, continents have deep roots inside the earth’s mantle. Plate tectonics and volcanism are an evidence of the dynamic plate that is Earth and its dynamics are confined to the crust and mantle layers. The lower mantle interacts with the outer core, which is liquid and composed of iron-nickel alloys and other elements. The lower mantle is heated from below and thus the outer core also contributes to mantle dynamics. It is the active convection of liquid metal in the outer core that generates the earth’s magnetic field, which extends into space and protects us from cosmic radiation, including charged particles of solar wind. The inner core has an identical composition to the outer core, but it is solid, as demonstrated by the propagation of seismic waves.

As tectonic plates move past oneanother—colliding, separating, or simply sliding sideways—they create the mountains, valleys, and other “scars” that we see on the surface of the earth. Where plates move apart, along mid-ocean ridges (1), magma ascends to the surface and cools to create new oceanic crust (2). When denser, heavier oceanic plates  collide (at 3) against less dense, lighter continental plates (4), the oceanic plates dive below the continental plates into the earth’s interior in a process called subduction (5).  Subduction (5) also occurs when two oceanic plates collide (at 7). As the oceanic plate descends into the mantle, it releases water, lowering the melting point of mantle rocks and generating magma, which ascends to the surface. This creates volcanic arcs on oceanic plates (8) and volcanos on continental margins (4). The collision between two continental plates (at 6) leads to the building of huge mountains, due to the shortening and thickening of the lithosphere. Mid-ocean ridges are often offset and connected by transform faults (9).  The scars of these faults on a plate are fracture zones (9). Most earthquakes are generated at plate boundaries.

The motion of the tectonic plates that we see at the surface is related to mantle convection. As dense and cold oceanic plates subduct into the mantle (2), they force a downward flow (3). Some plates stagnate at mid-mantle discontinuities, whereas others continue all the way down to the core-mantle boundary (3). Upward flow of material is associated with rising plumes of hot, less dense mantle rocks (4). These plumes can rise both from shallow levels or from deep reservoirs. At mid-ocean ridges (1) plates diverge, opening space for the upwelling of mantle that will create new crust. The details of mantle convection and its interaction with surface tectonic movements remains a matter of active scientific investigation.

As tectonic plates move past one another—colliding, separating, or simply sliding sideways—they create the mountains, valleys, and other “scars” that we see on the surface of the earth. If we would empty the oceans of the world, most of the plate boundaries could be identified on the relief map.

When compared to the age of the earth (4,500 million years) the oceans are very young. The oldest extensive ocean domains are no older than 180 million years. The explanation lies in plate tectonics. All older oceans were already subducted and are now recycling the mantle.

Science Alive Centre of Estremoz

Science Alive Centre of Faro

What are tectonic plates?

Tectonic Plates and Moving Plates

A seismometer is an instrument that measures the motion of the ground. Ground shaking can be caused by earthquakes, volcanic eruptions, explosions, human or animal activities, ocean waves pounding against the surface, or any other forces applied to the earth.  

Initially, seismometers operated in independent observatories. Now, they work in networks of tens to thousands of sensors and we can see in real-time the earthquakes that happen all over the world.   

Seismometers are very sensitive and can measure the tinniest movement of the ground. In fact, they measure particle velocity when the ground is shaken. If the motion is stronger, seismometers reach their limits and we need less sensitive instruments, such as accelerometers. These measure the acceleration of the ground and stay on scale even during strong shaking.  

Seismograms showing ground motion caused by a regional earthquake 120 km away from the seismic station. To describe the ground motion completely, one seismic station needs three different sensors aligned along three orthogonal directions. These are usually the vertical (Z in the figure) and two horizontal directions, North-South (N in the figure) and East-West (E in the figure).

Seismometers (yellow arrow) are deployed on the ocean floor for long periods of time to record signals that are not caught by the permanent seismic stations on land. The photo shows the deployment of an Ocean Bottom Seismometer (OBS) for an experiment to record baleen whale vocalizations. 

Early seismometers, better identified as seismographs, were large and heavy pieces of hardware, so that they could record earthquakes from the other side of the earth. The figure shows one such model, the Wiechert seismograph, with the horizontal sensor on the left and the vertical sensor on the right. The oscillating mass is over 1 ton on the vertical sensor. 

Instituto Dom Luiz

Instituto Português do Mar e da Atmosfera

IGUC

What is a seismometer?

