DOCSLIB.ORG

Wait! How Do We Know This?? Earth!S Interior Was Uniform

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Wait! How Do We Know This?? Earth!S Interior Was Uniform Simplest assumption: suppose the Wait! How do we know this?? Earth!s interior was uniform... We measure the travel •! P wave, S wave, and times of seismic waves surface waves would from earthquakes, and arrive at all stations compare them with what we would expect •! we would compute from different layered their arrival times at models until we get a different seismometers match assuming a gradual increase in velocity due to pressure “inversion” of the seismic travel time •! this would work data for velocity perfectly structure If the Earth!s interior were uniform... Here is what we actually see! we could make a plot of P, S, and surface wave arrival time with distance “ ∆ “ surface ∆ = 50° S time (minutes) time P ∆ ∆ P wave velocities drop suddenly at 2900 km depth, “Seismic Phases” and S waves cannot pass through this layer • P,S : P,S waves in the mantle, e.g. PPP, SS • K : P-wave in the outer core, e.g. PKP, SKP • I,J : P,S waves in the inner core • c : reflection from CMB, e.g. PcP, ScS • i : reflection from IOB, e.g. PKiKS • SUMMARY: many phases are created by Earth’s stratification (reflections, and sometimes conversion from one wave type to another) Fig. 4.7 P wave shadow zone S wave shadow zone Fig. 4.8 Fig. 4.9 We get this picture by calculating arrival times for all seismic Why are there seismic velocity jumps inside phases in a stratified model of the Earth, and making sure they the mantle? match the observed arrival times at all points on the Earth • mantle has fairly uniform composition • same chemical elements arrange into different minerals at different depths • minerals that are stable dots (phase travel times) at great depth are the densest match curves (model- • seismic wave speeds predicted change as minerals change travel times) Engdahl and Kennett 1991 very well. • Low Velocity Zone: close to melting temperature We can get seismic wavespeeds, but does this tell us the chemical composition of the layers? Meteorites: analogues to METEORITES composition of the Earth!s interior? Abundances of elements in the solar system are estimated from meteorites, solar corona, etc. Crust and mantle: too little iron (and nickel and lead etc.) relative to oxygen, silicon, etc. Missing: we need lots of What is the Earth!s Core iron and nickel inside the Earth a small amount of made of? • liquid outer core: must be a liquid at the P,T + + conditions deep in the Earth • magnetic field generation: it must be a metal “Primordial” (never melted or re-processed) • densities: the core must be dense 4.5 BY old meteorite – Earth’s average: 5.5 g/cm^3 ~ – crust, mantle: 2.7-3.3 g/cm^3 (85% by volume) = Likely close to average – therefore, core: 10+ g/cm^3 composition of the Earth • meteorites suggest Fe core with trace O, Si, Ni, S Core - mantle boundary CORE-MANTLE BOUNDARY • dramatic density and seismic velocity change • slab graveyard • mantle plume birthplace • site of “anti-crust’’ (ULVZ) and “anti- lithosphere’’ (D’’ layer) • is the core reacting with the mantle? or is it melting the mantle? D’ ’ layer and ultra low velocity zone (ULVZ): what’s going on? Rising plumes, sinking slabs... The Earth Seismic tomography: let!s find is not exactly radially symmetric Texas seismographs suppose that the Earth is flat and that seismic waves travel unusually slowly through earthquakes Texas... Finding Texas. Global Seismic Tomography on-time late late late seismic velocity anomalies usually on-time differ by less than 2% from surroundings Global Seismic Tomography upper mantle • similar to CAT scan in medical imaging • compares real travel times with travel times predicted by the radially symmetric Earth model • small differences in travel times are translated to seismic velocity variations • Blue (cold) is fast & Red (hot) is slow • snapshots of mantle convection: hot core - mantle material rises and cold material sinks boundary Van der Hilst et al., 1998 Subducting slabs: stronger and colder Giant superplume rising from the CMB than their surroundings under the Pacific Ocean Do they all sink to the core-mantle boundary or not? There!s another one of these under Africa too. Van der Hilst et al., 1998 Seismic Tomography of the Mantle Tomography at a finer scale - P wave a “snapshot” of current mantle velocity velocity anomalies at 100 km depth • primary influences: • composition Subducting • temperature slab Yellowstone hot spot Strong mantle Maximum 3-D image: under the Sierra perturbation • orange-red: hot Nevada batholith (upward convection) is 2% Tomography at a finer scale - the Yellowstone hot spot they* could not resolve the bottom of this low-velocity feature small plumes associated with hot spots are too small to show up on the global tomographic images *Derek Schutt, Eugene Humphreys, Rebecca Salzer (P and S wave studies).
