Land-level Changes Research Articles

A new digital database of Ellen Louise Mertz’s 1924 ‘Overview of late- and postglacial elevation changes in Denmark’

Data from an important historic article on late- and postglacial land-level changes in Denmark and the accompanying map are presented here in a new digital format. The original data were compiled in 1924 by Ellen Louise Mertz and comprise field observations of the marine limit in Denmark made over the late 19th and early 20th centuries. The original tables have been transcribed and expanded into a digital database consisting of 658 entries. The original map sheet has been georeferenced and 392 mapped points have been assigned coordinates. The points are linked to their attributes in the digital data table, making them newly amenable to geospatial analysis in a Geographic Information System. To demonstrate, we briefly present one such application, namely a reproduction and verification of the isolines of raised beach elevation from the original 1924 map.

20thto 21stCentury Relative Sea and Land LevelChanges in Northern California:Tectonic Land Level Changes and theirContribution to Sea-Level Rise, Humboldt BayRegion, Northern California

Sea-level changes are modulated in coastal northern California by land-level changes due to the earthquake cycle along the Cascadia subduction zone, the San Andreas plate boundary fault system, and crustal faults. Sea-level rise (SLR) subjects ecological and anthropogenic infrastructure to increased vulnerability to changes in habitat and increased risk for physical damage. The degree to which each of these forcing factors drives this modulation is poorly resolved. We use NOAA tide gage data and ‘campaign’ tide gage deployments, Global Navigation Satellite System (GNSS) data, and National Geodetic Survey (NGS) first-order levelling data to calculate vertical land motion (VLM) rates in coastal northern California. Sea-level observations, highway level surveys, and GNSS data all confirm that land is subsiding in Humboldt Bay, in contrast to Crescent City where the land is rising. Subtracting absolute sea-level rate (~1.99 mm/year) from Crescent City (CC) and North Spit (NS) gage relative sea-level rates reveals that CC is uplifting at ~2.83 mm/year and NS is subsiding at ~3.21mm/year. GNSS vertical deformation reveals similar rates of ~2.60 mm/year of uplift at Crescent City. In coastal northern California, there is an E-W trending variation in vertical land motion that is primarily due to Cascadia megathrust fault seismogenic coupling. This interseismic subsidence also dominates the N-S variation in vertical land motion in most of the study region. There exists a second-order heterogeneous N-S trend in vertical land motion that we associate to crustal fault-related strain. There may be non-tectonic contributions to the observed VLM rates.

20th to 21st Century Relative Sea and Land Level Changes in Northern California: Tectonic Land Level Changes and their Contribution to Sea-Level Rise, Humboldt Bay Region, Northern California

20th to 21st Century Relative Sea and Land Level Changes in Northern California: Tectonic Land Level Changes and their Contribution to Sea-Level Rise, Humboldt Bay Region, Northern California

High-Resolution Marine Seismic Imaging of the Seattle Fault Zone: Near-Surface Insights into Fault Zone Geometry, Quaternary Deformation, and Long-Term Evolution

ABSTRACTThe Seattle fault zone (SFZ) is a north-directed thrust fault system that underlies the greater Seattle metropolitan area. Evidence of past land level changes, landslides, liquefaction, and a local tsunami indicate that this 70-km-long fault system can host up to M 7–7.5 earthquakes. Both the geometry and earthquake recurrence of the SFZ are debated and surveys of the shallow subsurface have not yet been incorporated into deeper crustal-scale structural interpretations, especially where the SFZ cuts across marine portions of the Puget Lowland. Here we use a new high-resolution marine seismic reflection dataset to image fault-related deformation in Quaternary sediments and Tertiary bedrock throughout Puget Sound and Lake Washington. We use this perspective of shallow geology as a link between existing crustal-scale geophysical insights into fault geometry at depth and paleoseismological observations of faulting at the surface and propose a refined structural model for the SFZ. We interpret that our new seismic reflection data in the Rich Passage area of Puget Sound images evidence of an inactive, south-dipping strand of the SFZ, which is overprinted by Quaternary folding and slip along north-dipping backthrusts within the hanging wall of a blind, south-dipping fault located 6 km farther north. To explain these results, we propose that the SFZ is a normal sequence fault propagation fold that has stepped northward through time, and we show the plausibility of this model through trishear forward modeling. Growth strata and faulting imaged in Quaternary sediments in Lake Washington and Rich Passage are consistent with the spatial distribution of folding and backthrusting that occurred during an M 7–7.5 earthquake in A.D. 900–930, corroborating existing evidence that the SFZ has been active throughout the Quaternary.

