Transcript Seismology
Geophysics 50 Introduction to Geophysics and Planetary Physics (3) Seismology Geophysics 51 Tectonic Plates Eurasian Plate North American Plate Eurasian Plate African Plate Pacific Plate South American Plate IndoAustralian Plate Antarctic Plate The seven major lithospheric plates and their boundaries (Press & Siever). Geophysics 52 Earthquakes and Plate Boundaries Worldwide distribution of earthquakes with Magnitude > 5 („Richter Scale“) from 1980 until 1990. Most earthquake foci are located at plate boundaries (Source: USGS). Geophysics 53 Stress and Stress Relief Tectonic earthquakes occur, when plate movement leads to sufficient stress (stored elastic energy) to overcome the local rock strength and to drive fracture that propagates along a fault plane. The (elastic) rebound causes stress in the surrounding – a reason for aftershocks. Thrust faults (top left) – responsible for the most powerful earthquakes – are usually caused by convergent plate movement (compression), normal faults (middle) by divergent plate movement (tension). Strike-slip faults (bottom) are the result of lateral movement with little or no vertical motion (shearing at transform boundaries). Geophysics 54 Seismic Waves Source: http://www.physik.uni-muenchen.de Geophysics 55 P- and S-Waves (Weak) Seismic waves can be – and are triggered – with a hammer. The (schematic) picture on the left shows the excitation of primary and secondary waves. Geophysics 56 Body Waves vP 4 K G 3 The speed of body waves depends on few material constants: K is the bulk modulus (or the modulus of incompressibility) ρ is the density. G is the shear modulus (or the modulus of rigidity). Since G = 0 in liquids, secondary waves cannot propagate there. vS G Geophysics 57 Sunquake Seismic waves can even be observed on the Sun – the SOHO satellite picture (left) shows surface waves (propagating more than 100 000 km) that have been caused by a solar flare (white) on July 6, 1996 – corresponding to a magnitude 11.3 Above: Solar flare on May 2, 1998 (Source: SOHO, ESA/NASA). Geophysics 58 Focus (Hypocenter) and Epicenter http://www.physik.uni-muenchen.de Geophysics 59 Travel-Time Curve Seismogram A Seismogram B Seismogram C S-Wave 11 Minutes P-Wave Epicenter Since P-Waves are almost twice as fast as S-Waves, the interval between their travel-time curves increases with distance. Measuring the time interval yields the distance to the epicenter. A time interval of 11 minutes (Seismogram C, left) corresponds to a distance of 8 600 km. The Epicenter must therefore be on a circle around the seismometer station (with 8 600 km radius). Seismograms from two stations leave two possible locations (at the intersections of the two circles). With the seismogram of a third station, the exact location of the epicenter can be determined („Allgemeine Geologie“, Press & Siever). Geophysics 60 Seismograph Seismometers measure ground movements, in particular due to seismic waves. Seismographs (directly) record such movements. The Amplitude of the Seismogram depends on the instrument type (e.g. the amplification). The measurement principle is always similar (left): An internal mass tends not to move because of its inertia, the relative motion is then recorded. The B/W picture shows a seismogram from the Austrian station Kremsmünster, revealing the large earthquake in San Francisco, 1906 (Source: ZAMG) . Geophysics 61 Dragon – Seismoscope The Chinese Geographer and Astronomer Chang Heng developed (132 AD) the first seismic instrument, a device for indicating ground motion – the famous Dragon-Seismoscope. Unfortunately there is no original left – just descriptions. Apparently it was a 2 m bronze vessel, with eight dragons, holding bronze balls. An arriving seismic wave triggered an internal mechanism that released on of those bronze balls, which then fell into the mouth of a frog underneath – indicating the direction to the epicenter. Geophysics 62 Richter–Magnitude (1) (2) (3) (4) Steps to determine the Richter-Magnitude with the aid of a Nomogram: (1) Read the maximum amplitude (SWaves), (2) read the time-delay between the first P-Wave and the first S-Wave, (3) connect the two points, (4) read the magnitude, done (Source: B. A. Bolt)