Frequency-magnitude Relation Research Articles

On certain aftershock and foreshock parameters in the area of Greece

Published information (l2) on the aftershocks and foreshocks of many principal shallow earthquakes occurred in the area of Greece between 1911 and 1973 constitutes a more or less homogeneous and complete sample of data in respect to some properties of these seismic sequences. These data have been used to determine certain parameters of these sequences. The value of the decay parameter p, in the time distribution law of aftershocks, is independent of the magnitude range and varies between 0.7 and 1.9 in this area. The smallest value was found for the aftershock sequence of an earthquake believed to be associated with the Kremasta artificial lake, while the largest value was found for the aftershock sequence of an earthquake occurred in the volcanic part of the Hellenic arc. The probability, N, that the largest aftershock will occur Tl days after the main shock or later is given by a relation of the form N = c — k log Tl. Representative values of the parameter b, in the frequency-magnitude relation, have been found for the foreshocks as well as for the aftershocks of the same main shocks, by a proper grouping of the data. This value is equal to 0.67 for foresliocks and equal to 0.92 for the corresponding aftershocks. The difference in magnitude between the main shock and the largest aftershock is almost independent of the magnitude of the main shock. The relation M0 — = 1.1 holds on an average.

Fractal dimension and b-value mapping in the Andaman-Sumatra subduction zone

The Andaman-Sumatra subduction zone is seismically one of the most active and complex subduction zones that produced the 26 December 2004 mega thrust earthquake (Mw 9.3) and large number of aftershocks. About 8,000 earthquakes, including more than 3,000 aftershocks (M ≥ 4.5) of the 2004 earthquake, recorded during the period 1964–2007, are relocated by the EHB method. We have analysed this large data set to map fractal correlation dimension (Dc) and frequency-magnitude relation (b-value) characteristics of the seismogenic structures of this ~3,000-km-long mega thrust subduction zone in south-east Asia. The maps revealed the seismic characteristics of the Andaman-Sumatra-Java trenches, West Andaman fault (WAF), Andaman Sea Ridge (ASR), Sumatra and Java fault systems. Prominent N–S to NW–SE to E–W trending fractal dimension contours all along the subduction zone with Dc between 0.6 and 1.4 indicate that the epicentres mostly follow linear features of the major seismogenic structures. Within these major contours, several pockets of close contours with Dc ~ 0.2 to 0.6 are identified as zones of epicentre clusters and are inferred to the fault intersections as well as asperity zones along the fault systems in the fore arc. A spatial variation in the b-value (1.2–1.5) is also observed along the subduction zone with several pockets of lower b-values (1.2–1.3). The smaller b-value zones are corroborated with lower Dc (0.5–0.9), implying a positive correlation. These zones are identified to be the zones of more stress or asperity where rupture nucleation of intermediate to strong magnitude earthquakes occurred.

Seismic cluster analysis for the Burmese–Andaman and West Sunda Arc: insight into subduction kinematics and seismic potentiality

The Burmese–Andaman Arc System (BAAS) and the West Sunda Arc (WSA) in NE Indian Ocean are well known for their high seismic hazard and tsunami potentiality. Seismicity is caused by eastward subduction of the Indian plate to intermediate focal depths below the BAAS, but the penetration depth goes even deeper to about 500 km below the WSA. The seismicity map and its correlation to crustal and mantle faults for this extensive plate margin are presented. This is achieved by using frequency–magnitude relationship to select larger (m b ≥ 5.0) and comparatively well-recorded events from the available earthquake catalogue that span for a period of little more than a century (1906–2008). Barely 14% of the events qualify the treatment, and the events so selected are subjected to cluster analysis using a statistical function ‘point density’. The clusters found for the arc demonstrate significant relationship to subduction geometry in their respective areas; 11 out of a total of 13 clusters commonly originate below the fore arc. Earthquakes within the individual clusters have linear fractal geometry consistent with the traces of seismogenic surfaces that actually produce them. Correlation of clusters to seismologic depth sections and the composite results derived from 518 CMT solutions of earthquakes establish a close spatial relationship between the shape and orientation of the clusters with stress axes and regional tectonics. This provides a three-dimensional perspective on the stress distribution within the respective clustered seismic zones. Seismic potentiality for five most conspicuous clusters is also inferred.

