Ross D. Hartleb

The Surface Rupture and Slip Distribution of the 17 August 1999 İzmit Earthquake (M 7.4), North Anatolian Fault

The 17 August 1999 Izmit earthquake occurred on the northern strand of the North Anatolian fault zone. The earthquake is associated with a 145-km-long surface rupture that extends from southwest of Duzce in the east to west of Hersek delta in the west. Detailed mapping of the surface rupture shows that it consists of five segments separated by releasing step-overs; herein named the Hersek, Karamursel-Golcuk, Izmit-Sapanca Lake, Sapanca-Akyazi, and Karadere segments from west to east, respectively. The Hersek segment, which cuts the tip of a large delta plain in the western end of the rupture zone, has an orientation of N80°. The N70°-80°E-trending Karamursel-Golcuk segment extends along the linear southern coasts of the Izmit Gulf between Karamursel and Golcuk and produced the 470-cm maximum displacement in Golcuk. The northwest-southeast-striking Golcuk normal fault between the Karamursel-Golcuk and Izmit-Sapanca segments has 2.3-m maximum vertical displacement. The maximum dextral offset along the Izmit-Sapanca Lake segment was measured to be about 3.5 m, and its trend varies between N80°E and east-west. The Sapanca-Akyazi segment trends N75°-85°W and expresses a maximum displacement of 5.2 m. The Karadere segment trends N65°E and produced up to 1.5-m maximum displacement. The Karadere and Sapanca-Akyazi segments form fan-shape or splaying ruptures near their eastern ends where the displacement also diminished.

Surface Rupture and Slip Distribution of the 12 November 1999 Düzce Earthquake (M 7.1), North Anatolian Fault, Bolu, Turkey

The 12 November 1999 earthquake ( M 7.1) occurred on the Duzce fault, a splay of the North Anatolian fault in the Bolu basin approximately three months after the 17 August 1999 ( M 7.4) earthquake. The surface rupture was 40 km long, and the maximum right-lateral offset was 500 ± 5 cm, averaging 300 cm. The 9 km of the westernmost part of the rupture along the southern margin of the Eften Lake had a 350-cm maximum vertical displacement (normal faulting), some of which was already ruptured during the 17 August 1999 event with few tens of centimeters. The surface rupture has a generally simple narrow deformation zone of 0.5-5 m, however, in some places, it widens to 50 m. Transtensional and transpressional structures were observed within releasing and restraining step-over areas respectively. The larger dextral offsets on some streams indicate previous events. The dextral slip measurements along the rupture reflect a symmetric distribution. The eastern connection of this rupture zone with the main trace of North Anatolian fault remains unclear because the Bakacak and Elmalik fault, which are connecting faults, had no surface rupture. Manuscript received 14 November 2000.

The 1999 Izmit, Turkey, earthquake: A 3D dynamic stress transfer model of intraearthquake triggering

Before the August 1999 Izmit (Kocaeli), Turkey, earthquake, theoretical studies of earthquake ruptures and geological observations had provided estimates of how far an earthquake might jump to get to a neighboring fault. Both numerical simulations and geological observations suggested that 5 km might be the upper limit if there were no transfer faults. The Izmit earthquake appears to have followed these expectations. It did not jump across any step-over wider than 5 km and was instead stopped by a narrower step-over at its eastern end and possibly by a stress shadow caused by a historic large earthquake at its western end. Our 3D spontaneous rupture simulations of the 1999 Izmit earthquake provide two new insights: (1) the west- to east-striking fault segments of this part of the North Anatolian fault are oriented so as to be low-stress faults and (2) the easternmost segment involved in the August 1999 rupture may be dipping. An interesting feature of the Izmit earthquake is that a 5-km-long gap in surface rupture and an adjacent 25° restraining bend in the fault zone did not stop the earthquake. The latter observation is a warning that significant fault bends in strike-slip faults may not arrest future earthquakes. Manuscript received 30 August 2000.