Seismometer

Seismic waves are caused by the energy released when rocks fracture under deforming forces. There are four types of seismic waves, which fall into two categories: body and surface waves. 

P-waves are the fastest seismic waves and can cross through any material—solid, liquid, or gas. They are sound waves (or acoustic waves), also called compressional waves, as rocks compress and extend repeatedly before returning to their original shape after the wave passes. 

S-waves are slower than P-waves but still faster than surface waves. They cannot travel through liquids or gases; therefore they cannot travel through the earth’s outer core, which is made of molten metal. 

Rayleigh and Love waves travel parallel to the ground. 

They are less rapid than body waves but lose their energy more slowly. Rayleigh waves have a vertically elliptical motion and are typically described as a rocking movement, similar to the waves you feel on a boat. Love waves result from the interaction of S-waves, moving the ground sideways in a horizontal plane and causing no vertical motion.

Sketch of the ray path (long arrows), particle motion (short arrows), and wave front (rectangular plane) associated to P- and S-body waves (arrows in blue), and surface waves (arrows in red). The particle motion of S-waves can be on the horizontalplane (SH) or on the vertical plane (SV). S-particle motion will be a combination of the two.

Sketch of the ray path, particle motion, and wave front associated to P- and S-body waves (arrows in blue) and surface waves (arrows in red). Surface waves propagate parallel to the surface, while body waves propagate along curved rays through the interior of a heterogeneous earth, illustrated by red and blue colours. Dashes show the location of two discontinuities in the earth’s mantle.

Did you know that in 1939, Jeffreys established travel-time tables for the most important seismic phases, which say how much time each seismic wave takes to travel a given distance, and today they are still precise to 0.2%? The figure shows a recent and more comprehensive version of the Jeffreys travel-time curves. The variety of seismic waves is a consequence of the body waves inside the earth undergoing reflection and refraction at the major interfaces.

We observe a variety of seismic waves in our planet because body waves travelling inside the Earth undergo reflection and refraction at the major interfaces. The type of wave and the layer where each ray travels is identified by capital letters, P, S, K, I, J. Small cap letters show reflections, c and i.

A seismogram is the recording of the ground motion caused by an earthquake. When we are very close to the earthquake, the seismometer (accelerometer) records the waves generated progressively from the whole fault. It is usually impossible to identify clearly where the S-waves begin. Surface waves are not yet visible.

For smaller events at close range to the source, P- and S-waves can be clearly identified on the seismogram. Surface waves may, or may not, be present.

At large distances from large earthquakes the seismograms are complex, and many seismic waves can be identified due to the heterogeneous structure of the Earth,which leads to reflections and/or refractions. For shallow earthquakes, at large distances from earthquakes, the largest amplitude (and longest period) waves are the surface waves.

For very large earthquakes, the surface waves arriving at one station travel around the earth along opposite directions and circle the globe several times. At large distances from the earthquake, surface waves have the greatest amplitude.

The four types of seismic waves

P&S Waves

There are three main types of faults—normal, strike-slip, and reverse. The forces applied on the brittle rock define the motion on the fault, which in turn defines the type of fault. Normal faults, found mainly on divergent plate boundaries, are caused by horizontal stresses that move the two blocks apart. Reverse faults, found mainly on convergent plate boundaries, are caused by horizontal compression that moves the two blocks closer together. Strike-slip faults, found mainly on transform plate boundaries, are caused by horizontal compression and tension stresses of similar magnitudes, resulting in lateral block motion.  

Faults are not always easy to identify. The erosion and deposition of sediments actively shape the landscape, sometimes erasing the surface traces of active faults. Other faults are buried, and they don’t leave a noticeable trace on the earth’s surface. They’re called blind faults. This makes the mapping of active faults—where future earthquakes will occur—a very challenging task for geologists and geophysicists, especially in regions where tectonic activity rates are low due to slow plate motion or distributed fault zones.

There are three main types of faults—normal, strike-slip, and reverse. The forces that are applied on the brittle rock define the motion on the fault, which in turn defines the type of fault. Normal faults, found mainly on divergent plate boundaries, are caused by horizontal stresses, moving the two blocks apart. Reverse faults, found mainly on convergent plate boundaries, are caused by horizontal compression, moving two blocks closer together. Strike-slip faults, found mainly on transform plate boundaries, are caused by horizontal compression and tension stresses of similar magnitudes, moving blocks laterally.

If the active fault ruptures the surface of the earth, it will generate a fault trace. The image shows the fault rupture of the Chi Chi earthquake, Taiwan, in 1999. 

