Geophysics 48 (3) Seismology Introduction to Geophysics and Planetary Physics

Tectonic Plates The seven major lithospheric plates and their boundaries (Press & Siever). Pacific Plate Eurasian Plate African Plate North American Plate South American Plate Indo- Australian Plate Antarctic Plate Geophysics 49 Eurasian Plate

Earthquakes and Plate Boundaries Worldwide distribution of earthquakes with Magnitude > 5 („Richter Scale“) from 1980 until Most earthquake foci are located at plate boundaries (Source: USGS). Geophysics 50

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 51

Seismic Waves Source: Geophysics 52

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 53

Sunquake Seismic waves can even be observed on the Sun – the SOHO satellite picture (left) shows surface waves (propagating more than 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 54

Focus (Hypocenter) and Epicenter Geophysics 55

Travel-Time Curve 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 km. The Epicenter must therefore be on a circle around the seismometer station (with 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). Seismogram A Seismogram B Seismogram C P-Wave S-Wave 11 Minutes Epicenter Geophysics 56

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 57

Dragon – Seismoscop e 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 58

Richter–Magnitude Steps to determine the Richter-Magnitude with the aid of a Nomogram: (1) Read the maximum amplitude (S- Waves), (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) (1) (2) (3) (4) Geophysics 59