Load more

Recommended publications
  • STRUCTURE of EARTH S-Wave Shadow P-Wave Shadow P-Wave
    STRUCTURE OF EARTH Earthquake Focus P-wave P-wave shadow shadow S-wave shadow P waves = Primary waves = Pressure waves S waves = Secondary waves = Shear waves (Don't penetrate liquids) CRUST < 50-70 km thick MANTLE = 2900 km thick OUTER CORE (Liquid) = 3200 km thick INNER CORE (Solid) = 1300 km radius. STRUCTURE OF EARTH Low Velocity Crust Zone Whole Mantle Convection Lithosphere Upper Mantle Transition Zone Layered Mantle Convection Lower Mantle S-wave P-wave CRUST : Conrad discontinuity = upper / lower crust boundary Mohorovicic discontinuity = base of Continental Crust (35-50 km continents; 6-8 km oceans) MANTLE: Lithosphere = Rigid Mantle < 100 km depth Asthenosphere = Plastic Mantle > 150 km depth Low Velocity Zone = Partially Melted, 100-150 km depth Upper Mantle < 410 km Transition Zone = 400-600 km --> Velocity increases rapidly Lower Mantle = 600 - 2900 km Outer Core (Liquid) 2900-5100 km Inner Core (Solid) 5100-6400 km Center = 6400 km UPPER MANTLE AND MAGMA GENERATION A. Composition of Earth Density of the Bulk Earth (Uncompressed) = 5.45 gm/cm3 Densities of Common Rocks: Granite = 2.55 gm/cm3 Peridotite, Eclogite = 3.2 to 3.4 gm/cm3 Basalt = 2.85 gm/cm3 Density of the CORE (estimated) = 7.2 gm/cm3 Fe-metal = 8.0 gm/cm3, Ni-metal = 8.5 gm/cm3 EARTH must contain a mix of Rock and Metal . Stony meteorites Remains of broken planets Planetary Interior Rock=Stony Meteorites ÒChondritesÓ = Olivine, Pyroxene, Metal (Fe-Ni) Metal = Fe-Ni Meteorites Core density = 7.2 gm/cm3 -- Too Light for Pure Fe-Ni Light elements = O2 (FeO) or S (FeS) B.
    [Show full text]
  • Characteristics of Foreshocks and Short Term Deformation in the Source Area of Major Earthquakes
    Characteristics of Foreshocks and Short Term Deformation in the Source Area of Major Earthquakes Peter Molnar Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, Massachusetts 02139 USGS CONTRACT NO. 14-08-0001-17759 Supported by the EARTHQUAKE HAZARDS REDUCTION PROGRAM OPEN-FILE NO.81-287 U.S. Geological Survey OPEN FILE REPORT This report was prepared under contract to the U.S. Geological Survey and has not been reviewed for conformity with USGS editorial standards and stratigraphic nomenclature. Opinions and conclusions expressed herein do not necessarily represent those of the USGS. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS. Appendix A A Study of the Haicheng Foreshock Sequence By Lucile Jones, Wang Biquan and Xu Shaoxie (English Translation of a Paper Published in Di Zhen Xue Bao (Journal of Seismology), 1980.) Abstract We have examined the locations and radiation patterns of the foreshocks to the 4 February 1978 Haicheng earthquake. Using four stations, the foreshocks were located relative to a master event. They occurred very close together, no more than 6 kilo­ meters apart. Nevertheless, there appear to have been too clusters of foreshock activity. The majority of events seem to have occurred in a cluster to the east of the master event along a NNE-SSW trend. Moreover, all eight foreshocks that we could locate and with a magnitude greater than 3.0 occurred in this group. The're also "appears to be a second cluster of foresfiocks located to the northwest of the first. Thus it seems possible that the majority of foreshocks did not occur on the rupture plane of the mainshock, which trends WNW, but on another plane nearly perpendicualr to the mainshock.
