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Daníel Freyr Jónsson Esther Ruth Gudmundsdóttir Gudrún Larsen Bergrún Arna Óladóttir Egill Erlendsson Sigrún Dögg Eddudóttir Olgeir Sigmarsson 《第四纪科学杂志》2020,35(3):410-421
Large Plinian eruptions from Hekla volcano, Iceland, produce compositionally zoned tephra used as key markers in tephrochronology. However, spatial variations in chemical composition of a tephra layer may complicate its identification. An example is the 5950–6180 cal a bp Hekla Ö tephra layer, which shows compositional spread from rhyolite, dacite and andesite to basalt. In soil sections north of Hekla, the SiO2 content of the tephra glass reaches 76 wt% in the lowest unit of the Hekla Ö deposit and decreases to 62–63 wt% in the uppermost unit. Intermingled within the whole deposit are basalt tephra grains having 46–47 wt% SiO2. The composition of the basalt glass includes primitive basalt and a more evolved basalt (MgO >6 and <6 wt%, respectively). Together with literature data, the Hekla Ö tephra and the so-called T-Tephra/Hekla-T are most likely from contemporaneous eruptions of different vents on the Hekla volcanic system, forming a single important marker tephra (Hekla ÖT) deposited over 80% of Iceland. Identification is complicated by its spatial compositional heterogeneity, such as systematic decrease in SiO2 content from the east to the west of Hekla volcano. Consequently, an individual tephra layer from a large explosive eruption can have different composition at different locations. © 2020 John Wiley & Sons, Ltd. 相似文献
2.
Egill Hauksson Joann Stock Kate Hutton Wenzheng Yang J. Antonio Vidal-Villegas Hiroo Kanamori 《Pure and Applied Geophysics》2011,168(8-9):1255-1277
The El Mayor-Cucapah earthquake sequence started with a few foreshocks in March 2010, and a second sequence of 15 foreshocks of M?>?2 (up to M4.4) that occurred during the 24?h preceding the mainshock. The foreshocks occurred along a north?Csouth trend near the mainshock epicenter. The M w 7.2 mainshock on April 4 exhibited complex faulting, possibly starting with a ~M6 normal faulting event, followed ~15?s later by the main event, which included simultaneous normal and right-lateral strike-slip faulting. The aftershock zone extends for 120?km from the south end of the Elsinore fault zone north of the US?CMexico border almost to the northern tip of the Gulf of California. The waveform-relocated aftershocks form two abutting clusters, each about 50?km long, as well as a 10?km north?Csouth aftershock zone just north of the epicenter of the mainshock. Even though the Baja California data are included, the magnitude of completeness and the hypocentral errors increase gradually with distance south of the international border. The spatial distribution of large aftershocks is asymmetric with five M5+ aftershocks located to the south of the mainshock, and only one M5.7 aftershock, but numerous smaller aftershocks to the north. Further, the northwest aftershock cluster exhibits complex faulting on both northwest and northeast planes. Thus, the aftershocks also express a complex pattern of stress release along strike. The overall rate of decay of the aftershocks is similar to the rate of decay of a generic California aftershock sequence. In addition, some triggered seismicity was recorded along the Elsinore and San Jacinto faults to the north, but significant northward migration of aftershocks has not occurred. The synthesis of the El Mayor-Cucapah sequence reveals transtensional regional tectonics, including the westward growth of the Mexicali Valley and the transfer of Pacific?CNorth America plate motion from the Gulf of California in the south into the southernmost San Andreas fault system to the north. We propose that the location of the 2010 El Mayor-Cucapah, as well as the 1992 Landers and 1999 Hector Mine earthquakes, may have been controlled by the bends in the plate boundary. 相似文献
3.
Seismic tomography shows that upwelling beneath Iceland is confined to the upper mantle 总被引：3，自引：0，他引：3
4.
