DOCSLIB.ORG

BY JOHNATHAN EVANS What Exactly Is an Earthquake?

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
BY JOHNATHAN EVANS What Exactly Is an Earthquake? Earthquakes BY JOHNATHAN EVANS What Exactly is an Earthquake? An Earthquake is what happens when two blocks of the earth are pushing against one another and then suddenly slip past each other. The surface where they slip past each other is called the fault or fault plane. The area below the earth’s surface where the earthquake starts is called the Hypocenter, and the area directly above it on the surface is called the “Epicenter”. What are aftershocks/foreshocks? Sometimes earthquakes have Foreshocks. Foreshocks are smaller earthquakes that happen in the same place as the larger earthquake that follows them. Scientists can not tell whether an earthquake is a foreshock until after the larger earthquake happens. The biggest, main earthquake is called the mainshock. The mainshocks always have aftershocks that follow the main earthquake. Aftershocks are smaller earthquakes that happen afterwards in the same location as the mainshock. Depending on the size of the mainshock, Aftershocks can keep happening for weeks, months, or even years after the mainshock happened. Why do earthquakes happen? Earthquakes happen because all the rocks that make up the earth are full of fractures. On some of the fractures that are known as faults, these rocks slip past each other when the crust rearranges itself in a process known as plate tectonics. But the problem is, rocks don’t slip past each other easily because they are stiff, rough and their under a lot of pressure from rocks around and above them. Because of that, rocks can pull at or push on each other on either side of a fault for long periods of time without moving much at all, which builds up a lot of stress in the rocks. When they finally do move past each other, so much pressure has been built up that it causes an earthquake. Where do earthquakes happen at? Most earthquakes,(including the largest and most powerful) commonly occur near the edge of oceanic and continental plates(the earth’s crust is made up of several pieces, called plates. The plates under the oceans are called oceanic plates and the rest are called continental plates.) How powerful are earthquakes? Earthquake strength is measured by the Richter scale. The Richter scale measures these earthquakes from small earthquakes such as a 2.3 earthquake or a powerful one such as a 8.5 earthquake. Earthquakes are measured using a machine called a Seismometer which produces a seismograph. A Richter scale is normally numbered 1-10, though there is no upper limit. Pangea Pangea was a super continent that existed about 250million years ago and consisted of every single continent that exists today. About 175million years ago, There was a massive earthquake in Pangea which broke it apart into all the normal continents we have today *This is just what people think Pangea might have looked like. Mount Tai earthquake The mount Tai earthquake is very important because it was the first recorded earthquake in history. It occurred at Mount Tai during the 7th year of the reign of King Fa from the Xia dynasty between circa 2205 and 1600 BCE. Mount Tai earthquake cont. Mount Tai is located in western Shandong, just north of the city of Tai'an and to the south of the provincial capital Jinan. The mount tai earthquake has been dated to 1831 or 1731 BCE. Sparta earthquake The Sparta earthquake had a magnitude of 7.2 and occurred along the Sparta fault in the year 464 BCE and destroyed much of what was Sparta and many other City-states in Ancient Greece. Sparta earthquake cont. Historical sources suggest that the Sparta earthquake may have had a casualty toll of almost 20,000 although modern scholars suggest that this figure is likely an exaggeration. Sparta was located on what is now called the Hellenic arc which has a high amount of large seismic activity. Rhodes earthquake The Rhodes earthquake occurred at 3:00 in the morning on may 3rd, 1481 and was very powerful, having a magnitude of 7.1. The earthquake destroyed a statue of The colossus of Rhodes which was one of the tallest statues of the ancient world. Rhodes earthquake cont. The Rhodes earthquake also caused many casualties because it triggered a small tsunami, Which caused local flooding. It was the largest of a series of earthquakes that affected Rhodes, starting on 15 march 1481, continuing until January 1482. Tsunamis Tsunamis are enormous tidal waves that occur because of strong earthquakes (6.5 or more) occurring below or close enough to the ocean to cause the shifting of tectonic plates below the water. Tsunamis are very dangerous because they can travel as far as 10 miles inland, depending on the shape and slope of the shoreline. California Earthquakes. The reason California has so many earthquakes is that the earthquakes there are caused by the movement of huge blocks of the earth's crust called the northern and pacific plates. The pacific plate is constantly moving northwest so it is scraping horizontally past North America at a speed of about 50 millimeters (or 2 inches) per year. Largest earthquake ever. The world's largest earthquake (with an instrumentally documented magnitude) occurred on May 22, 1960 near Valdivia, in southern Chile.it had a magnitude of 9.5 and is referred to as the "Great Chilean Earthquake" and the "1960 Valdivia Earthquake. The United States Geological Survey reports this event as the "largest earthquake of the 20th Century." Largest earthquake ever cont. The worlds largest earthquake also produced a powerful tsunami that traveled at a speed of about 200 miles per hour across the Pacific Ocean. Other earthquakes in history might have been larger however this is the largest earthquake that has occurred since accurate estimates of magnitude became possible in the early 1900s..
