Average Recurrence Interval Research Articles

A modelling approach for estimating the frequency of sea level extremes and the impact of climate change in southeast Australia

An efficient approach for evaluating storm tide return levels along the southeastern coastline of Australia under present and future climate conditions is described. Storm surge height probabilities for the present climate are estimated using hydrodynamic model simulations of surges identified in recent tide gauge records. Tides are then accounted for using a joint probability method. Storm tide height return levels obtained in this way are similar to those obtained from the direct analysis of tide gauge records. The impact of climate change on extreme sea levels is explored by adding a variety of estimates of mean sea level rise and by forcing the model with modified wind data. It is shown that climate change has the potential to reduce average recurrence intervals of present climate 1 in 100 year storm tide levels along much of the northern Bass Strait coast to between 1 and 2 years by the year 2070.

Contributions of dust to phosphorus cycling in tropical forests of the Luquillo Mountains, Puerto Rico

The input of phosphorus (P) through mineral aerosol dust deposition may be an important component of nutrient dynamics in tropical forest ecosystems. A new dust deposition calculation is used to construct a broad analysis of the importance of dust-derived P to the P budget of a montane wet tropical forest in the Luquillo Mountains of Puerto Rico. The dust deposition calculation used here takes advantage of an internal geochemical signal (Sr isotope mass balance) to provide a spatially integrated longer-term average dust deposition flux. Dust inputs of P (0.23 ± 0.08 kg ha−1 year−1) are compared with watershed-average inputs of P to the soil through the conversion of underlying saprolite into soil (between 0.07 and 0.19 kg ha−1 year−1), and with watershed-average losses of soil P through leaching (between 0.02 and 0.14 kg ha−1 year−1) and erosion (between 0.04 and 1.38 kg ha−1 year−1). The similar magnitude of dust-derived P inputs to that of other fluxes indicates that dust is an important component of the soil and biomass P budget in this ecosystem. Dust-derived inputs of P alone are capable of completely replacing the total soil and biomass P pool on a timescale of between 2.8 ka and 7.0 ka, less than both the average soil residence time (~15 ka) and the average landslide recurrence interval (~10 ka).

Application of Floodplain Stratigraphy to Determine the Recurrence of Ice-Jam Flooding Along the Lower Peace and Athabasca Rivers, Alberta

Multi-proxy reconstructions of ice-jam flooding have increased significantly in the last several decades, providing important estimates of changes in ice-jam activity over time. Here, the floodplain sedimentary archive is investigated as a stratigraphic proxy indicator of ice-jam flooding along the lower Peace and Athabasca Rivers, Alberta. This relatively new method involves stratigraphic logging and interpretation of floodplain sediments resulting from successive ice jam floods and radiocarbon dating of inter-flood organic material for chronology (Livingston et al., 2009). At six sites the method is used to extend the record of ice-jam flooding by up to 1500 years, providing the longest temporal framework for assessing the recurrence of ice-jam flooding in the Peace-Athabasca Delta (PAD) to date. Chronostratigraphic logs indicate that the average long-term recurrence interval of ice-jam flooding varies between sites, from approximately once in 13 years to once in 23 years (4–8% probability of bankfull exceedence in any given year). This is near the range interpreted from oxbow lake sediment in the PAD spanning the last 180-300 years (5–14%), but much lower than the probability of recurrence of ice-jam floods (AD 1826–1995) inferred from historic archives (16%). Although spatio-temporal incongruencies amongst proxy records make it difficult to resolve differences among flood recurrence estimates for the PAD, they collectively indicate a range of 4–16%. The temporal resolution of ice jam-flood chronologies derived from floodplain sediment depends largely on the availability, reliability and sampling frequency of radiocarbon-dated organic material and its calibration to calendar years. Changes in the frequency of ice jams at a particular site can be determined when organic material suitable for radiocarbon dating is present at multiple levels in the floodplain sedimentary archive. Alternative absolute dating methods should be used to constrain ice-jam flood events in the last 350 years or when organic material suitable for radiocarbon dating is limited. Even with limited chronometric control the average long-term flood recurrence can be determined, providing a first approximation of ice-jam flood activity at a particular site. Thus, in the absence of other proxy archives, and given the simplicity of the method, we suggest that floodplain stratigraphy provides an invaluable archive of ice-jam flooding along northern rivers.

