Scarp Height Research Articles

Paleoseismic activity on Wujiahe segment of Serteng piedmont fault, Inner Mongolia

Geomorphic study on Wujiahe segment of Serteng piedmont fault, Inner Mongolia is made. Through analysis of the available data in combination with the results of predecessors’ studies it can be obtained that average vertical displacement rate is 0.48–0.75 mm/a along the Wujiahe segment since the late Pleistocene (14.450–22.340 ka BP) and 0.56–0.88 mm/a since the early-middle Holocene (5.570–8.830 ka BP). Analyzing paleoseismic phenomena revealed in the excavated 5 trenches in combination with the results of predecessors’ studies of paleoearthquakes on the fault, we determine five paleoseismic events on the Wujiahe segment of Serteng piedmont fault since 27.0 ka BP and the recurrence interval to be about 4.300–4.400 ka. A cluster of paleoearthquakes occurred probably during 8.000–9.000 ka BP and two paleoseismic events in 10.000–20.000 ka BP may be missed. A comparison between height of fault scarps and sum of displacement caused by paleoseismic events revealed in trenches, and recurrence interval of paleoseismic events obtained from average displacement rate along the fault and the dislocation by one event suggest that three paleoseismic events are absent in Alagaitu trench. Two paleoseismic events may be absent on the whole active fault segment.

Application of high-resolution, interferometric DEMs to geomorphic studies of fault scarps, Fish Lake Valley, Nevada–California, USA

Recent developments in interferometric radar remote sensing provide a method for deriving detailed topographic and slope information for fault-scarp detection. In October 1996, the airborne TOPSAR instrument [IEEE Trans. Geosci. Remote Sens., 30 (1992) 933] was flown over southwest Nevada and east-central California. Topography calculated from TOPSAR data are in the form of a high-resolution (5-m spatial grid) digital elevation model (DEM). In this study, we focus upon the large, steep fault scarps that cut alluvium and alluvial fans in Fish Lake Valley, east of the White Mountains of Nevada–California. A series of topographic profiles extracted from the DEM reveal that the larger fault scarps are greater than 40 m in height and that the average midsection slope angle for all measured scarps is approximately 23°. These large scarps are the product of multiple offsets rather than a single event. Other relevant geomorphic features present in the digital topography include splays and benches along the main fault, levees, cutbanks, gullies incising fault scarp slopes, shutterridges, offset drainage, and small normal faults with scarp heights of only 4–6 m. Field work corroborated general geomorphologic landforms, confirmed fault-scarp morphometry, and aided the evaluation of the accuracy of the DEM. We are also able to assess fault segmentation models that divide the Fish Lake Valley fault zone (FLVFZ) into discrete segments based upon surface-rupture characteristics.

Faulting patterns near 19°30′S on the East Pacific Rise: Fault formation and growth at a superfast spreading center

Structural mapping from high‐resolution sonar and bathymetric data collected near 19°30′S on the southern East Pacific Rise (EPR) reveals multiple packets of small, closely spaced outward‐dipping faults that form antithetic to larger inward‐dipping faults. The mean spacing and height of inward‐dipping scarps are ∼1.6 and ∼1.9 times greater than those of outward‐dipping faults, and the median spacing and height of inward‐dipping scarps are ∼2.0 and ∼1.7 times greater, respectively. Accordingly, inward‐dipping faults are less abundant (35–45%) but accommodate more (50–55%) of the roughly ∼4.0% strain. Although previous studies have shown that EPR fault populations exhibit exponential size‐frequency distributions, one‐dimensional throw and spacing data from the 19°30′S region are not described well by this model. The first major (>40 m scarp height) fault is observed within 1–4 km of the ridge crest and invariably dips inward. Further off‐axis, a transition from greater inward‐dipping to greater outward‐dipping fault abundance constrains the minimum half width of the active plate boundary zone to be ∼5–15 km. Inspection of additional data sets from 16° and 18°30′S indicates a pattern of large inward‐dipping faults with associated hanging wall deformation similar to that observed in the 19°30′S region. This pattern is distinct from the horst and graben morphology observed along the fast spreading northern EPR. The pervasive hanging wall deformation observed on the flanks of the southern EPR suggests master fault geometries that are more sharply curved or kinked at depth, relative to their abyssal hill equivalents on the northern EPR. Boundary element modeling indicates the pattern and geometry of faulting observed along the southern EPR is consistent with fault nucleation above a shallow, sill‐like magma chamber with internal overpressure. Since the influence of the sill on fault growth is strongly dependent on the depth of the sill, the different patterns of faulting observed along the northern and southern EPR may reflect the small difference in axial magma chamber depth between these two regions, with a slightly shallower chamber beneath the southern EPR.