Load more

Recommended publications
  • Keck Combined Strong-Motion/Broadband Obs
    KECK COMBINED STRONG-MOTION/BROADBAND OBS WHOI Ocean Bottom Seismograph Laboratory W.M. Keck Foundation Award to Understand Earthquake Predictability at East Pacific Rise Transform Faults. • In 2006, the W.M. Keck Foundation awarded funding to Jeff McGuire and John Collins to determine the physical mechanisms responsible for the observed capability to predict, using foreshocks, large (Mw ~6) transform-fault earthquakes at the East Pacific Rise, and to gain greater insight into the fundamentals of earthquake mechanics in general. • Achieving this objective required building 10 OBS of a new and unique type that could record, without saturation, the large ground motions generated by moderate to large earthquakes at distances of less than ~10 km. W.M. Keck OBS: Combined Broadband Seismometer and Strong-Motion Accelerometer • The broadband OBS available from OBSIP carry a broadband seismometer whose performance is optimized to resolve the earth’s background noise, which can be as small as 10-11 g r.m.s. in a bandwidth of width 1/6 decade at periods of about 100 s. Thus, these sensors are ideal for resolving ground motions from large distant earthquakes, necessary for structural seismology studies. • Ground motions from local, moderate to large earthquakes can generate accelerations of up a g or more, which would saturate or clip these sensors. Currently, there are no sensors (or data-loggers) that have a dynamic range to resolve this ~220 dB range of ground motion. • To overcome this limitation, we used Keck Foundation funding to build 10 OBS equipped with both a conventional broadband seismometer and a strong- motion accelerometer with a clip level of 2.5 g.
    [Show full text]
  • Seismic Rate Variations Prior to the 2010 Maule, Chile MW 8.8 Giant Megathrust Earthquake
    www.nature.com/scientificreports OPEN Seismic rate variations prior to the 2010 Maule, Chile MW 8.8 giant megathrust earthquake Benoit Derode1*, Raúl Madariaga1,2 & Jaime Campos1 The MW 8.8 Maule earthquake is the largest well-recorded megathrust earthquake reported in South America. It is known to have had very few foreshocks due to its locking degree, and a strong aftershock activity. We analyze seismic activity in the area of the 27 February 2010, MW 8.8 Maule earthquake at diferent time scales from 2000 to 2019. We diferentiate the seismicity located inside the coseismic rupture zone of the main shock from that located in the areas surrounding the rupture zone. Using an original spatial and temporal method of seismic comparison, we fnd that after a period of seismic activity, the rupture zone at the plate interface experienced a long-term seismic quiescence before the main shock. Furthermore, a few days before the main shock, a set of seismic bursts of foreshocks located within the highest coseismic displacement area is observed. We show that after the main shock, the seismic rate decelerates during a period of 3 years, until reaching its initial interseismic value. We conclude that this megathrust earthquake is the consequence of various preparation stages increasing the locking degree at the plate interface and following an irregular pattern of seismic activity at large and short time scales. Giant subduction earthquakes are the result of a long-term stress localization due to the relative movement of two adjacent plates. Before a large earthquake, the interface between plates is locked and concentrates the exter- nal forces, until the rock strength becomes insufcient, initiating the sudden rupture along the plate interface.
