Jochen Zschau, Andreas Küppers, Early Warning Systems for Natural Disaster Reduction, Springer, Berlin, Heidelberg, 2003, ISBN 978-3-642-63234-1
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Seismic images of the inner Earth structure
Ancient geography and modern seismology mixed in an old map of the world. Inner depth regions inside the planet Earth (depth ≤ 100 km) are imaged by seismic tomography as low (in red) and high (in blue) seismic wave velocity zones.
This image has been obtained by processing and modelling arrival time and waveform data from the world-wide seismic network GEOSCOPE.
The GEOSCOPE Observatory is a Global Network of Broad Band Seismic Stations operated by the Institut de Physique du Globe de Paris.
The GEOSCOPE Observatory provides data and information for earthquakes with magnitudes larger than 5.5-6. Similar information may be provided for smaller earthquakes, for example those located in France or in the European-Mediterranean region.
Earth is a continuous evolving, dynamic planet
The Earth is a dynamic physical system continuously evolving in space and time. Plate Tectonics explains the continental drift which has been first confirmed during 1950s and 1960s by the seafloor spreading evidence at mid-oceanic ridges. The Earth’s litosphere is fractured into rigid plates moving relatively to each other. The plate motion is caused by large-scale convection phenomena occurring in the deep mantle layer. The hot, deep mantle melt rises by buoyancy to the Earth surface and outpoures at the ridge creating the new oceanic crust. The accreted material cools down drifts away at a speed of few to several cm/year due to ridge-push and slab-pull forces. At the consumption edges of the plate, the cold, thermally contracted material become dense, and sinks in the process of subduction usually at an ocean trench.
Earthquakes reflect the state of deformation of the Earth
Earthquakes , e.g. the Earth shaking due to seismic waves, are caused by extended fractures (or faulting) phenomena within the fragile portion of the crust. They mostly occur at the plate boundaries being generated by the accumulation and release of the tectonic stress which is driven by the relative speed of plate motion.
Indeed, earthquake epicenters (yellow, orange and red circles in figures) are unevenly distributed over the planet surface and delineate the plate boundaries, where the relative plate motion induce a high tectonic stress concentration and increase causing faults to form and slip.
Here we have Earthquakesmaps from the ANSS catalog - Advanced National Seismic System - USA.
Earthquake mostly originate at the plate boundaries - convergent, divergent, transcurrent
There are three main types of plate boundary: divergent, convergent and transform.
At mid-ocean ridges, plates move apart from each other and the boundary is called “divergent”.
Earthquakes at a mid-ocean ridge are frequent, they occur at shallow depths (<20 km) and have small to moderate sizes (magnitude < 6).
A “convergent” boundary is where two plates collides. At subduction zones, in particular, one plate sinks beneath another due to its higher density and tectonic forces as the "slab-pull" and "ridge-push". Subduction zones are the sites of the largest magnitude earthquakes (M > 8.5), with events occurring down to several hundred km depth along the subducting slab.
Plates slides past each other at “transform” boundaries. The most famous examples of this type of boundary are the San Andreas Fault in California and the North-Anatolian fault in Turkey. Moderate to large earthquakes (M=6-7.5) occur at these boundaries, mostly at shallow depths (<20 km).
The Earth Structure
A typical cross section of the Earth illustrates the simplified one-dimensional structure of the planet, with the inner/outer core, the mantle, and the shallow crustal shell.
The enlarged inset figure shows the composition of the lithosphere (comprising the continental crust, oceanic crust, and solid upper mantle) which behaves elastically on time scales of thousands of years or greater. The underlying asthenosphere has a visco-elastic behavior, e.g. exhibits both elastic and viscous characteristics depending on the time-scale of the applied stress.
The fractures that generate earthquakes mainly occur in the shallow crustal layer, between 0 and 20-30 km of depth. At these depths the crust behaves as a «brittle» material, e.g. it breaks when subjected to physical stresses overcoming the ultimate material strenght.
The interior of the Earth
The knowledge about the inner Earth’s structure before 1900 was poor, based on direct observations and measurements at its surface. The figure shows a simplified Earth model obtained by calculations of the Earth moment of inertia and rotation speed.
In mid-thirties of the past century, our knowledge of the Earth interior significantly improved thanks to the contribution of global seismology observations. The data of travel times and paths of seismic waves, acquired at the Earth surface, generated by earthquakes and propagating through the planet allowed the reconstruction of a reliable, one-dimensional model of the inner Earth structure. This describes the variation of elastic properties, in terms of P and S wave velocities and density, as a function of the Earth’s radius.
