Bulletin of the Seismological Society of America, 92, 1, pp. 1–18, February 2002

The Seismic Activity of the Sea Region over the Last 2000 Years by N. Ambraseys

Abstract We examined the seismicity of the region of the Sea of Marmara in northwest over the last 2000 yr using the historical record of the region. We found no evidence for truly large earthquakes of a size comparable to those in the North Anatolian Fault zone in the Marmara Basin. Events are smaller in keeping with the known fault segmentation of the Basin. Over the last 2000 yr seismic mo- ment release accounted for the known right-lateral shear velocity across the observed by Global Positioning System measurements. Its average rate is relatively constant and varies between 1.6 and 2.4 cm/yr. The long-term seismicity in the Marmara Sea region shows that large earthquakes are less frequent than pre- dicted from the 100-yr-long instrumental period.

Introduction The Marmara Sea region is a densely populated and great importance. The Roman land route called the Via Eg- fast-developing part of Turkey, roughly bounded by 39.5Њ N natia, the main trade route from Rome to the East, used by to 41.5Њ N and 26Њ Eto31Њ E (Fig. 1). One of the most the Byzantines and later by the Ottomans (S¸ol Qol), was tectonically active on the continents, this region has been built ca. 130 B.C. It crossed the Balkans to and unusually active during the twentieth century with two earth- followed the Thracian coast of the Sea of Marmara via Thes- quakes of MS 7.3 and 7.4 occurring 240 km apart at its two saloniki, (Enos), and Tekirdag˘ (Raedestos) to . extremities in 1912 and 1999 (Fig. 2). A question that must From Istanbul, the trade route ran along the north coast of be addressed in any realistic assessment of the earthquake the Gulf of I˙zmit to I˙zmit (), the main route cross- hazard in this populous area that includes Istanbul, a mega- ing over the Gulf of I˙zmit at Hersek (Helenopolis) to Iznik city of 11 million inhabitants, concerns the long-term seis- (Nicaea). From there, one branch turned to the west, passing micity of the region. from Bursa to C¸ anakkale and the Dardanelles, thus encir- To answer this question, Ambraseys and Jackson (2000) cling the Sea of Marmara, with the sea routes connecting examined the long-term seismicity of the region over the last most of the ports around the Sea and thus supplementing 500 yr. In this article the investigation is extended to the past the communication of information with the interior (e.g., 2000 yr for which both instrumental and macroseismic Taeschner, 1926). events are appraised and historical data relating to earth- At the center of the region, Byzantium, later Constan- quakes in previous centuries are critically studied. These tinople, or Istanbul, was the thriving, populous capital of the studies of past earthquakes, which are still in progress, pro- Lower Roman, Byzantine, and later, Ottoman Empires, with vide a practical context within which early and modern a sustained tradition of historiography. The historical record events in the region can be calibrated on a uniform basis and of this region over the past 20 centuries, as provided by permit creation of yardsticks in terms of qualitative infor- literary sources, is remarkably full and relatively continuous. mation against which early earthquakes can be classified ac- The Roman, and in particular, the Byzantine, tradition, was cording to their size (Ambraseys, 2002). In the process of necessarily waned with the Ottoman conquest about six cen- acquiring and classifying this information, new earthquakes turies ago but was superseded by the fairly rich, but not so were found and known ones were discarded as spurious. accessible, Ottoman historiography, supplemented by occi- dental sources. The historical record of the Marmara Sea Background region regarding information about earthquakes is rivaled only by that of a small part of northeast China. The benefit Although scattered indications of earthquakes in the of being able to use observations over a period 20 times Marmara Sea region go back as far as the third century B.C., longer than the 100 yr of instrumental seismology is obvious. adequate documentary coverage of individual events for lo- cation and magnitude does not begin until the first century Tectonics A.D. The coastal area of the Marmara Sea region was always a major trade route, and for more than 20 centuries it was The active tectonics of the Marmara Sea region in north- the crossroads between the West and the East, a region of east Turkey is dominated by the right-lateral North Anato-

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