CHAPTER 3 Seismic Sources and Source Parameters Peter Bormann, Michael Baumbach, Günther Bock, Helmut Grosser, George L. Choy and John Boatwright 3.1 Introduction to seismic sources and source parameters (P. Bormann) 3.1.1 Types and peculiarities of seismic source processes Fig. 3.1 depicts the main kinds of sources which generate seismic waves (see Chapter 2). Seismic waves are oscillations due to elastic deformations which propagate through the Earth and can be recorded by seismographic sensors (see Chapter 5) . The energy associated with these sources can have a tremendous range and, thus, can have a wide range of intensities (see Chapter 12) and magnitudes (see 3.2 below) . SEISMIC SOURCES Natural Events Man-Made Events Tectonic Controlled Sources Earthquakes ( Explosions, Vibrators...) Volcanic Tremors Reservoir Induced and Earthquakes Earthquakes Rock Falls / Collapse Mining Induced of Karst Cavities Rock Bursts / Collapses Cultural Noise Storm Microseisms ( Industry, Traffic etc.) Fig. 3.1 Schematic classification of various kinds of events which generate seismic waves. 1 3. Seismic Sources and Source Parameters 3.1.1.1 Tectonic earthquakes Tectonic earthquakes are caused when the brittle part of the Earth’s crust is subjected to stress that exceeds its breaking strength. Sudden rupture will occur, mostly along pre-existing faults or sometimes along newly formed faults. Rocks on each side of the rupture "snap" into a new position. For very large earthquakes, the length of the ruptured zone may be as much as 1000 km and the slip along the fault can reach several meters. Laboratory experiments show that homogeneous consolidated rocks under pressure and temperature conditions at the Earth's surface will fracture at a volume strain on the order of 10 -2 - 10 -3 (i.e., about 0.1 % to 1% volume change) depending upon their porosity. Rock strength is generally smaller under tension or shear than under compression. Shear strains on the order of about 10 -4 or less may cause fracturing of solid brittle rock. Rock strength is further reduced if the rock is pre-fractured, which is usually the case in the crust. The strength of pre-fractured rock is much less than that of unbroken competent rock and is mainly controlled by the frictional resistance to motion of the two sides of the fault. Frictional resistance, which depends on the orientation of the faults with respect to the stress field and other conditions (see Scholz, 1990), can vary over a wide range. Accordingly, deformations on the order of only 10 -5 to 10 -7, which correspond to bending of a lithospheric plate by about 0.1 mm to 1 cm over a distance of 1 km, may cause shear faulting along pre-existing zones of weakness. But the shear strength depends also on the composition and fabric (anisotropy) of rock, its temperature, the confining pressure, the rate of deformation, etc. as well as the total cumulative strain. More details on the physics of earthquake faulting and related geological and seismotectonic conditions in the real Earth can be found in Scholz (1990) and in section 3.1.3 on Source representation. Additional recommended overview articles on the rheology of the stratified lithosphere and its relation to crustal composition, age and heat flow were published by Meissner and Wever (1988), Ranalli and Murphy (1987) and Wever et al. (1987). They also explain the influence of these parameters on the thickness and maximum depth of the seismogenic zone in the crust, i.e., the zone within which brittle fracturing of the rocks is possible when the strains exceed the breaking strength or elastic limit of the rock (see Fig. 2.1). The break-up of the lithosphere into plates due to deformation and stress loading is the main cause of tectonic earthquakes. The plates are driven, pushed and pulled by the slow motion of convection currents in the more plastic hot material of the mantle beneath the lithosphere. These relative motions are in the order of several cm per year. Fig. 3.2 shows the global pattern of earthquake belts and the major tectonic plates. There are also numerous small plates called sub- or micro-plates. Shallow earthquakes, within the upper part of the crust, take place mainly at plate boundaries but may also occur inside plates (interplate and intraplate earthquakes, respectively). Intermediate (down to about 300 km) and deep earthquakes (down to a maximum of 700 km depth) occur under ocean trenches and related subduction zones where the lithosphere plates are thrusted or pulled down into the upper mantle. The major trenches are found around the Circum-Pacific earthquake and volcanic belt (see Fig. 3.2). However, intermediate and deep earthquakes may occur also in some other marine or continental collision zones (e.g., the Tyrrhenian and Aegean Sea or