EAS 116 Earthquakes and Volcanoes

Transcription

1 EAS 116 Earthquakes and Volcanoes J. Haase Lecture 05 - The Earthquake cycle The probability that an earthquake can be esbmated by looking at how ofen earthquakes have occurred in the past on the fault. This assumes that there is a relabvely regular cycle in earthquake occurrence. 1

2 Fig. 3-14, p.43 How far the crust can be deformed before it breaks is described by the shear modulus K = bulk modulus ~5x10 10 N/m 2 µ= shear modulus ~3.3x10 10 N/m 2 V P = K µ! V S = µ! 2

3 The amount of force being applied to the rock The distance the rock deforms Fig. 3-16, p.44 We will use a sliding sandpaper block as a model of a fault, to invesbgate whether a repeabng cycle is a good way to describe earthquake occurrence. 3

4 In our sliding block experiment : the distance the block jumps represents the The length of the block represents the The length of the long board represents the The Bme between jumps represents the choices are: A) Bme between earthquakes, B) offset, C) fault rupture length D) fault What was each of the following represent? The stretching of the rubber band The velocity the pencil moves The slow steady force moving the pencil The properbes of the system that make it more difficult to move/rupture (weight of the rock, sandpaper roughness) choices are: A) offset, B) tectonic plate mobon forces, C) tectonic plate velocity, D) fault rupture length, E) shear modulus, F) energy stored in the deforming rock, G) fault Everyone fills out this form as we go A B C D E F G H I J Time 1 pencil distanceblock distance Jumps pencil pos pencil pos block position since start since start time of jump time since last block position jump size 1/5 cm/sec (cm) (cm) (cm) (cm) (sec) jump (sec) (cm) (cm)

5 In our sliding block experiment : the distance the block jumps represents the The length of the block represents the The length of the long board represents the The Bme between jumps represents the choices are: A) Bme between earthquakes, B) offset, C) fault rupture length D) fault In our sliding block experiment : The stretching of the rubber band represents the The velocity the pencil moves represents The slow steady force moving the pencil represents the The properbes of the system that make it more difficult to move/rupture (weight of the rock, sandpaper roughness) represent the choices are: A) offset, B) tectonic plate mobon forces, C) tectonic plate velocity, D) fault rupture length, E) shear modulus, F) energy stored in the deforming rock, G) fault 5

6 2/23/12 This goes on forever* because of steady tectonic plate mobon In the real case, we want to know how ofen these earthquakes occur ON AVERAGE. On real faults, we look for evidence that earthquakes have occurred again and again. 6

7 Fig. 3-13a, p.42 7

8 Offset = 20cm Offset = 20cm 8

9 Offset = 20cm Offset = 20cm+20cm Layer 3 (young) Offset = 20cm Offset = 20cm+20cm 9

10 Layer 3 (young) Offset = 10cm Offset = 20cm+10cm Offset = 20cm+20cm+10cm Layer 4 Layer 3 (young) Offset = 10cm Offset = 20cm+10cm Offset = 20cm+20cm+10cm 10

11 Layer 4 Offset = 2cm Layer 3 (young) Offset = 10cm+2cm Offset = 20cm+10cm+2cm Offset = 20cm+20cm+10cm+2cm Layer 5 Layer 4 Offset = 2cm Layer 3 (young) Offset = 10cm+2cm Offset = 20cm+10cm+2cm Offset = 20cm+20cm+10cm+2cm 11

12 677 AD Offset = 20cm 12

13 Offset = 20cm Offset = 20cm Offset = 20cm+20cm 13

14 Layer 3 (young) Offset = 20cm Offset = 20cm+20cm Layer 3 (young) Offset = 10cm Offset = 20cm+10cm Offset = 20cm+20cm+10cm 14

15 Layer 4 Layer 3 (young) Offset = 10cm Offset = 20cm+10cm Offset = 20cm+20cm+10cm Layer 4 Offset = 2cm Layer 3 (young) Offset = 10cm+2cm Offset = 20cm+10cm+2cm Offset = 20cm+20cm+10cm+2cm 15

16 Layer 5 Layer 4 T=1857 AD Offset = 2cm Layer 3 (young) T=1812 AD T=1480 AD Offset = 10cm+2cm Offset = 20cm+10cm+2cm T=1346 AD Offset = 20cm+20cm+10cm+2cm The San Andreas Fault at Carrizo plain hip://pubs.usgs.gov/gip/dynamic/san_andreas.html 16