    [Show full text]
  • PEAT8002 - SEISMOLOGY Lecture 13: Earthquake Magnitudes and Moment
    PEAT8002 - SEISMOLOGY Lecture 13: Earthquake magnitudes and moment Nick Rawlinson Research School of Earth Sciences Australian National University Earthquake magnitudes and moment Introduction In the last two lectures, the effects of the source rupture process on the pattern of radiated seismic energy was discussed. However, even before earthquake mechanisms were studied, the priority of seismologists, after locating an earthquake, was to quantify their size, both for scientific purposes and hazard assessment. The first measure introduced was the magnitude, which is based on the amplitude of the emanating waves recorded on a seismogram. The idea is that the wave amplitude reflects the earthquake size once the amplitudes are corrected for the decrease with distance due to geometric spreading and attenuation. Earthquake magnitudes and moment Introduction Magnitude scales thus have the general form: A M = log + F(h, ∆) + C T where A is the amplitude of the signal, T is its dominant period, F is a correction for the variation of amplitude with the earthquake’s depth h and angular distance ∆ from the seismometer, and C is a regional scaling factor. Magnitude scales are logarithmic, so an increase in one unit e.g. from 5 to 6, indicates a ten-fold increase in seismic wave amplitude. Note that since a log10 scale is used, magnitudes can be negative for very small displacements. For example, a magnitude -1 earthquake might correspond to a hammer blow. Earthquake magnitudes and moment Richter magnitude The concept of earthquake magnitude was introduced by Charles Richter in 1935 for southern California earthquakes. He originally defined earthquake magnitude as the logarithm (to the base 10) of maximum amplitude measured in microns on the record of a standard torsion seismograph with a pendulum period of 0.8 s, magnification of 2800, and damping factor 0.8, located at a distance of 100 km from the epicenter.
    [Show full text]
  • Waveform Analysis of the 1999 Hector Mine Foreshock Sequence Eva E
    GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 8, 1429, doi:10.1029/2002GL016383, 2003 Waveform analysis of the 1999 Hector Mine foreshock sequence Eva E. Zanzerkia and Gregory C. Beroza Department of Geophysics, Stanford University, Stanford, CA, USA John E. Vidale Department of Earth and Space Sciences, UCLA, Los Angeles, CA, USA Received 2 October 2002; revised 5 December 2002; accepted 6 February 2003; published 23 April 2003. [1] By inspecting continuous Trinet waveform data, we [4] Although the Hector Mine foreshock sequence find 42 foreshocks in the 20-hour period preceding the 1999 occurred in an area of sparse instrumentation, we are able Hector Mine earthquake, a substantial increase from the 18 to obtain precise locations for 39 of the 42 foreshocks by foreshocks in the catalog. We apply waveform cross- making precise arrival time measurements from waveform correlation and the double-difference method to locate these data even at low signal to noise ratio (snr) and double- events. Despite low signal-to-noise ratio data for many of difference relocation. After relocation we find that the the uncataloged foreshocks, correlation-based arrival time foreshocks occurred on the mainshock initiation plane and measurements are sufficient to locate all but three of these that the extent of the foreshock zone expands as the time of events, with location uncertainties from 100 m to 2 km. the mainshock approaches. We find that the foreshocks fall on a different plane than the initial subevent of the mainshock, and that the foreshocks spread out over the plane with time during the sequence as 2.
    [Show full text]
  • GEOL 460 Solid Earth Geophysics Lab 5: Global Seismology Part II Questions 1
    GEOL 460 Solid Earth Geophysics Lab 5: Global Seismology Name: ____________________________________________________ Date: _____________ Part I. Seismic Waves and Earth’s Structure While technically "remote" sensing, the field of seismology and its tools provide the most "direct" geophysical observations of the geology of Earth's interior. Seismologists have discovered much about Earth’s internal structure, although many of the subtleties remain to be understood. The typical cross‐ section of the planet consists of the crust at the surface, followed by the mantle, the outer core, and the inner core. Information about these layers came from seismic travel times and analysis of how the behavior of seismic waves changes as they propagate deeper into the Earth. Today we are going to examine seismic waves in the Earth and we will take a look at how and what seismologists know about Earth’s core. Here is a summary of the current understanding of Earth structure: To look into what seismic waves can tell us about the core, we need to know how they travel in the Earth. While seismic energy travels as wavefronts, we often depict them as rays in figures and sketches. A ray is an idealized path through the Earth and is drawn as a line traveling through the Earth. A wavefront is a surface of energy propagating through the Earth. We’ll begin by looking at some videos of wavefronts and rays to get a handle on how these things behave in the Earth. These animations were made by Michael Wysession and Saadia Baker at Washington University in St. Louis.
