3. Earthquake Belts at Plate Boundaries
Belts- a geographic zone on the Earth’s surface along which most
earthquake activity occurs
Ring of Fire- one of the best known belts surrounds the Pacific
Ocean
Shallow-Focus Earthquakes at Divergent Boundaries
Deep-Focus Earthquakes at Convergent Boumdaries
Shallow Focus Earthquakes within Plates
5. Deep-Focus
Earthquakes at
Convergent
Boundaries
These are earthquakes that originate at
depths greater than about 100 km
Found to coincide with continental margins or
island chains that are adjacent to ocean
trenches and young volcanic mountains
Examples of such areas are West coast of South America and the chains of
islands that make up Japan and Philippines
7. Earthquake Destructiveness
Each year 800 000 little tremors that
are not felt by humans are not recorded
by instruments around the world
100 earthquakes occur each year with
Richter magnitude between 6 and 7
Once every 5 to 10 years great
earthquakes occur with a Richter
magnitude exceeding 8
Destructive earthquakes are even more
frequent in Japan than in California
10. Fires ignited by ruptured gas lines or downed electrical power lines
11. Power seismic waves also take their toll on land forms and
underlying soils
Avalanches
Liquefaction
12. Tsunami
• sea wave that is triggered by undersea event such as an earthquake
or landslide or eruption of an oceanic volcano
• Popularly known as tidal waves
• It travels across the ocean at speeds of up to 800 km per hour and
form walls of water that can be higher than 20 m when they reach the
coast
13. Generation of a tsunami by fault movements caused by an earthquake
on the seafloor.
18. 1. Izmit, Turkey(1999)
2. Papua New Guinea
3. Northern Iran
4. Windward Islands
5. Sakhalin Island
6. Kobe, Japan
7. Kuril Islands
8. Northern Bolivia
9. Northridge CA
10. Southern India
11. Republic of South
12. Flores region
13. Switzerland
14. Northern Colombia
15. Landers, CA
16. Northern India
17. Luzon, Philippines
18. Western Iran
19. Loma Prieta, CA
21. *These are the possible premonitory indicators being examined by some
scientists:
• A rapid tilting of the ground or other forms of surface deformation
• An unusual aseismic slip on a fault
• An episode of stretching of the crust across fault
• Changes in the physical properties of rock in the vicinity of a fault
• Changes in the level of water in wells
• An usual increase in the frequency of smaller earthquakes in a region before a
main shock
Early-Warning Indicators
22. Seismic gap
a segment of an active fault known to produce significant
earthquakes, that has not slipped in an unusually long time when
compared with other segments along the same structure
23. Seismic gap hypothesis/theory
states that, over long periods of time, the displacement on any
segment must be equal to that experienced by all the other parts of
the fault.[1] Any large and longstanding gap is therefore considered
to be the fault segment most likely to suffer future earthquakes.
27. Exploring the Interior with Seismic Waves
• Any differences detected for different paths can be used to infer the
properties of materials the waves have been encountered along these
paths
• A boundary between two materials, some of the waves bounce off
and others are transmitted into the second material
• There are waves bend or refract when cross the boundary between
two materials
28. Two beams of light enter the
bowl of water from the top
29. Paths of Seismic Waves in the Earth
• Shadow Zone
A zone where no P wave reaches the surface between 105° and 142°.
Discovery of it led geologists to surmise that the Earth has a core made
up of a material different from than that of overlying mantle.
*Waves traveling through the Earth
30. The pattern of P-wave paths through earth’s interior The larger S-wave shadow zone
31. *Waves traveling through the Earth
• PcP wave- bounces off the core; used to
determine the depth of the core
• SS wave- S waves that reflect back into the Earth
• PKP wave- P waves that penetrate the outer core
• PKIP wave- P wave that penetrate the inner core
*Waves reflected in the Earth
32.
33. Composition and
Structure of the
Interior
Changes in P- and S- wave velocities
with depth in the Earth reveal the
sequence of layers that make up
Earth’s interior
34. *The Crust
• It varies in thickness- thin under oceans (about 5 km), thicker under
continents ( about 40 km) and thickest under high mountains ( ranging up to
65 km).
• P waves move through crustal rocks at about 6-7 km per second
37. Principle of Isostasy
• The idea that continents are less dense than the mantle and float on it
38.
39. *The Mantle
It is mainly made up of olivine and pyroxene, the two silicates of magnesium
and iron
40.
41. • lithosphere- outermost zone, is a slab up to 100 km thick in which the
continents are embedded tectonic plates are large fragments of the
lithosphere . S waves pas through easily without being absorbed
42. • asthenosphere- zone of weakness; a partially fluid solid .It rises close to the
surface at mid-ocean ridges where plates separate and is found at depths up
to about 100 km and ends at a about 200 km where the velocity of S waves
increases to a value that fits that of solid peridotite.
43. • From about 200 to 400 km, the velocity of S waves increases gradually with
depth
• About 400 km below the surface waves increases rapidly
• In the region from 450 to 650 km, properties change little as depth increases
• Near 670 km, the velocity increases again
• From 700 km to the core at a depth of 2900 km, changes little in
composition and crystal structure with depth
44. • *The Core
• It is 2900 km from the surface
• P waves that penetrate to depths of 5100 km suddenly speed up, a discovery made by a
Danish seismologist, Inge Lehman
• Composition of the core derived from astronomical data, and seismological data
• To be consistent with the that the core is made up material that sank during the initial
formation and differentiation of the Earth, geologists searched for the substances that were
dense
• Because the core contains one- third of Earth’s mass, geologists considered substances that
were abundant in the universe
45. • Geologists concluded that Earth’s core is composed mostly of iron, molten
in the inner core and solid in the inner core