Neotectonics and catastrophic failure of mountain fronts in the southern intra-Andean Puna Plateau, Argentina

New cosmogenic nuclide exposure ages of rock-avalanche deposits in front of Sierra Laguna Blanca in the arid Argentine Puna Plateau demonstrate the effects of westward propagation of reverse faulting and the influence of active faulting on slope oversteepening and subsequent gravitational collapse. The 21 Ne exposure ages of 8 superposed landslides with volumes >0.3 km 3 are between ca. 150 and 430 ka, and indicate an average recurrence interval of large gravitational mountain-front failures of ∼27.5 k.y. for the ultimate 7 landslides. Landsliding did not occur after 150 ka along this sector of the Sierra Laguna Blanca. The termination of landsliding activity correlates with reverse-fault propagation into the piedmont area of the Sierra Laguna Blanca. The exposure age of ca. 130 ka of a tectonically uplifted terrace postdates initiation of fault activity in the piedmont region, and the fault scarp height of 120 m indicates important continued uplift in the piedmont; no further offsets occurred at the mountain front. Due to this lack of tectonic activity at the mountain front, no further collapse has occurred, indicating that in this environment catastrophic mountain-front failure is mainly controlled by tectonically driven slope oversteepening.

New constraints on the width of the zone of active faulting on the East Pacific Rise 8° 30′ N – 10° 00′ N from Sea Beam Bathymetry and SeaMARC II Side-scan Sonar

Sea Beam bathymetry and SeaMARC II side-scan sonar data are used to constrain the width of the zone of active faulting (plate boundary zone) to be ∼90 km (∼0.8 Ma) wide along the East Pacific Rise 8° 30′ N – 10° 00′ N. Fault scarps, identified on the basis of contoured, shaded relief and slope intensity maps of bathymetry, are measured. These scarp measurements, used in conjunction with data from a separate near-axis study, show that both inward- and outward-facing fault scarps increase in height away from the ridge axis, reaching average heights of ∼100 m at 0.8±0.2 Ma, 45±10 km from the ridge axis. Beyond this distance, there is no significant increase in scarp height. Earlier studies had suggested that the width of the zone of active faulting for outward-dipping faults might be significantly narrower than for inward-dipping faults. A lower crustal decoupling zone between brittle crust and strong upper mantle is predicted to exist out to ∼20–200 km from the ridge based on previously published lithospheric models. Such a decoupling zone may explain why outward-dipping faults continue to be active as far off-axis as inward-dipping faults. If the width of the zone of active faulting is controlled by the width of a lower crustal decoupling zone, our observations predict an ∼90 km wide decoupling zone in the lower oceanic crust at this location.

Active oblique extension in the central Apennines (Italy): evidence from the Fucino region

The Fucino Basin is a flat Quaternary depression within the central Apennines. It is surrounded by active normal faults with small oblique-slip components. Surface faulting was observed along the east side of the basin during the Ms = 7.0 Avezzano earthquake of 1915. In order to understand the kinematics of recent strain better and to assess the seismic hazard of this region, we carried out a detailed geomorphic and structural study of the faults around the basin. Fault scarp heights were accurately measured with total station profiles, and vertical slip rates were estimated assuming slope offsets to post-date the end of periglacial abrasion (14 ± 4 kyr BP). To the north, the most prominent fault, the Magnola fault, appears to have an average postglacial throw rate of 0.7 ± 0.3 mm yr−1 and, together with the Velino fault, to be capable of generating earthquakes of maximum magnitudes of 6.9–7.3 with recurrence intervals of 1000–3000 yr. East of the basin, the Serrone, Parasano and Ventrino faults form a right-stepping horsetail of the Giovenco fault. Current vertical slip rates on the three former faults appear to be between 0.5 and 1.4, 0.5 and 1.0, and 0.3 and 1.1 mm yr−1 , respectively. Infrequent maximum-magnitude earthquakes on them may also exceed 7, particularly in the case of coupled rupture, with loosely constrained recurrence intervals (up to several thousand years). The right-lateral slip components implied by the most recent slickensides and by geomorphic offsets on the NW–SE-trending normal faults of the area suggest that the blocks they bound rotate counterclockwise, consistent with oblique left slip on the NNW–SSE-trending Giovenco and Ovindoli faults to the east. Sinistral shear parallel to the latter faults, the maximum relief across the Magnola fault, and the postglacial slope offsets measured suggest that the extension rate across the central Apennines might be of the order of 6 ± 3 mm yr−1 in a N20° ± 10°E direction, more northerly than hitherto inferred.