Aftershock sequences of two great Sumatran earthquakes of 2004 and 2005 and simulation of the minor tsunami generated on September 12, 2007 in the Indian Ocean and its effect

We present the seismic energy, strain energy, frequency–magnitude relation (b-value) and decay rate of aftershocks (p-value) for the aftershock sequences of the Andaman–Sumatra earthquakes of December 26, 2004 (M w 9.3) and March 28, 2005 (M w 8.7). The energy released in aftershocks of 2004 and 2005 earthquake was 0.135 and 0.365% of the energy of the respective mainshocks, while the strain release in aftershocks was 39 and 71% for the two earthquakes, respectively. The b-value and p-value indicate normal value of about 1. All these parameters are in normal range and indicate normal stress patterns and mechanical properties of the medium. Only the strain release in aftershocks was considerable. The fourth largest earthquake in this region since 2004 occurred in September 2007 off the southern coast of Island of Sumatra, generating a relatively minor tsunami as indicated by sea level gauges. The maximum wave amplitude as registered by the Padang, tide gauge, north of the earthquake epicenter was about 60 cm. TUNAMI-N2 model was used to investigate ability of the model to capture the minor tsunami and its effect on the eastern Indian Coast. A close comparison of the observed and simulated tsunami generation, propagation and wave height at tide gauge locations showed that the model was able to capture the minor tsunami phases. The directivity map shows that the maximum tsunami energy was in the southwest direction from the strike of the fault. Since the path of the tsunami for Indian coastlines is oblique, there were no impacts along the Indian coastlines except near the coast of epicentral region.

Seismicity and focal mechanism of some recent earthquakes in North-east India and neighbourhood

The seismicity of northeast India has been studied in thelight of the known geotectonic features based on the data from 1881 to 1972.Now focal mechanism solutions of earthquakes using double couple hypothesishave been discussed with reference to the plate theory of global tectonicsand the geological faults.It was found that the constant b in Gutenberg-Ricliter's frequencymagnitude relationship was nearly I for both the periods of observationsnamely 1920-1972 and 1963-1972 which was in agreement with the otherhighly seismic regions of the world. Most of the new fault plane solutionsshow significant strike slip movements with thrust faulting in this region.Near the main boundary fault in northeast Himalaya, the pressures areoriented almost along the strike of the fault while along the Manipur-Burinaborder region, the pressures are acting at right angles or at acute angles tothe mountains and are shallow dipping.

A generalization of the frequency-magnitude relation in the hypothesis of a maximum regional magnitude

In this work a new non-linear theoretical cumulative frequency-magnitude relation is proposed, that better fits the experimental data in the high magnitude range than the classical Gutenberg-Richter's relation does. The parameters of the relation are fi, which is connected with the parameter b of Gutenberg-Richter's relation, and the value of maximum possible regional magnitude Mp. The relation is very close to the Gutenberg-Richter's one when the value of Mp is very high. Finally some application examples are given.

Contribution of dendrochronology to the determination of magnitude–frequency relationships for landslides

The determination of magnitude–frequency (M–F) relationships is necessary for landslide hazard assessment. Ideally, this is achieved by using inventories of past landslide events recorded by technical units or aerial photo interpretation. However, direct sources of landslide data are often unavailable. Dendrogeomorphological techniques allow us to determine the frequency of events, but few attempts have been made to estimate landslide magnitude by analysing the spatial distribution of disturbed trees. This paper addresses M–F determination for rockfalls, debris flows and complex slides obtained by dendrogeomorphology. The cases presented require the appropriate understanding of both the geomorphological context and the disturbance caused by landslide motion on trees. Both small and large rockfalls can be identified and dated by reconstructing the trajectories in the forest stand. The larger the rockfall size, the longer the path, the higher the number of affected trees and the greater the area of deposition of rock fragments. The area of damaged trees is proposed as an indicator of the rockfall magnitude. The frequency and extent of a series of debris flows were reconstructed by mapping their deposits using relative dating criteria and by dating damaged trees. Local and global mudslide reactivations were deduced by sampling various tree generations and by analysing the location of the disturbed trees in the landslide units. Magnitude–cumulative frequency curves were determined for the three cases.