Prehistoric seismicity-induced liquefaction along the western segment of the strike-slip Kunlun fault, northern Tibet

Abstract The 2001 M w 7.8 Kunlun earthquake occurred in northern Tibet, and produced a 450-km-long surface rupture zone along the western segment of the strike-slip Kunlun fault. There are, however, no historic or instrumental records of large earthquakes in this fault segment. Field investigations of liquefaction structures and radiocarbon dating results reveal that at least three large earthquakes, including the 2001 earthquake, occurred in the western segment of the Kunlun fault during the past seven to nine centuries. Liquefaction structures formed in alluvial deposits composed of sand-gravel yielding 14 C ages of 679–901 yr bp are observed on the current stream channel which is sinistrally offset 75–82 m, including 3–6 m displacement produced by the 2001 event. On the basis of the field investigations and 14 C dating results, we conclude that the liquefaction structures and subsequent faulting events were caused by at least two large earthquakes of M>7 prior to the 2001 earthquake and the average recurrence interval of large earthquakes is estimated to be about 400 years in the late Holocene.

Multi-year streamflow drought in eastern Australia

Streamflow drought characteristics were calculated for 123 coastal and inland catchments in eastern Australia. The characteristics for each catchment were determined by stochastic generation of2000 years of daily rainfalls, which were converted to streamflows using the AWBM rainfall-runoff model. Droughts were calculated from the 2000 years of generated streamflow for durations of 1, 2, 3, 5 and 10 years, and expressed as a percentage of mean annual flow to facilitate comparisons among the catchments. There were six river basins with five or more catchments in the basin - three coastal basins in northern New South Wales and three inland basins in the Murray-Darling drainage division. These showed more variation within each basin than between basins. A long sequence of 100,000 years of streamflow was generated on one catchment, which showed that the severity of droughts increased with average recurrence interval to the limit of the generated data. There was no evidence of any regional influence on or geographical pattern of drought severity. The main result was the large variability of drought severity from catchment to catchment even within river basins.

Evaluation of Greenwaste Mulch to Control Runoff Quality from Landfill Sites During Frequent Storms

This paper describes a preliminary evaluation of two types of greenwaste (fresh and aged) used as a mulch layer to control runoff from disturbed landfill areas. Fresh greenwaste refers to woody and herbaceous garden waste that has been recently collected, chopped and shredded. Aged greenwaste is greenwaste which has been stockpiled for 18 months. We used rainfall simulator tests to investigate two aspects: (1) the performance of greenwaste mulch in reducing runoff during designed storm events with a high frequency of occurrence and (2) the release of pollutants via runoff as total suspended solids (TSS) and total organic carbon (TOC) during rain. Rainfall of <5-year average recurrence interval (ARI) was generally applied, consistent with stormwater compliance requirements for many Australian landfills. TOC released from fresh greenwaste material was higher in concentration than from aged greenwaste. However, when used as a 10-cm-deep mulch layer, fresh greenwaste was able to completely prevent runoff, even when tested under rainfalls of up to 50-year ARI duration. An equivalent mulch layer of aged greenwaste was also effective in reducing runoff volume and TSS concentration compared with the bare soil during a 3.5-year ARI rainfall, but mean TOC concentration was higher. Based on these preliminary results, fresh greenwaste mulching of bare soils is an attractive option to control runoff and erosion from areas subject to intermittent landfill operations and worthy of further investigations.

Structural Configuration of the Otates Fault (Southern Basin and Range Province) and Its Rupture in the 3 May 1887 MW 7.5 Sonora, Mexico, Earthquake

