The Reelfoot fault is an east vergent, reverse fault underlying the Lake County uplift, a low-amplitude, late Holocene anticline bordered on the east by the 32-km-long Reelfoot scarp. Fluvial deposits across the scarp define an 8-m-high, east facing monocline. Most near-surface deformation along the scarp is accommodated via folding rather than faulting. We interpret the scarp as a fault-propagation fold developed over a west dipping reverse fault interpreted from shallow seismic reflection data. Trench exposures provide evidence for three episodes of deformation along the Reelfoot fault within the past approximately 2400 years, between A.D. 780 and 1000, between A.D. 1260 and 1650, and during A.D. 1812. Our best estimate of the average recurrence interval for deformation along the scarp is 400-500 years. Each episode of deformation had a slightly different style. The third most recent event produced a small graben a few tens of centimeters deep in the hanging wall of the reverse fault. The second most recent earthquake produced about 1.3 m of throw in the graben, as well as folding along the updip projection of the reverse fault and development of the scarp. These relations suggest that graben development increased through time concomitant with growth of the monocline or that the events are of different magnitude. The 1811-1812 episode of deformation produced abundant liquefaction, prominent folding of fluvial strata along the scarp, and minor faulting in the graben.

The northern Calaveras fault traverses a heavily populated area in the eastern San Francisco Bay region and has not had a large earthquake in more than 130 years. To obtain data on the number, timing, and recurrence of large paleoearthquakes, we conducted paleoseismologic investigations at Leyden Creek, which crosses the fault in the rugged southern East Bay Hills. The site is characterized by a prominent west facing scarp and five fluvial terraces on the western (upstream) side of the fault. On the eastern (downstream) side of the fault, the creek flows through a narrow bedrock canyon that constricts the modern valley and has constrained the location of a late Pleistocene paleovalley. The margin of a buried bedrock valley west of the fault trends nearly perpendicular to the fault and is offset 54 (+ 18, -14) m in a right-lateral sense from the narrow bedrock canyon. Based on radiocarbon ages for alluvial sediments predating and postdating this paleovalley margin, we estimate an age of 11.5 (+3, -1) ka for the valley margin and a Holocene slip rate of 5 _ 2 mm/yr for the fault at Leyden Creek. Slickensides exposed in multiple trenches across the fault show that the most recent movement was predominantly lateral with a minor component of down-to-the-west slip. Multiple displaced scarp-derived colluvial deposits are interpreted as results of five or six surface ruptures within the past 2500 years. Twenty-one radiocarbon samples from scarpderived colluvium and interfingered alluvial deposits suggest an average interval between surface rupture earthquakes of 250 to 850 years. Introduction The Calaveras fault is a major, northwest striking component of the San Andreas fault system and is a significant potential seismic hazard to the heavily populated eastern San Francisco Bay region. The fault is composed of three sections: the southern, central, and northern Calaveras faults (Figure !). The 48-km-long northern Calaveras fault extends from Calaveras Reservoir to the northern San Ramon Valley, where Holocene slip either dies out or is transferred to the Concord and/or Hayward faults [Simpson et al., 1992]. The boundary between the northern and central Calaveras faults coincides with the complex, poorly understood zone of intersection between the Calaveras and southern Hayward faults as well as with a 7-km-long releasing double bend in the Calaveras fault at Calaveras Reservoir (Figure 1). A prominent trend of microseismicity that diverges to the northwest from near Calaveras Reservoir toward the Hayward fault [Wong and Hemphill-Haley, 1992] probably reflects partial transfer of strain between the central Calaveras and southern Hayward faults [Andrews et al., 1993; Kelson et al., 1993]. Historical and contemporary seismicity data suggest that the northern Calaveras fault is seismologically distinct from the central and southern sections of the fault. South of Calaveras Reservoir, several moderate earthquakes have occurred on the fault since the early 1900s [Oppenheimer et al., 1990; Du and Aydin, 1992], including the 19...