Toward an Integrative Geological and Geophysical View of Cascadia Subduction Zone Earthquakes

The Cascadia subduction zone (CSZ) is an exceptional geologic environment for recording evidence of land-level changes, tsunamis, and ground motion that reveals at least 19 great megathrust earthquakes over the past 10 kyr. Such earthquakes are among the most impactful natural hazards on Earth, transcend national boundaries, and can have global impact.Reducing the societal impacts of future events in the US Pacific Northwest and coastal British Columbia, Canada, requires improved scientific understanding of megathrust earthquake rupture, recurrence, and corresponding hazards. Despite substantial knowledge gained from decades of research, large uncertainties remain about the characteristics and frequencies of past CSZ earthquakes. In this review, we summarize geological, geophysical, and instrumental evidence relevant to understanding megathrust earthquakes along the CSZ and associated uncertainties. We discuss how the evidence constrains various models of great megathrust earthquake recurrence in Cascadia and identify potential paths forward for the earthquake science community. ▪ Despite outstanding geologic records of past megathrust events, large uncertainty of the magnitude and frequency of CSZ earthquakes remains. ▪ This review outlines current knowledge and promising future directions to address outstanding questions on CSZ rupture characteristics and recurrence. ▪ Integration of diverse data sets with attention to the geologic processes that create different records has potential to lead to major progress.

The 21 May 1960 Mw 8.1 Concepción Earthquake: A Deep Megathrust Foreshock That Started the 1960 Central-South Chilean Seismic Sequence

AbstractOne of the most notable seismic sequences in modern times was recorded in May 1960 along the southern Chilean subduction zone. The sequence started on 21 May with the Mw 8.1 Concepción earthquake; 33 hr later the Mw 9.5 Valdivia megathrust earthquake occurred, the largest ever recorded in the instrumental period. These events changed the geomorphology of the coast along more than 1000 km, generated extensive structural damage in the main cities of central-south Chile, and triggered a Trans-Pacific tsunami. Observed land-level changes due to both earthquakes were reported in 1970. These observations were ascribed to both events but have been used to study only the general source properties of the 22 May Valdivia mainshock. Here, we separate these data to constrain for the first time the slip distribution of the 21 May Concepción earthquake, applying a Bayesian approach that considers uncertainties in the data. Our results show that the Mw 8.1 Concepción earthquake ruptured a deep segment of the megathrust, concentrated in a compact zone below the Arauco peninsula between depths of 20 and 50 km. Tsunami generation from this deep source agrees well with the tsunami arrival times and small amplitudes recorded by tide gauges along the Chilean coast. Our study highlights the importance of the 21 May 1960 Concepción earthquake in the context of large historical Chilean earthquakes.

Relative sea-level change and climate change in the Northeastern Adriatic during the last 1.5 ka (Istria, Croatia)

A new high-resolution relative sea-level (RSL) reconstruction is presented for the past 1500 years based on four bio-constructions formed by alga Lithophyllum byssoides (algal rims). Two algal structures have been studied on the southern (Premantura site) and two on the eastern Istrian coast (Uboka and Brseč sites) in the Northeastern Adriatic. The data from the algal rims (47 radiocarbon data points) enabled the distinction of four major phases of RSL change which corresponds to periods of climate change. RSL between AD 400 and 800 during the Dark Ages Cold Period (DACP), was almost stable. After AD 800, during the Medieval Climate Anomaly (MCA) the RSL increased up to ∼0.8 mm/yr. The following Little Ice Age period, (LIA) interval I (AD 1400 till 1600) is again characterised by RSL stability (RSL slowed down) which allows the rims at the southern coast to reach the width of ∼40–80 cm at their uppermost part and up to 20 cm for those along the eastern coast. Between AD ∼1600 and 1750, during the colder LIA II interval, algal rims do not form, as LIA II is assumed to be a period of RSL fall. Algal rims reveal that from the second part of the 19th century the RSL rose by 13–15 cm at the Premantura location and around 10 cm at the Brseč and Uboka areas, providing rates between 1 and 0.7 mm/yr respectively for the Current Warm Period (CWP).The sea-level trends were quantitatively defined using an Errors-In-Variables Integrated Gaussian Process (EIV-IGP) model, with full consideration of the available uncertainty. Following correction for the total land-level change (assumed to be around −0.4 mm/yr), four successive trends in sea-level change were confirmed. Sea-level dropped during the DACP at a mean rate of −0.4 mm/yr and increased to 0.5 mm/y as a consequence of Medieval warmth. Thereafter it was relatively stable during LIA I, fell up to −0.1 mm/yr during LIA II interval and has been slowly rising again during the CWP. Moreover, L. byssoides δ18O records show that these periods of sea-level changes are consistent with changes in temperature and thus with periods of rapid climate change.