The interior of the Earth 2
Using millions of arrival time data, powerful computers and sophisticated modelling techniques (e.g. seismic tomography), the first high resolution, three-dimensional images of the Earth are obtained around mid-eighties. Seismic tomography is an image reconstruction method which uses the arrival times of seismic waves travelling through the planet, to locate and define the geometry of regions where seismic velocities are higher or lower than a reference value.
The method is comparable to that of the CT scan that uses X-ray intensities to image the interior of the human body. The tomographic images of seismic velocity anomalies therefore represent the variation of elastic properties within the Earth.
The Earth is far from being a homogeneous body but has progressively differentiated into a metallic core (radius of about 3500 km) and a more rocky mantle (about 2900 km thick). Starting from the surface, we find the less dense rocks ejected from the mantle at mid-ocean ridges known as the crust, which varies in thickness from about 5km to about 60 km. The larger crustal thicknesses are related to more ancient and rigid continental plates.
The radial elastic Earth model shows a continuous increase of P and S seismic velocities and density in the crust and mantle. The '660 km' discontinuity is a phase change, and possibly a compositional change, in the silicate mantle. At about 2900 km depth, a sharp increase of P-velocity and density, but a vanishing S wave velocity occur, which reveals the presence of a fluid materials (outer core). In 1970-1975, first proofs of the solidity of the inner core (radius of about 1220 km) were derived, from the measure of frequencies associated with the free Earth oscillations triggered the great 1964 M 9.2 Alaska earthquake.
The figures show an example of three-dimensional tomographic images of the Earth interior, at depths corresponding to the upper and lower mantle. The images show the distribution of P-wave velocity pertubations (measured in percent relative to the reference model) at the depths of 200 km and 1325 km. The model has been elaborated based on P-wave arrival times recorded at the global, worldwide seismic network.
As we go deeper into the Earth the image resolution decreases. This effect is clearly visible as the appearance of large, uniform colored pixels in the 1325 km depth image. The resolution of seismic tomography images depends on the azimuthal coverage and sampling of seismic rays, whose number sharply rarefies as P-waves travel through the deep Earth.
The first direct evidence of the subduction zones, originally predicted by Plate Tectonics Theory, comes from the depth location of earthquakes and the seismic velocity images of the Earth interior inferred by seismic tomography. The massive scale of the subducting plates and of the related deformation can cause enormous earthquakes, such as the magnitude 9.5 in Chile in 1960, the magnitude 9.2 in Alaska in 1964 and the magnitude 9.0 in Japan in 2011.
Simulation of time evolution of the mantle thermal structure
Thermal Structure from Models of Mantle Convection with Surface Plates, Temperature-dependent and Radially Stratified Viscosity. This animation shows the evolution of thermal structure within the mantle over a period of about 4 transit times (equivalent to ~200 Ma). The scale of thermal structure is controlled by surface plates. Downwelling sheets and upwellings plumes are the predominant structures. Thermal anomalies below spreading centers only extend to a depth of about 200 km.
Watch the Video Animation on the evolution of thermal structure within the mantle.
Observation scales of the phenomena
In the following sections of this introductory lectures we investigate three main aspects which characterize the observations in earthquake seismology:
Dimension and extent of earthquake sources
- How big can an earthquake be?
- How to measure its size?
- What it iis ts frequency-magnitude distribution?
Size and geometry of observation networks
- How are earthquakes detected and recorded?
- What about the seismic monitoring at local, regional and global scale?
- What is the next generation of earthquake observation systems?
Amplitude and frequency content of recorded signals
- What is the frequency band of seismic phenomena?
- What is a seismogram?
The size of an earthquake
Earthquakes can be naturally (faulting phenomena) or artificially (explosions) generated. Their size is commonly represented by three different quantities:
- It measures the moment of the couple of forces causing the fault slip
- It is an adimensional quantity related to the maximum observed amplitude of the seismic wave motion
- It describes qualitatively the damaging effects of an earthquake, from the received shaking to the building collapse. It is expressed in degrees
Frequency and magnitude of earthquakes
The magnitude defines the size of an earthquake and it is measured from the maximum recorded displacement along the seismogram.
The U.S. Geological Survey estimates that several million earthquakes occur in the world each year. Many go undetected because they hit remote areas or have very small magnitudes. The US earthquake agency now locates about 50 earthquakes each day, or about 20,000 a year. As more and more seismographs are installed in the world, more earthquakes can be and have been located. However, the number of large earthquakes (magnitude 6.0 and greater) has stayed relatively constant during the past century.