    [Show full text]
  • Foreshock Sequences and Short-Term Earthquake Predictability on East Pacific Rise Transform Faults
    NATURE 3377—9/3/2005—VBICKNELL—137936 articles Foreshock sequences and short-term earthquake predictability on East Pacific Rise transform faults Jeffrey J. McGuire1, Margaret S. Boettcher2 & Thomas H. Jordan3 1Department of Geology and Geophysics, Woods Hole Oceanographic Institution, and 2MIT-Woods Hole Oceanographic Institution Joint Program, Woods Hole, Massachusetts 02543-1541, USA 3Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-7042, USA ........................................................................................................................................................................................................................... East Pacific Rise transform faults are characterized by high slip rates (more than ten centimetres a year), predominately aseismic slip and maximum earthquake magnitudes of about 6.5. Using recordings from a hydroacoustic array deployed by the National Oceanic and Atmospheric Administration, we show here that East Pacific Rise transform faults also have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. The high ratio of foreshocks to aftershocks implies that such transform-fault seismicity cannot be explained by seismic triggering models in which there is no fundamental distinction between foreshocks, mainshocks and aftershocks. The foreshock sequences on East Pacific Rise transform faults can be used to predict (retrospectively) earthquakes of magnitude 5.4 or greater, in narrow spatial and temporal windows and with a high probability gain. The predictability of such transform earthquakes is consistent with a model in which slow slip transients trigger earthquakes, enrich their low-frequency radiation and accommodate much of the aseismic plate motion. On average, before large earthquakes occur, local seismicity rates support the inference of slow slip transients, but the subject remains show a significant increase1. In continental regions, where dense controversial23.
    [Show full text]
  • Earthquake Measurements
    EARTHQUAKE MEASUREMENTS The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs1. The zig-zag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument. From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released. Seismograph/Seismometer Earthquake recording instrument, seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free2. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is Seismograph what is recorded. Measuring Size of Earthquakes The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was. A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake2. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.
    [Show full text]
  • Source Parameters of the 1933 Long Beach Earthquake
    Bulletin of the Seismological Society of America, Vol. 81, No. 1, pp. 81- 98, February 1991 SOURCE PARAMETERS OF THE 1933 LONG BEACH EARTHQUAKE BY EGILL HAUKSSON AND SUSANNA GROSS ABSTRACT Regional seismographic network and teleseismic data for the 1933 (M L -- 6.3) Long Beach earthquake sequence have been analyzed, Both the teleseismic focal mechanism of the main shock and the distribution of the aftershocks are consistent with the event having occurred on the Newport-lnglewood fault. The focal mechanism had a strike of 315 °, dip of 80 o to the northeast, and rake of - 170°. Relocation of the foreshock- main shock-aftershock sequence using modern events as fixed refer- ence events, shows that the rupture initiated near the Huntington Beach-Newport Beach City boundary and extended unilaterally to the northwest to a distance of 13 to 16 km. The centroidal depth was 10 +_ 2 km. The total source duration was 5 sec, and the seismic moment was 5.102s dyne-cm, which corresponds to an energy magnitude of M w = 6.4. The source radius is estimated to have been 6.6 to 7.9 km, which corresponds to a Brune stress drop of 44 to 76 bars. Both the spatial distribution of aftershocks and inversion for the source time function suggest that the earthquake may have consisted of at least two subevents. When the slip estimate from the seismic moment of 85 to 120 cm is compared with the long-term geological slip rate of 0.1 to 1.0 mm I yr along the Newport-lnglewood fault, the 1933 earthquake has a repeat time on the order of a few thousand years.
    [Show full text]
  • Chapter 2 the Evolution of Seismic Monitoring Systems at the Hawaiian Volcano Observatory
    Characteristics of Hawaiian Volcanoes Editors: Michael P. Poland, Taeko Jane Takahashi, and Claire M. Landowski U.S. Geological Survey Professional Paper 1801, 2014 Chapter 2 The Evolution of Seismic Monitoring Systems at the Hawaiian Volcano Observatory By Paul G. Okubo1, Jennifer S. Nakata1, and Robert Y. Koyanagi1 Abstract the Island of Hawai‘i. Over the past century, thousands of sci- entific reports and articles have been published in connection In the century since the Hawaiian Volcano Observatory with Hawaiian volcanism, and an extensive bibliography has (HVO) put its first seismographs into operation at the edge of accumulated, including numerous discussions of the history of Kīlauea Volcano’s summit caldera, seismic monitoring at HVO HVO and its seismic monitoring operations, as well as research (now administered by the U.S. Geological Survey [USGS]) has results. From among these references, we point to Klein and evolved considerably. The HVO seismic network extends across Koyanagi (1980), Apple (1987), Eaton (1996), and Klein and the entire Island of Hawai‘i and is complemented by stations Wright (2000) for details of the early growth of HVO’s seismic installed and operated by monitoring partners in both the USGS network. In particular, the work of Klein and Wright stands and the National Oceanic and Atmospheric Administration. The out because their compilation uses newspaper accounts and seismic data stream that is available to HVO for its monitoring other reports of the effects of historical earthquakes to extend of volcanic and seismic activity in Hawai‘i, therefore, is built Hawai‘i’s detailed seismic history to nearly a century before from hundreds of data channels from a diverse collection of instrumental monitoring began at HVO.