Abstract During the great 3 May 1887 Sonoran earthquake (surface-rupture end-to-end length: 101.8 km; M W 7.5±0.3), an array of three north–south-striking Basin and Range Province faults (from north to south: Pitaycachi, Teras, and Otates) slipped along the western margin of the Sierra Madre Occidental Plateau of northern Mexico. This detailed field survey of the previously undocumented 1887 earthquake rupture zone along the Otates fault includes mapping the rupture scarp and measurements of surface deformation at 25 sites. The surface rupture coincides with the mapped trace of the Otates fault, dips 68° W, and has an end-point-to-end-point length of 18.2 km. The lateral rupture terminations are controlled by cross faults. Striation measurements and the style of rupture indicate extensional dip slip without significant lateral displacement. The average shortening ( P ) and extension ( T ) kinematic axes inferred from the striations are similar to published composite focal mechanisms of the microseismicity recorded in the epicentral region of the 1887 earthquake. Based on surface deformation, maximum slip along the Otates fault in the 1887 earthquake was 270 cm, and the average slip was 189 cm. A quadratic best fit to the along-rupture surface offsets indicates an asymmetric distribution with the maximum offset near the northern end of the rupture segment, which can be explained by the mechanical interaction of the Teras and Otates surface-rupture segments through their stress fields. The average recurrence interval of 1887-sized earthquakes on the Otates fault is 30–42 kyr based on the long-term geologic dip-slip rate of the fault (0.06 mm/yr) and its amount of slip in the 1887 earthquake.

GEOLOGIC EVIDENCE FOR THREE GREAT EARTHQUAKES IN THE PAST 3400 YEARS OFF MYANMAR

Tectonic environments, recent stress and crustal strain observations, and historical descriptions of geomorphological changes and eyewitness accounts of the 1762 Bengal earthquake suggest that great earthquakes (M 8.0 or larger) can occur along the northward continuation of the 2004 Sumatra-Andaman earthquake. We describe marine terraces along the Rakhine coast of Myanmar as evidence for three great earthquakes in the past 3400 years. Radiocarbon dating of coral remains suggests that the oldest terrace emerged three times, during 1395–740 BC, AD 805–1220 and AD 1585–1810. We assign the youngest age to the 1762 earthquake, which reportedly raised parts of the Burmese coast by 3–7 m. These indicate that the great subduction-zone earthquakes have repeatedly occurred west off Myanmar with an average recurrence interval of about 1000–2000 years. The time since the last earthquake, ~ 250 years, is much shorter than the average interval, hence the chance of next earthquakes in the near future may be considered as low. However, the variability in both uplift amounts and recurrence intervals suggests the next great earthquake could happen sooner or later than would be expected from the average interval.

Does the 2007 Noto Hanto earthquake reveal a weakness in the Japanese national seismic hazard map that could be remedied with geological data?

AbstractThe Noto Hanto earthquake struck one of the lowest earthquake probability regions on the national seismic hazard map of Japan. To contribute to future updates of the hazard map, we examined the predictability of the 2007 earthquake on the basis of geological data that were available before it occurred. Sonic prospecting profiles of active faulting and the absence of an onshore fault could have limited the potential rupture length to 12–15 km, similar to the 2007 source. Empirical relationships between magnitude and fault length would have given usMj= 6.6–6.8 andMw= 6.3–6.4. The emergence of one marine terrace, which inclines to the south and reaches an altitude of approximately 50 m, can be dated to 120–130 ka and yields an uplift rate of approximately 0.4 mm/year.Mw-displacement empirical relationships and examples of recent blind fault events that have occurred at various locations around the world suggest that the conceivable maximum coseismic uplift of such shocks is 40–70 cm. Together with the uplift rate, we would have obtained an average recurrence interval of 1000–2000 years and, consequently, a 1.5–3.0% time-independent (Poisson) probability for 30 years. In addition, the significant inclination of the marine terraces—3.2 per mille (0.32%)—is better explained by the accumulation of frequent southward tilting as large as that of the 2007 type event with approximately 1600-year intervals, without any significant contributions from other seismic sources. We therefore conclude that the Noto Hanto earthquake source would have been better evaluated and identified if we had taken into account not only major active faults but also the active tectonics of moderate-size faults and their associated scale and rate.