The left image compares the size of the 2001 Tohoku-Oki Japan's earthquake with other destructive events. Japan has a long and notorious earthquake history. About 1,500 earthquakes strike the island nation every year. Minor tremors occur on a nearly daily basis. Japan has such a large potential for earthquakes and disaster because the nation sits atop a huge subduction zone, whose relative motion trigger deadly earthquakes, like the 9.0-magnitude quake that struck on 11 March 2011.
The giant earthquakes (M>8.5)
During the past century about 1 event with magnitude larger that 8 has occurred on average per year. Very large magnitude earthquakes (M >8.5) are denoted as «Giant», they are rare fracture phenomena occurring at subduction zones and involve faults which extend for several hundred km. In the period 1900-2010 five major earthquakes occurred clustered in time. The debate was about a possible link between huge earthquakes.
Do giant earthquakes occurr clustered in time?
The observation of huge event clustering in the middle of the past century suggested that a mutual interaction may occur between distant, large size events, so that the occurrence of one may trigger the others .
Michael A.J. in 2011 has demonstrated that the hypothesis of mutual triggering of giant earthquakes at great distances from each other is not verified statistically. The clustered occurrence in time of huge events can be the manifestation of a random process. This conclusion was based on the statistical modelling of the 100 year worldwide earthquake catalogue.
Earthquakes: relations between seismic moment and frequency of the signal
The earthquakes are generated by fracture phenomena whose duration at the source is controlled by the spatial extent of the fault surface.
The emitted seismic signals show a frequency spectrum (the plot of amplitude vs frequency of the Fourier’ sinusoids components of the time signal), with a characteristic frequency (the corner frequency), which is related to the inverse of the earthquake source duration.
Since the earthquake source duration scales with the magnitude or the seismic moment, in many seismic regions worldwide it is commonly observed that the characteristic earthquake frequency decreases with the earthquake size (left figure). It ranges between 1Hz and several thousands Hz for microearthquakes and laboratory-induced rock fractures. Frequencies of moderate to very large events span over the range 0.001-0.1 Hz. This relation represents one of the most relevant scaling laws of the earthquake process.
Scaling laws of the earthquake process
A scaling law is an empirical/theoretical relationship linking a given quantity of the physical process to some parameter which define its size or intensity. As concerning the earthquake process, the reference size parameter is the magnitude or the seismic moment. The analysis of worldwide recordings of earthquakes with different sizes allowed to determine the scaling relations for source parameters like the fault length, the average fault slip, the source duration and the corner frequency.
Fault length and average slip
One main observational evidence is the increase of fault dimensions and average slip with the magnitude of the earthquake. The fault length is the largest linear fault dimension, assuming a rectangular rupture surface. The slip is the amount of relative displacement between blocks which move relatively each other across the fault plane (top figure).
The fault length is nearly linear correlated to the seismic moment (magnitude) in a log-log scale. The slope of the linear trend is proportional to the stress drop, e.g. the difference between the stress causing the slip across the fault before and after an earthquake.
Watch the Video Animation here.
Damages are produced by seismic waves
The large majority of observed damages during earthquakes are not produced by the earthquake rupture process itself, but by seismic waves which are radiated by the source and propagate to the Earth surface. As an effect of the seismic wave geometrical spreading and Earth attenuation properties, the amplitude of seismic waves decrease as a function of the distance. This is the reason why most of damages are confined within a region close to the epicenter. The size of the damage zone depends however on the earthquake magnitude, being larger for high magnitude events. As an example, the macroseismic intensity field (represented as degrees of the Mercalli-Cancani-Sieberg scale ) of the April 6th, 2009, Central Italy earthquake (Mw 6.3) at L’Aquila show a damage zone (I>7-8) extending 25-30 km out of the epicenter.
Not always the amount of damage after an earthquake is correlated to the earthquake size. The quality, robustness and earthquake-resistance of buildings is the most important factor to secure lives and things from damages caused by earthquakes.
A study by Ambraseys & Bilham published on Nature in 2005 shows that 83% of victims due to the building collapse during destructive earthquakes of the last 30 years has been produced in countries with the highest level of political and administrative corruption.
In the figure (corruption perception index vs income per capita) , a regression line (dashed) divides nations that are perceived as more corrupt (below the line) than might be expected from the average income per capita from those that are less corrupt (above the line).
Named countries have lost citizens in building collapse caused by earthquakes since 1980. It can be noted that most of the countries where the earthquakes have produced fatalities in the last three decades are perceived as the ones with the highest corruption level. This correlation suggests that where corruption is extreme, its effect are manifest in the poor quality of buildings and more in general in the unreliability of the building industry.