Late Quaternary slip rate and seismic hazards of the West Klamath Lake fault zone near Crater Lake, Oregon Cascades

Crater Lake caldera is at the north end of the Klamath graben, where this N10°W-trending major Basin and Range structure impinges upon the north-south–trending High Cascades volcanic arc. East-facing normal faults, typically 10–15 km long, form the West Klamath Lake fault zone, which bounds the graben on its west side. The fault zone terminates on the south near the epicentral area of the September 1993 Klamath Falls earthquakes. It continues north past Crater Lake as the Annie Spring fault, which is within ∼1 km of the west caldera rim, and Red Cone Spring fault. We have determined a long-term vertical slip rate of 0.3 mm/yr for these two faults using high-precision K-Ar and 40 Ar/ 39 Ar age measurements on offset lava flows ranging in age from ca. 35 to 300 ka. Holocene offset reported by Hawkins et al. and epicenters of eight M W 2 earthquakes in 1994 and 1995 indicate that the West Klamath Lake fault zone is active. Empirical relations between earthquake magnitudes and scarp heights or fault lengths suggest that the fault zone is capable of producing earthquakes as large as M W 7¼. Earthquakes on these or other faults of the zone could trigger landslides and rockfalls from the walls of the caldera, possibly resulting in large waves on Crater Lake.

Whitemans Valley Fault: A newly discovered active second order fault near Wellington, New Zealand—implications for regional seismic hazard

A NNE‐SSW‐trending linear scarp, trenched in Whitemans Valley near Upper Hutt, proved to be a fault scarp upthrown to the west. The trench, in the steepest part of the 8–9 m high fault scarp, revealed two reverse fault planes dipping at c. 45° to the west. Sediments displaced by the faults include weathered fan gravel, three loess/paleosol couplets including the Porewan, Ratan and Ohakean loesses, Kawakawa Tephra (22.6 ka), and topsoil. The fan gravel represents the youngest alluvial sedimentation at the site and, at c. 80 000 yr (ka), it forms the oldest displaced unit seen in the trench. With the exception of the topsoil, each unit in the trench has been displaced vertically 1.4–2.1 m (equivalent to true dip‐slip displacement of c. 3 m), which we interpret as a single‐rupture displacement. A possible dextral displacement component is suggested by greater displacement of younger units in the trench than older units and possible dextral displacement of a stream channel. The rupture cuts and deforms a soil B horizon developed in the Ohakean loess, which is considered to be c. 10 ka or younger, but the topsoil is undeformed. Displacement on the fault strands in the trench is insufficient to account for the total vertical displacement represented by the 8–9 m scarp height, indicating the existence of a further fault strand(s) immediately to the west of the trench. The vertical displacement of c. 2 m observed in the trench is our best estimate of displacement for the most recent surface rupture earthquake, and total vertical displacement on the scarp may represent four or five events. Ifso, rupture recurrence interval is estimated at 15–20 000 yr. The c. 3 m, single event, dip‐slip displacement is probably a minimum net single‐event displacement. On the basis of the minimum net slip of c. 3 m per event, and the mappable length of c. 20 km, surface rupture of the Whitemans Valley Fault is thought to result in M 7.0 ± 0.3 earthquakes. The Whitemans Valley Fault is just one of conceivably 10 similar active “second order” faults in the Wellington region, suggesting that collectively these faults may make a noticeable contribution to the region's seismic hazard.