Yih-Min Wu Hiroo Kanamori Richard M. Allen Egill Hauksson 《Geophysical Journal International》2007,170(2):711-717
We explore a practical approach to earthquake early warning in southern California by determining a ground-motion period parameter τ c and a high-pass filtered displacement amplitude parameter Pd from the initial 3 s of the P waveforms recorded at the Southern California Seismic Network stations for earthquakes with M > 4.0. At a given site, we estimate the magnitude of an event from τ c and the peak ground-motion velocity ( PGV ) from Pd . The incoming three-component signals are recursively converted to ground acceleration, velocity and displacement. The displacements are recursively filtered with a one-way Butterworth high-pass filter with a cut-off frequency of 0.075 Hz, and a P -wave trigger is constantly monitored. When a trigger occurs, τ c and Pd are computed. We found the relationship between τ c and magnitude ( M ) for southern California, and between Pd and PGV for both southern California and Taiwan. These two relationships can be used to detect the occurrence of a major earthquake and provide onsite warning in the area around the station where onset of strong ground motion is expected within seconds after the arrival of the P wave. When the station density is high, the methods can be applied to multistation data to increase the robustness of onsite early warning and to add the regional warning approach. In an ideal situation, such warnings would be available within 10 s of the origin time of a large earthquake whose subsequent ground motion may last for tens of seconds. 相似文献
5.
Climate change and human impact in a sensitive ecosystem: the Holocene environment of the Northwest Icelandic highland margin 总被引：1，自引：0，他引：1 
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下载免费PDF全文 Sigrún D. Eddudóttir Egill Erlendsson Leone Tinganelli Guðrún Gísladóttir 《Boreas: An International Journal of Quaternary Research》2016,45(4):715-728
Complex interactions of climate and volcanic activity have shaped the environment of Iceland during the Holocene. Palaeoecological records from Iceland offer a unique look at a Holocene environment that was uninhabited by humans and free of mammal herbivores until about AD 870. We present a new reconstruction of Holocene vegetation and landscape dynamics from a small lake, Barðalækjartjörn, located near the highland margin in Northwest Iceland. A multi‐proxy approach was used to reconstruct vegetation based on pollen and plant macrofossil analysis and landscape stability based on lithological proxies. The record covers the period c. 10 300–200 cal. a BP. For the first two millennia aeolian processes probably played a part in vegetation development. This period is characterized by high input of minerogenic material into the lake and a vegetation assemblage in which plants tolerant of aeolian deposition are prominent. Betula pubescens woodland reached a maximum between c. 7400 and 6500 cal. a BP. Betula nana‐dominated dwarf shrub heath replaced woodland after c. 4000 cal. a BP, following the onset of Neoglaciation. Land use following human settlement caused an environmental shift at the highland margin. Betula pubescens probably disappeared from the vicinity of the lake soon thereafter. Large‐scale soil erosion began at c. 1000 cal. a BP in the wake of human activities, such as introduction of grazing livestock and woodcutting. This study offers an important long‐term perspective of the development of the highland ecosystem under both wholly natural and human‐influenced conditions. 相似文献
6.
The study presents the effect of soil erosion on vegetation, soil accumulation (SA), SA rate (SAR), soil quality, soil mass, and the soil organic carbon (SOC) pool in Brown Andosols and Histosols in a 24-km2 area in southwest Iceland. Undisturbed prehistoric soils were distinguished from disturbed historic soils using tephrochronology. Soil erosion has been severe during historic time (last 1135 yr), resulting in the increase of the soil mass deposited in soils covered by vegetation by a factor of 7.3-9.2 and net loss of soil in unvegetated areas. The SAR correlated positively with SOC sequestration. SOC is easily transported and, given the extensive accumulation of soil, the net effect of burial and subsequent reduction in decomposition is to increase SOC storage. Nevertheless, the increased accumulation and soil depletion has decreased soil quality, including the SOC, and reduced soil resistance to erosion with the depleted SOC contributing to enrichment of atmospheric CO2. The initial terrestrial disturbance was triggered by anthropogenic land use during the Medieval Warm Period, followed by volcanic activity approximately three centuries later. The combination of harsh climate during the Little Ice Age and drastic anthropogenic perturbations has led to land degradation at a catastrophic scale. 相似文献
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8.
