1 Natural Disasters Earthquakes & Their Damages

2 San Francisco M = 7.8; 3,000 killed

3 What is an Earthquake? Ground shaking usually accompanied by sudden movement on a fault

4 What Causes Earthquakes? Elastic Rebound Theory –movement on 2 sides of a fault leads to bending of rock until it snaps to release the bending strain

5 Stress vs. Strain Stress – forces imposed on a rock Strain – change in shape of a rock in response to the imposed tress

6 Types of Strain Elastic – Rocks revert to their original shape when force is relieved Plastic – Rocks permanently change shape Brittle – Rocks fracture or break

7 Stress vs. Strain With increasing stress, a rock deforms elastically, then plastically, before ultimately fracturing

8 Where Do Earthquakes Occur? Faults – fractures in Earth’s crust where rocks on one side move past those on the other side

9 Types of Faults

10 Fault Terminology Hanging wall – Fault block above the fault – Fault block below the fault Footwall

11 Normal Fault Hanging wall moves down relative to footwall Exhibits lengthening or extension of the crust

12 Reverse Fault Hanging wall moves up relative to footwall Exhibits shortening or compression of crust Have dips > 45° If dip’s < 45°, then thrust fault

13 Strike-Slip Fault Horizontal displacement – Right lateral – As you face the fault, opposite side of plate moves right Types of Strike Slip: – Left lateral – opposite side of plate moves left

14 San Andreas Fault

15 What Type of Fault is This? Reverse Fault

16 What Type of Fault? Normal Fault

17 Type of Fault? Thrust Fault

18 Measuring Earthquakes Seismograph –An instrument that measures Earth’s movement Seismogram –Records seismic waves

19 Locating Earthquakes Focus (hypocenter) –Location within the Earth where movement originates Epicenter –Point on Earth’s surface directly above the focus Energy radiates in all directions from the focus Energy is in the form of “seismic waves”

20 Seismic Waves Types of Seismic Waves –P-waves (primary) Push-pull motion Travel fastest Travel through solids and liquids –S-waves (secondary) Side-to-side motion Slower than p-waves Travel through solids only –Surface waves (Love) Travel on Earth’s surface Slowest Causes the most damage

21 Step 1 – Locating Epicenter Measure the distance between P- and S-waves –This is the time difference in arrival times

22 Step 2 - Locating Epicenter Plot the time difference on y-axis (time interval) Trace plot to blue line to determine distance to epicenter The greater the difference, the farther the epicenter

23 Step 3 - Locating Epicenter Measure the distance around each seismic station Epicenter may be located anywhere on that line Minimum of 3 seismic stations needed to determine epicenter Triangulation 200 km

24 Measuring Earthquakes Magnitude - measure of the strength of an earthquake or the amount of energy released

25 Determining Magnitude Measure amplitude of the strongest wave Amplitude is the height on paper Plot distance between p- and s-wave Plot amplitude Connect plots to determine magnitude

26 Mercalli Intensity Scale Intensity – measures two things: 1.how strongly people feel the shaking 2.The severity of the damage Higher Roman #’s depict greater intensities

27 Intensity Map Typically, intensity decreases as you move away from epicenter Distribution not perfectly circular from epicenter –Elongate parallel to fault –Intensities increase over loose wet muds i.e. near epicenter  intensity = 8 Oakland  intensity = 9

28 Loma Prieta, 1989 October 17 th, 5:04 p.m. San Francisco Giants vs. Oakland A’s Also called “Earthquake Series”

29 Loma Prieta, M; 63 dead; >13,000 injured, >1,000 homes destroyed, >23,000 damaged $5.9 billion damage

30 Liquefaction Occurs when water-saturated ground is reorganized due to violent shaking Sediment collapses, expels water, and causes ground to subside

31 Northridge, 1994 January 17 th, 4:30 a.m. 6.7 M, 67 deaths Blind thrust – 11 miles deep

32 Damage Control Collapsed structures –Reinforcement Northridge Meadows

33 Damage Control Collapsed Structures –Reinforce garages & windows

34 Damage Control Bridges collapse

35 Steel Reinforced Concrete Steel sheathing to reinforce columns

36 Damage Control Houses slip off foundations –Anchor foundations

37 Damage Control Taller Buildings –Anchor together –Build far apart

38 Damage Control Safeguarding your home