Uplift of the Longmen Shan range and the Wenchuan earthquake

The 12 May 2008 Wenchuan earthquake (M s =8.0) struck on the Longmen Shan foreland thrust zone. The event took place within the context of long-term uplift of the Longmen Shan range as a result of the extensive eastward-extrusion of crustal materials from the Tibetan plateau against the rheologically strong crust of the Sichuan Basin. The Longmen Shan range is characterized by a Pre-Sinian crystalline complex constrained by the Maoxian-Wenchuan-Kangding ductile detachment at the western margin and the Yingxiu-Beichuan-Luding ductile thrust at the eastern margin. The Longmen Shan uplift was initiated by intracontinental subduction between the Songpan-Ganzi terrane and the Yangtze block during the Pre-Cenozoic. The uplift rate was increased considerably by the collision between the Indian and Eurasian plates since ∼50 Ma. The Wenchuan earthquake resulted in two major NE-striking coseismic ruptures (i.e., the ∼275 km long Yingxiu- Beichuan-Qingchuan fault and the ∼100 km long Anxian-Guanxian fault). Field investigations combined with focal solutions and seismic reflection profiles suggest that the coseismic ruptures are steeply dipping close-topure reverse or right reverse oblique slip faults in the ∼15 km thick upper crust. These faults are unfavorably oriented for frictional slip in the horizontally compressional regime, so that they need a long recurrence interval to accumulate the tectonic stress and fluid pressure to critically high levels for the formation of strong earthquakes at a given locality. It is also found that all the large earthquakes (M s >7.0) occurred in the fault zones across which the horizontal movement velocities measured by the GPS are markedly low (<3 mm/yr). The faults, which constitute the northeastern fronts of the enlarging Tibetan plateau against the strong Sichuan Basin, Ala Shan and Ordos blocks, are very destructive, although their average recurrence intervals are generally long.

Postwildfire hydrological response in an El Niño–Southern Oscillation–dominated environment

The rainfall‐runoff events following five fires that occurred within a 40‐year period in eucalypt forests of the Nattai catchment, southeastern Australia, were investigated to quantify the postwildfire hydrological response and to provide context for lower than expected erosion and sediment transport rates measured after wildfires in 2001. Daily rainfall and hourly instantaneous discharge records were used to examine rainfall‐runoff events in two gauged subcatchments (&gt;100 km2) for up to 3 years after fire and compared with nonfire periods. Radar imagery, available from 2001, was used to determine the intensity and duration of rainfall events. Wildfires in the study catchment appear to have no detectable impact on surface runoff at the large catchment scale, regardless of fire severity, extent or time after fire. Instead, the magnitude of postfire runoff is related to the characteristics of rainfall after fire. Rainfall is highly variable in terms of annual totals and the number, size, and type of events. Rainfall events that cause substantial surface runoff are characterized by moderate‐high intensity falls lasting one or more days (≥1 year average recurrence interval). These are triggered by synoptic‐scale weather patterns, which do not reliably occur in the postfire window and are independent of broad‐scale climate dominated by the El Niño–Southern Oscillation (ENSO). This study highlights the importance of considering the characteristics of rainfall, as well as local factors, in interpreting the postfire hydrological response.

Detecting Vertical Faulting Event Horizons from Holocene Synfaulting in Shallow Marine Sediments on the Western Margin of the Nobi Plain, Central Japan

Abstract Holocene activity of the Kuwana fault was reconstructed on the basis of 82 accelerator mass spectrometry (AMS) dates and stepwise changes in the depositional rates of shallow marine sediments in four cores drilled to examine the regularity of the recurrence interval of ruptures of this active intraplate fault. In our model, depositional rates on the footwall and hanging wall are nearly equal during interseismic periods, but depositional rates on the footwall become markedly larger than those on the hanging wall just after vertical displacement of the fault. To detect the timing of each faulting event more precisely, we compared changes in the depositional rates in cores from both the downthrown (footwall) and uplifted (hanging-wall) sides of the fault. As a result, six probable and one possible paleoseismic events, including the last two historical earthquakes, were inferred to have occurred during the last 7000 yr on the basis of stepwise changes in the depositional rate on the two sides of the fault. The timing of the faulting events was estimated from depositional rate curves of four cores. Probable ages of seismic events were approximately 6200, 5700, 4000, 3600, and 2100 calendar years before present and A.D. 745 and 1586. The average earthquake recurrence interval on the Kuwana fault calculated from these data is about 1000 yr. The average vertical slip rate of the fault has been at least 1 mm/yr during the last 7000 yr.