Pattern recognition of major asperities using local recurrence time in Alborz Mountains, Northern Iran

In this study, seismic data recorded during the period 01/01/1996 to 09/01/2009 has been used to evaluate the seismic hazard potential along the Alborz region, Northern Iran. The technique of mapping local recurrence time, TL, is used to map major asperities, which are considered as the areas with maximum hazard. We calculated TL from a and b values which are in turn derived from the frequency–magnitude relation constants within a radius of 30 km about every corner point of a 10-km spacing grid. Since b value is inversely related to applied stress, the areas with lowest b values and/or shortest TL are interpreted to locate the asperities or the areas of maximum seismic hazard. To test this method, we computed TL map using seismic catalogues before and after the 2004 Baladeh earthquake of Mw 6.2. The local recurrence time map before the earthquake shows anomalously short TL in the epicentral region of the Baladeh earthquake a decade before its occurrence. The TL map after the earthquake indicates that this large event has redistributed the applied stress in the Alborz region. The microseismicity of the region after the Baladeh earthquake, however, suggests that there are two anomalies in TL map positioned in Alborz. The places where these anomalies are observed can be considered as the areas with maximum seismic hazard for future large earthquake in the Alborz region.

Rock-glacier dynamics and magnitude–frequency relations of debris flows in a high-elevation watershed: Ritigraben, Swiss Alps

A widespread risk in high mountains is related to the accumulation of loose sediments on steep slopes, which represent potential sources of different types of geomorphic processes including debris flows. This paper combines data on 50 yr of permafrost creep at the Ritigraben rock glacier (Valais, Swiss Alps) with magnitude–frequency (M–F) relationships of debris flows recorded in the Ritigraben torrent originating at the rock-glacier front. Debris production and volumetric changes at the rock-glacier front are compared with debris-flow activity recorded on the cone and potential couplings and feedbacks between debris sources, channel processes and debris sinks. The dataset existing for the Ritigraben rock glacier and its debris-flow system is unique and allows prime insights into controls and dynamics of permafrost processes and related debris-flow activity in a constantly changing and warming high-altitude environment. Acceleration in rock-glacier movement rates is observed in the (1950s and) 1960s, followed by a decrease in flow rates by the 1970s, before movements increase again after the early 1990s. At a decadal scale, measured changes in rock-glacier movements at Ritigraben are in concert with changes in atmospheric temperatures in the Alps. Geodetic data indicates displacement rates in the frontal part of the rock glacier of up to 0.6–0.9 m yr − 1 since the beginning of systematic measurements in 1995. While the Ritigraben rock glacier has always formed a sediment reservoir for the associated debris-flow system, annual horizontal displacement rates of the rock-glacier body have remained quite small and are in the order of decimeters under current climatic conditions. Sediment delivery from the rock-glacier front alone could not therefore be sufficient to support the 16 debris flows reconstructed on the cone since 1958. On the contrary, debris accumulated at the foot of the rock glacier, landslide and rockfall activity as well as the partial collapse of oversteepened channel walls have to be seen as important sediment sources of debris flows at Ritigraben and would represent 65–90% of the material arriving on the Ritigraben cone. There does not seem to exist a direct coupling between displacement rates of and sediment delivery by the rock-glacier body and the frequency of small- and medium-magnitude debris flows. In contrast, a direct link between source and sink processes clearly exists in the case of active-layer failures. In this case, failure processes at the rock-glacier snout and debris-flow events in the channel occur simultaneously and are both triggered by the rainfall event.