Interseismic Hanging Wall Uplift on Teton Normal Fault, Grand Teton National Park Wyoming, 1988-1997, Measured by Precise Leveling

The 55 km-long Teton normal fault at the eastern base of the Teton Range, Wyoming, has one the highest rates of Holocene slip of any fault in the Basin-Range, but it is seismically dormant at the M2 + level and presently lies in the center of a 50 km-long seismic gap (Byrd et al, 1993). Analyses of trenching, fault scarp heights, and fault proftles indicate earthquakes on the Teton fault are non­Poissonian, with from 5 to 10 M >7 earthquakes occurring from 7,900 to 14,000 years ago, but only two such events between 5,000 and 7,900 years ago, and none in the last 5,000 years (Byrd et al., 1994).

Direct geological evidence for prior earthquakes on the 1981 Corinth Fault (central Greece)

We present the preliminary results of geomorphic and trench investigations at the eastern end of a scarp produced by the 1981 Corinth earthquake sequence. At this site, the 1981 rupture produced average vertical displacements of 0.6–0.7 m (maximum 1.3 m) that in part occurred along a cumulative scarp which displaces the fan surface by up to 5 m. Based on this cumulative scarp displacing the Holocene fan surface, we estimate a vertical slip rate of ∼1 mm/yr that translates to 1 mm/yr N‐S extension. We found evidence for two individual pre‐1981 earthquakes which occurred, on the basis of preliminary radiocarbon dating on two paleosols, shortly after A.D. 590 and A.D. 1295. Each of these events produced a vertical displacement ranging between 0.5 and 1.2 m, hence comparable to the 1981 scarp height. This suggests that coseismic slip may have been characteristic at this location over the past three seismic cycles. Following this hypothesis and assuming a periodic strain release, a maximum recurrence interval of ∼700 yr is calculated. These results indicate that the 1981 surface‐breaking faults accommodate only part of the regional extension deduced from the geodetic measurements.

Sea Beam, SeaMARC II and ALVIN-based studies of faulting on the East Pacific Rise 9�20? N?9�50? N

A study of Sea Beam bathymetry and SeaMARC II side-scan sonar allows us to make quantitative measures of the contribution of faulting to the creation of abyssal hill topography on the East Pacific Rise (EPR) 9°15′ N–9°50′ N. We conclude that fault locations and throws can be confidently determined with just Sea Beam and SeaMARC II based on a number of in situ observations made from the ALVIN submersible. A compilation of 1026 fault scarp locations and scarp height measurements shows systematic variations both parallel and perpendicular to the ridge axis. Outward-facing fault scarps (facing away from the ridge axis), begin to develop within ∼2 km of the ridge and reach their final average height of ∼60 m at 5–7 km. Beyond these distances, outward-dipping faults appear to be locked, although there is some indication of continued lengthening of outward-facing fault scarps out to the edge of the survey area. Inward-facing fault scarps (facing toward the ridge axis), initiate ∼2 km off axis and increase in height and length out to the edge of our data at 30 km, where the average height of inward fault scarps is 60–70 m and the length is ∼30 km. Continued slip on inward faults at a greater distance off axis is probable, but based on fault lengths, ∼80% of the lengthening of inward fault scarps occurs within 30 km of the axis (>95% for outward faults). Along-strike propagation and linkage of these faults are common. Outward-dipping faults accommodate more apparent horizontal strain than inward ones within 10 km of the ridge. The net horizontal extension due to faulting at greater distances is estimated as 4.2–4.3%, and inward and outward faults contribute comparably. Both inward- and outward-facing fault scarps increase in height from north to south in our study area in the direction of decreasing inferred magma supply. Average fault spacing is ∼2 km for both inward-dipping and outward-dipping faults. The azimuths of fault scarps document the direction of ridge spreading, but they are sensitive to local changes in least compressive stress direction near discontinuities. Both the ridge trend and fault scarp azimuths show a clockwise change in trend of ∼3–5° from 9°50′ N to 9°15′ N approaching the 9° N overlapping spreading center.

