GREAT THRUST EARTHQUAKES AND ASEISMIC SLIP ALONG THE PLATE BOUNDARY OF THE MAKRAN SUBDUCTION ZONE

The Makran subduction zone of Iran and Pakistan exhibits strong variation in seismicity between its eastern and western segments and has one of the world's largest forearcs. We determine the source parameters for 14 earthquakes at Makran including the great (Mw 8.1) earthquake of 1945 (the only instrumentally recorded great earthquake at Makran); we determine the loci of seismic and aseismic slip along the plate boundary, and we assess the effects of the large forearc and accretionary wedge on the style of plate boundary slip. We apply body waveform inversions and, for small-magnitude events, use first motions of P waves to estimate earthquake source parameters. For the 1945 event we also employ dislocation modeling of uplift data. We find that the earthquake of 1945 in eastern Makran is an interplate thrust event that ruptured approximately one-fifth the length of the subduction zone. Nine smaller events in eastern Makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock. Seaward of these thrust earthquakes lies the shallowest 70–80 km of the plate boundary; we find that this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes. This aseismic zone, as in other subduction zones, lies within that part of the accretionary wedge that consists of largely uconsolidated sediments (seismic velocities less than 4.0 km/s). The existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern Makran become sufficiently well consolidated and de watered about 70 km from the deformation front or older, lithified rocks are present within the forearc so that stick-slip sliding behavior becomes possible. This study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes. In contrast to the east, the plate boundary in western Makran has no clear record of historic great events, nor has modem instrumentation detected any shallow thrust events for at least the past 25 years. Most earthquakes in western Makran occur within the downgoing plate at intermediate depths. The large change in seismicity between eastern and western Makran along with two shallow events that exhibit right-lateral strike-slip motion in central Makran suggest segmentation of the subduction zone. Two Paleozoic continental blocks dominate the overriding plate. The boundary between them is approximately coincident with the transition in seismicity. Although relative motion between these blocks may account for some of the differing seismic behavior, the continuity of the deformation front and of other tectonic features along the subduction zone suggests that the rate of subduction does not change appreciably from east to west. The absence of plate boundary events in western Makran indicates either that entirely aseismic subduction occurs or that the plate boundary is currently locked and experiences great earthquakes with long repeat times. Evidence is presently inconclusive concerning which of these two hypotheses is most correct. The presence of well-defined late Holocene marine terraces along portions of the coasts of eastern and western Makran could be interpreted as evidence that both sections of the arc are capable of generating large plate boundary earthquakes. If that hypothesis is correct, then western Makran could produce a great earthquake or it could rupture as a number of segments in somewhat smaller-magnitude events. Alternatively, it is possible that western Makran is significantly different from eastern Makran and experiences largely aseismic slip at all times. A knowledge of the velocity structure and nature of the state of consolidation or lithification of rocks at depth in the interior portion of the forearc of western Makran should help to ascertain whether that portion of the plate boundary moves aseismically or ruptures in large to great earthquakes. A resolution of this question has important implications for seismic hazard not only for western Makran but also for other margins, such as the Cascadia subduction zone of western North America, where historical thrust events have not occurred.

[1]  F. Vita C-14 dating of Recent crustal movements in the Persian Gulf and the Iranian Makran. , 1980 .

[2]  T. Jordan,et al.  Earth structure from fundamental and higher-mode waveform analysis , 1983 .

[3]  K. Kadinsky-Cade,et al.  Seismotectonics of southern Iran: The Oman Line , 1982 .

[4]  J. Nábělek,et al.  Rupture process of the Andreanof Islands earthquake of May 7, 1986 , 1988 .

[5]  A. Şengör,et al.  Origin and assembly of the Tethyside orogenic collage at the expense of Gondwana Land , 1988, Geological Society, London, Special Publications.

[6]  G. Plafker,et al.  Alaskan Earthquake of 1964 and Chilean Earthquake of 1960: Implications for Arc Tectonics , 1972 .

[7]  R. Quittmeyer Seismicity variations in the Makran region of Pakistan and Iran: Relation to great earthquakes , 1979 .

