Numerous observations pertaining to the magnitude 9.0 2011 Tohoku-oki earthquake (offshore Japan) have led to new understanding of subduction zone earthquakes. By synthesizing published research results and our own findings, we explore what has been learned about fault behavior and Earth rheology from the observation and modeling of crustal deformation before, during, and after the earthquake. Before the earthquake, megathrust locking models based on land-based geodetic observations correctly outlined the along-strike location of the future rupture zone. Their incorrect definition of the locking pattern in the dip direction demonstrates the need to model the effects of interseismic viscoelastic stress relaxation and stress shadowing. The observation of decade-long accelerated slip downdip of the future rupture zone raises new questions on fault mechanics. During the earthquake, seafloor geodetic measurements revealed huge coseismic displacements (up to 31 m). Modeling of bathymetry difference before and after the earthquake suggests >60 m of coseismic slip of the most seaward 40 km of the fault in the main rupture area, with the slip peaking at the trench. Large differences in shallow slip between published rupture models are due mainly to the near absence of near-trench deformation measurements, but model simplifications in fault and seafloor geometry also bear large responsibility. After the earthquake, seafloor geodetic measurements provided unambiguous evidence for the dominance of viscoelastic relaxation in short-term postseismic deformation. There is little deep afterslip in the fault area where the decade-long pre-earthquake slip acceleration is observed. Investigating the physical processes responsible for the complementary spatial distribution of pre-slip and afterslip calls for new scientific research.
Static stress drop distribution and its average value over the rupture area contain important information on the mechanics of large earthquakes. Here we derive static stress drop distributions from 40 published rupture models for the 2011 Mw 9 Tohoku‐oki earthquake that are based on various multidisciplinary observations. Average stress drop value over the fault area encompassed by the 5 m coseismic slip contour is not unusually large for each rupture model; the mean for the 40 models is 2.3 ± 1.3 MPa, assuming a uniform rigidity 40 GPa. The value for the entire rupture zone and with a more realistic rigidity structure will be even lower. In the majority of the models, local stress drop in parts of the rupture zone well exceeds 20 MPa. The heterogeneous stress change distribution, with large stress drop being accompanied by large stress increase, leads to the small average for the earthquake.
Rupture‐zone averaged static stress drop in the 2011 M=9 Tohoku‐Oki earthquake was less than 5 MPa, but it caused a stress reversal in most of the offshore forearc, although the reversal is less well constrained far offshore by earthquake mechanisms because of 20‐ to 30‐km errors in event depths. Using a finite element model of force balance, we demonstrate that the stress reversal unambiguously indicates (1) a very weak subduction megathrust and (2) very low differential stresses in the forearc. Prior to the reversal, the upper limit of megathrust strength could not be determined from forearc stresses. In the forearc, effects of megathrust friction and gravity are in a fragile balance, and stresses fluctuate around a neutral state in earthquake cycles. If most of the offshore forearc is to be compressive before but extensional after the earthquake, the effective friction coefficient of the megathrust must be ~0.032. Under low differential stresses associated with megathrust weakness, the forearc is generally well below yielding. Applying the concepts of dynamic Coulomb wedge, we show that the inner wedge, and by inference farther landward, stays stable throughout earthquake cycles. The outer wedge is stable most of the time but may occasionally enter a critical state during great earthquakes; its geometry suggests that complete stress drop of the underlying shallow megathrust is unlikely to have happened. We reason that the occurrence of earthquakes and active faulting under low stress in the stable forearc is due to heterogeneities in structure, stress, and/or pore fluid pressure.
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. American Geographical Society is collaborating with JSTOR to digitize, preserve and extend access to Geographical Review. http://www.jstor.org This content downloaded from 169.229.32.137 on Thu, 8 May 2014 23:56:04 PM All use subject to JSTOR Terms and Conditions THE GEOGRAPHICAL REVIEW THE GEOGRAPHICAL REVIEW THE GEOGRAPHICAL REVIEW and development, and the economy in monetary terms are given an appropriate setting in a brief resume of the land and the people and their recent history. The summary and critical assessment and the recommendations set out in concise and convenient form points where foreign aid may be desirable. There are many valuable statistical tables. LES AMERIQUES ET L'EMPIRE DES AIRS. By HENRY MOURER. 318 pp.; maps. Les Editions Internationales, Paris, I949. 690 fr. 94 x 6 inches. The striking parallel between the emergence of the United States as one of the world's great powers and its rise to world primacy in air transport is the main theme of M. Mourer's volume. Displaying a thorough knowledge of air transportation all over the world, and in the Americas in particular, the author discusses the role of aviation in the concept of "inter-American defense." He emphasizes the necessity of fast and reliable means of communication between North and South America, the lack of adequate connecting roads and railroads, and the consequent importance of air transportation. He describes the development of aviation in Latin America, the international rivalry for predominance, and the final emergence of the United States as the principal carrier. The problems of aviation in Canada and the North American Arctic and of American overseas air lines, and those raised by our participation in World War II, are clearly presented.It is the author's belief that the airplane has revolutionized the concept of empire. On the one hand the security and defense of nations, on the other free air navigation and free
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