Stratigraphic and structural relations exposed in a 90-m-long trench across the Reelfoot scarp in the central New Madrid Seismic Zone (NMSZ) provide data to assess the style and timing of late Holocene tectonic surficial deformation in the central NMSZ. Near-surface deposits exposed in the trench include natural levee, overbank, colluvial, and liquefaction-related deposits. The levee deposits consist of fine-grained, cross-bedded sands and silty sands and are overlain by clayey overbank and scarp-derived colluvial deposits. Liquefaction-related features include sand dikes and sills that intrude into the levee and overbank deposits, and a possible older extrusive sand deposit. Charcoal and archaeological artifacts from a deposit inset into and overlying the levee deposits suggest that the levee deposits are older than about A.D. 800 to 900. Charcoal from the overbank deposits yielded an age estimate of cal. A.D. 1310 ± 90; charcoal from the overlying colluvial deposits yielded an age estimate of cal. A.D. 1540 ± 90. Distinct marker beds within the levee deposits define a broad monocline that parallels the ground surface and exhibits more than 5 m of down-to-the-east vertical separation. This fold consists, in part, of four smaller-scale flexures each having amplitudes of about 1 m. Associated with these flexures are numerous west-dipping normal faults that have a total net vertical separation of about 0.4 m in a down-to-the-west sense, which is opposite in sense to that exhibited by the scarp and the silty marker beds. We interpret that these faults are related to extension in the crest of the monocline, and that the monocline represents deformation above a west-dipping reverse fault that reaches or approaches the ground surface east of the trench and the base of the scarp. At the trench site, this interpretation places the surface projection of the fault near the western margin of Reelfoot Lake. Stratigraphic relations exposed in the trench and several shallow boreholes permit identification of at least one and possibly three late Holocene earthquakes. Stratigraphic relations and age-estimates best support the interpretation that the unweathered liquefaction related features exposed in the trench are a result of the 1811–12 earthquakes, and that the scarp-derived colluvial deposits are a result of a prior event. Radiocarbon analyses show that this penultimate event occurred between about A.D. 1310 and A.D. 1540. Stratigraphic evidence of a third event prior to about A.D. 900 is present but equivocal. Given that the most-recent event occurred in A.D. 1812, we estimate that the time between the two most-recent earthquakes large enough to produce liquefaction and/or surface deformation was about 200 to 600 years.

The Working Group on California Earthquake Probabilities estimated that the northern Hayward fault had the highest probability (0.28) of producing a M7 Bay Area earthquake in 30 years (WGCEP, 1990). This probability was based, in part, on the assumption that the last large earthquake occurred on this segment in 1836. However, a recent study of historical documents concludes that the 1836 earthquake did not occur on the northern Hayward fault, thereby extending the elapsed time to at least 220 yr ago, the beginning of the written record. The average recurrence interval for a M7 on the northern Hayward is unknown. WGCEP (1990) assumed an interval of 167 years. The 1996 Working Group on Northern California Earthquake Potential estimated ~210 yr, based on extrapolations from southern Hayward paleoseismological studies and a revised estimate of 1868 slip on the southern Hayward fault. To help constrain the timing of paleoearthquakes on the northern Hayward fault for the 1999 Bay Area probability update, we excavated two trenches that cross the fault and a sag pond on the Mira Vista golf course. As the site is on the second fairway, we were limited to less than ten days to document these trenches. Analysis was aided by rapid C-14 dating of more than 90 samples which gave near real-time results with the trenches still open. A combination of upward fault terminations, disrupted strata, and discordant angular relations indicates at least four, and possibly seven or more, surface faulting earthquakes occurred during a 1630-2130 yr interval. Hence, average recurrence time could be <270 yr, but is no more than 710 yr. The most recent earthquake (MRE) occurred after AD 1640. Preliminary analysis of calibrated dates supports the assumption that no large historical (post-1776) earthquakes have ruptured the surface here, but the youngest dates need more corroboration. Analyses of pollen for presence of non-native species help to constrain the time of the MRE. The earthquake recurrence estimates described in this report are preliminary and should not be used as a basis for hazard estimates. Additional trenching is planned for this location to answer questions raised during the initial phase of trenching.

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