Long Period Ground Motion at Bedrock Level in Delhi City from Himalayan Earthquake Scenarios

Delhi, the capital of India, is prone to severe seismic hazards, not only from local events but also from Himalayan earthquakes at distances of 250–300 km. Standard techniques are not sufficiently reliable to completely characterize the seismic hazards in this case due to the difficulty of predicting the occurrence of earthquakes (frequency–magnitude relations) and of properly treating the propagation of their effects (attenuation laws), especially their long-period components. In order to give a sound description of the seismic ground motion due to an earthquake in such a given range of distances (and magnitudes), we use modelling techniques developed from physics of the seismic source generation and propagation processes. Such models take into account the directivity effect of rupture propagation and the attenuation of (long-period) ground motions. The generated ground motion scenarios permit us to build a very important knowledge base to be fruitfully used by civil engineers, since long period ground motions, especially if amplified by deep sedimentary basins, can represent a severe threat for large scale structures (e.g. lifelines and bridges) and tall buildings, which are widespread in fast-growing megacities. In this study, we simulate the ground motion, at bedrock level, in Delhi city, for an earthquake scenario corresponding to a source of Mw = 8.0 located in the central seismic gap of Himalayas, at an epicentral distance of about 300 km from Delhi city. By means of several parametric studies, we simulate the time histories using Size Scaled Point Source, Space and Time Scaled Point Source and Extended Source models. Together with the complete time histories (displacements, velocities and accelerations, from which the peak amplitudes have been extracted), we have also used the displacement response spectrum to characterize the seismic input at Delhi. Not only is the displacement response spectrum of great significance to modern displacement-based design engineering approaches, but it is probably the best parameter by which to characterize the destructiveness potential of earthquakes located at such great distances from the target sites (of the order of 300 km), since the energy of the seismic input is mainly concentrated at long periods (in general, greater than 1 s) and it cannot be determined by straightforward integration of velocity or acceleration response spectra.

Ecogeomorphic feedbacks in regrowth of travertine step-pool morphology after dam decommissioning, Fossil Creek, Arizona

The linkages between fluvial geomorphology and aquatic ecosystems are commonly conceptualized as a one-way causal chain in which geomorphic processes create the physical template for ecological dynamics. In streams with a travertine step-pool morphology, however, biotic processes strongly influence the formation and growth of travertine dams, creating the potential for numerous feedbacks. Here we take advantage of the decommissioning of a hydroelectric project on Fossil Creek, Arizona, where restoration of CaCO 3-rich baseflow has triggered rapid regrowth of travertine dams, to explore the interactions between biotic and abiotic factors in travertine morphodynamics. We consider three conceptual frameworks, where biotic factors independently modulate the rate of physical and chemical processes that produce travertine dams; combine with abiotic factors in a set of feedback loops; and work in opposition to abiotic processes, such that the travertine step-pool morphology reflects a dynamic balance between dominantly-biotic constructive processes and dominantly-abiotic destructive processes. We consider separately three phases of an idealized life cycle of travertine dams: dam formation, growth, and destruction by erosive floods. Dam formation is catalyzed by abiotic factors (e.g. channel constrictions, and bedrock steps) and biotic factors (e.g. woody debris, and emergent vegetation). From measurements of changes over time in travertine thickness on a bedrock step, we find evidence for a positive feedback between flow hydraulics and travertine accrual. Measurements of organic content in travertine samples from this step show that algal growth contributes substantially to travertine accumulation and suggest that growth is most rapid during seasonal algal blooms. To document vertical growth of travertine dams, we embedded 252 magnets into nascent travertine dams, along a 10 km stretch of river. Growth rates are calculated from changes over time in the magnetic field intensity at the dam surface. At each magnet we record a range of hydraulic and travertine composition variables to characterize the dominant mechanism of growth: abiotic precipitation, algal growth, trapping of organic material, or in situ plant growth. We find: (1) rapid growth of travertine dams following flow restoration, averaging more than 2 cm/year; (2) growth rates decline downstream, consistent with loss of dissolved constituents because of upstream travertine deposition, but also parallel to a decline in organic content in dam surface material and a downstream shift in dominant biotic mechanism; (3) biotic mechanisms are associated with faster growth rates; and (4) correlations between hydraulic attributes and growth rates are more consistent with biotic than abiotic controls. We conclude that the strong influence of living organisms on rates of travertine growth, coupled with the beneficial effects of travertine on ecosystem dynamics, demonstrate a positive feedback between biology and geomorphology. During our two-year study period, erosive flood flows occurred causing widespread removal of travertine. The temporal distribution of travertine growth and erosion over the study period is consistent with a bimodal magnitude–frequency relation in which growth dominates except when large, infrequent storms occur. This model may be useful in other systems where biology exerts strong controls on geomorphic processes.