    [Show full text]
  • Lecture 6: Seismic Moment
    Earthquake source mechanics Lecture 6 Seismic moment GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake magnitude Richter magnitude scale M = log A(∆) - log A0(∆) where A is max trace amplitude at distance ∆ and A0 is at 100 km Surface wave magnitude MS MS = log A + α log ∆ + β where A is max amp of 20s period surface waves Magnitude and energy log Es = 11.8 + 1.5 Ms (ergs) GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Seismic moment ß Seismic intensity measures relative strength of shaking Moment = FL F locally ß Instrumental earthquake magnitude provides measure of size on basis of wave Applying couple motion L F to fault ß Peak values used in magnitude determination do not reveal overall power of - two equal & opposite forces the source = force couple ß Seismic Moment: measure - size of couple = moment of quake rupture size related - numerical value = product of to leverage of forces value of one force times (couples) across area of fault distance between slip GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Seismic Moment II Stress & ß Can be applied to strain seismogenic faults accumulation ß Elastic rebound along a rupturing fault can be considered in terms of F resulting from force couples along and across it Applying couple ß Seismic moment can be to fault determined from a fault slip dimensions measured in field or from aftershock distributions F a analysis of seismic wave properties (frequency Fault rupture spectrum analysis) and rebound GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY
    [Show full text]
  • Localized Amplification of Seismic Waves and Correlation with Damage Due to the Northridge Earthquake: Evidence for Focusing in Santa Monica
    Bulletin of the Seismological Society of America, Vol. 86, No. 1B, pp. $209--$230, February 1996 Localized Amplification of Seismic Waves and Correlation with Damage due to the Northridge Earthquake: Evidence for Focusing in Santa Monica by S. Gao, H. Liu, P. M. Davis, and L. Knopoff Abstract The analysis of seismograms from 32 aftershocks recorded by 98 seis- mic stations installed after the Northridge earthquake in the San Fernando Valley, the Santa Monica Mountains, and Santa Monica, California, indicates that the en- hanced damage in Santa Monica is explained in the main by focusing due to a lens structure at a depth of several kilometers beneath the surface and having a finite lateral extent. The diagnosis was made from the observation of late-arriving S phases with large amplitudes, localized in the zones of large damage. The azimuths and angles of incidence of the seismic rays that give rise to the greatest focusing effects correspond to radiation that would have emerged from the lower part of the rupture surface of the mainshock. Thus the focusing and, hence, the large damage in Santa Monica were highly dependent on the location of the Northridge event, and an earth- quake of similar size, located as little as one source dimension away, would not be likely to repeat this pattern. We show from coda wave analysis that the influence of surface geology as well as site effects on damage in Santa Monica is significantly smaller than are the focusing effects. Introduction During the 17 January 1994 Mw = 6.7, depth = 19 km concentrated damage in Santa Monica is unlikely to be re- Northridge earthquake (USGS and SCEC, 1994), Sherman lated to this effect.
    [Show full text]
  • Page -  Lab 09 - Seismology
    Page - Lab 09 - Seismology Every year earthquakes take a tremendous toll on human life and property throughout the world. Fires from broken gas lines, flooding by large tsunamis (tidal waves caused by seaquakes), and the collapsing of buildings and other artificial structures are just a few of the devastating results of a major earthquake event. Most of the damage caused by an earthquake occurs at its geographic origin, or epicenter. It is, therefore, imperative that the epicenter of an earthquake be rapidly located, so that emergency relief personnel can be rushed to the area as quickly as possible. In this exercise we will learn more about how earthquakes are formed and how they may be rapidly located through the science of seismology. An earthquake has its origin below the earth’s surface when rocks that have been placed under extreme pressure are suddenly released from the pressure and move rapidly. The position below the earth’s surface where this rapid movement takes place is called the focus. Energy released at the focus propagates through the ground as seismic waves. Much of this energy is concentrated at the geographic point that lies directly above the focus. This point is called the epicenter of the earthquake, and is nearly always the point where most of the earthquakes’ devastation is concentrated. There are two major classes of seismic waves. Body waves move through the interior of the earth and are capable of penetrating the entire earth. Surface waves move up to the epicenter of the earthquake and spread out along the surface. It is the surface waves that provide the energy causes earthquake devastation.
