unprecedented evidence for deep convection hydrating the tropical stratosphere 1 we report on in situ and remote sensing measurements of ice particles in the tropical stratosphere found during the geophysica campaigns troccinox and scout o3 we show that the deep convective systems penetrated the stratosphere and deposited ice particles at altitudes reaching 420 k potential temperature these convective events had a hydrating effect on the lower tropical stratosphere due to evaporation of the ice particles in contrast there were no signs of convectively induced dehydration in the stratosphere

gully systems under undisturbed indigenous forest east coast region new zealand abstract gully erosion is commonly associated with agricultural landscapes where vegetation clearance and farming practices increase runoff leading to fluvial incision however gully erosion can also occur in forests that have undergone some form of disturbance either natural or resulting from human impacts this paper reports on recent gully development within areas of undisturbed indigenous forest as a result of a high magnitude rainfall event on the east coast of new zealand s north island this region through a combination of crushed and sheared rock types steep topography and tectonic and climatic setting has high natural rates of erosion exacerbated by european deforestation in the late 19th and early 20th centuries sequential air photographs spanning a 58 year period between 1939 and 1997 were used to classify and document the growth and recovery of gully systems in the 14 1 km 2 headwaters of the mangaoporo catchment following a severe cyclone in 1988 with a rainfall of 535 mm there were 21 active gully systems within the indigenous forest on photography prior to 1988 only four gully systems were present during this period there were 8 major rainfall events 150 250 mm despite further 5 rainfall events of 150 250 mm between 1988 and 1997 all gully systems showed signs of recovery with a combined reduction in active area of 37 the nature and location of these features is strongly influenced by lithology orientation of jointing and bedding and to a topographic threshold defined by catchment slope and catchment area

the oaxaca mexico earthquake of 1931 lithospheric normal faulting in the subducted cocos plate a study of the great earthquake in oaxaca mexico on 15 january 1931 shows that it was a normal fault earthquake striking east west the lack of evidence of surface faulting near the source region1 and the focal depth of 40 km inferred from body wave modelling suggest that the earthquake occurred within the subducted slab the large size of the earthquake ms 8 0 strongly suggests that the rupture broke through the entire lithosphere of the subducted cocos plate other large normal fault earthquakes in the japanese2 aleutian3 peruvian4 and sumba5 arcs show lithospheric faulting seawards of the trench or very close to it the 1931 event however occurred farther down dip in the slab 150 km landward of the trench below the bottom of what appears to be the seismogenic plate interface we speculate here that large although infrequent normal fault earthquakes such as the 1931 oaxaca earthquake may have a role in decoupling at depth the subducted slab from the upper part of the slab and perhaps from the overriding mexican continental plate the epicentral location of the 1858 michoacan earthquake6 ms 7 5 suggests that it may represent another example of lithospheric normal faulting in the middle american arc such earthquakes for which the periods of recurrence are unknown constitute an extremely high seismic risk to population centres of mexico

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

high frequency earthquake ground motion scaling in utah and yellowstone velocity seismograms from the university of utah seismograph stations are used to quantify high frequency ground motion scaling for the yellowstone region and the wasatch front in utah this study uses a data set consisting of over 3000 waveforms from 93 stations and 506 regional earthquakes and mining associated seismic events that occurred in the years 1995 2001 all events lie within the upper crust with magnitudes less than 5 5 the signals were processed to examine the peak ground velocity and fourier velocity spectra in the frequency range of 1 0 16 0 hz regression results for utah are characterized by rapid decreases of amplitude at short distances which requires low q and a rapid geometrical spreading g r for the forward modeling q f 160 f 0 75 eff 0 05 sec and 300 bars are selected to predict the excitation terms when combined with the derived geometrical spreading the frequency dependent excitation of mining associated events differs significantly from that of earthquakes because of either an additional attenuation due to shallow source depth or apparent low stress drops associated with the slow collapse of long tunnels a q f 140 f 0 70 and geometrical spreading less rapid than r 1 0 at short distance ranges are used to describe the wave propagation distance dependence in the yellowstone region the yellowstone attenuation functional d r f is very different from that of utah perhaps because a shallow wave guide in the caldera traps more energy the yellowstone excitation spectra were modeled using 30 bars and eff 0 04 sec manuscript received 24 october 2003