17 2/23/12 hip://earthquake.usgs.gov/research/geology/paleoseis/slideshow/index.php#dig Hayward fault hip://earthquake.usgs.gov/research/geology/paleoseis/slideshow/index.php#dig 17

18 Paleoseismology Trench across earthquake fault Locate disturbed units Carbon- 14 dabng of the unbroken and broken layers hip://earthquake.usgs.gov/research/geology/paleoseis/slideshow/index.php#dig 18

19 Layer 5 Layer 4 T=1857 AD Offset = 2cm Layer 3 (young) T=1812 AD T=1480 AD Offset = 10cm+2cm Offset = 20cm+10cm+2cm T=1346 AD Offset = 20cm+20cm+10cm+2cm Recurrence Interval Recurrence interval = average Bme between earthquakes for a specific segment of a fault Earthquake history Intervals between earthquakes interval 1 = = 46 yrs interval 2 = = 332 yrs interval 3 = =134 7rs Average interval = ( )/3 = 171 years Probability of an earthquake this year on this segment of the San Andreas: 1 in 171 = 1/171 = = 0.58% 19

20 Hazard vs Risk Natural hazard - an event resulbng from natural environmental processes that has the potenbal to cause harm to a populabon or a community Natural hazard assessment - evaluabng the geographic extent and probability/likelihood that a natural hazard will occur Hazard vs Risk Natural hazard - an event resulbng from natural environmental processes that has the potenbal to cause harm to a populabon or a community Natural hazard assessment - evaluabng the geographic extent and probability/likelihood that a natural hazard will occur Risk - quanbficabon of the populabon or property that is at risk of being affected by a natural hazard Natural hazard risk assessment - evaluabng the potenbal effects of a natural hazard on a populabon or a community and the likelihood that those economic/ social effects would occur. 20

21 You can t dig a trench if you can t see the fault at the surface 21

22 The Loma Prieta, California, Earthquake of October 17, LiquefacFon hip://pubs.usgs.gov/pp/pp1551/pp1551b/ 22

23 1964 Niigata earthquake h"p://earthquake.usgs.gov/regional/world/events/images/1964_06_16_01.jpg Loma Prieta earthquake liquefacbon 23

24 h"p://web.mst.edu/~rogersda/nmsz 24

25 hip:// Patrick J. Munson, Stephen F. Obermeier, Cheryl Ann Munson, and Edwin R. Hajic; 1997; in Seismological Research LeIers, Volume 68, Number 4 25

26 How ofen do they occur? Patrick J. Munson, Stephen F. Obermeier, Cheryl Ann Munson, and Edwin R. Hajic; 1997; in Seismological Research LeIers, Volume 68, Number 4 How ofen do they occur? Time between earthquakes: = 3500 yrs =2400 yrs =2100 yrs =2000 yrs Average = ( )yrs/4 =2500 yrs Patrick J. Munson, Stephen F. Obermeier, Cheryl Ann Munson, and Edwin R. Hajic; 1997; in Seismological Research LeIers, Volume 68, Number 4 26

27 New Madrid earthquake New Madrid earthquakes Dec 16, 1811; Jan 23, 1812; Feb 7, 1812 Magnitude Mw , 7.0, Modified Mercalli Intensity Peak Ground Acceleration I < 0.17 %g II-III %g IV %g V %g VI %g VII %g VIII %g IX %g X+ > 124 %g Stover & Coffman, 1993; Nu"li 1973 Wald, 1999b; Trifunac & Brady,

28 Earthquakes in: A.D /- 100 years A.D /- 150 years Recurrence interval: =550 yrs =361 yrs Average= ( )yrs/2 =455 yrs Hazard vs Risk Natural hazard - an event resulbng from natural environmental processes that has the potenbal to cause harm to a populabon or a community Natural hazard assessment - evaluabng the geographic extent and probability/likelihood that a natural hazard will occur Risk - quanbficabon of the populabon or property that is at risk of being affected by a natural hazard Natural hazard risk assessment - evaluabng the potenbal effects of a natural hazard on a populabon or a community and the likelihood that those economic/ social effects would occur. 28