GIS-based risk analysis of debris flow: an application in Sichuan, southwest China

Debris flow is a serious geologic hazard in China. It is estimated that nationally debris flows cause up to 2 billion RMB (250 million US$) in damages and 300-600 deaths and injuries annually. To mitigate debris flow hazards, it is necessary to map, model, and identify zones of debris flow hazards and vulnerability as to inform the local people about the potential risk with a geographic information system. This research presents a regional scale case study modeling debris flow risk (hazard and vulnerability) in Sichuan Province, Southwestern China. In this area, 3,290 debris flows have been identified and the spatial-temporal distribution and activity characteristics of them have been documented. Based on the available meteorological data, a Digital Elevation Model with the rate of 1:250,000 and a regional geological map, the 24-hr rainfall threshold (y) for debris flow occurrence is closely related (significant at 99% confidence level) to the index (x) defined using a geology factor (rock hardness: a) and a topographical factor (channel gradient: d) where y = 21 + 10200 / x, in which x = 2.7 × e a + 1000 × d. The discipline is constructive in developing the rainfall threshold for debris flow activity in remote mountainous areas that lack data. For a given watershed, a four-level debris flow hazard map is developed by comparing the rainfall threshold to the design rainfall intensities with 50-, 20-, and 5-year average recurrence intervals, respectively. The degree of debris flow vulnerability is determined by the watershed socio-economic conditions. A four-class debris flow risk map, at the final phase of the research, is generated by combining debris flow hazards and vulnerability. With the debris flow risk assessment, the Sichuan Province is classified into the slight, moderate, severe and very severe regions, which accounts for 36%, 19%, 20% and 25% of total area respectively.

Late Quaternary paleoseismic evidence on the Munébrega half-graben fault (Iberian Range, Spain)

The Munebrega Plio-Quaternary half-graben is a NW-SE trending neotectonic depression located in the central sector of the intraplate Iberian Range (NE Spain). The master fault of the half-graben offsets an Upper Pleistocene pediment deposit, forming an upslope-facing scarp. A trench dug across the fault scarp exposed a 25-m wide deformation zone consisting of graben and horst fault blocks with fissures in the upper part of the scarp, and a monoclinal flexure affected by normal and reverse faults in the lower part of the scarp. We infer a minimum of three faulting events over the past 72 ka, yielding an average (maximum) recurrence interval of 24 ka. The oldest event (72–41 ka) produced an antislope scarp on the relict pediment surface, confining deposition to the downthrown block. Cross-cutting faults affecting sedimentary units deposited in the sediment trap produced by the first event provide evidence for at least two younger events (33–19? ka). The measured cumulative vertical displacement (7.4 m) yield a minimum vertical slip rate of 0.10 ± 0.01 mm/year (2σ error) for the past 72 ka. If the paleoearthquakes ruptured the whole mappable length of the fault (ca. 20 km), they probably had moment magnitudes ca. 6.9 (Stirling et al. Bull Seismol Soc Am, 2002). Such earthquakes would have been more than a magnitude unit larger than the largest ones recorded historically in the Iberian Range. These results suggest that the official seismic hazard assessments, based solely on the historic and instrumental record, may underestimate the seismic hazard in the area.

The impact of wind on trees in Australian tropical savannas: lessons from Cyclone Monica

Abstract The role of tropical cyclones in structuring tropical rainforests has long been recognized in the Americas and northern Australia. However, their role in the dynamics of Australian savannas has received little attention compared with the effects of fire and drought. In April 2006, Tropical Cyclone Monica caused trees to be snapped or uprooted across a swath about 50 km wide and 130 km inland in northern Australia. The proportion of trees damaged was assessed using on‐ground surveys and interpretation of aerial photographs along the cyclone path and along several perpendicular transects. From these data, a relationship between the damage to trees and estimated maximum gust speed was developed. From knowledge of gust recurrence, curves were then derived describing the changes with distance inland in the average recurrence interval for tree windthrow events. These relationships were consistent with the historical record of tree damage in the northwest of the Northern Territory. It was concluded that windthrow has profound, but hitherto unrecognized implications for ecological processes such as tree dynamics and fluxes of carbon and water in Australian tropical savannas.