    [Show full text]
  • Short-Term Foreshocks As Key Information for Mainshock Timing and Rupture: the Mw6.8 25 October 2018 Zakynthos Earthquake, Hellenic Subduction Zone
    sensors Article Short-Term Foreshocks as Key Information for Mainshock Timing and Rupture: The Mw6.8 25 October 2018 Zakynthos Earthquake, Hellenic Subduction Zone Gerassimos A. Papadopoulos 1,*, Apostolos Agalos 1 , George Minadakis 2,3, Ioanna Triantafyllou 4 and Pavlos Krassakis 5 1 International Society for the Prevention & Mitigation of Natural Hazards, 10681 Athens, Greece; [email protected] 2 Department of Bioinformatics, The Cyprus Institute of Neurology & Genetics, 6 International Airport Avenue, Nicosia 2370, P.O. Box 23462, Nicosia 1683, Cyprus; [email protected] 3 The Cyprus School of Molecular Medicine, The Cyprus Institute of Neurology & Genetics, 6 International Airport Avenue, Nicosia 2370, P.O. Box 23462, Nicosia 1683, Cyprus 4 Department of Geology & Geoenvironment, National & Kapodistrian University of Athens, 15784 Athens, Greece; [email protected] 5 Centre for Research and Technology, Hellas (CERTH), 52 Egialias Street, 15125 Athens, Greece; [email protected] * Correspondence: [email protected] Received: 28 August 2020; Accepted: 30 September 2020; Published: 5 October 2020 Abstract: Significant seismicity anomalies preceded the 25 October 2018 mainshock (Mw = 6.8), NW Hellenic Arc: a transient intermediate-term (~2 yrs) swarm and a short-term (last 6 months) cluster with typical time-size-space foreshock patterns: activity increase, b-value drop, foreshocks move towards mainshock epicenter. The anomalies were identified with both a standard earthquake catalogue and a catalogue relocated with the Non-Linear Location (NLLoc) algorithm. Teleseismic P-waveforms inversion showed oblique-slip rupture with strike 10◦, dip 24◦, length ~70 km, faulting depth ~24 km, velocity 3.2 km/s, duration 18 s, slip 1.8 m within the asperity, seismic moment 26 2.0 10 dyne*cm.
    [Show full text]
  • Topic Magnitude Calibration Formulas and Tables, Comments on Their Use
    Datasheet DS 3.1 Topic Magnitude calibration formulas and tables, comments on their use and complementary data Author Peter Bormann (formerly GFZ German Research Centre for Geosciences, Potsdam, Telegrafenberg, D-14473 Potsdam, Germany; E-mail: [email protected] Version January 2012; DOI: 10.2312/GFZ.NMSOP-2_DS_3.1 1 Local magnitude Ml The classical formula for determining the local magnitude Ml is, according to Richter (1935), Ml = log Amax - log A0 (1) with Amax in mm of measured zero-to-peak trace amplitude in a Wood-Anderson seismogram. The respective corrections or calibration values –logA0 are given in Table 1 as a function of epicentral distance ∆. Table 1 The classical tabulated calibration function -logA0(∆) for local magnitudes Ml according to Richter (1958). A0 are the trace amplitudes in mm recorded by a Wood- Anderson Standard Torsion Seismometer from an earthquake of Ml = 0. ∆ (km) –logA0 ∆ (km) –logA0 ∆ (km) –logA0 ∆ (km) –logA0 0 1.4 90 3.0 260 3.8 440 4.6 10 1.5 100 3.0 280 3.9 460 4.6 20 1.7 120 3.1 300 4.0 480 4.7 30 2.1 140 3.2 320 4.1 500 4.7 40 2.4 160 3.3 340 4.2 520 4.8 50 2.6 180 3.4 360 4.3 540 4.8 60 2.8 200 3.5 380 4.4 560 4.9 70 2.8 220 3.65 400 4.5 580 4.9 80 2.9 240 3.7 420 4.5 600 4.9 Different from the above, the IASPEI Working Group on Magnitude Measurement now recommends, as approved by the IASPEI Commission on Seismic Observation and Interpretation (CoSOI), the following standard formula for calculating Ml (quote from IASPEI, 2011, which uses the nomenclature ‘ML’ instead of ‘Ml’): “For crustal earthquakes in regions with attenuative properties similar to those of Southern California, the proposed standard equation is ML = log10(A) + 1.11 log10R + 0.00189∗R - 2.09, (2) where A = maximum trace amplitude in nm that is measured on output from a horizontal- component instrument that is filtered so that the response of the seismograph/filter system replicates that of a Wood-Anderson standard seismograph but with a static magnification of 1; 1 Datasheet DS 3.1 R = hypocentral distance in km, typically less than 1000 km.
