Dip-slip Rate Research Articles

Paleoseismology along the 1980 surface rupture of the Irpinia Fault: Implications for earthquake recurrence in the southern Apennines, Italy

The Irpinia fault was the source of the Ms 6.9 1980 Irpinia earthquake and produced the first unequivocal historical surface faulting in Italy. Trenching of the 1980 fault scarp at Piano di Pecore, a flat intermontane basin about 5 km south of the 1980 instrumental epicenter, provides the first data on earthquake recurrence intervals, slip per event, and slip rate on a major normal fault in the Southern Apennines fault zone. The trenches exposed evidence of four pre‐1980 paleoearthquakes that occurred during the past 8600 years. A best estimate average recurrence interval is 2150 years, although the time interval between individual events varies by as much as a factor of 2. Each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation, which suggests that the 1980 event is characteristic for the Irpinia fault. Slip per event values average 61 cm. The net vertical displacement of 2.12–2.36 m since 8600 cal year B.P. observed in the trenches gives a vertical slip rate of 0.25–0.35 mm/yr, a dip slip rate of 0.29–0.40 mm/yr, and an extension rate of 0.14–0.20 mm/yr. Although fault behavior data are only available for the Irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern Apennines. They suggest that (1) fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude (M7) earthquakes and (2) the Holocene rate of extension across the Apennines is ≤1 mm/yr. The 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern Apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation.

Radiocarbon Dating of Paleoseismicity Along an Earthquake Fault in Southern Italy

On 23 November 1980, a major earthquake (Ms = 6.9) struck a large area of the southern Apennines (Campania and Lucania regions, southern Italy). This seismic event, the largest in Italy over the last 80 years, almost completely destroyed 15 villages and caused extensive damage to other towns, including Naples. The quake produced the first well-documented example in Italy of surface dislocation, represented by a fault scarp 38 km long. We undertook a study that included 14C dating of organic materials from layers displaced by paleoseismic events to assess the seismologic hazard for the area. We collected peat and charred wood samples from the walls of two trenches excavated across the 1980 fault at Piano di Pecore di Colliano, Salerno, where the sedimentary suite is faulted and warped by five quakes (including that of 1980). This produced comparable vertical throw and deformation patterns. Chronological data for pre-1980 events, coupled with detailed stratigraphic analysis, yielded a dip-slip rate and a recurrence interval of 0.4 mm/yr and 1700 yr, respectively.

Hope fault, Jordan thrust, and uplift of the Seaward Kaikoura Range, New Zealand

In the northern South Island of New Zealand, displacement at the Pacific-Indian plate boundary is accommodated by the east-north-east-striking, right-lateral strike-slip Marlborough fault system. The southernmost Marlborough fault is the Hope fault; the late Pleistocene-Holocene horizontal slip rate on this fault is 20-25 mm/yr, about half of the rate of Pacific-Australian plate motion. Near the eastern end of the Hope fault, most displacement is transferred to the north-northeast-striking Jordan thrust, but the average dip-slip rate at the surface trace of this thrust is less than 4 mm/yr. We propose that most slip takes place on a blind thrust, expressed at the surface by the fault-propagation folding of the Seaward Kaikoura Range, and that the rate of uplift of this range is as high as that of the Southern Alps, 6 to 10 mm/yr. The major restraining bend of the Alpine fault has the same average slip rate as the Wairau fault, 4-6 mm/yr. Even though the Alpine fault is an east-dipping, reverse-separation fault at the restraining bend, this low slip rate results in uplift of the Spenser Mountains east of the bend at a rate lower than that of the Southern Alps and Seaward Kaikoura Range.

Neotectonics of the San Cayetano fault, Transverse Ranges, California

Study of late Quaternary alluvial deposits cut by the San Cayetano reverse fault indicates that the dip-slip rate of faulting increases from 1.05 ± 0.2 mm/yr at Sisar Creek near the western end of the fault eastward to 1.35 ± 0.4 mm/ yr at Bear Canyon and 2.35 ± 0.55 to 4.15 ± 0.85 mm/yr at Mud Creek. At Timber Canyon near the central part of the fault, a minimum slip rate of 3.6 ± 0.4 mm/yr is suggested by fanhead segmentation; however, a higher slip rate of 8.75 ± 1.95 mm/yr is suggested by stratigraphic evidence. Stratigraphic separation of 7.5 km in less than 1 m.y. across the fault between Fillmore and Piru indicates a slip rate of at least 7.5 mm/yr for the eastern section of the fault. Recent activity is shown by faulted Holocene or latest Pleistocene fan alluvium and colluvium at Bear Canyon and the Silverthread oil field, respectively, and inferred at Timber Canyon by fanhead deposition. Active folding of the footwall is demonstrated by tilted and faulted Holocene and Pleistocene alluvial fans: present tilt rates approach 1°/10 ka yr, and associated bedding plane faulting rates are about 0.5 to 1.5 mm/yr. The hanging wall, however, does not appear to be involved in active deformation, inasmuch as Pleistocene deposits as old as several hundred thousand years are not tilted or affected by bedding-plane faulting. The only exception is in the Silverthread field area, where hanging-wall deformation is directly relatable to fault geometry, not folding of the hanging-wall rocks. The lack of documented moderate- to large-magnitude historical earthquakes on this fault is in strong contrast with its demonstrated late Quaternary activity. Based on its length, slip rate, overall geometry, and segmentation into two parts, a recurrence interval of between 200 and 600 yr for the western segment and about 230 yr for the eastern segment is suggested.

Displacement history of oceanic fracture zones

Oceanic crust on opposite sides of a fracture zone is generally of different ages ( t ) and is therefore at different depths dependent on √t. Subsidence rates are proportional to 1/√t, so that the younger, higher side of a fracture zone is subsiding faster than the older, lower side. Thus, over all of its length, including the aseismic extensions, a fracture zone has a small component of dip-slip motion. Calculated dip-slip rates vary from 2 mm/yr in the vicinity of a spreading ridge with an axial valley to 10−3 mm/yr at a distance from the ridge where the younger side of the fracture zone is 50 m.y. old. A similar qualitative argument suggests that strike-slip motion may also occur because of differential thermal contraction of oceanic crust parallel with the fracture zone. Cumulative effects of this very slow dip-slip and strike-slip motion could contribute to (1) off-ridge seismicity, (2) a recognizable pattern of scarp height versus age that could be useful in identifying ambiguous ocean-floor features as fossil fracture zones, (3) distinctive fabrics on the younger side of the aseismic portions of fracture zones, which have never passed through the transform domain, and (4) development of complex stratigraphic and structural relationships within fracture-zone troughs.