Frequency-Magnitude Characteristics Down to Magnitude -4.4 for Induced Seismicity Recorded at Mponeng Gold Mine, South Africa

This article aims to investigate the frequency-magnitude characteristics and lower magnitude limits of the microseismic catalog recorded with a seismic network sensitive to high frequencies at Mponeng mine, South Africa. The network, composed of one three-component accelerometer and eight acoustic emission sensors, is located at a depth of 3.5 km below the surface and covers the limited volume of approximately 300×300×300 m. The three-component accelerometer was used to estimate the moment magnitude for the limited number of 135 events ( M w ranged from -4.1 to -0.3) well recorded by the network. We use the relation between the moment magnitude estimated from accelerometer data and radiated energy/moment magnitude estimated from acoustic emission sensors to extend the catalog to lower magnitudes. The magnitude of completeness of selected spatiotemporal subsets of the catalog was estimated for: (1) an aftershock sequence of an M w 1.9 event that occurred approximately 30 m from the network, and (2) postblasting activity during working days, located more than 80 m from the network. The data follow the Gutenberg–Richter (GR) frequency-magnitude relationship with no visible deviation from self-similar behavior of seismicity between M w -4.4 and -1.9 for the aftershock sequence and between -3.5 and -1.5 for the postblasting dataset. We estimated the magnitude of completeness of selected subset as low as -4.3 ( b =1.26) for the aftershock sequence and -3.4 ( b =1.17) for the postblasting activity. Differences in magnitude of completeness are attributed to location of recorded activity and site effects.

Probability of Peak Ground Horizontal and Peak Ground Vertical Accelerations at Tehran and Surrounding Areas

The active faults near Tehran are capable of producing Mw magnitudes of 6.62–7.23; at epicentral distances of 25 km from the active faults, and Mw = 7.23, the peak ground horizontal acceleration, PGH, is between 386 and 730 cm/s2 and peak ground vertical acceleration, PGV, is between 192 and 261 cm/s2; the historic earthquake of the fourth century BC, Mw = 7.16 produced the highest estimated PGH acceleration in Tehran, between 254 and 479 cm/s2 and PGV acceleration between 127 and 173 cm/s2. Earthquakes from 1909 to 2008, within 300 km from Tehran, are used for calculation of magnitude frequency relation, and results are applied to estimate PGH and PGV accelerations as a function of magnitudes for a set of fixed epicentral distance and site conditions; also as a function of epicentral distances for a set of fixed magnitudes and site conditions. Poisson’s distribution is used for probability calculation of PGH and PGV accelerations for several exposure times, site conditions and epicentral distances; accelerations with very high probability, near 1, are presented. At an epicentral distance of 10 km and exposure time of 450 years, in the northern part of Tehran, close to Mosha and the Northern Tehran faults, PGH acceleration is 800–420 cm/s2 and PGV is 400–220 cm/s2 with high probability. At an epicentral distance of 25 km and 1,000 years exposure time, PGH is 610–320 cm/s2 and PGV is 310–160 cm/s2 with high probability, where larger values are for soft soil and smaller values are for hard rock.