Implementation of Routine Regional Moment Tensor Analysis in New Zealand

The active tectonics of New Zealand are dominated by three main features (figure 1). Beneath the North Island there is active subduction of the Hikurangi trough where the Pacific plate is undergoing oblique subduction beneath the Australian plate at ∼ 45 mm/yr ( e.g. , Demets et al. 1990; Demets et al. 1994). The Hikurangi trough extends from the Kermadec trench in the north and terminates in the upper South Island. Large subduction zone earthquakes of around Mw 8.0 in the lower North Island and around Mw 6.9 farther north are expected (Reyners 1998). Slow slip events have also been observed on the Hikurangi subduction interface ( e.g. , Wallace and Beavan 2006) similar to those documented at other Pacific Rim subduction margins ( e.g. , Dragert et al. 2001; Obara et al. 2004; Larson et al. 2004). A number of dextral strike-slip faults are also present throughout the North Island and the upper South Island and have produced a number of M 6.0–7.5 earthquakes in the past century (Doser and Webb 2003). The Wellington region is cut by five active dextral faults that have average recurrence intervals of meter-scale surface rupture that range from ∼ 500 to 5,000 years (Van Dissen and Berryman 1996). The Wairarapa fault is the only one of these to have ruptured in historic times, in 1855, in an event estimated at Mw 8.2 (Darby and Beanland 1992). ▴ Figure 1. Overview of the tectonic setting of New Zealand, which is dominated by the interaction between the Pacific plate and the Australian plate. In the North Island the Pacific plate is subducting beneath the Australian plate at the Hikurangi trough and Kermadec trench. The Alpine fault delineates a dextral strike-slip fault through most of the South Island, and south of the South Island the …

GPS constraints on 34 slow slip events within the Cascadia subduction zone, 1997–2005

Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the Cascadia subduction zone from 1997 through 2005. Timing of transient onset is determined with wavelet transformation of geodetic time series. Thirty continuous stations are included in this study, ranging from northern California to southwestern British Columbia. Our improvements in analysis better resolve the largest creep events and also identify many smaller events. At 48.5°N latitude, a 14‐month average recurrence interval has been observed over eight events since 1997. Farther north along Vancouver Island a host of smaller events with a distinct 14‐month periodicity also occurs. In southern Washington State, some of the largest transient displacements are observed but lack any obvious periodicity in their recurrence. Along central Oregon, an 18‐month recurrence is evident, while in northern California an 11‐month periodicity continues through 2005. We invert GPS offsets of the 12 best recorded events for thrust slip along the plate interface using a cross‐validation scheme to derive optimal smoothing parameters. These 12 events have equivalent moment magnitudes between 6.3 and 6.8 and have 2–3 cm of slip. Unlike other subduction zones, no long‐duration events are observed, and cumulative surface deformation is consistently less than 0.6 cm. The many newly resolved smaller transient events in Cascadia show that slow slip events occur frequently with GPS best capturing only the largest events. It is likely that slow slip events occur more frequently at levels not detectable with GPS.

Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone

We relate the late Holocene northern San Andreas fault (NSAF) paleo- seismic history developed using marine sediment cores along the northern California continental margin to a similar dataset of cores collected along the Cascadia margin, including channels from Barclay Canyon off Vancouver Island to just north of Mon- terey Bay. Stratigraphic correlation and evidence of synchronous triggering imply earthquake origin, and both temporal records are compatible with onshore paleoseis- mic data. In order to make comparisons between the temporal earthquake records from the NSAF and Cascadia, we refine correlations of southern Cascadia great earth- quakes, including the land paleoseismic record. Along the NSAF during the last ∼2800 yr, 15 turbidites, including one likely from the great 1906 earthquake, establish an average repeat time of ∼200 yr, similar to the onshore value of ∼240 yr. The combined land and marine paleoseismic record from the southern Cascadia subduction zone includes a similar number of events during the same period. While the average recurrence interval for full-margin Cascadia events is ∼520 yr, the southern Cascadia margin has a repeat time of ∼220 yr, similar to that of the NSAF. Thirteen of the 15 NSAF events were preceded by Cascadia events by ∼0-80 yr, averaging 25-45 yr (as compared to ∼80-400 yr by which Cascadia events follow the NSAF). Based on the temporal association, we model the coseismic and cumulative post- seismic deformation from great Cascadia megathrust events and compute related stress changes along the NSAF in order to test the possibility that Cascadia earth- quakes triggered the penultimate, and perhaps other, NSAF events. The Coulomb fail- ure stress (CFS) resulting from viscous deformation related to a Cascadia earthquake over ∼60 yr does not contribute significantly to the total CFS on the NSAF. However, the coseismic deformation increases CFS on the northern San Andreas fault (NSAF )b y up to about 9 bars offshore of Point Delgada, most likely enough to trigger that fault to fail in north-to-south propagating ruptures.

