Hugh Cowan

A comparison of historical and paleoseismicity in a newly formed fault zone and a mature fault zone, North Canterbury, New Zealand

The timing of large Holocene prehistoric earthquakes is determined by dated surface ruptures and landslides at the edge of the Australia-Pacific plate boundary zone in North Canterbury, New Zealand. Collectively, these data indicate two large (M > 7) earthquakes during the last circa 2500 years, within a newly formed zone of hybrid strike-slip and thrust faulting herein described as the Porters Pass-to-Amberley Fault Zone (PPAFZ). Two earlier events during the Holocene are also recognized, but the data prior to 2500 years are presumed to be incomplete. A return period of 1300-2000 years between large earthquakes in the PPAFZ is consistent with a late Holocene slip rate of 3-4 mm/yr if each displacement is in the range 4-8 m. Historical seismicity in the PPAFZ is characterized by frequent small and moderate magnitude earthquakes and a seismicity rate that is identical to a region surrounding the structurally mature Hope fault of the Marlborough Fault System farther north. This is despite an order-of-magnitude difference in slip rate between the respective fault zones and considerable differences in the recurrence rate of large earthquakes. The magnitude-frequency distribution in the Hope fault region is in accord with the characteristic earthquake model, whereas the rate of large earthquakes in the PPAFZ is approximated (but over predicted) by the Gutenberg-Richter model. The comparison of these two fault zones demonstrates the importance of the structural maturity of the fault zone in relation to seismicity rates inferred from recent, historical, and paleoseismic data.

Trench investigation along the Merida section of the Bocono fault (central Venezuelan Andes), Venezuela

Abstract The Bocono fault is a major NE–SW-trending, dextral fault that extends for about 500 km along the backbone of the Venezuelan Andes. Several large historical earthquakes in this region have been attributed to the Bocono fault, and some of these have been recently associated with specific parts through paleoseismologic investigations. A new trench study has been performed, 60 km to the northeast of Merida in the central Venezuelan Andes, where the fault forms a releasing bend, comprising two conspicuous late Holocene fault strands that are about 1 km apart. The southern and northern strands carry about 70% and 30% (respectively) of the 7–10 mm/yr net slip rate measured in this sector, which is based on a 40 vs. 85–100 m right-lateral offset of the Late Pleistocene Los Zerpa moraines. A trench excavated on the northern strand of the fault (near Morros de los Hoyos, slightly northeast of Apartaderos) across a twin shutter ridge and related sag pond exposed two main fault zones cutting Late Pleistocene alluvial and Holocene peat deposits. Each zone forms a shutter ridge with peat deposits ponded against the uplifted block. The paleoearthquake reconstruction derived from this trench allows us to propose the occurrence of at least 6–8 earthquakes in the past 9000 yr, yielding a maximum average recurrence interval of about 1100–1500 yr. Based on the northern strands average slip rate (2.6 mm/yr), such an earthquake sequence should have accommodated about 23 m of slip since 9 ka, suggesting that the maximum slip per event ranges between 3 and 4 m. No direct evidence for the large 1812 earthquake has been found in the trench, although this earthquake may have ruptured this section of the fault. Further paleoseismic studies will investigate the possibility that this event occurred on the Bocono fault, but ruptured mainly its southern strand in this region.

Late Holocene Earthquakes on the Aeropuerto Fault, Managua, Nicaragua

Managua, capital of Nicaragua, is built on the shore of Lake Managua, within a densely faulted graben at a major discontinuity in the Central American volcanic chain. Shallow moderate earthquakes ( M s 6-6.2) ruptured faults with devastating effect at the heart of urban Managua in 1931 and 1972, and damaging earthquakes are cataloged in the earlier history of the surrounding region. The Aeropuerto fault is a major structure in the Managua Graben, but like other faults in this area its behavior is little understood. Paleoseismic investigations now suggest that the most recent large earthquake on this fault occurred sometime during the interval A.D. 1650-1810. An earlier earthquake on this fault occurred prior to A.D. 1390 and possibly around 2000 B.P. On the basis of stratigraphic correlations we estimate the ages of two shorelines associated with former high stands of Lake Managua to be less than 6.4 ka and approximately 2 ka, respectively. Deformation of these abandoned shorelines adjacent to the Aeropuerto fault implies a vertical slip rate of 0.3 to 0.9 mm/yr. Strike-slip movement on this fault is also expected, but no direct measurement could be performed. By comparison with faults of similar geometry in the Managua area that ruptured in 1931 and 1972, we suspect a left-lateral component of horizontal slip that is higher than the vertical one but less than 5 mm/yr. Additional data on slip rate and timing of paleoearthquakes are needed to better assess the Holocene behavior of the Managua faults and to investigate the influence of magmatic processes on the nature of faulting in the Managua Graben. Manuscript received 15 January 2002.

High resolution seismic imaging of faults beneath Limón Bay, northern Panama Canal, Republic of Panama

High-resolution seismic reflection profiles from Limon Bay, Republic of Panama, were acquired as part of a seismic hazard investigation of the northern Panama Canal region. The seismic profiles image gently west and northwest dipping strata of upper Miocene Gatun Formation, unconformably overlain by a thin (<20 m) sequence of Holocene muds. Numerous faults, which have northeast trends where they can be correlated between seismic profiles, break the upper Miocene strata. Some of the faults have normal displacement, but on many faults, the amount and type of displacement cannot be determined. The age of displacement is constrained to be Late Miocene or younger, and regional geologic considerations suggest Pliocene movement. The faults may be part of a more extensive set of north- to northeast-trending faults and fractures in the canal region of central Panama. Low topography and the faults in the canal area may be the result of the modern regional stress field, bending of the Isthmus of Panama, shearing in eastern Panama, or minor deformation of the Panama Block above the Caribbean subduction zone. For seismic hazard analysis of the northern canal area, these faults led us to include a source zone of shallow faults proximal to northern canal facilities. D 2003 Elsevier B.V. All rights reserved.

Folding and the development of small sedimentary basins along the New Zealand plate boundary

The relationships between folding and small (10–60 km long, < 30 km wide and < 2 km deep) sedimentary basins are examined using outcrop and seismic reflection data from the New Zealand plate boundary. Folding has a significant effect on the geometry and formation of these basins, which are thickest above asymmetric synclines with dimensions approximately equal to fault (or fold) length and 40–50% of fold wavelength (fault spacing). Increases in fold-limb dip are associated with a decrease in basin width and basins may become more elongate with increased shortening. The relationships between deformation and sedimentation, which are rarely constant, are also critical and basins form where local fold-related subsidence causes the ground surface to be warped below the base level of erosion. Fluctuations in the levels of sedimentation and erosion markedly increase or decrease basin dimensions. Basins initiate after as much as 6% shortening and are characteristically shallow ephemeral structures with depositional histories of < 1 Ma.