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Fracturing and seismicity of the lithosphere and goals of computer modeling
S.I. Sherman
Institute of the Earth’s Crust SB RAS, Irkutsk, 664033, Russia
Seismicity within tectonically active territories is controlled by zones of recent fracturing of the lithosphere. Being composed by seismically active faults of various hierarchic ranks, these zones are structures of the highest hierarchic level; they have a “tree-like” shape pattern in a vertical cross-section and complex fracturing patterns on the surface. As exemplified by the Baikal rift system, the report discusses methods to identify the zone of recent fracturing of the lithosphere and regularities of seismic process at faults of various ranks that compose the zone. Seismic process in the zone is a complex manifestation of spatial and temporal migration of earthquake epicenters of various magnitudes in areas of dynamic influence of faults of various ranks in the zone of recent fracturing of the lithosphere [1]. Modern computer modeling methods provide for revealing complex space-and-time regularities of the process as a whole within the seismic zone. Structural and physical parameters of the lithosphere, faults and seismicity, which should be taken into account in computer modeling of seismic process, are considered.
1. Introduction
Seismic process within tectonically active territories is controlled by faults of various hierarchic ranks. Structurally, they compose the zone of recent fracturing of the lithosphere, and its area of dynamic influence predetermines seismicity. Establishing regularities of seismicity is a challenging problem due to the extremely huge difference in the duration of the geological development of faults, which compose the fracturing zone, and the period of instrumental measurements of seismicity. G.A. Sobolev, A.V. Ponomarev [2] and some other researchers consider the regularities of the seismic process in seismically active zones in terms of the space-and-time succession of seismic event. They attach particular significance to physical modeling which provides for a deeper insight in earthquake prediction indicators, though only in part addresses problems of general regularities of seismic process in tectonically active areas. Computer modeling can considerably widen the capacities of available research methods to study space-and-time regularities of seismic process, since it processes numerous parameters describing complex changes of seismicity both in space and time.
Geological and seismological data, parameters of seismically active faults, areas of dynamic influence of faults, principal aspects of fractured medium behavior as a quasi-vis-cous body and some other issues are considered as exemplified by the Baikal rift system (BRS). The above mentioned information is converted into values employed by computer modeling of processes of faulting and seismicity in order to reveal space-and-time regularities of seismic process and
geological and geophysical factors for a medium-scale prediction of earthquakes.
2. The Baikal rift system in the fault-block structure of Central Asia
The BRS stretches for over 2000 km from the northwestern Mongolia through mountainous structures of East Siberia to the Southern Yakutia. Its basement is a heterogeneous and heterochronous fold belt which ceased in its development in the early Paleozoic. The orogenic complex of the given territory was formed from the end of the early Paleozoic through the Cenozoic. In most of its length, the BRS is controlled by structural lithospheric suture between the Siberian and Amur megablocks of the Eurasian plate; the suture formation commenced in early Proterozoic, and during the Phanerozoic it separated the lithospheric blocks that were significantly different both in structure and development [3, 4]. The ancient suture predetermines the recent generally S-shaped structural plan of the BRS that is characterized by a relatively regular fault pattern [5-8 and others]. The faults formed through the whole history of the geological development of the BRS, since the early Paleozoic through the Cenozoic. During rifting, the faults controlled positions of rift basins and functioned as normal faults in the central BRS and normal strike-slip faults in its flanks (Fig. 1). However, the earthquake epicenter field does not always correspond to the known faults of the given region. In cases when distribution patterns of earthquake foci and tectonic faults and block in the BRS are inconsistent, there
© S.I. Sherman, 2004
are grounds to suggest that the faults mapped by geological and geological-geophysical methods do not fully describe the recent process of fracturing in the BRS. To reveal regularities in distribution patterns, the earthquake epicenter field is considered relative to the zone of the recent fracturing of the lithosphere. It also taken into account that an earthquake focus is actually a fracture, as proved by many researches [9 and others].
3. Zone of fracturing in the lithosphere as a leading structure controlling seismicity
According to S. Sherman [10], ensembles of fault systems may form a hierarchically higher taxones in the litho-spheric structure, such as zones of fracturing in the lithosphere that are characterized by higher rates of disunity, states of intensive stresses, higher deformation rates of the medium, and contrasting variations in the geophysical fields. Being the most unstable areas of the Earth’s surface, such zones can be classified as territories of high seismicity and
potential natural and technogenic hazards. The zone of the recent fracturing of the lithosphere has been identified in the BRS for the first time [11, 12]. Hierarchic subordinations are established: (1) between the length of the whole zone of fracturing on the surface and magnitudes of strong earthquakes M LH, that always tend to occur at the axial line of the zone; and (2) between lengths of separate faults of lower ranks, that compose the structure of the given zone, and magnitudes of weak earthquakes. All strong earthquakes, M > 6.0, known from instrumental and historical data, were located close to the axial line of the established zone of the recent fracturing of the lithosphere. Their epicenters evidently “migrate” from SW to NE and backwards along the strike of the zone of fracturing. The “oscillation” migration of seismic events from one termination of the fault to another has been reliably revealed for many seismically active faults through the world [13, 14 and others]. However, the recurrence of events in various parts of the BRS is different. The oscillation migration of epicenters of weaker earthquakes is recorded for faults of other hierarchic ranks that compose
Fig. 1. A map of major faults of the Baikal rift zone. Author S.I. Sherman. Materials of S.I. Sherman, K.G. Levi, V.A. San’kov, A.S. Gladkov et al., “Fault Map of the Southern East Siberia” (1982), other geophysical and seismological data are used. Legend: 1 — zone of recent destruction of the lithosphere including transform faults; 2 — zone of recent destruction of the lithosphere; 3 — seismic active faults; 4 — non-active faults; 5 — normal faults; 6 — reverse faults; 7 — strike-slip faults; 8 — displacement sign not identified; 9 — basins of the Baikalian type: Tunka (1); Barguzin (2); Upper Angara (3); Muya (4); Chara (5); Tokka (6)
Fig. 2. Space-time migration of seismic events of M > 6 along the strike of the zone of the recent fracturing of the lithosphere (left) and the strike from West to East of the Tunka transform fault (right). Circles show magnitude of earthquakes
elements of the zone of the recent fracturing of the lithosphere.