Measuring decadal vertical land-level changes from SRTM-C (2000) and TanDEM-X ( ∼ 2015) in the south-central Andes

Abstract. In the arctic and high mountains it is common to measure vertical changes of ice sheets and glaciers via digital elevation model (DEM) differencing. This requires the signal of change to outweigh the noise associated with the datasets. Excluding large landslides, on the ice-free earth the land-level change is smaller in vertical magnitude and thus requires more accurate DEMs for differencing and identification of change. Previously, this has required meter to submeter data at small spatial scales. Following careful corrections, we are able to measure land-level changes in gravel-bed channels and steep hillslopes in the south-central Andes using the SRTM-C (collected in 2000) and the TanDEM-X (collected from 2010 to 2015) near-global 12–30 m DEMs. Long-standing errors in the SRTM-C are corrected using the TanDEM-X as a control surface and applying cosine-fit co-registration to remove ∼1/10 pixel (∼3 m) shifts, fast Fourier transform (FFT) and filtering to remove SRTM-C short- and long-wavelength stripes, and blocked shifting to remove remaining complex biases. The datasets are then differenced and outlier pixels are identified as a potential signal for the case of gravel-bed channels and hillslopes. We are able to identify signals of incision and aggradation (with magnitudes down to ∼3 m in the best case) in two >100 km river reaches, with increased geomorphic activity downstream of knickpoints. Anthropogenic gravel excavation and piling is prominently measured, with magnitudes exceeding ±5 m (up to >10 m for large piles). These values correspond to conservative average rates of 0.2 to >0.5 m yr−1 for vertical changes in gravel-bed rivers. For hillslopes, since we require stricter cutoffs for noise, we are only able to identify one major landslide in the study area with a deposit volume of 16±0.15×106 m3. Additional signals of change can be garnered from TanDEM-X auxiliary layers; however, these are more difficult to quantify. The methods presented can be extended to any region of the world with SRTM-C and TanDEM-X coverage where vertical land-level changes are of interest, with the caveat that remaining vertical uncertainties in primarily the SRTM-C limit detection in steep and complex topography.

Late Holocene paleoseismology of Shuyak Island, Alaska

We report stratigraphic evidence of land-level changes along the eastern portion of the Alaska-Aleutian megathrust. Four marshes on Shuyak Island record variable amounts of coseismic deformation during four pre-20th century earthquakes. We combine these data with paleoseismic evidence from across the Kodiak, Kenai and Prince William Sound segments of the megathrust. These indicate that in the last 2000 years, AD 1964 was the only one to rupture all three segments simultaneously and generate a Mw 9.2 earthquake. The Kodiak segment ruptured independently on four further occasions with magnitudes > Mw 8.0; in AD 1788 and c.400 (440–320) BP, and independently but around the times of great earthquakes in the Prince William Sound segment c.850 and c.1500 BP.

Back to full interseismic plate locking decades after the giant 1960 Chile earthquake

Great megathrust earthquakes arise from the sudden release of energy accumulated during centuries of interseismic plate convergence. The moment deficit (energy available for future earthquakes) is commonly inferred by integrating the rate of interseismic plate locking over the time since the previous great earthquake. But accurate integration requires knowledge of how interseismic plate locking changes decades after earthquakes, measurements not available for most great earthquakes. Here we reconstruct the post-earthquake history of plate locking at Guafo Island, above the seismogenic zone of the giant 1960 (Mw = 9.5) Chile earthquake, through forward modeling of land-level changes inferred from aerial imagery (since 1974) and measured by GPS (since 1994). We find that interseismic locking increased to ~70% in the decade following the 1960 earthquake and then gradually to 100% by 2005. Our findings illustrate the transient evolution of plate locking in Chile, and suggest a similarly complex evolution elsewhere, with implications for the time- and magnitude-dependent probability of future events.