Paleoseismology along the range‐front fault of Helan Mountains, north central China

The NNE trending range‐front fault of the Helan Mountains along the western margin of Yinchuan graben, northcentral China, is a 13‐km‐long high‐angle normal and right‐lateral strike‐slip fault. It cuts late Pleistocene to Holocene alluvial fans, forming a SE facing scarp, and offsets the Great Wall of the Ming Dynasty (about 400 years B.P.) right laterally for 1.45 m and vertically for 0.95 m. The offset of the Great Wall is probably a result of the M = 8 Yinchuan‐Pingluo earthquake of 1739. The 1739 earthquake formed an 88‐km‐long discontinuous surface rupture zone along the fault scarps. Both the length and height of the fault scarps are largest at Hongguozi in the north and Suyukou in the south. We have prepared large‐scale topographic and geologic maps along the Hongguozi and Suyukou scarps, surveyed 88 profiles across the scarps, and excavated 14 trenches across the fault, in order to study Holocene paleoseismicity and recurrence intervals of large earthquakes along the fault. Four levels of terraces were developed on the upthrown block of the fault along some gullies that dissected the alluvial fans. The older terraces, such as T4 and T3, are tilted to the west, whereas the younger terraces are less tilted. This indicates that the terraces may have been tilted and rotated by multiple faulting events. The height difference between the two adjacent terraces is about 1–2.7 m. The elevation differences between the lower original surface and the T3, T2, and T1 terraces were measured in 64 topographic profiles across the scarps. The results show that the elevation of T3 is highest, averaging 8.4 ± 1.9 m and 5.5 ± 1.1 m along the Suyukou and Hongguozi fault scarps, respectively. The elevations of T2 at the two sites are 5.8 ± 1.4 m and 3.4 ± 0.4 m, respectively, whereas elevations of T1 are 3.1 ± 1.2 m and 1.2 ± 0.3 m, respectively. Ages of terrace risers between T3 and T2, as well as between T2 and T1, are 3600–4600 years and 2000–2800 years, respectively, as calculated using a diffusion model of scarp development. The morphology of the fault scarps is complicated. Two or three bevels were developed on some fault scarps that were active in an earlier period, and the upper and the lower slope sections correspond to T3 and T2 alluvial terraces, respectively. A free face exists only on T1 terrace. The slope angles of the upper and lower slope sections of the scarp at Suyukou are 6–22° and 17–38°, respectively. The difference between the two in the same profiles is consistently in the range of 9–21°. In the place where the fault scarp is high, the free face, with a slope angle up to 60°, may be a result of the M = 8 earthquake of 1739. We infer that the other two bevels represent two paleoearthquake events. Eight and six trenches have been excavated across the fault scarps at Suyukou and Hongguozi, respectively. All the trenches reveal normal faults, multiple offsets, and colluvial and filled wedges along the scarps. In these trenches, we found colluvial wedges formed in four different periods indicating multiple faulting events. On the basis of this evidence, four Holocene seismic events have been identified. The timing of the events was determined through 14C dating samples collected from the trenches. On the basis of terrace offset, scarp morphology, and paleoseismic trenching, we conclude that the four large earthquakes along the fault occurred 8400, 4600–6300 (or 5700), 2600, and 256 years B.P. The latest event was the M = 8 Yinchuan‐Pingluo earthquake of 1739. The recurrence interval of these earthquakes is 2300–3000 years. We infer that the magnitudes of these events were probably ∼8, because the thickness of each colluvial wedge and the height of terrace risers produced by these events are similar to those produced by the 1739 earthquake.

Geometric relations of dip slip to a faulted ground surface: new nomograms for estimating components of fault displacement

The throw, heave, and dip-slip components of displacement on a fault scarp can be estimated from two easily collected field measurements when fault dip is known or assumed: the vertical separation and slope angle of the original ground surface displaced across the fault. Nomograms allow graphical determination of slip components for cases where fault dip is in the same or in an opposite direction as the slope of the ground surface. The technique has several advantages over a previous method that requires three field measurements; those of scarp height, scarp slope angle, and the slope of the original ground surface. The geometric relations of a fault scarp illustrate that although differences between throw and vertical separation of the ground surface are small in the cases of steep fault dips or flat ground surfaces, these differences can be greater than commonly assumed.

