Yasuo Awata

Slip Distribution, Fault Geometry, and Fault Segmentation of the 1944 Bolu-Gerede Earthquake Rupture, North Anatolian Fault, Turkey

The 1944 Bolu-Gerede earthquake ( M s 7.3) occurred along the North Anatolian fault in northern Turkey and ruptured for about 180 km of the fault. The surface slip distribution and fault geometry, which provide fundamental data on detailed fault segmentation and recurrence of faulting during a multisegment rupture, are poorly known for this event. We conducted geomorphological and geological investigations and eyewitness interviews to determine cumulative offsets through several earthquake cycles. The 1944 rupture comprised five fault segments based on slip distribution and fault step-overs and bends. From west to east, they are called the Bolu, Yenicaga, Gerede, Ismetpasa, and Bayramoren segments. The segments were 21 to 46 km long, and the average slip was 1.9–4.3 m, similar to well-examined segments of the 1999 Izmit rupture. In contrast, the fault jogs were relatively small, suggesting that the 1944 earthquake rupture process was relatively smooth and of short duration. Proposed fault segmentation is consistent with the extent of surface ruptures and damage intensity associated with historical earthquakes in 967, 1035, and 1050 a.d. Historical records indicate that the Bolu and Yenicaga segments ruptured in 967 a.d., and the Gerede, Ismetpasa, and possibly, Bayramoren segments ruptured in 1035 a.d. Therefore, the 1944 rupture occurred as a consequence of multisegment rupture, which consists of at least two distinct earthquake segments. Along the Gerede segment, which slipped 3 to 6 m in 1944, cumulative offsets of 6 to 25 m were observed. These offsets are double, triple, and quadruple multiples of the 1944 slip, implying a similar slip distribution during the four most recent large earthquakes along this segment and supporting the concept of characteristic slip, even though the rupture length varied greatly during the last four earthquake cycles.

Active faults in the Gulf of Izmit on the North Anatolian Fault, NW Turkey: a high-resolution shallow seismic study

The northern strandof the North Anatolian Fault enters the Sea of Marmara through the Gulf of Izmit which was affectedby the Mw = 7.4 I zmit earthquake on August 17, 1999. A significant segment of the fault rupturedin between the area west of Go « lcu « k andeast of Lake Sapanca d uring the earthquake. In the southeastern corner of the gulf andfrom Go« lcu « k westwardthe active trace plunges into the sea andis lost to view. We investigatedthe Gulf of I zmit by means of high-resolution shallow seismic data acquired during a post-August 17, 1999 earthquake to locate and map the active faults. In the area, considering the neotectonic features around the gulf, two different sets of faults were differentiated: an earlier and now mostly inactive set of faults that are responsible for the formation of the large pull-apart depression in which the Gulf of Izmit is locatedanda younger, secondset of active faults in the gulf that cut through the former set. The second set was further divided into two groups: the main fault and the secondary faults. The main fault, which extends roughly in an E^W direction as rather throughgoing and longer segments, was tracedbetween the southeast endof the gulf in the east andsouth of Dar|ca in the west where it joins the Sea of Marmara Fault. The secondary faults, on the other hand, lie along the main fault; they are shorter and their strikes are controlled by the bathymetry. Two main seismic sequences were identified in the gulf according to the seismic profiles and existing borehole data: Holocene marine sediments and pre-Holocene fluvio-lacustrine sediments. > 2002 Elsevier Science B.V. All rights reserved.

Average Slip Rate and Recurrence Interval of Large-Magnitude Earthquakes on the Western Segment of the Strike-Slip Kunlun Fault, Northern Tibet

Interpretations of satellite remote sensing images, field and trench excavation investigations, and radiocarbon dates constrain the Holocene slip rate and average recurrence interval of large earthquakes on the western segment of the strike- slip Kunlun fault related with the 2001 M w 7.8 Central Kunlun earthquake, northern Tibet. Streams and gullies developed on the alluvial fans having an average 14 C age of ∼7000 years are sinistrally offset by up to 115 m along the Kunlun fault. This constrains a slip rate of 16.4 mm/yr for the past ∼7000 years. Trenches and 14 C ages reveal that at least four seismic faulting events occurred in the past 6200 years and that the penultimate event prior to the 2001 M w 7.8 earthquake occurred during the past 400 years with an average left-lateral offset of 4–5 m. Coupling the slip rate of 16.4 mm/yr with the average offset of 4–5 m produced by individual large earthquakes, it is estimated that the average recurrence interval of large earthquakes is 300–400 years on the western segment of the Kunlun fault. Our results confirm that the Kunlun fault plays an important role as a major strike-slip fault in accommodating the horizontal eastward extrusion of Tibet.