Subduction zone slip variability during the last millennium, south-central Chile

The Arauco Peninsula (37°-38°S) in south-central Chile has been proposed as a possible barrier to the along-strike propagation of megathrust ruptures, separating historical earthquakes to the south (1960 AD 1837, 1737, and 1575) and north (2010 AD, 1835, 1751, 1657, and 1570) of the peninsula. However, the 2010 (Mw 8.8) earthquake propagated into the Arauco Peninsula, re-rupturing part of the megathrust that had ruptured only 50 years earlier during the largest subduction zone earthquake in the instrumental record (Mw 9.5). To better understand long-term slip variability in the Arauco Peninsula region, we analyzed four coastal sedimentary sections from two sites (Tirúa, 38.3°S and Quidico, 38.1°S) located within the overlap of the 2010 and 1960 ruptures to reconstruct a ∼600-year record of coseismic land-level change and tsunami inundation. Stratigraphic, lithologic, and diatom results show variable coseismic land-level change coincident with tsunami inundation of the Tirúa and Quidico marshes that is consistent with regional historical accounts of coseismic subsidence during earthquakes along the Valdivia portion of the subduction zone (1960 AD and 1575) and coseismic uplift during earthquakes along the Maule portion of the subduction zone (2010 AD, 1835, 1751). In addition, we document variable coseismic land-level change associated with three new prehistoric earthquakes and accompanying tsunamis in 1470–1570 AD, 1425–1455, and 270–410. The mixed record of coseismic subsidence and uplift that we document illustrates the variability of down-dip and lateral slip distribution at the overlap of the 2010 and 1960 ruptures, showing that ruptures have repeatedly propagated into, but not through the Arauco Peninsula and suggesting the area has persisted as a long-term impediment to slip through at least seven of the last megathrust earthquakes (∼600 years).

Postseismic coastal development in Aceh, Indonesia - Field observations and numerical modeling

We model postseismic changes to the shoreline of West Aceh, Indonesia, a region largely affected by the December 2004 Sumatra-Andaman earthquake and ensuing Indian Ocean tsunami, using a cross-shore morphodynamic model. Subsidence of 0.5–1.0m and tsunami scouring during the 2004 event caused the complete destruction of the beach and the landward displacement of the western coast of Aceh by an average of 110m. Comparing a series of satellite images and topographic surveys, we reconstruct the build-up of a new beach ridge along a 6km long stretch of coastline in the years following the event. We then use the cross-shore model UNIBEST-TC developed for a wave-dominated sandy shoreline to determine the controlling factors of shoreline recovery. Input parameters include bathymetric data measured in 2015, grain size characteristics of offshore sediment samples, modeled wave data, tidal elevations from a nearby tide-gauge station as well as measured and modeled postseismic uplift data. After establishing a cross-shore profile in equilibrium with the prevailing hydrodynamic conditions, we simulate the post-tsunami recovery, the effect of the monsoon seasons, as well as the influence of postseismic land level changes for up to 10years and compare them to the observed coastal development. Our modeling results indicate that the recovery of the western Acehnese shoreline after the 2004 tsunami was quick with littoral sediment transport normalizing to pre-tsunami conditions within two to four years following the event. However, field data shows that the shoreline stabilized 50–90m landward of its pre-2004 tsunami position, most likely due to the build-up of a prominent higher beach ridge in response to coseismic subsidence. Observed variability in shoreline position in the order of a few tens of meters since 2009 can be attributed to seasonal wave climate variability related to the monsoon cycle. The effect of postseismic uplift on shoreline position is small and in the order of only a few meters over 10years, which is 3 to 5 times smaller than long-term coastal progradation rates that are driven by abundant sediment supply to the littoral zone. This overall progradational trend will promote preservation of seismically modified beach ridges, which can serve as paleoseismic indicators.