Holocene paleoseismicity of the Tunka fault, Baikal rift, Russia

The Tunka fault is a major normal‐oblique transform fault within the NNE trending Baikal rift that displays geomorphic evidence of recurrent Quaternary movement. A flight of six fanhead terraces of the Kyngarga River is displaced by several parallel faults and has scarps up to 32 m high at Arshan. The main fault zone is exposed in an abandoned gravel quarry about 1 km east of the Kyngarga River. Thirteen radiocarbon dates from the quarry, a shallow trench in a graben, and natural streamcuts constrain the timing of the latest two or three Holocene paleoearthquakes. The latest earthquake is bracketed by the 1024–1315 calendar years (cal. year) B.P. age of an unfaulted terrace and by the 1947–2179 cal. year B.P. age of a soil buried by scarp colluvium in the graben trench. The penultimate earthquake is also relatively well constrained, assuming that the displacement event slightly younger than 7091–7867 cal. year B.P. in the upper quarry exposure is the same as the fissuring event slightly older than 6733–7385 cal. year B.P. in the lower quarry exposure. Evidence for earlier event(s) between 9.2–12.7 ka depends on ambiguous stratigraphic evidence in the lower quarry exposure. On the basis of only the two well‐dated earthquakes, the recurrence interval at Arshan may range from 2.9 to 6.8 ka for earthquake displacements of at least 1.3 m or a slip rate of 0.19–0.44 mm/yr. Intermediate‐term (100 ka) and long‐term (circa 500 ka) slip rates computed from terrace age estimates and fault scarp heights are 0.08–0.16 mm/yr and 0.07–0.11 mm/yr, respectively, rates that are considerably lower than the late Holocene rate and the approximately 0.5 mm/yr that might be inferred from the tectonic geomorphology of the Tunka Range front.

Ridge offsets, normal faulting, and gravity anomalies of slow spreading ridges

We develop a model relating mid‐ocean ridge normal faulting and crustal structure by examining recently available high‐resolution gravity and multibeam bathymetric data of the Mid‐Atlantic Ridge and other spreading centers. Results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major Atlantic offsets studied, especially along the inside corner side of the offsets. Individual residual gravity anomaly spikes (local peaks), which have amplitudes of up to 20 mGal and typical across‐axis spacing of 10–30 km, often coincide with individual major fault scarps, suggesting crust thinned by normal faulting. Theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive, ridge‐parallel low‐angle faults that originate episodically at inside corners of ridge offset intersections. Fault scarp heights and the amounts of crustal thinning (as inferred from gravity anomalies) are consistently larger at inside corners than at outside corners, supporting a model in which tectonic extension near ridge offsets is asymmetric with low‐angle faults occurring preferentially at inside corners. These results on spatial variations in seafloor morphology and gravity anomalies further support a three‐dimensional tectonic faulting model at oceanic spreading centers with three major characteristics: (1) Low‐angle faults form preferentially at inside corners, where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets; (2) low‐angle faults decrease in throw toward midpoints of long ridge segments, where large low‐angle faults may not be sustained by a weak lithosphere; and (3) the residual gravity peaks are statistically larger at transform than near nontransform offsets, indicating that the length of a ridge offset and its tectonic style control the development of low‐angle faults.

Topography of Closed Depressions, Scarps, and Grabens in the North Tharsis Region of Mars: Implications for Shallow Crustal Discontinuities and Graben Formation