Comment on "Is There a Basis for Preferring Characteristic Earthquakes over a Gutenberg-Richter Distribution in Probabilistic Earthquake Forecasting" by Tom Parsons and Eric L. Geist

In their short note Parsons and Geist (2009) claim that the use of a Gutenberg–Richter relationship is justified for describing the magnitude–frequency relationship of single faults. I will show that their conclusions are biased. The assumption implicitly made by the authors that single faults can produce an uncountable number of earthquakes of different magnitudes directly leads to the consequence that a Gutenberg–Richter relationship describes such a data set (created by simulation) reasonably well.

Appearance of extreme monsoonal rainfall events and their impact on erosion in the Himalaya

Monsoonal rainfall in the Himalaya dominates erosion and sediment transport through the fluvial system. In addition to the strong seasonal nature of the Indian summer monsoon, striking interannual variations in monsoonal strength characterize longer records. For example, during any given year, rain may penetrate further into the orogen, even though peak rainfall amounts almost always occur at topographic barriers in regions with high relief, regardless of overall monsoonal strength. Tropical Rainfall Measurement Mission (TRMM) product 3B42 rainfall is first used to document the spatial rainfall distribution and then the TRMM time series are used to identify temporal and seasonal rainfall variations. A simple, but robust magnitude-frequency relation for each rainfall pixel is used to show that rainfall greater or equal to the 90th percentile occurs at least twice as often in mountainous terrain as in low-elevation regions. Previous field-based observations and measurements show that this significantly higher number of extreme events leads to higher erosion volumes and greater fluvial-mass transport rates. The spatiotemporal context of these extreme events helps to predict occurrences of high sediment flux and could underpin the strategic development of preventative measures. Improved statistics for extreme events are key to mitigating the filling of hydropower reservoirs and abrasion of hydropower turbines, as well as to sustaining infrastructure and successful agriculture in the downstream section of the Himalaya.

Volcanic ashfall in New Zealand – probabilistic hazard modelling for multiple sources

The airfall ash component of the New Zealand Probabilistic Volcanic Hazard Model (PVHM) estimates the likelihood of volcanic ash deposits of any given thickness at any site, based on the frequency-magnitude relations of all the significant volcanic sources in New Zealand and the wind distribution statistics. The main source of error in these models is the uncertainty in the historic ash production records of the volcanoes. Another source of error is that the current wind pattern may not be the same as during the period that the observed ash record was deposited. The spatial distribution and magnitude/frequency curves of volcanoes are very different from those of earthquakes, and this is reflected in the hazard models. Whereas a typical probabilistic earthquake hazard model will have the expected acceleration increase by a factor of 3 to 5 between 500 and 10,000 year return periods, the expected ash thickness can change by up to a factor of 1000 over these periods. The Auckland Volcanic Field is now occupied by New Zealand's largest city, so the comparatively small and infrequent eruptions potentially represent a major social and financial hazard. Our modelling of lake core ash records in the Auckland area suggests that the known vents and eruptions probably represent less than half of the total volcanic activity from the field, and that the hazard from local events is comparable to that from more distant sources. An area needing improvement is the hazard in Auckland from local eruptions, with more ash thickness data and better basaltic eruption models.

On the Influence of the Rheological Properties of a Medium upon some Regularities of its Seismic Regime

In connection with the problems of the seismic regime a phenomenological model of a heterogeneous elastico-viscous medium is offered in which two compensating processes take place: the fractures increase and the strains relax. The results of the model investigation are compared with laboratory experiments. Tt is ascertained that the ratio between the number and dimension of fractures, developing within a deformating medium, is determined by the ratio between its elastic, relaxation and strength properties and also by the statistical distribution of heterogeneities and deformation velocity. These factors influence the ratio between the number of fractures and the value of elastic energy released by each one of them, and also the maximum value of elastic energy, that can be released while forming a single fracture (characteristics, analogous to the ‘magnitude-frequency relation’ and the ‘maximum intensity’ of earthquakes). The comparison of the theoretical conclusions with the field seismic data can only be made yet qualitatively. Nevertheless there is no doubt the parameters of the seismic regime in nature depend on rheological properties of the Earth's material as well as on the rate of its deformation in a given region, and we may suppose that our study can give some idea of explaining the physical character of this dependence.

