Earthquake Terminology & Diagrams

Transcription

1 Earthquake Terminology & Diagrams School Programs Resources TERMINOLOGY Alluvial fan: Cone or fan-shaped deposit of alluvium made by a stream where it runs out onto a level plain. Once deposited the stream may continue to cut channels across the fan (Fig. 3) Anticline & syncline (geological folds): Anticline: A fold in rocks in which the strata dip outward from both sides of the axis forming an inverted U-shaped ( ) feature in profile. Commonly adjacent to a syncline (see below). Syncline: A fold in rocks in which the strata dip inward from both sides towards the axis forming a U-shaped feature in profile. Basin: A segment of the crust which has been downwarped with intermittent raisings and sinkings. The Los Angeles basin is a combination of a structural and a sedimentary basin (i.e., formed by tectonic movements and filled with thick sedimentary layers). Crust: The outermost major layer of the Earth, ranging from 10 to 65 km in thickness worldwide and about 20 km thick in coastal California. Epicenter: The point on the Earth s surface vertically above the point (focus or hypocenter) in the crust where rupture initiates (Fig 2). Fault: A fracture or fracture zone along which there has been offset of the sides relative to one another (Fig. 1) Active: A fault that is considered likely to undergo renewed movement within a period of concern to humans. Lateral: Strike-slip faults are also known as lateral faults and the San Andreas Fault is a prime example of a right lateral strike-slip fault. This means that an object across the fault from you would move to your right when the fault moves. Reverse: A reverse, or thrust fault, is a compressional feature where older rocks are faulted above younger rocks. Reverse and Normal faults are examples of dip-slip faults The Sierra Madre Fault is a good example of a reverse fault as are subduction zone faults. Normal: These faults are extensional features that lengthen the crust and are common in the Basin and Range Province of Nevada. Oblique-Slip: Normal and reverse oblique-slip faults have both strike- and dip-slip components.

2 Earthquake Terminology Fault trace: Intersection of a fault with the ground surface. Also the line commonly plotted on geologic or topographic maps to represent a fault (Fig. 2). Formation: A body of rock that consists dominantly of a certain lithologic type or combination of types. This rock body must be mappable, have a defined top and bottom, and be fairly widespread. Mantle: The part of the Earth s interior between the crust and the metallic core. Plate tectonics: A well-proven theory that considers the Earth s crust and upper mantle to be composed of large, thin, relatively rigid plates that move relative to one another. Interaction along their boundaries commonly results in earthquake and volcanic activity. Sag Pond: A small body of water in an enclosed depression or sag where active faulting has impounded drainage. Scarp: A line of cliffs produced by faulting. Also referred to as fault trace (Fig 2). Stepover: Area where a fault ends and another en echelon fault (or the same fault) of the same orientation begins. Strike & dip: Strike: The horizontal bearing of inclined beds or structure perpendicular to dip. Dip: The vertical bearing of inclined beds or structure perpendicular to strike. Tectonic: Refers to crustal rock-deforming processes that affect relatively large areas. Water Gap: A pass through a ridge where a stream currently flows. Wind Gap: A low depression or notch in a ridge where a stream once flowed.

3 Earthquake Diagrams Figure 1. Fault types common in the Los Angeles basin. We will mostly observe strike slip and reverse (or thrust faults) during our excursion (from Ziony, 1985).

4 Earthquake Diagrams Figure 2. Fault plane diagram showing where an earthquake originates and releases energy towards the surface along the fault plane causing surface faulting (from Ziony, 1985).

5 Figure 3. Block diagram showing fault trace buried by alluvial fan deposits. We will traverse several of these fan deposits from the Santa Monica Mountains during our excursion in the Los Angeles Basin (from Ziony, 1985). Earthquake Diagrams

6 Earthquake Diagrams Figure 4. Block diagram of the Los Angeles Basin showing main faults and some geologic features that we will observe during our excursion, particularly in the northern portions of the Southwestern, Central, and Northeastern blocks south of the Santa Monica Mountains (from Yerkes et al., 1965).

7 Figure 5. Relative motions of crustal blocks in southern California. Blocks in the foreground move northwest with the Pacific plate and represent the Peninsular Ranges. Blocks representing the Transverse Ranges (shaded) generally bounded by reverse faults and prominent scarps. Our excursion will traverse the central area of the Transverse Ranges (shaded area) east to the San Gabriel Valley (from Ziony, 1985). Earthquake Diagrams

8 Figure 6. The fifteen major tectonic plates of the Earth (from Lynch, 2006). Earthquake Diagrams

9 References Cited references and suggested reading Brumbaugh, D.S Earthquakes: Science and society. 2 nd edition. Prentice Hall: New York, NY. viii p., figs Collier, M A land in motion: California s San Andreas Fault. Golden Gate National Parks Conservancy: San Francisco, California. 118 p., numerous unnumbered figs. Hough, S.E Fault finding in California: An earthquake tourist s guide. Mountain Press Publishing Company: Missoula, Montana. viii p., numerous unnumbered figs. Harden, D.R California geology. 2 nd edition. Pearson Prentice Hall: Upper Saddle River, NJ. xxi p., figs Iacopi, R Earthquake country, revised edition. Lane Books: Menlo Park, California. 160 p., numerous unnumbered figs. Lynch, D.K Field guide to the San Andreas Fault. Thule Scientific: Topanga, California. x p. numerous unnumbered figs. Meldahl, K.H Rough-hewn Land. University of California Press: Berkeley, CA. xviii p., figs A.1 A.7. Norris, R.M. & Webb, R.W Geology of California. 2 nd edition. John Wiley & Sons, Inc.: New York. xiii p., figs Sharp, R.P. & Glazner, A.F Geology underfoot in southern California. Mountain Press Publishing Company: Missoula, Montana. viii p., numerous unnumbered figs. Southern California Earthquake Center Putting down roots in earthquake Country. 32 p. [pdf available at ]. Yeats, R.S Living with earthquakes in California: A survivors guide. Oregon State University Press: Corvallis, OR. x p., figs. 2-1 to Yerkes, R.F., McCulloh, T.H., Schoellhamer, J.E., & Vedder, J.G Geology of the Los Angeles basin, California An introduction. United States Geological Survey Professional Paper 420-A: vi + A1-A57, figs. 1-14, pls Ziony, J.I. (ed.) Evaluating earthquake hazards in the Los Angeles Region An earth-science perspective. United States Geological Survey Professional Paper 1360: xii , figs figs. + numerous unnumbered figs.