Using Viking Orbiter images, detailed photoclinometric profiles were obtained across 10 irregular depressions, 32 fretted fractures, 49 troughs and pits, 124 solitary scarps, and 370 simple grabens in the north Tharsis region of Mars. These data allow inferences to be made on the shallow crustal structure of this region. The frequency modes of measured scarp heights correspond with previous general thickness estimates of the heavily cratered and ridged plains units. The depths of the flat-floored irregular depressions (55-175 m), fretted fractures (85-890 m), and troughs and pits (60-1620 m) are also similar to scarp heights (thicknesses) of the geologic units in which these depressions occur, which suggests that the depths of these flat-floored features were controlled by erosional base levels created by lithologic contacts. Although the features have a similar age, both their depths and their observed local structural control increase in the order listed above, which suggests that the more advanced stages of associated fracturing facilitated the development of these depressions by increasing permeability. If a ground-ice zone is a factor in development of these features, as has been suggested, our observation that the depths of these features decrease with increasing latitude suggests that either the thickness of the ground-ice zone does not increase poleward or the depths of the depressions were controlled by the top of the ground-ice zone whose depth may decrease with latitude. Deeper discontinuities are inferred from fault-intersection depths of 370 simple grabens (assuming 60° dipping faults that initiate at a mechanical discontinuity) in Tempe Terra and Alba Patera and from the depths of the large, flat-floored troughs in Tempe Terra. The frequency distributions of these fault-intersection and large trough depths show a concentration at 1.0-1.6 km depth, similar to data obtained for Syria, Sinai, and Lunae Plana. The consistency of these depth data over such a large region of western Mars suggests that a discontinuity or a process that transcends local and regional geology is responsible for the formation of these features. If this discontinuity is represented by the base of the cryosphere, its uniform depth over 55° of latitude suggests that the cryosphere did not thicken poleward. Alternatively, the concentration of depths at 1.0-1.6 km may represent the upper level of noneruptive dike ascent (lateral dike propagation) of Mars, which is controlled by gravity and atmospheric pressure and magma and country-rock characteristics, and was probably controlled, in part, by ground ice. Fault-intersection depths in the north Tharsis region locally extend down to a depth of 5-7 km. The depth data between 2 and 3 km are attributed to the discontinuity at the interface of megaregolith and basement or to the upper limit of noneruptive dike ascent of magma with a high volatile content. Intersection depths greater than 3 km, which were found at Alba Patera, may be due to the megaregolith-basement discontinuity, which was buried and depressed by volcanic loading, or to the upper level of noneruptive dike ascent of magma with a low volatile content. The near absence of narrow simple grabens with fault-initiation depths less than 0.6-1.0 km in this study area, as well as in most of western Mars, suggests that this depth represents the minimum depth that normal faults can initiate; at shallower depths tension cracks or joints would form instead. This hypothesis is supported by the application of the Griffith failure criterion to this minimum depth of normal fault initiation, which suggests that shallow crustal materials have a tensile strength of 2-4 MPa throughout most of western Mars, in close agreement with previous estimates of tensile strength of martian basaltic rock.

Considerations on the significance of some post-glacial fault scarps in the Abruzzo Apennines (Central Italy)

Abstract In this paper the tectonic and palaeoseismological significance of the fault scarps present in the massifs of the Velino and of Mount Greco, and in the plains adjacent to these massifs, forming part of the Abruzzo Apennines, is discussed. The basic features of post-glacial fault scarps already mentioned in the literature are illustrated (found in the Fucino Plain, in the Ovindoli-Piani di Pezza area, on the Mount Magnola, in the Valle Majelama alluvial fan and in the Piano di Aremogna). Others not previously mentioned, surveyed by the author, in the Mount Ocre-Cefalone area, in the Piana di Corvaro and on the Mount Greco-Toppe del Tesoro, are also discussed. On the basis of the characteristics of these scarps and of the chronological framework of the last phases of activity of the faults which gave rise to the scarps described, the reliability of the assumptions made for palaeoseismological studies in other places is discussed for the Abruzzo area. The studies performed in the area of the Velino and Greco massifs suggest that, to establish the possible co-seismical activity of the faults, the use of morphological indices (and also of the results obtained with the excavation of trenches) has to be very cautiously applied. It is considered that the following can be stated: It is probable that the majority of the mentioned fault scarps are co-seismic. In view of the height of some fault scarps, it cannot be excluded that they were formed during the course of earthquake sequences lasting not more that 2–3000 years. It is possible that some of the above mentioned scarps were produced during seismic events of great magnitude. It does not seem possible at present to calculate a precise slip-rate: the amount of the offset often changes along the same fault, even over short distances. Moreover, there are many horst and graben type structures, produced by local effects which contribute towards making an assessment of the fault offsets complicated. It is also clear that erosion can change the aspect of the scarps: an evaluation of the displacements caused by the faults producing them must therefore be made extremely cautiously. Caution is needed in assessing the magnitude of past earthquakes and in assuming recurrence intervals. The dating of the phases of fault activity is approximate, and there is great difficulty in recognizing with any certainty the duration, the sort of activity and the length of the fault segment concerned in the activity. Moreover, the existence in the past of aseismic creep and of movements linked to post-glacial isostatic rebound cannot be excluded with any certainty. These considerations must be regarded for the two massifs examined, since there have been the epicentres of rare earthquakes of low intensity. The only large earthquake known in history, which caused surface ruptures, occurred in the Fucino area in 1915. The fault scarps reported in this paper seem, however, interpretable, in general, as indications of post-glacial tectonic activity, the intensity of which was unsuspected, until a few years ago.