    [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]
  • By Chieh-Hung Chen Et Al
    Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2020-47-AC3, 2020 NHESSD © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Interactive comment Interactive comment on “Wide sensitive area of small foreshocks” by Chieh-Hung Chen et al. Chieh-Hung Chen et al. [email protected] Received and published: 10 July 2020 The manuscript presents results which, in my opinion, can be very relevant for the fore- casting challange. However I find that they are not well presented and the discussion appears quite confusing for the following reasons: The first part of the manuscript is devoted to study spatio-temporal patterns of seismic activity before and after events in a given magnitude range, for Taiwan and Japan. There are many papers which report a similar increase of seismic activity before large earthquakes. The key point is if the observed increase can have a prognostic value or it can be explained within normal aftershock triggering. Printer-friendly version Reply: Discussion paper The authors fully agree with the comment. The results for the increase of seismic C1 activity close to the epicenter observed in Figs. 1 and 2 are consistent with the obser- vation in the previous studies (Lippiello et al., 2012, 2017, 2019; de Arcangelis et al., NHESSD 2016). The associated statements have been added in the manuscript (lines 171-174). The agreement suggests that the increase of seismic activity in Figs. 1 and 2 is not contributed by aftershocks but a prognostic value. Interactive comment I just suggest some papers where this point is detailed discussed, other references can be find therein (Lippiello et al., Scientific Reports 2012, de Arcangelis et al.
    [Show full text]
  • Mamuju–Majene
    Supendi et al. Earth, Planets and Space (2021) 73:106 https://doi.org/10.1186/s40623-021-01436-x EXPRESS LETTER Open Access Foreshock–mainshock–aftershock sequence analysis of the 14 January 2021 (Mw 6.2) Mamuju–Majene (West Sulawesi, Indonesia) earthquake Pepen Supendi1* , Mohamad Ramdhan1, Priyobudi1, Dimas Sianipar1, Adhi Wibowo1, Mohamad Taufk Gunawan1, Supriyanto Rohadi1, Nelly Florida Riama1, Daryono1, Bambang Setiyo Prayitno1, Jaya Murjaya1, Dwikorita Karnawati1, Irwan Meilano2, Nicholas Rawlinson3, Sri Widiyantoro4,5, Andri Dian Nugraha4, Gayatri Indah Marliyani6, Kadek Hendrawan Palgunadi7 and Emelda Meva Elsera8 Abstract We present here an analysis of the destructive Mw 6.2 earthquake sequence that took place on 14 January 2021 in Mamuju–Majene, West Sulawesi, Indonesia. Our relocated foreshocks, mainshock, and aftershocks and their focal mechanisms show that they occurred on two diferent fault planes, in which the foreshock perturbed the stress state of a nearby fault segment, causing the fault plane to subsequently rupture. The mainshock had relatively few after- shocks, an observation that is likely related to the kinematics of the fault rupture, which is relatively small in size and of short duration, thus indicating a high stress-drop earthquake rupture. The Coulomb stress change shows that areas to the northwest and southeast of the mainshock have increased stress, consistent with the observation that most aftershocks are in the northwest. Keywords: Mamuju–Majene, Earthquake, Relocation, Rupture, Stress-change Introduction mainshock from two of the nearest stations in Mamuju On January 14, 2021, a destructive earthquake (Mw 6.2) and Majene are 95.9 and 92.8 Gals, respectively, equiva- between Mamuju and Majene, West Sulawesi, Indonesia, lent to VI on the MMI scale (Additional fle 1: Figure S1).