- Ambraseys & Bilham (2011). Corruption kills, Nature, 469, 153–155, doi:10.1038/469153a
- Aki K. (1966). Generation and propagation of G waves from the Niigata earthquake of June 16, 1964, Part 2: estimation of earthquake moment, released energy, and stress-strain drop from the G wave spectrum. Bull. Earthq. Res. Inst. 44, 73-88.
- Bolt B.A. (1982). Inside the Earth. Freeman& Co. Publisher.
- Bijwaard H. and W. Spakman (2000). Nonlinear global P-wave tomography by iterated linearised inversion. Geophys. J. Int. 141, 71-82.
- Larroque C. et J. Virieux (2001). Physique de la Terre solide. Gordon and Breach Science Publisher.
- Zollo, Emolo (2011). Terremoti e Onde. Metodi e pratica della sismologia moderna, Liguori Ed., Napoli, ISBN-13 978-88-207-3585-2.
Risorse della lezione
- Observation scales of the earthquake process
- Quiz: Quiz Lesson 1 - Observation scales of the earthquake process
- Size and dimensions of the seismological networks
- Quiz: Quiz Lesson 2 - Size and dimensions of the seismological networks
- The seismograms: amplitude and frequency content as a function of the earthquake size and recording distance
- Quiz: Quiz Lesson 3 - The seismograms: amplitude and frequency content as a function of the earthquake size and recording distance
- Digital signal processing: discrete-time signals, Fourier analysis
- Quiz: Quiz Lesson 4 - Digital signal processing: discrete-time signals, Fourier analysis
- Digital signal processing: aliasing, windowing, convolution, filtering
- Quiz: Quiz Lesson 5 - Digital signal processing: aliasing, windowing, convolution, filtering
- Earthquake Source and wave propagation contributions in seismograms
- Quiz: Quiz Lesson 6 - Earthquake Source and wave propagation contributions in seismograms
- The «point source» approximation of the earthquake rupture process: the seismic moment
- Quiz: Quiz Lesson 7 - The «point source» approximation of the earthquake rupture process: the seismic moment
- The «point source» approximation: The magnitude
- Quiz: Quiz Lesson 8 - The «point source» approximation of the earthquake rupture process: The magnitude
- Point source approximation: The earthquake location
- Quiz: Quiz Lesson 9 - The «point source» approximation of the earthquake rupture process: The earthquake location
- Point source approximation: the focal mechanisms
- Quiz: Quiz Lesson 10 - The «point source» approximation of the earthquake rupture process: the focal mechanisms
- Extended source: the Haskell rupture model
- Extended source: rupture directivity, circular fault
- Quiz: Quiz Lessons 11 & 12 - The «extended source» models of the earthquake rupture process
- Real-time seismology and early warning - 1
- Real-time seismology and early warning – 2
- Real-time seismology and early warning - 3
- Quiz: Quiz Lessons 13 & 14 & 15 - Real-time seismology and early warning
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- Dark/light areas, represent high/low velocity regions relative to a one-dimensional reference velocity model. Earth Model at 1985
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- Earth structure: - Using seismic waves to image Earth's internal structure
- Section view of the Earth structure
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- Earthquake depth distribution is shown for several subduction zones. The earthquake locations delineate the Wadati–Benioff zone, a nearly-planar zone of seismicity corresponding with the down-going slab in a subduction zone
- Tomographic P-wave velocity model along several cross-sections cutting the Italian peninsula at different latitudes. The white dots indicate major earthquakes (magnitude greater than 4.8) which delineate the shape of the subducting slab. The subduction plate is imaged as a relatively high-velocity zone (blue colored) gently dipping within the upper mantle inside a relatively low-velocity volume(red colored). The dotted lines indicate the discontinuity in the mantle at about 410 km and 660 km depth
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- The table illustrates the range of variability of the source size and observations for natural and artificial seismic sources
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- Global earthquake record (top) and a sample random simulation without clustering displayed (bottom) as cumulative seismic moment (left) and magnitude versus time for events with M ≥ 8 (right)
- M ≥ 8.5 earthquakes since 1900 earthquakes since 1900 with magnitudes and years of M ≥ 9 events annotated
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- Scaling law of the characteristic frequency of the earthquake signal
- SIn wave composition of a seismic signal
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- Intensity decrease as the wave amplitude by increasing the distance from the source
- Map of L’Aquila Earthquake with seismogenteic box
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