Late Holocene slip rate for the North Anatolian fault, Turkey, from cosmogenic 36Cl geochronology: Implications for the constancy of fault loading and strain release rates

Geomorphologic mapping and cosmogenic radionuclide (36Cl) dating of an offset fluvial terrace yield a preferred late Holocene slip rate of 20.5 ± 5.5 mm/yr for the central part of the North Anatolian fault, Turkey; an independent slip rate constrained by 14C ages is 20.5 ± 8.5 mm/yr. These rates are generally similar to, but possibly slightly slower than, the short-term rate of elastic strain storage of 25 ± 1 measured geodetically across this major strike-slip fault ([Reilinger et al., 2006][1]), suggesting that loading and strain release on this part of the North Anatolian fault have been relatively constant when averaged over the past ∼2–2.5 k.y. We attribute this consistency to the relatively simple structure of the Anatolia-Eurasia plate boundary in north-central Turkey, where almost all plate boundary strain is accommodated along the North Anatolian fault. The absence of other moderate to high slip rate faults (and the earthquakes they produce) leads to a relatively simple stress evolution for the fault dominated by steady tectonic loading. [1]: #ref-10

A 2500-yr-long paleoseismologic record of large, infrequent earthquakes on the North Anatolian fault at Çukurçimen, Turkey

Paleoseismologic trenches excavated across the North Anatolian fault near the village of Cukurcimen in north-central Turkey yield a complete record of surface ruptures for the past 2500 yr. The trenches provide mutually consistent evidence for the timing of the five most recent surface ruptures at the site, as well as at least two older events. These are interpreted as: (1) the historic 1939 M w 7.9 earthquake; (2) the historic 1254 A.D. earthquake; (3) the historic 1045 A.D. earthquake; (4) an earthquake that probably occurred late in the interval between 250 and 540 A.D., possibly the historic 499 A.D. earthquake; and (5) an earthquake that occurred sometime between 770 and 50 B.C. (and probably between 230 and 50 B.C.). One additional earthquake occurred sometime between 1450 and 800 B.C., and at least one other surface rupture occurred between 2880 and 200 B.C., but the stratigraphic section at the site was not completely exposed for sediments older than -2500 yr. As a result, it is unclear whether the oldest event horizons represent a single earthquake, or multiple events. Our findings, when coupled with other published results and the historical record, enable us to construct a space-time history of earthquakes along the North Anatolian fault. The most striking aspects of this analysis are: (1) the rarity of earthquakes at any given place along the fault, suggesting that the fault typically ruptures in large, infrequent events; (2) earthquake occurrence is relatively temporally regular, with inter-event times that range from -200 to <900 yr, and vary by a factor of only 3-4; and (3) almost the entire length of the North Anatolian fault ruptured in short-lived clusters of activity in the seventeenth to eighteenth centuries and in the tenth to twelfth centuries, similar to the twentieth-century sequence. The magnitude and sequence of the individual ruptures, however, have varied considerably between clusters. Moreover, the paleoseismologic and historical data indicate that the North Anatolian fault does not always rupture in sequences similar to the twentieth-century cluster. These results suggest that the relatively regular recurrence of rare, large-magnitude earthquakes may be the expected mode of earthquake occurrence on mature strike-slip faults in settings where such faults are structurally isolated from other major seismic sources.

Surface Rupture and Slip Distribution along the Karadere Segment of the 17 August 1999 İzmit and the Western Section of the 12 November 1999 Düzce, Turkey, Earthquakes

On 17 August 1999 the Mw 7.5 I zmit, Turkey, earthquake produced surface rupture in excess of 120 km, and perhaps as much as 200 km, with up to 5 m of dextral slip, along a western portion of the North Anatolian fault zone. The 12 November 1999 Mw 7.1 Duzce, Turkey, earthquake produced a 40-km-long sur- face rupture, including 9 km of rupture overlap with the eastern end of the August event. Our mapping focused on the 40-km-long Karadere rupture segment, the east- ernmost segment of the August event, as well as on the western 20 km of the No- vember rupture. Maximum dextral slip along the Karadere segment is approximately 1.5 m, and the average slip on this segment is close to 1 m. Although slip along the Karadere segment is considerably less than that on segments to the west, this segment is of particular interest for three reasons: (1) the western boundary of the Karadere segment is defined by the most striking structural discontinuity along the entire Au- gust surface rupture (i.e., a 5-km-wide zone of no surface rupture, as well as a 25 change in trend from E-W to ENE), and such a discontinuity may have important implications for rupture dynamics; (2) surface rupture terminates at the east end of the Karadere rupture segment at a 1.5- to 3-km-wide extensional step-over at Eften Lake; and (3) the 12 November 1999 Duzce earthquake reruptured the easternmost 9 km of the Karadere segment, raising interesting questions about rupture dynamics and interactions between events on adjacent fault segments. The details of the 17 August 1999 and l2 November 1999 surface rupture traces suggest that rupture may have partially propagated across the Eften Lake extensional step-over, although this step-over seems to have acted as an effective barrier to rupture propagation.