Based on the above, there are grounds to conclude that, in the BRS, seismic events of various magnitudes that accompany the recent process of rifting are controlled by the zone of fracturing of the elastic lithosphere. The realization of the given genetic relationship occurs with respect to fracturing structures varying in ranks. Strong earthquakes are controlled by the whole zone of the recent fracturing of the lithosphere as a uniform structural unit; whereas, weaker events are controlled by its fragments. Generally, the process of seismicity is represented by a summarized manifestation of space-and-time migrations of seismic events of various magnitudes in the areas of dynamic influence of faults of various hierarchic ranks that compose the zone of the recent fracturing of the lithosphere. Structurally, this zone can be viewed as a specific fault “tree” in the vertical cross-section of the lithosphere; the major trunk is an echeloned combination of faults of various ranks; the fault network is most dense along the axial line of the zone of the recent fracturing of the lithosphere (see Fig. 1). The trunk is combined with faults of lower hierarchic ranks that are represented in geological maps as faults systems of various ranks of the NE strike. They are sub-vertical or steeply dipping towards the axis of the zone of the recent fracturing of the lithosphere. Each of the faults has its own area of dynamic influence with similar oscillation migrations of earthquake foci within the given area (Fig. 2). The recent seismic process is characterized by the occurrence of earthquake foci varying in space, time and strength. Which is a model ofthe given process, and what are the abilities for its computer modeling?
4. A principal model of the seismically active zone of the lithosphere in the BRS and goals of its computer modeling
Most of the well known models of the seismic process in huge territories are based on principles of the fault-block divisibility of the lithosphere and its quazi-plastic state and even flow [15-17 and others]. Several models ofthe seismic process in the BRS have been proposed, including those based on results of physical modeling of active fault zones [18]. These models are unable to consider the multi-staged hierarchy of fault structures and their formation within the given uniform zone when “accidental” merging of two random dislocations of one and the same rank leads to the transformation of the whole, already available pattern of the fault ‘tree”. Considering the above phenomena is possible by new models of the seismic process in fault zones that have been elaborated by computer modeling based on the available geological and geophysical data on the BRS. Which medium, structures and known regularities of the seismic process should be used as a basis for the computer modeling?
The necessity to construct a double-layered model is evidenced by experiences of employing computer modeling methods [5] to formulate and resolve problems of irregular seismic activity manifestation during the formation of shear zones in the one-layered models of the lithosphere. The following parameters are used:
Medium: the double-layered Earth’s crust (the upper elastic layer’s thickness is about 20-25 km; Young’s modulus is (0.7-0.8)-105 MPa; Poisson’s ratio is 0.25; the compression strength is 400-500 MPa; the extension strength is 20-40 MPa; the internal friction angle is 40°; the cohesion
strength is 40 MPa; the thickness of the lower viscoelastic layer (the Maxwell body) is 15-20 km; viscosity is 1020-21 Pa- s ). Structures: recent active faults of the BRS of three or even four hierarchic ranks: the zone of fracturing of the lithosphere, major and local faults. It needs to be taken into account that they are reactivated at the background of the existence of the inter-plate boundary between the Siberian and Transbaikalian plates of the lithosphere that predetermined the initial position and development of the BRS. The inter-plate boundary can be regarded as a fault of the highest hierarchic rank. Faults decrease the quazi-viscosity of the medium as a whole. The internal friction angle in faults zones is 30°; viscosity is 1018 Pa - s; the cohesion strength is 25 MPa [19]. These parameters should change according to the hierarchic rank of faults. In general, a seismically active structure under modeling should be similar to the zone of the recent fracturing of the lithosphere; in the vertical cross-section, a combination of faults varying in ranks is represented by a “tree-shaped” structure along and across the strike; the projection of its trunk as an axial zone is given in Fig. 1.
The model of the seismic process in the above described structure should describe space-and-time regularities of the occurrence of earthquake foci varying in magnitudes within areas of dynamic influence of faults varying in ranks in the zone of the recent fracturing of the lithosphere. From spatial and temporal variations of earthquake foci, it may be possible to establish regularities of the recent seismic process by applying modern methods of computer modeling.
The research was supported by the RAS Presidium Program 13 (Project 12), the Russian Foundation for Basic Research (grant 04-05-64348), Integration Project of the SB RAS, No. 2003-101, and the Ministry of Education (grant E02-8-45).
References
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[3] S.I. Sherman and K.G. Levi, Transform Faults of the Baikal Rift Zone and Seismicity of its Flanks, in Tectonics and Seismicity of Continental Rift Zones, Nauka, Moscow (1978) 7.
[4] V.D. Matz, G.F. Ufimtsev, M.M. Mandelbaum et al., The Cenozoic Period of the Baikal Rift Basin, Structure and Geological History, Publishing House of the SB RAS, “GEO”, Novosibirsk, 2001.
[5] S.I. Sherman, K.Zh. Seminsky, S.A. Bornyakov, et al., Faulting in the Lithosphere, Vol. 1-3, Nauka, Novosibirsk, 1991, 1992, 1994.
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