    [Show full text]
  • High Probability of Foreshock Occurrence and Significant Probability of Multiple Events Associated with Magnitude Greater Than O
    High Probability of Foreshock Occurrence and Significant Probability of Multiple Events Associated with Magnitude ≥6 Earthquakes in Nevada, U.S.A. by Craig M. dePolo INTRODUCTION moment magnitudes (M w) when possible; otherwise they are catalog or historical values (M; dePolo, 2013). Sixty percent of magnitude 5.5 and larger earthquakes in the western Cordillera were preceded by foreshocks (Doser, 1989). M ≥6 M FORESHOCKS PRIOR TO NEVADA Foreshocks also preceded the 2008 Wells ( w 6.0; Smith et al., EARTHQUAKES 2011) and the 2008 Mogul (M w 5.0; Smith et al., 2008; de- Polo, 2011) earthquakes in Nevada. Understanding foreshocks There have been several studies of earthquake foreshocks, in- and their behavior is important because of their potential use cluding a classic paper on foreshocks along the San Andreas for earthquake forecasting and foreshocks felt by communities fault system by Jones (1984), which concluded that 35% of act as a natural alarm that can motivate people to engage in M ≥5 earthquakes were preceded by an immediate foreshock seismic mitigation. A majority of larger earthquakes in Nevada within one day and 5 km of the mainshock. Considering a two- had foreshocks, and several were multiple earthquakes of mag- ≥6 month time window and a 40 km radius, Doser (1989) studied nitude . Multiple major earthquakes can shake a community M ≥5:5 earthquakes in the western Cordillera and found that with damaging ground motion multiple times within a short 60% of these events had foreshocks and that 58% of these fore- period of time, such as happened in Christchurch, New shocks occurred within 24 hours of their mainshock (35% of Zealand, in 2010 and 2011 (Gledhill et al., 2011; Bradley the total).
    [Show full text]
  • Seismicity Rate Immediately Before and After Mainshock Rupture from High-Frequency Waveforms in Japan Zhigang Peng, John E
    Seismicity rate immediately before and after mainshock rupture from high-frequency waveforms in Japan Zhigang Peng, John E. Vidale, Miaki Ishii, A. Helmstetter To cite this version: Zhigang Peng, John E. Vidale, Miaki Ishii, A. Helmstetter. Seismicity rate immediately before and after mainshock rupture from high-frequency waveforms in Japan. Journal of Geophysical Research, American Geophysical Union, 2007, 112, pp.B03306. 10.1029/2006JB004386. hal-00195470 HAL Id: hal-00195470 https://hal.archives-ouvertes.fr/hal-00195470 Submitted on 11 Dec 2007 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Seismicity rate immediately before and after mainshock rupture from high-frequency waveforms in Japan Zhigang Peng1,2, John E. Vidale1, Miaki Ishii3, Agnes Helmstetter4 1. Department of Earth & Space Sciences, University of California, Los Angeles, CA, 90095-1567, USA 2. Now at School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332 3. Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, 02138, USA 4. Observatoire des Sciences de l'Univers de Grenoble, Laboratoire de Geophysique Interne et Tectonophysique, BP 53 Grenoble Cedex 9 38041, France Running Title: Seismicity rate before/after mainshock Received xx/xx/2006; revised xx/xx/2006; accepted xx/xx/2006; published xx/xx/2006.
    [Show full text]
  • Precise Aftershock Distribution of the 2011 Off the Pacific Coast of Tohoku
    Earth Planets Space, 64, 1137–1148, 2012 Precise aftershock distribution of the 2011 off the Pacific coast of Tohoku Earthquake revealed by an ocean-bottom seismometer network Masanao Shinohara1, Yuya Machida1, Tomoaki Yamada1, Kazuo Nakahigashi1, Takashi Shinbo1, Kimihiro Mochizuki1, Yoshio Murai2, Ryota Hino3, Yoshihiro Ito3, Toshinori Sato4, Hajime Shiobara1, Kenji Uehira5∗, Hiroshi Yakiwara6, Koichiro Obana7, Narumi Takahashi7, Shuichi Kodaira7, Kenji Hirata8, Hiroaki Tsushima8, and Takaya Iwasaki1 1Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan 2Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan 3Graduate School of Science, Tohoku University, Sendai 980-8578, Japan 4Graduate School of Science, Chiba University, Chiba 263-8522, Japan 5Institute of Seismology and Volcanology, Kyushu University, Shimabara 855-0843, Japan 6Faculty of Science, Kagoshima University, Kagoshima 892-0871, Japan 7Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan 8Meteorological Research Institute, Japan Meteorological Agency, Tsukuba 305-0052, Japan (Received February 3, 2012; Revised August 25, 2012; Accepted September 5, 2012; Online published January 28, 2013) The 2011 off the Pacific coast of Tohoku Earthquake occurred at the plate boundary between the Pacific plate and the landward plate on March 11, 2011, and had a magnitude of 9. Many aftershocks occurred following the mainshock. Obtaining a precise aftershock distribution is important for understanding the mechanism of earthquake generation. In order to study the aftershock activity of this event, we carried out extensive sea- floor aftershock observations using more than 100 ocean-bottom seismometers just after the mainshock. A precise aftershock distribution for approximately three months over the whole source area was obtained from the observations.
    [Show full text]