    [Show full text]
  • 1. Introduction
    SEISMIC WAVES AND EARTHQUAKE LOCATION J.R. Kayal Geological Survey of India, 27, J.L. Nehru Road Road, Kolkata – 700 016 email : [email protected] SEISMIC WAVES Two basic types of elastic waves or seismic waves are generated by an earthquake; these are body waves and surface waves. These waves cause shaking that is felt, and cause damage in various ways. These waves are similar in many important ways to the familiar waves in air generated by a hand-clap or in water generated by a stone thrown into water. Body Waves The body waves propagate within a body of rock. The faster of these body waves is called Primary wave (P-wave), or longitudinal wave or compressional wave, and the slower one is called Secondary wave (S-wave) or shear wave. P-wave The P-wave motion, same as that of sound wave in air, alternately pushes (compresses) and pulls (dilates) the rock (Fig. 1). The motion of the particles is always in the direction of propagation. The P-wave, just like sound wave, travels through both solid rock such as granite and liquid material such as volcanic magma or water. It may be mentioned that, because of sound like nature, when P-wave emerges from deep in the Earth to the surface, a fraction of it is transmitted into atmosphere as sound waves. Such sounds, if frequency is greater than 15 cycles per second, are audible to animals or human beings. These are known as earthquake sound. 1 Fig 1 : Seismic wave propagation The relation between compressional or P-wave velocity (Vp) and the elastic constants E (Young's modulus), σ (Poisson's ratio), K (bulk modulus), µ (rigidity modulus), λ (Lame's constant) and density ρ is given as follows: 1 ⎧(λ + 2µ)⎫ 2 V p = ⎨ ⎬ (1) ⎩ ρ ⎭ S-wave It is known that the S-wave or the shear wave shears the rock sideways at right angle to the direction of propagation (Fig.
    [Show full text]
  • 1 Earthquakes and Seismo-Tectonics Chapter 5 HW Answers 1. a Seismic
    Earthquakes and Seismo­tectonics Chapter 5 HW answers 1. A seismic recording station receives the first S arrival 8 minutes after the P one. What is the epicentral angle to the source. Because P‐waves and S‐waves go a different wave speed, this fact can be exploited to measure the distance (not direction) to an earthquake via measurement of the time interval between the arrival times of the P‐ and S‐wave. The accuracy of the distance to the earthquake is dependent upon the accuracy of the P‐ and S‐wave speeds and the raypath taken by the earthquake waves. See Figure 5.4. 2. The length of rupture on a fault plane associated with a large earthquake may be hundreds of kilometers long and tens of kilometers deep. How then can we refer to the hypocenter of the earthquake. The hypocenter is where the point where the earthquake slip nucleated for a large earthquake. 3. If the long­term average rate of displacement is roughly the same along the length of a long fault, how can some places experience large earthquakes yet other escape then. While most faults only slip during an earthquake, a few faults (e.g., central San Andreas fault) actually mostly ‘creep’. This does NOT mean that there are no earthquakes along the creeping sections of a fault; actually as there are many small (Mb < 3) earthquakes (see Fig. 5.20). But, given that the earthquake scale is logarithmic a large number of Mb < 3 earthquakes do NOT amount to any significant displacement on the fault compared to a Mb > 5 or 6 earthquake.
    [Show full text]