GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Transcription

1 Tectonics Lecture 10 Global Seismotectonics

2 Rigid plate translation A Map of the World s Fracture Zones Magnetic anomalies and fracture zones form the basic building blocks for the construction of isochron charts.

3 Rigid plate translation Isochrons - used to calculate the relative positions of North America with respect to Northwest Africa

4 Plate translation on a sphere The relative position of a plate i with respect to a reference plate j at time t can be mathematically described in terms of finite rotation of i about an axis a by an angle ω: longitude shift, rotation about equatorial axis, rotation about vertical axis. A 3x3 matrix describes this rotation. Definition: Euler's fixed point theorem states that any motion of a rigid body on the surface of a sphere may be represented as a rotation about an appropriately chosen rotation pole, called an Euler pole. The axis about which two plates, i and j, rotate with respect to each other is called the rotation axis. Since vectors in general are additive, the (unknown) rotation vector for any plate may be found by combining the (known) rotation vectors from two other plates: ω = ω + ω jk ji ik

5 Plate translation on a sphere Transcurrent and transform tectonic boundaries allow direct calculation of finite rotations by a combination of geologic data and kinematic methods The strike-slip fault is modelled as a small circle arc about axis α The corresponding Euler pole e is calculated by fitting the modelled arc to plate boundary data The rotation angle Ω, it must be determined geologically The timing of displacement is estimated stratigraphically or other indirect methods Sketch map illustrating the method of computation of finite rotations associated to strike-slip boundaries

6 Plate translation on a sphere Euler's fixed point theorem: Any motion of a rigid body on the surface of a sphere may be represented as a rotation about an Euler pole. The rotation axis passes through the centre of the Earth, and pierces the surface of the Earth at the two Euler poles; equivalently, the Euler poles are the sites on the Earth's surface where the angular velocity vector is located. The linear velocity vector for any point may be found from: v ji = ω r sinθ ji Its magnitude v is equal to ω a sin θ where θ is the angle between the rotation pole and a is the radius of the Earth. Further reading: Fowler

7 NUVEL-1 GLOBAL RELATIVE PLATE MOTION MODEL COMBINES DATA (277 SPREADING RATES, 121 TRANSFORM DIRECTIONS, 724 EARTHQUAKE SLIP VECTORS) FROM AROUND THE WORLD (mm/yr) further reading: Vita-Finzi, Monitoring the Earth

8 Back arc Volcano Continent x x x x or Accretionary wedge Compression or tension Intermediate earthquakes km Deep focus earthquakes x x km x Lower mantle km Subduction x x x P Fore arc Trench x x x x x x P x T Highly schematic subduction zone Outer rise x x x T x x x T P T P Down dip tension Down dip compression T P Shallow thrusting Shallow normal faults Compression Focal mechanisms in section

9 Subduction Back arc Trench Fore arc Shallow normal faults Deep Intermediate Shallow thrusting Compression at greater depth Going down Benioff zone Fore arc compression or tension parallel to plate motion Focal mechanisms in plan

10 Intermediate & deep focus events Phase transitions Olivine spinel at 400km exothermic: heat source for back arc, accounts for high heat flow Olivine spinel increases density increase in down dip tension and affects intermediate depth earthquakes Spinel perovskite at 670km endothermic: absorbs heat (as removing energy) and leads to down dip compression. No earthquakes below ~700km Kirby (1987) proposed that the solid-solid phase transition occurs preferentially in a deviatoric stress field, so accounting for double-couple mechanism Seismic tomography and attenutation

11 North American seismotectonics Dot-dash lines shows small circle, and thus direction of plate motion, about the Pacific- North American Euler pole. The variation in the boundary type along its length from extension, to transform, to convergence, is shown by the focal mechanisms. The diffuse nature of the boundary zone is shown by the seismicity, focal mechanisms, topography and vectors showing the motion of GPS n VLBI sites with respect to the stable North America