Freddy J. Gathorne-Hardy Egill Erlendsson Peter G. Langdon Kevin J. Edwards 《Journal of Paleolimnology》2009,42(3):413-426
Ecosystem variability must be assessed over a range of timescales in order to fully understand natural ecosystem processes.
Long-term climate change, at millennial and centennial scales, is a major driver of natural ecosystem variability, but identifying
evidence of past climate change is frequently confounded by human-induced impacts on the ecosystem. Iceland is a location
where it is possible to separate natural from anthropogenic change in environmental archives, as the date of settlement is
accepted to be around AD 874, prior to which the island was free from proven human impacts. We used a lake sediment core from
Breieavatn, near Reykholt, a major farm of the Norse period in western Iceland, to examine landscape development. A change
in pollen concentration in the sediments, especially the decline in Betula, indicated initial landscape degradation immediately post-settlement, whereas the chironomid fauna and reconstructed temperatures
were relatively complacent during this period. The pollen evidence is corroborated by 14C analyses, which indicate an increase in older carbon entering the lake, inferred to have been caused by increased erosion
following settlement. Further decreases in Betula pollen occurred around AD 1300, pre-dating a drop in chironomid-inferred temperatures (CI-T) of ~1°C over 100–200 years.
The CI-T reconstruction also shows a significant cooling after ~AD 1800, likely indicative of the coldest phase of the Little
Ice Age. The evidence suggests that the chironomid record was relatively unaffected by the increased landscape degradation
and hence reveals a temperature reconstruction independent of human impact. 相似文献
9.
Human impacts have been severe on Icelandic soils and vegetation. In order to assess human impact on soils soil quality, soil organic C (SOC), soil bulk density (BD), soil moisture content (SMC), soil mass, and SOC sequestration were measured from two Histosol cores in West Iceland. The cores cover a period from around 665 BC to present, capturing the initial human settlement of Iceland in AD 871. Tephrochronology allowed for a reliable correlation and comparison between the two cores. The initial settlement had profound impacts on the soil quality, causing decreased SOC concentration and SMC, and increased vegetation degradation, soil exposure, eolian deposition, and BD. The total SOC pool was 34.6 kg C m2 at one of the sites, of which 60.1% was formed during historic times, driven by increased soil mass deposition from surrounding eroded areas. The SOC pool was 43.7 kg C m2 at the other site, of which 31.4% was formed during historic time, constrained by water cycling and decomposition. 相似文献
10.
Tephra-dated, high-resolution pollen profiles from Ketilsstaðir, southern Iceland, indicate a largely unwooded pre-settlement environment, a probable consequence of the exposed coastal location. The degree of change associated with the Norse landnám is more limited than in many Icelandic pollen diagrams. There are three main periods of change in the post-settlement vegetational development of the area. Firstly, Norse settlement affected the hydrology of the bog, resulting in the near-disappearance of Sphagnum and agricultural activity led to a reduction of some species (e.g. Angelica spp. and, Salix). Secondly, the establishment of probable permanent settlement in the mid-11th century AD initiated expansion of such apophytic taxa as Plantago spp. Lactuceae, Ranunculus spp. and Pteridophytes. Thirdly, the ≥ 10 cm thick Katla tephra, deposited in AD 1357, enhanced drainage of the bog surface, favouring dryland taxa (e.g. Poaceae, Galium and Lactuceae). The tephra deposit and the associated drainage probably caused or contributed to the local extinction of the wetland beetle Hydraena britteni. The study has enabled a series of natural and humanly-related issues to be addressed including tephra-vegetation relationships, the anthropogenic reduction in plant diversity, and comparisons between historical and environmental settlement records. 相似文献