Earthquake: A vibration caused by the sudden breaking or frictional sliding of rock in the Earth. Fault: A fracture on which one body of rock slides

Transcription

1 Earthquake: A vibration caused by the sudden breaking or frictional sliding of rock in the Earth. Fault: A fracture on which one body of rock slides past another. Focus: The location where a fault slips during an earthquake (also called the hypocenter). Epicenter: The point on the surface of the Earth directly above the focus of an earthquake. Fault trace: The intersection between a fault and the ground surface. Footwall: The rock mass that lies below the fault plane. Hanging wall: The rock or sediment above an inclined fault plane. (b) and (c) are showing 2 epicenters and foci.

2 View of fault trace after 1906 SF earthquake. Note that the fracture is discontinuous.

3 Normal fault: hanging-wall block moves down the slope of the fault. Due to extension. Reverse fault: A steeply dipping fault on which the hanging-wall block slides up. Thrust fault: A gently dipping reverse fault; the hanging-wall block moves up the slope of the fault. Strike-slip or transform fault: one block slides horizontally past another, so there is no relative vertical motion. Can be left lateral or right lateral.

4 10_07.jpg Blind fault: fault that does not come to the surface.

5 Normal fault in South-central Texas

6 Reverse fault

7 Thrust fault north of Knoxville, TN

8 Strike-sip faults can have motion that is left lateral, or right lateral. This means that if you were standing on one side of a fault, and the other side moves to the left, than the fault is left lateral. Reversely, if the other side of the fault moves to the right, then the fault is right lateral. It does not matter which side of the fault you stand on. The motion will still be the same.

9 Offset of this fence was caused by the 1906 SF earthquake. Note that the fence suggest that the fault was right lateral in movement.

10 Road is offset by 20 feet during the 1906 SF earthquake. Again note that it is a right lateral sense of movement.

11 This shows the location of earthquakes on the surface of the earth. Note that there is a difference between deep and shallow earthquakes. This is due to the tectonic setting of the earthquakes.

12 Note that, with some exceptions, the locations of earthquakes coincide with the location of plate boundaries.

13 Deep focus earthquakes occur along Wadati-Benioff zone at convergent plate boundaries (A boundary at which two plates move toward each other so that one plate sinks (subducts) beneath the other; only oceanic lithosphere can subduct). Wadati-Benioff zone: A sloping band of seismicity defined by intermediate- and deepfocus earthquakes that occur in the downgoing slab of a convergent plate boundary. These types of earthquakes occur in the Pacific Northwest.

14 Oceanic transforms and mid-ocean ridges are common locations for shallow focus earthquakes. mid-ocean ridges: A submarine mountain belt that forms along a divergent oceanic plate boundary. transform plate boundary: A boundary at which one lithosphere plate slips laterally past another. divergent plate boundary: A boundary at which two lithosphere plates move apart from each other.

15 And as we discussed, volcanic activity, here shown as a result of one plate subducting under another, has shallow focus earthquakes associated with it.

16 Earthquakes that occur within plates occur along transform plate boundaries (such as the San Andreas fault), in areas of active extension (such as the western US basin and range an rift zones such as the east African rift) and in collisional zones (such as the Himalayan mountains).

17 Why do earthquakes occur? They occur as stress built up on opposite sides of a fault become greater than the friction between the two fault surfaces. The stress, in most cases, is due to the relative motion of tectonic plates past one another. We tend to think that constant motion occurs across plate boundaries; however, this is not the case. Usually faults are inactive, causing the stress to builds up due to the lack of motion. Faults that move all of the time are said to creep. Earthquakes that occur along these creeping faults are small, and cause little damage.

18 We tend to think of faults as sharp, well defined lines. But in reality they tend to be broad zones or systems of faults, that splay off of a central fault. The San Andreas is a good example of this idea.