12 Seismotectonics: oceanic spreading centre

13 Seismotectonics of transform faults Spreading ridge Transform fault Earthquake foci on transform faults < 30km depth Ridge offsets are sinistral Earthquake focal mechanisms show dextral displacement Focal mechanism suggests displacement on planes approximately parallel or perpendicular to trace of seismic zone. Parallel planes from geometry seem more likely to be real fault planes

14 Compare with normal transcurrent fault Transform fault Earthquake focal mechanisms show sinistral displacement

15 Slow and fast spreading centres The slow Mid-Atlantic ridge has earthquake both on the active transform and ridge segment. Strike-slip faulting on a plane parallel to the transform azimuth is characteristic. On the ridge segments, normal faulting with nodal planes parallel to the ridge are seen. The fast East Pacific Rise has only strike-slip earthquakes on the transform segments.

16 2003 Bam earthquake, Iran Magnitude SE IRAN 2003 December 26 01:56:52 UTC Government officials in Iran warned yesterday that the number of people who perished in Bam could reach 50,000, the highest toll from any earthquake for more than 25 years.

17 2003 Bam earthquake: seismotectonic history The Arabian plate is pushing northeastwards against the Asian plate in the Zagros region, producing earthquakes along the northern side of the Gulf. The Asian plate, in turn, is pushing the central Iranian block, moving north-east, deforming slowly along its north and eastern flanks.

18 2003 Bam earthquake, Iran The earthquake was caused by right-lateral strike-slip motion on a north-south oriented fault The earthquake occurred in a region within which major north-south, right-lateral, strike-slip faults had been previously mapped, and the epicentre lies near the previously mapped, north-south oriented, Bam fault. The December 26 earthquake is 100 km south of the destructive earthquakes of June 11, 1981 (magnitude 6.6, approximately 3,000 deaths) and July 28, 1981 (magnitude 7.3, approximately 1,500 deaths). These earthquakes were caused by a combination of reverse-motion and strike-slip motion on the north-south oriented Gowk fault.

19 Iran: peak ground acceleration and seismicity

20 2003 Central California earthquake Hearst Castle Magnitude CENTRAL CALIFORNIA 2003 December 22 19:15:56 UTC Paso Robles, San Luis Obispo County -- When the earth heaved and the 111- year-old building began to wobble, life or death came down to which door you picked to run through. The door onto 12th Street led to safety. The door onto Park Street did not.

21 2003 Central California earthquake

22 2003 Central California earthquake The earthquake occurred on the Oceanic fault zone in the Santa Lucia mountains of coastal Central California The earthquake was caused by reverse faulting. Rupture propagated to the southeast from the hypocenter over approximately 20 km. Previous shocks from the region have also been caused by reverse faulting or by oblique-reverse faulting. The broad-scale tectonics of coastal Central California are dominated by the northwestward motion of the Pacific plate with respect to the North American plate. Most of the relative plate motion is accommodated by slip on major strike-slip faults, the San Andreas fault. The reverse faulting that generated the recent earthquake was caused by the release of compressive stress that was generated by the motion of crustal blocks within the overall strike-slip plate-boundary zone.

23 Central California: peak accelerations and seismicity

24 The Sumatra earthquake and tsunami Worst tsunami disaster in history Casualties from tsunami Somalia 300 India 16,000 Thailand 8,300 Sri Lanka 35,000 Indonesia 160,000

25 Thailand Myanmar

26 Tsunami heights along Andaman Sea Japanese survey teams

27 Satellite Images April 12, 2004 January 2, 2005 submerged Gleebruk Village (South of Banda Aceh) Digital Globe s QuickBird satellite /images/tsunami June 3, 2004 December 30, 2004 emerged Sentinel Island (Andaman Is.) ENVISAT-ASAR /workpackages/20300/sentinel /new/images.html

