ALBERT

Description: New paleoseismic investigations on the western segment of the Garlock fault at Twin Lakes, California, reveal evidence for up to six surface ruptures in the past ~5600 years. Calibrated radiocarbon dates from accelerator mass spectrometer analysis of detrital charcoal constrain the timing of three well-defined events at Twin Lakes to post-A.D. 1450 (event A), 720–395 B.C. (event G), and 3425–2200 B.C. (event I); two probable events are also constrained to 625–1525 A.D. (event C), and 155 B.C.–A.D. 615 (event E), and a possible additional event to 3425–2200 B.C. and prior to event I. Our findings offer new insights into mid- to late-Holocene behavior of the western and central segments of the Garlock fault, and regional fault interactions. The timing of the most recent event (MRE) on the western segment likely correlates with the MRE at paleoseismic sites on the central segment, suggesting that both segments do sometimes rupture together during large earthquakes. Evidence for events during periods of seismic quiescence on adjacent segments demonstrates that the western and central segments also sometimes rupture independently of one another. The occurrence of event G during a lull in seismic strain release at 2–5 ka on faults in the eastern California shear zone (ECSZ), contrasts with other studies that suggest the Garlock fault ruptures in phase with ECSZ faults. Our data suggest that seismic strain release on the Garlock fault may actually be more in phase with moment release on the Mojave section of the San Andreas fault and the Transverse Ranges faults of the Los Angeles region. Online Material: Two input files written for the computer program OxCal 4.1.7 and two output files from the OxCal analyses. The first input file calibrates and re-weights probability density functions (PDFs) for radiocarbon ages based on stratigraphic ordering, and the second input file calculates PDFs for earthquake ages based on radiocarbon ages and stratigraphy bracketing the events.

Description: On 17 August 1999 the M (sub w) 7.5 Izmit, 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 M (sub w) 7.1 Duzce, Turkey, earthquake produced a 40-km-long surface 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 easternmost segment of the AUgust event, as well as on the western 20 km of the November 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 August surface rupture (i.e., a 5-km-wide zone of no surface rupture, as well as a 25 degrees change in trend from E-W to ENE), and such a discontinuity may have important implications for future 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 12 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.

Description: 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 (sub 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 〈 or =800 years (if there was not a late-fourth- to early-eleventh-century event) or approximately 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.

Description: New paleoseismic investigations on the western segment of the Garlock Fault at Twin Lakes, California, reveal evidence for up to six surface ruptures in the past approximately 5600 years. Calibrated radiocarbon dates from accelerator mass spectrometer analysis of detrital charcoal constrain the timing of three well-defined events at Twin Lakes to post-A.D. 1450 (event A), 720-395 B.C. (event G), and 3425-2200 B.C. (event I); two probable events are also constrained to 625-1525 A.D. (event C), and 155 B.C.-A.D. 615 (event E), and a possible additional event to 3425-2200 B.C. and prior to event I. Our findings offer new insights into mid- to late-Holocene behavior of the western and central segments of the Garlock fault, and regional fault interactions. The timing of the most recent event (MRE) on the western segment likely correlates with the MRE at paleoseismic sites on the central segment, suggesting that both segments do sometimes rupture together during large earthquakes. Evidence for events during periods of seismic quiescence on adjacent segments demonstrates that the western and central segments also sometimes rupture independently of one another. The occurrence of event G during a lull in seismic strain release at 2-5 ka on faults in the eastern California shear zone (ECSZ), contrasts with other studies that suggest the Garlock fault ruptures in phase with ECSZ faults. Our data suggest that seismic strain release on the Garlock fault may actually be more in phase with moment release on the Mojave section of the San Andreas fault and the Transverse Ranges faults of the Los Angeles region.