3.1. The earthquake source

Fig. 3.1.1 shows the global distribution of potentially damaging earthquakes from 1900 to 2009, color coded according to earthquake strength (magnitude). What is immediately obvious from Fig. 3.1.1 is that the spatial distribution of these earthquakes is highly non-uniform.

Figure 3.1.1       Global distribution of earthquakes from 1900-2009. Data source: ISC-GEM catalog (Storchak et a., 2013). You can turn the globe using the mouse.

Fig. 3.1.2 shows the same set of earthquake locations in map view. The non-uniform spatial distribution becomes even more obvious if the earthquake hypocenters (the locations at which the earthquake ruptures start) are separated into different depth ranges (shallow, intermediate, deep). In particular the intermediate and deep earthquakes (panels Intermediate and Deep in Fig. 3.1.2) are spatially concentrated. The dominant features of the spatial pattern of earthquakes can easily be understood within the scientific theory of plate tectonics (see e. g.  Press and Siever, 1986). Most seismic energy is released where tectonic plates (which move relative to each other  at speeds of upp to a few cm/year) meet. These regions are called plate boundaries. It is actually not surprising that the plate boundaries coincide with the regions of concentrated seismicity (panel Plate Boundaries in Fig. 3.1.2) since the spatial distribution of seismicity was an important piece of information for the definition of plate boundaries.

Figure 3.1.2       Global distribution of earthquake epicenters (projections of the hypocenters to the surface)  from 1900-2009. You can select different depth ranges for display. Data source: ISC-GEM catalog (Storchak et a., 2013). Plate boundaries after DeMets et al. (2010). Plate name abbreviations : AM, Amur; AN, Antarctica; AR, Arabia; AU, Australia; CA, Caribbean; CO, Cocos; CP, Capricorn; CR, Caroline; EU, Eurasia; IN, India; LW, Lwandle; NA, North America; NB, Nubia; NZ, Nazca; OK, Okhotsk; PA, Pacific; PS, Philippine Sea; SA, South America; SC, Scotia; SM, Somalia; SR, Sur; SU, Sundaland; SW, Sandwich; YZ, Yangtze.

The physical mechanisms behind the processes during individual earthquakes have been enigmatic up to not very long ago.  Over the course of human history, most cultures have developed their  own myths to explain the generation of earthquakes, mostly involving different kinds of animals. It was a single event at the beginning of the last century, the San Francisco earthquake of April 18, 1906, which has paved the way for our current understanding of earthquake physics. One of the most remarkable observations during that earthquake was that of of large horizontal ground dislocations, as for example documented in Fig. 3.1.3.

Figure 3.1.3       This picture shows a fence near Woodville in California that was offset by  2.6 m  during the the San Francisco earthquake of 18 April 1906. The photo was taken at Lat: 36,106, Lon: -119,199 and is looking northeast. Image source: NOAA/NGDC, G.K. Gilbert, U.S. Geological Survey. Photo has been colorized.

The analysis in the aftermath of this earthquake by Henry Fielding Reid of Johns Hopkins University has led to what is now called elastic rebound theory (Reid, 1910).  In a nutshell, the elastic rebound theory sees earthquakes as shear ruptures driven by previously stored elastic energy. Before an earthquake, the ground deforms in reaction to plate tectonic forces and deformation energy is stored in the rock volume surrounding the later hypocenter. Over time, deformation increases until the shear strength of the rock (or the frictional resistance of the fault which is  going to rupture) is reached and the rupture starts.  During the rupture process,  part of the stored deformation energy is released in form of seismic wave energy and  the accumulated stress is at least partially released. Subsequently, the process can start over. Fig. 3.1.4  illustrates the key aspects of the rupture phase and the initiation of the radiation of seismic waves.

Figure 3.1.4       The rupture phase of an earthquake. The Mathematica code to create this  figure is from the blogpost by Yu-Sung Chang  from Wolfram research (Chang, 2013; posted March 18, 2013).

Plate boundaries are the regions where most of the seismic energy is released on a global scale.  However, although visible in Fig. 3.1.1 and Fig. 3.1.2 only at second glance (e. g. in Australia), it needs to be emphasized  that the interior of tectonic plates is not free of earthquakes. Intraplate earthquakes, earthquakes which occur at considerable distances away from plate boundaries are called, are not restricted to small magnitudes and can pose considerable hazard  (Johnston and Kanter, 1990). Famous historical example are provided by the New Madrid earthquake sequence in the winter of 1811/12 with three earthquakes of magnitude all larger than 7.5  (Johnston and Schweig, 1996).

.............. 3.1. The earthquake source
.............. 3.1.1. Styles of faulting
.............. 3.1.2. Deformation pattern in the immediate vicinity of the earthquake source
.............. 3.1.3. Earthquake focal mechanism plots (beach ball plots)

Frank Scherbaum (2015), Fundamental concepts of Probabilistic Seismic Hazard Analysis, Hazard Classroom Contribution No. 001