Characteristics of the surface rupture associated with the 2004 Mid Niigata Prefecture earthquake, central Japan and their seismotectonic implications

The 2004 Mid Niigata Prefecture earthquake sequence produced a nearly 1-km-long, N-S to NNW-SSE-striking, west-side-up surface rupture along a previously unmapped fault at Obiro, Uonuma City, eastern margin of the epicentral region. The maximum vertical displacement is 20 cm. The topographic and geometric features of the surface rupture are indicative of reverse faulting with dip to the west, which is consistent with focal mechanism solutions and aftershock distribution. Along the major active faults and folds that form the framework of landforms in the epicentral region, systematic surface ruptures were not found. Exceptionally small rupture dimension and offset amount of the surface rupture compared with those of the modeled fault suggest that the fault slip appears to have been accommodated by both internal deformation in thick Neogene-Quaternary deposits and growth of the folds and blind thrusts around the epicentral region.

Coastal deformation associated with the 2007 Noto Hanto earthquake, central Japan, estimated from uplifted and subsided intertidal organisms

The March 25, 2007 Noto Hanto earthquake (Mj = 6.9, Mw = 6.7) generated vertical crustal movement along the northwestern coast of the Noto Peninsula, central Japan. Soon after the event, we estimated the pattern and amount of coseismic coastal movement based on uplifted and subsided intertidal sessile organisms. Our observations reveal a broad 20-km-wide asymmetric zone of surficial deformation above and across the south-dipping source fault, with a steep north-facing frontal limb and a gentle south-facing back limb. The maximum coseismic uplift was approximately 40 cm at the crest of the zone of deformation. The result of forward modeling suggests that the top of the south-dipping source fault is buried at a depth of approximately 2 km, and that 1.2 m of slip on the fault provides the best fit to our surface observations. Our results demonstrate that traditional field investigations should be combined with modern instrumental observations such as GPS and InSAR to obtain the most effective and reliable spatio-temporal estimates of crustal movement associated with large earthquakes.

Does the 2007 Noto Hanto earthquake reveal a weakness in the Japanese national seismic hazard map that could be remedied with geological data

The Noto Hanto earthquake struck one of the lowest earthquake probability regions on the national seismic hazard map of Japan. To contribute to future updates of the hazard map, we examined the predictability of the 2007 earthquake on the basis of geological data that were available before it occurred. Sonic prospecting profiles of active faulting and the absence of an onshore fault could have limited the potential rupture length to 12–15 km, similar to the 2007 source. Empirical relationships between magnitude and fault length would have given us Mj = 6.6–6.8 and Mw = 6.3–6.4. The emergence of one marine terrace, which inclines to the south and reaches an altitude of approximately 50 m, can be dated to 120–130 ka and yields an uplift rate of approximately 0.4 mm/year. Mw-displacement empirical relationships and examples of recent blind fault events that have occurred at various locations around the world suggest that the conceivable maximum coseismic uplift of such shocks is 40–70 cm. Together with the uplift rate, we would have obtained an average recurrence interval of 1000–2000 years and, consequently, a 1.5–3.0% time-independent (Poisson) probability for 30 years. In addition, the significant inclination of the marine terraces—3.2 per mille (0.32%)—is better explained by the accumulation of frequent southward tilting as large as that of the 2007 type event with approximately 1600-year intervals, without any significant contributions from other seismic sources. We therefore conclude that the Noto Hanto earthquake source would have been better evaluated and identified if we had taken into account not only major active faults but also the active tectonics of moderate-size faults and their associated scale and rate.