the Carpathians and Hindu Kush, respectively). Most earthquakes occur along the main plate boundaries. These boundaries constitute either zones of extension (e.g., in the up-welling zones of the mid-oceanic ridges or intra-plate rifts), transcurrent shear zones (e.g., the San Andreas fault in the west coast of North America or the 2 3.1 Introduction to seismic sources and source parameters North Anatolian fault in Turkey), or zones of plate collision (e.g., the Himalayan thrust front) or subduction (mostly along deep sea trenches). Accordingly, tectonic earthquakes may be associated with many different faulting types (strike-slip, normal, reverse, thrust faulting or mixed; see Figs. 3.32 and 3.33 in 3.4.2). The largest strain rates are observed near active plate boundaries (about 10 -8 to 3 ×10 -10 per year). Strain rates are significantly less in active plate interiors (about 5 ×10 -10 to 3 ×10 -11 per year) or within stable continental platforms (about 5 ×10 -11 to 10 -12 per year) (personal communication by Giardini, 1994). Consequently, the critical cumulative strain for the pre- fractured/faulted seismogenic zone of lithosphere, which is on the order of about 10 -6 to 10 -7, is reached roughly after some 100, 1000 to 10,000 or 10,000 to 100,000 years of loading, respectively. This agrees well with estimates of the mean return period of the largest possible events (seismic cycles) in different plate environments (Muir-Wood ,1993; Scholz, 1990). Fig. 3.2 Global distribution of earthquake epicenters according to the data catalog of the United States National Earthquake Information Center (NEIC), January 1977 to July 1997, and the related major lithosphere plates. Although there are hundreds of thousands of weak tectonic earthquakes globally every year, most of them can only be recorded by sensitive nearby instruments. But in the long-term global statistical average about 100,000 earthquakes are strong enough (M ≥ 3) to be potentially perceptible by humans in the near-source area. A few thousand are strong enough (M ≥ 5) to cause slight damage and some 100 with magnitude M > 6 can cause heavy damage, if there are nearby settlements and built-up areas; while about 1 to 3 events every year (with M ≥ 8) may result in wide-spread devastation and disaster. During the 20 th century the 1995 Great Hanshin/Kobe earthquake caused the greatest economic loss (about 100 billion US$), the 1976 Tangshan earthquake inflicted the most terrible human loss (about 243,000 people killed) while the Chile earthquake of 1960 released the largest amount of seismic 18 19 energy E S (see 3.1.2.2 below) of about 5 ⋅10 to 10 Joule. The latter corresponds to about 25 to 100 years of the long-term annual average of global seismic energy release which is about 1 - 2 × 10 17 J (Lay and Wallace, 1995) and to about half a year of the total kinetic energy 3 3. Seismic Sources and Source Parameters contained in the global lithosphere plate motion. The total seismic moment (see 3.1.2.3. below) of the Chile earthquake was about 3 ×10 23 Nm. It ruptured about 800 - 1000 km of the subduction zone interface at the Peru-Chile trench in a width of about 200 km (Boore 1977; Scholz 1990). In summary: about 85 % of the total world-wide seismic moment release by earthquakes occurs in subduction zones and more than 95 % by shallow earthquakes along plate boundaries. The other 5 % are distributed between intraplate events and deep and intermediate focus earthquakes. The single 1960 Chile earthquake accounts for about 25 % of the total seismic moment release between 1904 and 1986. It should be noted that most of the total energy release, E T, is required to power the growth of the earthquake fracture and the production of heat. Only a small fraction of E T = E S + E f (with Ef - friction energy) goes into producing seismic waves. The seismic efficiency, i.e., the ratio of E S/E T , is perhaps only about 0.01 to 0.1. It depends both on the stress drop during the rupture as well as on the total stress in the source region (Spence, 1977; Scholz, 1990). 3.1.1.2 Volcanic earthquakes Although the total energy released by the strongest historically known volcanic eruptions was even larger than E T of the Chile earthquake, the seismic efficiency of volcanic eruptions is generally much smaller, due to their long duration. Nevertheless, in some cases, volcanic earthquakes may locally reach the shaking strength of destructive earthquakes (e.g., magnitudes of about 6; see 3.1.2.2). Most of the seismic oscillations produced in conjunction with sub-surface magma flows are of the tremor type, i.e., long-lasting and more or less monochromatic oscillations which come from a two- or three-phase (liquid- and/or gas-solid) source process which is not narrowly localized in space and time.