Palaeoseismicity in the Gran Sasso Massif (Abruzzo, Central Italy)

Abstract The upper Maone and Venacquaro valleys, on the northern side of the Gran Sasso Massif (Apennines, Central Italy), have been studied in detail from the geological and geomorphological standpoints to date the activity of the faults that produced spectacular fault scarps in the Late Glacial and Holocene sediments and morphological features. The faults that produced the studied scarps have been active at least four times during the last 18,000 years, and each time, on the basis of the indications emerging from the study of the scarp heights, displacements of similar size were probably produced. The first faulting seems datable to a period slightly more recent than 18,000 years ago. The second faulting occurred after the moraines of the second Apennine Stage. These, preceding a lacustrine deposit dated 11,760 ± 160 BP, could be put in the older portion of the period between 16,000 and 13,000 years ago. The third faulting must be more recent than 6000–7000 years ago, and slightly older than 5450 ± 170 and 6110 ± 180 BP. A fourth phase is appreciably later than 3490 ± 160 BP and older than the year 1000 A.D. For the Venacquaro valley, the size of the displacements (more recent than 18,000 years ago), indicated by the height of the fault scarps and by geophysical data, is about 16–18 m; for the Val Maone the size of the displacements (more recent than the moraines of the second Apennine Stage) is 10–11 m. The respective slip-rates are 0.88–1 mm/year for the Venacquaro valley and approximately 0.67–0.78 mm/year for the Maone valley. It is very probable that the fault activity occurred during a seismic event or events that took place during a short time. The intervals between the seismic events or seismic periods (although these cannot be defined with precision) appear longer than 2500–3000 years and shorter than 6000–7000 years. It is possible that also in the Gran Sasso, as in other parts of the Abruzzo Apennines, seismic activity is characterized by events of great magnitude with long return periods.

Quantitative fault studies on the East Pacific Rise: A comparison of sonar imaging techniques

A synthesis is presented of published statistics on the lengths, scarp heights, and spacing of normal faults on the flanks of the East Pacific Rise (EPR) between 19°S and 21°N. Nine data sets are included which were collected at different times employing one of four different sonar mapping systems (the Scripps deep‐tow instrument, SeaMARC I, SeaMARC II, and GLORIA) with widely ranging data collection capabilities. The EPR in this region is morphologically quite similar, and in some cases the data sets were obtained from the same area of the seafloor. It is assumed, therefore, that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes. We try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made. We discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters. We also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution. The theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution. We also discuss the effects that surveying geometry can have on the interpretation of fault lengths. The data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3.5°S and 12°N on the EPR. The correlation, which we find to be linear, is used to estimate (1) the minimum operational fault target size for each of the four mapping systems, and (2) the topography due to faulting in an area where only fault lengths are known.

Past earthquake timing and magnitude along the Inglewood fault, Taranaki, New Zealand

Several active normal faults in the onshore and offshore regions of Taranaki are capable of generating large earthquakes and associated strong ground shaking. Historical earthquakes are concentrated offshore of Cape Egmont, and no significant earthquakes have been detected along the major onshore surface faults. The northeaststriking Inglewood fault is a major onshore, southward-dipping normal fault. It has a known length of c. 20 km and an average scarp height of c. 3 m on landforms less than about 15,000 yrs old. Three subsurface excavations at two sites along the Inglewood fault about 15 km from New Plymouth have revealed three surface fault displacements during the last c. 13,000 years. Earthquakes resulting in about 1.2 m of surface displacement occurred at c. 3,500 radiocarbon yrs BP; between 4,000 and 9,000 radiocarbon yrs BP; and between 10,000 and 13,000 radiocarbon yrs BP, judged by the amount of vertical offset of dated volcanic ash layers. Based on average single-event fault slip values of 1.2-3.0 m and a fault length of 20-30 km, the estimated earthquake magnitudes associated with these past movements range from Mw 6.7 to 7.2.