[8]  J. Leggett,et al.  Slip vectors and fault mechanics in the Makran Accretionary Wedge, southwest Pakistan , 1988 .

[9]  R. White,et al.  Tectonics of the western Gulf of Oman , 1979 .

[10]  Hiroo Kanamori,et al.  Seismic coupling and uncoupling at subduction zones , 1983 .

[11]  B. F. Atwater,et al.  Evidence for Great Holocene Earthquakes Along the Outer Coast of Washington State , 1987, Science.

[12]  Robert S. White,et al.  Sediment compaction and fluid migration in the Makran Accretionary Prism , 1989 .

[13]  J. Platt Thrust mechanics in highly overpressured accretionary wedges , 1990 .

[14]  G. Suárez,et al.  The 1983 Goodnow earthquake in the central Adirondacks, New York: Rupture of a simple, circular crack , 1989 .

[15]  P. Molnar Continental tectonics in the aftermath of plate tectonics , 1988, Nature.

[16]  D. Giardini,et al.  Global seismicity of 1982: centroid-moment tensor solutions for 308 earthquakes , 1983 .

[17]  N. Ambraseys,et al.  A history of Persian earthquakes , 1982 .

[18]  L. Sykes,et al.  Focal mechanisms of earthquakes in the Indian Ocean and adjacent regions , 1969 .

[19]  Steven N. Ward,et al.  The 1960 Chile earthquake: inversion for slip distribution from surface deformation , 1990 .

[20]  Syed Sirtajuddin Ahmed Tertiary Geology of Part of South Makran, Baluchistan, West Pakistan , 1969 .

[21]  R. E. Snead Active Mud Volcanoes of Baluchistan, West Pakistan , 1964 .

[22]  C. Demets,et al.  Present‐day motion along the Owen Fracture Zone and Dalrymple Trough in the Arabian Sea , 1989 .

[23]  Victor E. Camp,et al.  The Sistan suture zone of eastern Iran , 1983 .

[24]  R. Fairbanks A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation , 1989, Nature.

[25]  O. Pilkey,et al.  Sediments of the northern Arabian Sea , 1965 .

[26]  Thomas H. Heaton,et al.  Seismic potential associated with subduction in the northwestern United States , 1984 .

[27]  Wang-Ping Chen,et al.  The Makran earthquake of 1983 April 18: A possible analogue to the Puget Sound earthquake of 1965? , 1989 .

[28]  B. Romanowicz,et al.  Three‐dimensional structure of the upper mantle beneath the Atlantic Ocean inferred from long‐period Rayleigh waves: 1. Group and phase velocity distributions , 1989 .

[29]  B. Gutenberg,et al.  Seismicity of the Earth , 1970, Nature.

[30]  James Jackson,et al.  Active tectonics of the Alpine—Himalayan Belt between western Turkey and Pakistan , 1984 .

[31]  J. Leggett,et al.  Large-scale sediment underplating in the Makran accretionary prism, southwest Pakistan , 1985 .

[32]  R. Kidd,et al.  The Makran, Southeastern Iran: the anatomy of a convergent plate margin active from Cretaceous to Present , 1982, Geological Society, London, Special Publications.

[33]  R. S. White,et al.  Mud diapirism within Indus fan sediments: Murray Ridge, Gulf of Oman , 1990 .

[34]  A. Kafka,et al.  ARABIAN SEA FROM THE FOCAL MECHANISMS OF SMALL EARTHQUAKES , 1984 .

[35]  Paul G. Richards,et al.  Pulse distortion and Hilbert transformation in multiply reflected and refracted body waves , 1975, Bulletin of the Seismological Society of America.

[36]  J. Nábělek Geometry and mechanism of faulting of the 1980 El Asnam, Algeria, earthquake from inversion of teleseismic body waves and comparison with field observations , 1985 .

[37]  R. White,et al.  Heat flow and age of the Gulf of Oman , 1981 .

[38]  C. Vita-Finzi Recent coastal deformation near the Strait of Hormuz , 1982, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.

[39]  R. Coleman Tectonic setting for ophiolite obduction in Oman. , 1981 .