Influence of climate fluctuations and changes in catchment land use on Late Holocene and modern beach-ridge sedimentation on a tropical macrotidal coast: Keppel Bay, Queensland, Australia

Beach ridges at Keppel Bay, central Queensland, Australia, preserve a record of sediment accumulation from the historical period back to middle Holocene times. The ridges comprise fine, well-sorted, feldspar-rich quartz sand that was eroded from the Fitzroy River catchment, deposited in Keppel Bay during floods of the Fitzroy River, and reworked onshore into beach and foredune deposits by the prevailing currents, waves and wind. These floods have an average recurrence interval of at least 7 yr and are induced by the passage of cyclones onshore into the large Fitzroy catchment. The youngest series of beach ridges sit sub-parallel to the modern beach and comprise six accretional units, each unit formed by a set of ridges and delineated by prominent swales. Optically stimulated luminescence (OSL) ages of beach ridges in these units indicate they were deposited in periods of rapid progradation approximately 1500, 1000, 450 and 230 yr BP, when there was an enhanced supply of sediment to the beach from the Fitzroy River via Keppel Bay. Estimates of the mass of sediment stored in the beach-ridge strandplain show that it represents a significant sediment store, potentially trapping the equivalent of 79% of the estimated long-term (100 yr) average annual bedload of the Fitzroy River that is deposited in Keppel Bay. There has been a reduction in the rate of sediment accumulation in the strandplain since around 1000 yr BP, which is consistent with other coastal records in eastern Australia of a relatively wetter phase of climate in the late Holocene compared to the present. The youngest beach ridges (OSL ages < 100 yr BP) are tall relict foredunes that reflect a low rate of sediment accumulation. These ridges have a distinctive trace-element composition produced by a greater contribution from catchment areas with basaltic soils. The change in catchment provenance has likely been a consequence of erosion that followed clearing of native vegetation in these areas. Our findings demonstrate the important insights that beach-ridge deposits proximal to a river sediment source can provide into processes of sediment accumulation and the response to variations in climate in tropical coastal sedimentary systems.

Sediment trapping by a tree belt: processes and consequences for sediment delivery

AbstractRestoring belts of perennial vegetation in landscapes is widely recognized as a measure of improving landscape function. While there have been many studies of the transport of pollutants through grass filter strips, few have addressed sediment related processes through restored tree belts. In order to identify these processes and quantify their relative contribution to sediment trapping, a series of rainfall simulations was conducted on a 600 m2 hillslope comprising a pasture upslope of a 15 year old tree belt. Although the simulated events were extreme (average recurrence intervals ∼10 and 50 yr), the trapping efficiency of the tree belt was very high: at least 94% of the total mass of sediments was captured. All the size fractions were trapped with a minimum Sediment Trapping Ratio (STR) of 91% for the medium‐sized fragments. Fractions &lt; 1·3 µm and &gt; 182 µm were totally captured (STR = 100%). Through the joint analysis of sediment budgets and soil surface conditions, we identified different trapping processes. The main trapping process is the sedimentation (at least 62% of trapped sediment mass) with deposits in the backwater and as micro‐terraces within the tree belt. Modelling results show that the coarsest size fractions above 75 µm are preferentially deposited. Joint infiltration of water and sediments has also been noticed, however, this process alone cannot explain the selective trapping of the finest fractions. We suggest that the finest fractions transported by the overland flow may be trapped by adsorption on the abundant litter present within the tree belt. Copyright © 2008 John Wiley &amp; Sons, Ltd.