The sea‐level budget along the Northwest Atlantic coast: GIA, mass changes, and large‐scale ocean dynamics

AbstractSea‐level rise and decadal variability along the northwestern coast of the North Atlantic Ocean are studied in a self‐consistent framework that takes into account the effects of solid‐earth deformation and geoid changes due to large‐scale mass redistribution processes. Observations of sea and land level changes from tide gauges and GPS are compared to the cumulative effect of GIA, present‐day mass redistribution, and ocean dynamics over a 50 year period (1965–2014). GIA explains the majority of the observed sea‐level and land motion trends, as well as almost all interstation variability. Present‐day mass redistribution resulting from ice melt and land hydrology causes both land uplift and sea‐level rise in the region. We find a strong correlation between decadal steric variability in the Subpolar Gyre and coastal sea level, which is likely caused by variability in the Labrador Sea that is propagated southward. The steric signal explains the majority of the observed decadal sea‐level variability and shows an upward trend and a significant acceleration, which are also found along the coast. The sum of all contributors explains the observed trends in both sea‐level rise and vertical land motion in the region, as well as the decadal variability. The sum of contributors also explains the observed acceleration within confidence intervals. The sea‐level acceleration coincides with an accelerating density decrease at high latitudes.

Calibrating coseismic coastal land-level changes during the 2014 Iquique (Mw=8.2) earthquake (northern Chile) with leveling, GPS and intertidal biota

The April 1st 2014 Iquique earthquake (MW 8.1) occurred along the northern Chile margin where the Nazca plate is subducted below the South American continent. The last great megathrust earthquake here, in 1877 of Mw ~8.8 opened a seismic gap, which was only partly closed by the 2014 earthquake. Prior to the earthquake in 2013, and shortly after it we compared data from leveled benchmarks, deployed campaign GPS instruments, continuous GPS stations and estimated sea levels using the upper vertical level of rocky shore benthic organisms including algae, barnacles, and mussels. Land-level changes estimated from mean elevations of benchmarks indicate subsidence along a ~100-km stretch of coast, ranging from 3 to 9 cm at Corazones (18°30’S) to between 30 and 50 cm at Pisagua (19°30’S). About 15 cm of uplift was measured along the southern part of the rupture at Chanabaya (20°50’S). Land-level changes obtained from benchmarks and campaign GPS were similar at most sites (mean difference 3.7±3.2 cm). Higher differences however, were found between benchmarks and continuous GPS (mean difference 8.5±3.6 cm), possibly because sites were not collocated and separated by several kilometers. Subsidence estimated from the upper limits of intertidal fauna at Pisagua ranged between 40 to 60 cm, in general agreement with benchmarks and GPS. At Chanavaya, the magnitude and sense of displacement of the upper marine limit was variable across species, possibly due to species—dependent differences in ecology. Among the studied species, measurements on lithothamnioid calcareous algae most closely matched those made with benchmarks and GPS. When properly calibrated, rocky shore benthic species may be used to accurately measure land-level changes along coasts affected by subduction earthquakes. Our calibration of those methods will improve their accuracy when applied to coasts lacking pre-earthquake data and in estimating deformation during pre–instrumental earthquakes.

Late Pleistocene-Holocene climate and sea level changes inferred based on the tidal terrace sequence, Kachchh, Western India

An estuarine tidal terrace sequence at the mouth of the Kharod River, Kachchh, Western India is investigated for Pleistocene to Holocene climate and sea-level changes. Based on sedimentology, major element geochemistry, mineralogy, supported by optical dating, the study reports a lowered sea level during 13.8ka, followed by gradual rise in the sea level until around 6ka. This is ascribed to a combination of enhanced melt water discharge and strengthened Indian Summer Monsoon (ISM). The temporal changes in major elements (detrital proxies), suggest a fluctuating to an overall arid monsoon conditions during 5.7 to 3.1ka with a rapidly rising sea level. This is further supported by the dominance of fine textured sediment along with the occurrences of shell rich horizon. In the absence of any evidence of land-level changes, the study suggests that at around 6ka to 3ka, the sea was approximately 2m higher then present. The decline in the concentration of geochemical proxies and occurrence of Aeolian sand after 3ka is interpreted as the onset of aridity and present day like conditions. The three levels of sea stands, during – Early, Middle and Late Holocene are synchronous with various archeological (Harappan) settlements in the region.

