Download 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)