2. Locating the source of earthquakes Focus - the place within Earth where earthquake waves originate Epicenter on an earthquake– location on the surface directly above the focus Epicenter is located using the difference in velocities of P and S waves

3. Earthquake focus and epicenter Earthquake focus and epicenter

4. Locating the epicenter of an earthquake Three seismographs needed to locate an epicenter Each station determines the time interval between the arrival of the first P wave and the first S wave at their location A travel-time graph then determines each station’s distance to the epicenter On a map, draw a circle around the seismic station Radius of circle= distance to epicenter Radius of circle= distance to epicenter Repeat for other seismic stations (Triangulation) Repeat for other seismic stations (Triangulation) The three circles will meet at a point; that point is the epicenter The three circles will meet at a point; that point is the epicenter

5. Travel-time graph used to find distance to epicenter The difference in arrival time of the First P wave and S Wave in the Example is 5 minutes

6. Typical seismic record. Note the time interval (about 5 minutes) between the arrival of the first P waves and the arrival of the first S waves

7. The records obtained from seismographs, called seismograms, provide a great deal Of information about the behavior of seismic waves

8. Earthquake epicenter is located using the distance obtained from three seismic stations

9. Earthquake belts 95% of energy released by earthquakes originates in narrow zones that wind around the Earth These zones mark of edges of tectonic plates

10. Locations of Earthquakes Most occur along tectonic plate boundaries: –Around the Pacific Ocean (Circum-Pacific belt is major site of earthquakes) (الحزام المطوق للمحيط الهادي) –Mediterranean Sea area through Iran and on the Himalayas –Asia (Indonesia, Himalayan region) –Mid ocean ridges (مرتفعات وسط المحيطات) Some occur far from plate edges

11. Locations of earthquakes with magnitude equal to or greater than 5 for the period 1980 to 1990

12. Circum Pacific Belt

13. Oceanic ridge system

14. Depths of Earthquakes Earthquakes originate at depths ranging from 5 to nearly 700 kilometers Definite patterns exist –Shallow focus occur along oceanic ridges (less than 70 km) - Intermediate Earthquakes: 60-300 km –Deep earthquakes occur in western Pacific east of oceanic trenches (greater than 300 km than 300 km -90 % of earthquakes are found in a depth less than 100 km

15. Earthquake in subduction zones

16. Measuring the size of earthquakes Two measurements describe the size of an earthquake Intensity – a measure of earthquake shaking at a given location based on amount of damage Intensity of an earthquake depends on: –Distance to epicenter –Building materials and design –Type of ground material-soil, rock, etc. Magnitude – estimates the amount of energy released by the earthquake

17. Intensity scales Modified Mercalli Intensity Scale was developed using California buildings as its standard Ranges from I (felt by only a few) to XII (total destruction)

19. Magnitude scales Richter magnitude - concept introduced by Charles Richter in 1935 Richter scale – Measures earthquake magnitude – Determined by measuring the amplitude of the largest waves on the the seismogram – A logarithmic scale –Each number on the Richter Scale is tens times greater in wave amplitude. The wave amplitude of an earthquake of “magnitude 6”is 10 times more than the amplitude of a “ magnitude 5 earthquake” – Each number on the Richter Scale involves an energy release about 32 times as great. Thus, an earthquake with a magnitude of 6.5 releases 32 times more energy than one with a magnitude of 5.5

21. Magnitudes scales Moment magnitude was developed because Richter magnitude does not closely estimate the size of very large earthquakes –Derived from the amount of displacement that occurs along a fault and the area of the fault that slips

22. Earthquake destruction Amount of structural damage depends on Intensity and duration of vibrations Nature of the material upon which the structure rests (hard rock good, soft bad) Design of the structure

23. Results of Earthquakes Vibration of the ground-Causes damage to structures Thixotropic sediment causes ground to flow. Thixotropic is a term used to refer to a solid that flows when vibrated or jolted Tsunami-seismic sea waves FiresLandslides Well water levels fluctuate; 1964 Alaska caused wells in SE US to fluctuate by 3.5 meter

24. Tsunamis, or seismic sea waves Earthquakes under the ocean Destructive waves called “tidal waves” Result from “push” of fault block or under sea landslide on water In open ocean height is > 1 meter In shallow coast water wave can be > 30 meters Very destructive

25. Formation of a tsunami by displacement of the ocean floor The speed of a wave moving across the ocean correlates with the ocean depth.

26. Tsunami 1960, Hawaii

27. Can earthquakes be predicted Short-range predictions Goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame Research has concentrated on monitoring possible precursors – phenomena that precede a forthcoming earthquake such as measuring uplift, subsidence, and strain in the rocks

28. Earthquakes cannot be predicted Short-range predictions Currently, no method exists for making short-range earthquake predictions Long-range forecasts Calculates probability of a certain magnitude earthquake occurring over a given time period

29. Probability of a major earthquake from 1988 to 2018 on the San Andreas Fault

30. Major historical earthquakes 1811 and 1812-Three large earthquakes, the largest in U.S. history occurred near New Madrid, Missouri (7.9) 1886 Charleston, South Carolina (7.9) 1906 San Francisco, CA (7.8) 1964 Alaska (9.2). Strongest earthquake to hit US this century 1989 San Francisco, California (Loma Prieta) (6.1)

31. Importance of Earthquakes Earthquake waves provide information about the internal structure and composition of earth Earthquakes are indictors of present-day tectonic activity or earth movements