Detection limits of tidal-wetland sequences to identify variable rupture modes of megathrust earthquakes

Recent paleoseismological studies question whether segment boundaries identified for 20th and 21st century great, >M8, earthquakes persist through multiple earthquake cycles or whether smaller segments with different boundaries rupture and cause significant hazards. The smaller segments may include some currently slipping rather than locked. In this review, we outline general principles regarding indicators of relative sea-level change in tidal wetlands and the conditions in which paleoseismic indicators must be distinct from those resulting from non-seismic processes. We present new evidence from sites across southcentral Alaska to illustrate different detection limits of paleoseismic indicators and consider alternative interpretations for marsh submergence and emergence. We compare predictions of coseismic uplift and subsidence derived from geophysical models of earthquakes with different rupture modes. The spatial patterns of agreement and misfits between model predictions and quantitative reconstructions of coseismic submergence and emergence suggest that no earthquake within the last 4000 years had a pattern of rupture the same as the Mw 9.2 Alaska earthquake in 1964. From the Alaska examples and research from other subduction zones we suggest that If we want to understand whether a megathrust ruptures in segments of variable length in different earthquakes, we need to be site-specific as to what sort of geological-based criteria eliminate the possibility of a particular rupture mode in different earthquakes. We conclude that coastal paleoseismological studies benefit from a methodological framework that employs rigorous evaluation of five essential criteria and a sixth which may be very robust but only occur at some sites: 1 – lateral extent of peat-mud or mud-peat couplets with sharp contacts; 2 – suddenness of submergence or emergence, and replicated within each site; 3 – amount of vertical motion, quantified with 95% error terms and replicated within each site; 4 – syncroneity of submergence and emergence based on statistical age modelling; 5 – spatial pattern of submergence and emergence; 6 – possible additional evidence, such as evidence of a tsunami or liquefaction concurrent with submergence or emergence. We suggest that it is possible to consider detection limits as low as 0.1–0.2 m coseismic vertical change. • Revised methodological framework of 6 criteria for coastal paleoseismic studies. • New evidence from southcentral Alaska to illustrate different detection limits. • No earthquake within last 4000 years had same rupture pattern as 1964 earthquake. • Possible to consider detection limits of 0.1–0.2 m of coseismic land-level change.

Localised Effects of a Mega-Disturbance: Spatiotemporal Responses of Intertidal Sandy Shore Communities to the 2010 Chilean Earthquake.

Determining the effects of unpredictable disturbances on dynamic ecological systems is challenged by the paucity of appropriate temporal and spatial coverage of data. On 27 February 2010, an 8.8 Mw mega-earthquake and tsunami struck central Chile and caused coastal land-level changes, massive damage to coastal infrastructure, and widespread mortality of coastal organisms. Wave-exposed sandy beaches showed significant changes of species abundances from before to after the earthquake, but the highly dynamic biotic and abiotic conditions of these habitats make difficult to draw clear-cut conclusions from these patterns. Here, we analysed a beyond-BACI (Before-After Control-Impact) sampling design to test whether the effects of the Maule earthquake on sandy-shore species diversity, abundance, and structure were heterogeneous along the shore. Invertebrate species abundances were quantified before (i.e. February 2010) and after (i.e. March 2010, September 2010, and March 2011) the earthquake at three sandy shores randomly located within the earthquake rupture area and three sites within a “control” area located >400 km southward from epicentre. Immediately after the earthquake took place, the three sites located in the rupture area showed anomalous beach-profile uplifts that did not comply with the erosion (i.e. “negative” uplifts) that regularly occurs during late summer in the region. Species richness, abundance, and community structure significantly varied from before to after the strike, but these patterns of change varied among sites within both areas. Only the site with the strongest and persistent beach-profile uplift within the rupture area showed significant concomitant changes in species richness and community structure; after 13 months, this community showed a similar multivariate structure to the before-disturbance state. This site, in particular, was located in the section of the rupture area that received most of the impact of the after-earthquake tsunami. Therefore, our results suggest that the effects of the Maule mega-earthquake on the ecological communities were spatially heterogeneous and highly localised. We suggest that high mobility and other species’ adaptations to the dynamic environmental conditions of sandy beaches might explain the comparatively high resilience of these assemblages. With this work we hope to motivate further experimental research on the role of individual- and population-level properties in the response of sandy-beach communities to interacting sources of disturbances.