[40]  J. Smewing,et al.  The biostratigraphy of sediments in the volcanic unit of the Samail Ophiolite , 1981 .

[41]  NábÄ lek,et al.  Determination of earthquake source parameters from inversion of body waves , 1984 .

[42]  Douglas A. Wiens,et al.  Application of Modern Techniques to Analysis of Historical Earthquakes , 1987 .

[43]  S. Duda Secular seismic energy release in the circum-Pacific belt , 1965 .

[44]  John H. Woodhouse,et al.  Mapping the upper mantle: Three‐dimensional modeling of earth structure by inversion of seismic waveforms , 1984 .

[45]  James E. Wright,et al.  Uranium-lead isotopic ages of the Samail Ophiolite, Oman, with applications to Tethyan ocean ridge tectonics , 1981 .

[46]  R. White,et al.  The Makran Continental Margin: Structure of a Thickly Sedimented Convergent Plate Boundary: Convergent Margins: Field Investigations of Margin Structure and Stratigraphy , 1982 .

[47]  Eugene Herrin,et al.  Introduction to “1968 seismological tables for P phases” , 1968 .

[48]  R. C. Quittmeyer,et al.  Historical and modern seismicity of Pakistan, Afghanistan, northwestern India, and southeastern Iran , 1979 .

[49]  C. Scholz,et al.  Mechanism of underthrusting in southwest Japan: A model of convergent plate interactions , 1971 .

[50]  L. Sykes,et al.  Loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones , 1988 .

[51]  E. Okal A theoretical discussion of time domain magnitudes: The Prague formula for Ms and the mantle magnitude Mm , 1989 .

[52]  J. Suppe,et al.  Mechanics of fold-and-thrust belts and accretionary wedges , 1983 .

[53]  K. Abe Magnitudes of large shallow earthquakes from 1904 to 1980 , 1981 .

[54]  U. Chandra Focal mechanism solutions for earthquakes in Iran , 1984 .

[55]  A. Nowroozi,et al.  A reply to discussion on “Focal mechanism of earthquakes in Persia, Turkey, West Pakistan, and Afghanistan and plate tectonics of the Middle East” , 1973, Bulletin of the Seismological Society of America.

[56]  Richard G. Gordon,et al.  Current plate motions , 1990 .

[57]  Hiroo Kanamori,et al.  Importance of Historical Seismograms for Geophysical Research , 1988 .

[58]  B. Gutenberg,et al.  Seismicity of the Earth and associated phenomena , 1950, MAUSAM.

[59]  H. Kanamori,et al.  Magnitudes of great shallow earthquakes from 1904 to 1952 , 1977, Bulletin of the Seismological Society of America.

[60]  R. White Active and passive plate boundaries around the gulf of Oman, North-West Indian Ocean , 1984 .

[61]  R. Huene Tectonic Processes Along the Front of Modern Convergent Margins--Research of the Past Decade , 1984 .

[62]  Lloyd S. Cluff,et al.  Evidence for the Recurrence of Large-Magnitude Earthquakes Along the Makran Coast of Iran and Pakistan , 1979 .

[63]  J. G. Caldwell,et al.  Effects of accretion on the geometry of the descending lithosphere , 1976 .

[64]  R. White,et al.  Sediment dewatering in the Makran accretionary prism , 1985 .

[65]  Chris Marone,et al.  The depth of seismic faulting and the upper transition from stable to unstable slip regimes , 1988 .

[66]  D. Karig,et al.  Makran of Iran and Pakistan as an active arc system , 1977 .

[67]  H. Shimamura,et al.  Microearthquakes and crustal structure off the Makran Coast of Iran , 1980 .

[68]  R. White,et al.  Sediment deformation and plate tectonics in the Gulf of Oman , 1976 .

[69]  Jacques Talandier,et al.  Mm : A variable‐period mantle magnitude , 1989 .

[70]  G. Farhoudi A Comparison of Zagros Geology to Island Arcs , 1978, The Journal of Geology.

[71]  T. Seno,et al.  Factors affecting seismic moment release rates in subduction zones , 1984 .