Extreme value statistics and thermodynamics of earthquakes: aftershock sequences

The Gutenberg-Richter magnitude-frequency law takes into account the minimum detectable magnitude, and treats aftershocks as if they were independent and identically distributed random events. A new magnitude-frequency relation is proposed which takes into account the magnitude of the main shock, and the degree to which aftershocks depend on the main shock makes them appear clustered. In certain cases, there can be two branches in the order-statistics of aftershock sequences: for energies below threshold, the Pareto law applies and the asymptotic distribution of magnitude is the double-exponential distribution, while energies above threshold follow a one-parameter beta distribution, whose exponent is the cluster dimension, and the asymptotic Gompertz distribution predicts a maximum magnitude. The 1957 Aleutian Islands aftershock sequence exemplifies such dual behavior. A thermodynamics of aftershocks is constructed on the analogy between the non-conservation of the number of aftershocks and that of the particle number in degenerate gases.

Seasonal precipitation gradients and their impact on fluvial sediment flux in the Northwest Himalaya

Precipitation in the form of rain and snowfall throughout the Himalaya controls river discharge and erosional processes and, thus, has a first-order control on the fluvial sediment flux. Here, we analyze daily precipitation data (1998–2007) of 80 weather stations from the northwestern Himalaya in order to decipher temporal and spatial moisture gradients. In addition, suspended sediment data allow assessment of the impact of precipitation on the fluvial sediment flux for a 10 3-km 2 catchment (Baspa). We find that weather stations located at the mountain front receive ∼ 80% of annual precipitation during summer (May–Oct), whereas stations in the orogenic interior, i.e., leeward of the orographic barrier, receive ∼ 60% of annual precipitation during winter (Nov–Apr). In both regions 4–6 rainstorm days account for ∼ 40% of the summer budgets, while rainstorm magnitude–frequency relations, derived from 40-year precipitation time-series, indicate a higher storm variability in the interior than in the frontal region. This high variability in maximum annual rainstorm days in the orogenic interior is reflected by a high variability in extreme suspended sediment events in the Baspa Valley, which strongly affect annual erosion yields. The two most prominent 5-day-long erosional events account for 50% of the total 5-year suspended sediment flux and coincide with synoptic-scale monsoonal rainstorms. This emphasizes the erosional impact of the Indian Summer Monsoon as the main driving force for erosion processes in the orogenic interior, despite more precipitation falling during the winter season.

Magnitude–frequency relationships of debris flows — A case study based on field surveys and tree-ring records

Debris-flow activity in a watershed is usually defined in terms of magnitude and frequency. While magnitude–frequency (M–F) relations have long formed the basis for risk assessment and engineering design in hydrology and fluvial hydraulics, only fragmentary and insufficiently specified data for debris flows exists. This paper reconstructs M–F relationships of 62 debris flows for an aggradational cone of a small (< 5 km 2), high elevation watershed in the Swiss Alps since A.D. 1863. The frequency of debris flows is obtained from tree-ring records. The magnitude of individual events is given as S, M, L, XL, and derived from volumetric data of deposits, grain size distributions of boulders, and a series of surrogates (snout elevations, tree survival, lateral spread of surges). Class S and M debris flows (< 5 × 10 3 m 3) encompass a typical size of events and have mean recurrence intervals of 5.4 (SD: 3.2) and 7.4 years (SD: 6.7), respectively. Class XL events (10 4–5 ×10 4 m 3) are, in contrast, only identified three times over the past 150 years, and major erosional activity on the cone was restricted to two of these events in 1948 and 1993. A comparison of results with hydrometeorological records shows that class L and XL events are typically triggered by advective storms (rainfall > 50 mm) in August and September, when the active layer of the rock glacier in the source area of debris flows is largest. Over the past ∼ 150 years, climate has exerted control on material released from the source area and prevented triggering of class XL events before 1922. With the projected climatic change, permafrost degradation and the potential increase in storm intensity are likely to produce “ class XXL” events in the future with volumes surpassing 5 × 10 4 m 3 at the level of the debris-flow cone.