The 1988 Spitak earthquake, with a magnitude of M 6.8, took the life of thousands of people and caused widespread devastation in Armenia. It also ruptured the surface as shown in the figure.

The August 1999 Izmit earthquake, in Turkey, was caused by the rupture on a strike-slip fault. It caused a large lateral movement between the two sides of the fault clearly shown by the rails.

The continued movement of active faults displaces rivers, builds reliefs, and creates geographical features that can be seen from the air and even from satellites in space. The San Andreas fault trace shown in the figure is a good example.

The continued movement of active faults also creates reliefs on the sea bottom that are disclosed by high-resolution bathymetric surveys. The arrows show the ocean bottom fault trace of the Horseshoe Abyssal Plain Fault (SW Portugal mainland), which has a length over 100 km and shows a vertical displacement higher than 1000 m at places. It is one of the faults that has been suggested to be the source of the 1755 earthquake and tsunami.

Map of active (full lines) and probable active (dashed lines) faults identified in Portugal and the surrounding oceanic domain. On land, fault activity is very low, with rates under 0.5 mm/year. The offshore fault activity rates are largely unknown.

European Fault-Source Model 2020 (EFSM20) of seismogenic faults in Europe, displaying faults where earthquakes may originate in the near future. 

Fault lines and earthquakes

Earthquakes and Faults

About 250 million years ago, all continents were assembled as a single supercontinent, known as Pangea. Over time, the continents moved into the positions they have today.

The earth’s surface is like a puzzle of tectonic plates. The movements of continental and oceanic plates shape the landscape—generating volcanoes, earthquakes, mountains, and basins along plate boundaries.

Throughout the history of our planet, continents have roamed over the surface of our planet, sometimes amalgamating together to form supercontinents, sometimes drifting apart. Rodinia was a Neoproterozoic supercontinent that assembled 1.1-0.9 billion years ago and broke up 750-633 million years ago. Land masses came back together about 250 million years ago, forming the supercontinent Pangea. Over time, the continents have moved into the positions they have today, yet they continue to move with respect to one another. 

About 200 million years ago, continents came together to form the supercontinent Pangaea. From then until today, continents have been slowly drifting apart. Even at their current position, they continue to move.

Science Alive Centre of Faro (currently closed)

What is continental drift?

Continental Drift

Tsunamis are rare occurrences in Europe, and this fact makes the 1755 tsunami an even more extraordinary event, since it crossed the entire Atlantic. As a transoceanic tsunami, it can be compared to the 2004 Sumatra tsunami and other tsunamis that cross the Pacific. Tsunamis with these characteristics are usually generated by large faults with very large movements. It is believed that a fault like that broke in 1755. According to the studies developed on this field, the fault that broke and deformed the ocean floor in 1755 was a fault 100km long and 50km wide. This same fault would have affected the ocean floor, it is estimated, in about 20m high, thus causing a transoceanic tsunami.

The records that we can access today corroborate this analysis, with reports on the impact of the tsunami in England, France, Brazil, Canada and the Caribbean where, for example, 3m waves were observed. The biggest impact was indeed in Portugal, but also in Morocco and southern Spain. Historical reports show that in Cadiz the waves would have reached more than 10m. As for Mainland Portugal, 10 to 15m waves would have reached Sagres area and in Lisbon it is estimated that they have reached 5m.

One of the reports that can be studied today is from an English sea captain that was in Lisbon on the day of Earthquake. Bugio Fort, still visible today at the mouth of the Tagus River, was surrounded by the tsunami waters and the sea captain, on his boat, observed the following events: “… The Castle of Bugio was so far overcome by the water, that the garrison fired several guns as signals for help, and were obliged to retire to the upper part of the tower. By my best judgment, the water rose in five minutes about 16 feet (approximately 5 meters), and at two in the afternoon the tide returned to its natural course”.

It is known today that the waves of a tsunami travel on high seas at the speed of commercial airplanes, at about 800km per hour, but that speed is much slower than the speed of seismic waves, which travel at 8/9/10km per second. Thus, it is known that there is a time between the arrival of seismic waves and the waves of a tsunami. In 1755, the estimated arrival time of the tsunami waves at Bugio Fort is 30 minutes. As for Lisbon centre, since the water depth is much smaller, the waves propagated much slower, so it is estimated that it took perhaps 70 or 90 minutes for the tsunami waves to reach the city.

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