28 Tsunami surveys around Bengal Bay Andaman Is. < 5m Tsunami height, m Myanmar < 3 m Inferred tsunami source Thailand 5-15m Sri Lanka 5-15m Measured height (m) Indonesia Band Aceh, max 30 m Measured height (m)

29 Northern Andaman 1 m uplift Southeast Andaman 1m subsidence Crustal deformation near the source Coastal surveys and GPS observation detected meter-scale deformation along Sumatra to Andaman Is (~1,000 km) Source for crustal deformation AIST, Univ. Tokyo Sumatra Is. 1m shift to SW Uplift of Sentinel Island ENVISAT-ASAR June 3, 2004 December 30, 2004 Nagoya Univ.

30 Only four M9 events in 20 th century Cumulative seismic moment Harvard CMT

31 M=9.0 Sumatra Earthquake The interface between the subducting plate and overriding plate is a large thrust fault, often termed an interplate thrust or megathrust.

32 M=9.0 Sumatra Earthquake

33 Tsunami size: earthquake mechanism Epicenters of the December 26, 2004, northern Sumatra earthquake and its large aftershocks. The Harvard CMT (Centroid Moment Tensor) solutions are plotted as beach-balls.

34 Tectonic situation

35 Tectonic situation Dec.26 March 28

36 A Flying Start, Then a Slow Slip Slow slip only Fast and slow slips Tsunami source Fast slip only May 20 issue of Science

37 M=9.0 Sumatra Earthquake

38 Source Time Functions from Body Waves 1 st stage ~ 400 km rupture 2 nd stage slower rupture Mw 8.4 Y. Yagi BRI

39 Source Time Functions from Body Waves P wave time window: ~ 200 sec < 90 deg ~ 280 sec > 90 deg Source process time may be longer than the time window Larry Ruff U. Michigan

40 Three slip distribution models ~3 x Nm 6.5 x Nm 6.5 x Nm surface waves SH waves regional +surface waves telseismic body + regional +surface waves SH waves regional +surface waves telseismic body +regional +surface waves Ammon et al. (2005, Science)

41 Northward rupture propagation 1,300 km 2.8 km/s 8 minutes Ishii et al., 2005

42 Surface waves travel around the Earth

43 Sumatra Earthquake: Earth oscillations

44 Earth s free oscillation Football mode 53.7 min n (radial order) = 0 l (angular order) = 2 m (azimuthal) =±2, ±1, 0 Spectral amp: CMT x 2.6 (Mo = 10x Nm; Mw 9.3) m=1, -1 (zero at equator) too small Better fit for a source at 7.5ºN For T > 1000 s Amplitudes = x CMT Mw= (if dip = 8º) does not fit a simple sinc function Park et al. (2005, Science)

45 Tsunami Generation from the 2004 M=9.0 Sumatra Earthquake Tsunami wave field in the Bay of Bengal. View to the north west, focused near the earthquake epicenter (northern Sumatra)

46 Deep and shallow water waves

47 2004 Off-Sumatra Earthquake

48 Tsunami size: breaking surface

49 Forward and inverse problems

50 Tsunami source estimated from arrival times

51 JASON-1 altimeter on NASA s satellite Observed Sea Surface Heights (SSH) 120 min Estimated tsunami source

52 Tsunami size: earthquake magnitude The magnitude of the earthquake is, in most cases, the most important factor that determines the size of a tsunami.

53 Next earthquake? McCloskey et al., 2005 Nalbant et al., 2005

54 Past earthquakes along Sumatran trench

55 Past earthquakes along Sumatran trench Seih et al. (2004, AGU fall meeting) No giant earthquakes have struck the outer-arc islands of western Sumatra since the sequence of 1797, 1833 and Mw 9.0 slip 10 m uplift 2m Paleoseismic studies of coral microatolls reveal that failure of the subduction interface occurs in clusters of such earthquakes about every 230 years. Thus, the next such sequence may well be no more than a few decades away.