The Effects on Tsunami Hazard Assessment in Chile of Assuming Earthquake Scenarios with Spatially Uniform Slip

We investigated the effect that along-dip slip distribution has on the near-shore tsunami amplitudes and on coastal land-level changes in the region of central Chile (29°–37°S). Here and all along the Chilean megathrust, the seismogenic zone extends beneath dry land, and thus, tsunami generation and propagation is limited to its seaward portion, where the sensitivity of the initial tsunami waveform to dislocation model inputs, such as slip distribution, is greater. We considered four distributions of earthquake slip in the dip direction, including a spatially uniform slip source and three others with typical bell-shaped slip patterns that differ in the depth range of slip concentration. We found that a uniform slip scenario predicts much lower tsunami amplitudes and generally less coastal subsidence than scenarios that assume bell-shaped distributions of slip. Although the finding that uniform slip scenarios underestimate tsunami amplitudes is not new, it has been largely ignored for tsunami hazard assessment in Chile. Our simulations results also suggest that uniform slip scenarios tend to predict later arrival times of the leading wave than bell-shaped sources. The time occurrence of the largest wave at a specific site is also dependent on how the slip is distributed in the dip direction; however, other factors, such as local bathymetric configurations and standing edge waves, are also expected to play a role. Arrival time differences are especially critical in Chile, where tsunamis arrive earlier than elsewhere. We believe that the results of this study will be useful to both public and private organizations for mapping tsunami hazard in coastal areas along the Chilean coast, and, therefore, help reduce the risk of loss and damage caused by future tsunamis.

Geoarchaeological Perspectives on the “Millennial Series” of Earthquakes in the Southern Puget Lowland, Washington, USA

Surface-breaking ruptures on shallow crustal faults in the southern Puget Lowland in western Washington State about a millennium ago prompted abrupt changes in land level and triggered tsunamis in Puget Sound. The displacement on the Seattle fault most likely occurred in the 1050–1020 cal BP interval. Structures further south (the Tacoma and Olympia faults, and one or more faults in the southern Hood Canal zone) ruptured at about the same time, or slightly earlier. The low frequency of radiocarbon ages from archaeological sites in the region in the aftermath of the “millennial series” of earthquakes, when compared to bootstrapped samples from a database of 1255 ages from the Pacific Northwest as a whole, suggests that these very large earthquakes had significant socioeconomic consequences. The cultural record from coastal archaeological sites shows that although survivors camped on the shore in the aftermath, many coastal villages appear to have been abandoned, and were not reoccupied for several centuries. There is little evidence, however, to suggest that people migrated from southern Puget Sound to neighboring areas, and no evidence of social conflict in the adjacent areas that might have served as havens. DOI: 10.2458/azu_rc.57.18201

Stratigraphic evidence for earthquakes and tsunamis on the west coast of South Andaman Island, India during the past 1000 years

Stratigraphic records from west coast of South Andaman Island revealed evidence of three historical earthquakes and associated transoceanic tsunamis during past 1000yrs, in addition to the Mw 9.3 tsunamigenic earthquake of 26 December, 2004. Our finding suggests that along with Sumatran arc segment the Andaman–Arakan segment is also capable of generating mega-subduction zone earthquakes and transoceanic tsunamis. To study the near sub-surface stratigraphic succession we excavated shallow trenches and obtained geoslices from two sites around Collinpur (sites 1 and 2). The exposed succession comprised 11 lithounits (Unit a — youngest and k — oldest) of alternating sequence of coarser units overlain by peaty soils and some of these are indicative of deposition during paleo-tsunami events.Event I that predated AD 800, and is marked by a 35–40cm thick deposit of fine gravel to coarse sands along with broken shell fragments (Unit k). Event II dated around AD 660–800, is represented by 20–25cm thick coarse sand and broken shell fragments (Unit i). Based on stratigraphic evidences of land-level changes, this event is attributed to a near source rupture along Andaman–Arakan segment, accompanied by a transoceanic tsunami. Event III, occurred around AD 1120–1300, is marked by a 50cm thick sand deposit (Unit g). The 2004 tsunami resulted in deposition of 15cm thick medium to coarse sand at the same location. We infer that the 2004 tsunami and Event III resulted in different styles of sedimentation at the same site.Four events at Collinpur along with the record of a subsidence event of AD 1679 from the east coast of Andaman, close-to, Port Blair (Malik et al., 2011), suggest that mega-subduction zone earthquakes and associated tsunamis recur at an interval of 300–500 years at variable locations along the Sumatra–Andaman subduction zone.