19 P-waves: primary waves. Compressional waves.

20 S-waves: Secondary waves. Shear waves.

21 R-waves (Rayleigh waves) or surface waves: cause ground to ripple up and down.

22 L-waves (Love waves): cause ground to snake from side to side.

23 R- and L-waves die out at depth. This, however, is not the case for P- and S- waves.

24 P- and S-waves will travel throughout the entire earth. There is a P-wave shadow zone in the earth. This is caused by the refraction of P-waves through the earth s core.

25 The S-waves shadow zone differs from the P-wave zone because the physical properties of S-waves do not let them travel through liquids. Therefore, they cannot penetrate the liquid outer core of the earth. The inner core is solid.

26 The different physical properties of waves produced by earthquakes, cause them to travel at different rates. P = fastest, S = moderate, surface = slowest.

27 The different speed, or arrival times of earthquake waves, allows for the creation of seismographs (an instrument that can record the ground motion from an earthquake). Seismographs record the duration and intensity of earthquakes.

28 Seismographs are usually fixed to bed rock. They can record both vertical (a) and horizontal (b) ground motion.

29 10_16e.jpg After an earth quake occurs, seismographs can be used to tell how far away the epicenter is located (use the velocity of the waves, and the duration of the quake, and this gives a distance). However, it is impossible to tell what direction the earthquake came from. Epicenters are found using the intersection of three (or more) distances measured from seismographs

30 Richter scale, named after Charles Richter, is commonly used to determined earthquake magnitude. This scale gives the largest ground motion 100 km away from the epicenter of an earthquake.

31 Richter magnitude is determined by first calculating the interval between the S- and P-waves, and then determine the amplitude of the largest wave produced by the earthquake. A line is drawn on special graph paper between these two points, and where this line intersects the magnitude scale, can be read off. Thus giving us the magnitude of an earthquake.

32 The insert graph displays that as the magnitude of the graph increases, the number of quakes decrease. This means that there are more magnitude 1 earthquakes, than magnitude 8. the larger graph shows that the energy released by earthquakes increases greatly with increasing magnitude.

33 A second scale for earthquakes is the Mercalli intensity scale. This scale measures the amount of damage that an earthquake causes. I causes little damage, while XII = s total destruction. This scale is easier to use when dealing with historical earthquakes.

34 Location of some historically significant earthquakes within the lower 48 states.

35 Ground shaking can cause sand to become mobilized, and rise up in a row of sand volcanoes. It the sand layers are between mud layers, can become disrupted and broken.

36 Sand volcano, 1979, El Centro, California

37 The sand becomes mobilized due to water in the pore space. Just as we discussed before, the pore space opens up during shaking caused by the quake, and the sand grains no longer touch each other. Liquefaction: the process that occurs when the motion of an earthquake causes clay-rich sediment to become a slurry of clay and water. The sediment is clay-rich, meaning it contains clay, but is not only made up of clay.

38 Liquefaction caused the collapse of Union St after 1906 quake in SF

39 Loma Prieta, California, Earthquake October 17, Watsonville area. Ground shaking triggered liquefaction in a subsurface layer of sand.

40 Secondary cracks on filled land on Bluxom Street after the 1906 SF earthquake. Note the devastation in the background. This area had been developed on fill that was placed in the bay. When the quake occurred, the sediment lost all of its cohesion.

41 Landslide was triggered by 1906 earthquake.

42 Loma Prieta, California, Earthquake October 17, San Francisco and San Mateo County Coast. Large slide at Daly City. This is the largest slide encountered in San Mateo County. The base is about 152 meters (500 feet) across at its widest point, and it displaced approximately 36,700 cubic meters (48,000 cubic yards) of material.

43 Here are some geological signs that indicate earthquake activity. 1) disrupted bedding, 2) off set soil horizons, 3) sand volcanoes, 4) tilted trees, with asymmetric tree rings, also buried and more recent faults.

44 Truncated ridges, with triangular escarpments are good indicators of normal faults.