A 2000-Year-Long Paleoseismologic Record of Earthquakes along the Central North Anatolian Fault, from Trenches at Alayurt, Turkey

Paleoseismologic data from trenches excavated across the central part of the North Anatolian fault at Alayurt, Turkey, reveal evidence for at least four, and possibly five, surface ruptures during the past 2000 years, as well as one much older event. These surface ruptures, as recognized on the basis of upward fault terminations and colluvial gravel layers, include (1) the historic 1943 M w 7.7 Tosya earthquake; (2) an older event that is not well dated, but which we interpret as the great 1668 earthquake, which historical accounts suggest ruptured this part of the fault; (3) a late-eighth- to early-thirteenth-century surface rupture; (4) a surface rupture that occurred between the first and third centuries A.D., possibly the historic A.D. 236 event; (5) a possible surface rupture that occurred between the late fourth and early eleventh centuries A.D.; and (6) a much older event that occurred sometime between 4600 and 3550 B.C. Our documentation of a late-eighth- to early-thirteenth-century surface rupture at Alayurt is particularly important because, when taken in context with available historical and paleoseismologic data, it suggests the occurrence of a brief interval during which large earthquakes ruptured most of the North Anatolian fault. This interval resembles two other short-lived clusters of activity in the sixteenth to eighteenth and twentieth centuries. These brief intervals of activity are separated by much longer periods of relative quiescence that range from 250 years to ≤800 years (if there was not a late-fourth- to early-eleventh-century event) or ∼600 years (if there was a late-fourth- to early-eleventh-century event). The factor of 2–3 variation in inter-event times suggests quasiperiodic earthquake occurrence, which we attribute to the structural simplicity and relative isolation of the central North Anatolian fault from other earthquake-producing faults.

Paleoseismologic evidence for the relatively regular recurrence of infrequent, large-magnitude earthquakes on the eastern North Anatolian fault at Yaylabeli, Turkey

Paleoseismologic trenches excavated across the eastern part of the North Anatolian fault at Yaylabeli, Turkey, provide evidence for five surface ruptures during the last 2000 yr. We interpret these events as: (1) the historical 1939 M w 7.9 earthquake; (2) the historical 1254 A.D. earthquake; (3) the historical 1045 A.D. earthquake; (4) an earthquake that occurred between 660 A.D. and 1020 A.D., most probably between 717 A.D. and 844 A.D.; and (5) an earthquake that occurred between 302 A.D. and 724 A.D., possibly the historical 499 A.D. event. Although one of the interevent intervals we document is 685 yr long (between the 1254 A.D. and 1939 A.D. earthquakes), the other three intervals are between 200 and 350 yr long. Our results, which facilitate a rare opportunity to test the completeness of the paleoseismologic record at multiple sites, are generally similar to those from the nearby Cukurcimen trench site, located 2 km to the east, demonstrating reproducibility of the paleoearthquake record. However, the eighth- to ninth-century event (E4) that we document at Yaylabeli was not observed at Cukurcimen. The addition of this event facilitates the recognition of a previously unnoticed North Anatolian fault earthquake cluster, during which at least the eastern and central parts of the fault appear to have ruptured during a brief sequence in the eighth and ninth centuries. Addition of this possible cluster suggests that the North Anatolian fault commonly ruptures in brief, systemwide sequences, although the individual earthquakes in each sequence differ from cluster to cluster in terms of location, magnitude, and rupture sequence. These paleoearthquake data reinforce the idea of relatively regular recurrence of infrequent, large-magnitude earthquakes on the eastern section of the North Anatolian fault. We attribute this relatively simple behavior to the structural maturity of the North Anatolian fault and its relative isolation from other major seismic sources within the Anatolia-Eurasia plate boundary.