Lecture-10 Thursday. october 1lth Faulting Causes of earthquakes There are basically three types 口 VOLCANIC 口 TECTONIC 口 COLLAPSE (Also man-made due to nuclear explosions)
1 Lecture-10 Thursday, October 11th Faulting Causes of Earthquakes There are basically three types:- VOLCANIC TECTONIC COLLAPSE (Also man-made due to nuclear explosions)
Tectonic Earthquakes Most earthquakes occur at plate ins due tension, compression or shearing forces Rocks at plate margins are in constant motion and are being pushed, pulled, bent, twisted and ably at some pe crack to produce FaULTS!! Note- some rocks break much more easily than other What is a Fault A fault is a break or fracture between two blocks of rocks in response to stress Three types of stresses produce faults tEnsion 2)Compression One block has moved relative to the other block The surface along which the blocks move is alled a fault plar
2 Tectonic Earthquakes Most earthquakes occur at plate margins due to tension, compression or shearing forces. Rocks at plate margins are in constant motion and are being pushed, pulled, bent, twisted and folded. Inevitably at some point they must break or crack to produce FAULTS!! [Note – some rocks break much more easily than others] What is a Fault? • A fault is a break or fracture between two blocks of rocks in response to stress. • Three types of stresses produce faults 1) Tension 2) Compression 3) Shear • One block has moved relative to the other block. • The surface along which the blocks move is called a fault plane
There is a chicken and egg relationship between faults and earthquakes 1) It was initially thought that earthquakes caused faulting(but then what caused the earthquake?) 2) It was later realized that faulting produced the rthquak Thus earthquakes may occur because a rocks are initially broken to produce a fault b) Movement or re-activation of an already existing fault [Faults may therefore be thought of as fossil relicts of previous earthquakes Folding and Faulting TENSION COMPRESSION COMPRESSION - causes rocks to fold and ultimately break to produce a fault TENSION causes rocks to stretch and also break to produce a fault Faulting also occurs in response to shearing stresses
3 There is a chicken and egg relationship between faults and earthquakes 1) It was initially thought that earthquakes caused faulting (but then what caused the earthquake?) 2) It was later realized that faulting produced the earthquakes. Thus earthquakes may occur because:- a) Rocks are initially broken to produce a fault. b) Movement or re-activation of an already existing fault. [Faults may therefore be thought of as “fossil” relicts of previous earthquakes] Folding and Faulting COMPRESSION – causes rocks to fold and ultimately break to produce a fault. TENSION – causes rocks to stretch and also break to produce a fault. TENSION COMPRESSION Faulting also occurs in response to shearing stresses
Relationship of stresses types of faulting Example of folded rocks that have broken to produce a fault Some basic terminology (see page 156-161 in book) STRIKE-a horizontal line along the direction of a fault pl DIP-the angle between the fault plane and the horizontal
4 Relationship of stresses to types of faulting Example of folded rocks that have broken to produce a fault Some basic terminology STRIKE – a horizontal line along the direction of a fault plane. DIP – the angle between the fault plane and the horizontal. (see page 156-161 in book)
More terminology Hanging wall -is the block above the fault plane Foot wall is the block below the fault plane Foot wall Terminology comes from mining Hanging wall Normal faults Normal faults are produced by tensional forces The upper block(hanging wall) moves down relative to the lower block(foot wall)
5 More terminology Hanging wall – is the block above the fault plane. Foot wall is the block below the fault plane. Hanging wall Foot wall Terminology comes from mining Normal Faults Normal faults are produced by tensional forces. The upper block (hanging wall) moves down relative to the lower block (foot wall)
Two examples of normal faults Fault plane Fault plane Two scarps produced by normal faulting
6 Two examples of normal faults Fault plane Fault plane Two scarps produced by normal faulting
Horsts and grabens A series of paralle normal faults produces horsts and grabens GRABEN -valley formed as one block drops down HORST-hill formed as the other block moves up Reverse faults Reverse faults are produced by compressive forces The upper block(hanging w to the lower block(foot wall)
7 Horsts and Grabens A series of parallel normal faults produces horsts and grabens GRABEN - valley formed as one block drops down. HORST - hill formed as the other block moves up. Reverse Faults Reverse faults are produced by compressive forces. The upper block (hanging wall) moves up relative to the lower block (foot wall)
Small reverse fault associated with folded Fault plane Another small reverse fault A larger reverse fault Close-up of a reverse fault plane Note the two col ompletely different rock-types
8 Small reverse fault associated with folded rocks Another small reverse fault Fault plane A larger reverse fault Close-up of a reverse fault plane. Note the two completely different rock-types
Thrust Faults Chief Mountain a thrust (or overthrust)fault is a low-angle reverse fault again resulting from compressive forces Thrust faults are very common in highly folded mountain belts(the example is from Glacier National Park) Older rocks Fault plane Younger rocks a thrust fault may look a bit like an unconformity in that completely different rocks are separated by a plar surface. With a thrust fault, however, the older rocks have been pushed up on top
9 Thrust Faults A thrust (or overthrust) fault is a low-angle reverse fault, again resulting from compressive forces. Thrust faults are very common in highly folded mountain belts (the example is from Glacier National Park). Chief Mountain A thrust fault may look a bit like an unconformity in that completely different rocks are separated by a plane surface. With a thrust fault, however, the older rocks have been pushed up on top! Fault plane Older rocks Younger rocks
Transform Faults (Strike-Slip Faults) Transform faults move horizontally in response to shearing stresses They are also called strike-Slip faults because the Is along str Two examples of strike-Slip movement along transform faults(note lack of vertical movement)
10 Transform Faults (Strike-Slip Faults) Transform faults move horizontally in response to shearing stresses. They are also called strike-slip faults because the movement is along strike. Two examples of strike-slip movement along transform faults (note lack of vertical movement)