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

the lithosphere asthenosphere boundary in the tien shan karakoram region from s receiver functions evidence for continental subduction 1 significant s to p converted wave energy from the crust mantle and lithosphere asthenosphere boundaries the moho and the lab respectively has been observed in the tien shan and karakoram orogenic belts and used for the determination of the thickness of the crust and the mantle lithosphere the moho lies at depths of 45 65 km in the tien shan and adjacent regions and deepens to 70 km depth in pamir and karakoram the thickness of the lithosphere varies between 90 and 120 km underneath the tien shan and increases to a value of 160 km beneath the tarim basin to the south of the tarim basin the lower boundary of the asian lithosphere can be clearly followed to a depth of 270 km beneath central pamir and karakoram while the indian lithosphere is observed beneath the karakoram dipping from 130 km to 170 km towards the north the observations indicate a scenario of continental collision and subduction consistent with seismicity and the surface wave tomography results

fault kinematics in northern central america and coupling along the subduction interface of the cocos plate from gps data in chiapas mexico guatemala and el salvador new gps measurements in chiapas mexico guatemala and el salvador are used to constrain the fault kinematics in the north america na caribbean ca and cocos co plates triple junction area the regional gps velocity field is first analysed in terms of strain partitioning across the major volcano tectonic structures using elastic half space modelling then inverted through a block model we show the dominant role of the motagua fault with respect to the polochic fault in the accommodation of the present day deformation associated with the na and ca relative motion the na ca motion decreases from 18 22 mm yr 1 in eastern guatemala to 14 20 mm yr 1 in central guatemala assuming a uniform locking depth of 14 28 km down to a few millimetres per year in western guatemala as a consequence the western tip of the ca plate deforms internally with 9 mm yr 1 of east west extension 5 mm yr 1 across the guatemala city graben alone up to 15 mm yr 1 of dextral motion can be accommodated across the volcanic arc in el salvador and southeastern guatemala the arc seems to mark the northern boundary of an independent forearc sliver ar pinned to the na plate the inversion of the velocity field shows that a four block na ca co and ar model that combines relative block rotations with elastic deformation at the block boundaries can account for most of the gps observations and constrain the overall kinematics of the active structures this regional modelling also evidences lateral variations of coupling at the co subduction interface with a fairly high coupling 0 6 offshore chiapas and low coupling 0 25 offshore guatemala and el salvador

non reversible geosystem destabilisation at 4200 cal bp sedimentological geochemical and botanical markers of soil erosion recorded in a mediterranean alpine lake a 144 cm long core was obtained in lake petit 2200 m a s l mediterranean french alps in order to reconstruct past interactions between humans the environment and the climate over the last five millennia using a multidisciplinary approach involving sedimentological geochemical and botanical analyses we show a complex pattern of environmental transformation from 4800 to 4200 cal bp podzol type soils progressively developed under forest cover this stable situation was interrupted by a major detrital pulse at 4200 cal bp that we consider as a tipping point in the environmental history at this point pedogenetic processes drastically regressed leading to the development of moderately weathered soils more frequent detrital inputs are recorded since 3000 cal bp ad 1050 as the human impact significantly increased in the catchment area we conclude that destabilisation of the environment was triggered by climate and exacerbated by human activities to a stage beyond resilience

imaging the moho and subducted oceanic crust at the isthmus of tehuantepec mexico from receiver functions using teleseismic data recorded along a transect which we call veox for veracruz oaxaca seismic line of 46 broadband stations installed across the isthmus of tehuantepec in southern mexico we obtained receiver functions and stacked them to study the moho topography and back projected them to visualize the subducted slab geometry beneath the isthmus we observed a back azimuth dependent moho thickness across the transect particularly beneath the los tuxtlas volcanic field also we observed the cocos plate which subducts with an angle of 26 between 140 and 310 km from the trench comparison with regional seismicity indicates that it occurs below the oceanic crust

lg coda q variation across eurasia and its relation to crustal evolution an extensive set of broadband and short period seismograms from regional seismic events has been used to derive a tomographic image of broad scale variations of lg coda q for most of eurasia the image indicates that lg coda q at a frequency of 1 hz varies between about 200 and 1000 lowest q values 800 are found in four regions the east european shield the siberian shield the indian shield and the southwestern and oldest portion of the altaid belt the pattern of lg coda q variation across eurasia suggests that crustal q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large scale compressional tectonic activity or upper mantle heating that occurred there we interpret the low q as resulting largely from hydrothermal fluids generated by tectonic activity or heating that now reside in cracks and permeable portions of the eurasian crust crustal q increases with time after that activity as fluids are lost to the surface or absorbed by retrograde metamorphism

thin surface soil layers attributable to rain flow transportation on low angle slopes an example from semi arid tropical queensland australia particle size analyses have been made on surface soils and a thin overlying sandy pavement at 16 sites along a gently sloping transect at torrens creek in north queensland the transect is 1590 m long and has a maximum gradient of 0 018 less than 1 the sandy pavement is enriched in coarse components due to the sorting and differential downslope transport of the finer soil fractions particle movement is ascribed to the action of raindrops impacting very shallow flowing water the process referred to as rain flow transportation is active wherever overland flow is too weak to generate rills and channels and occurs in areas of very low slope with permeable surface soils that allow rapid infiltration during storms rain flow transportation is discussed with particular reference to the development of soil profiles in an old subdued landscape of semi arid tropical australia

rainfall runoff relationship over encrusted dune surfaces nizzana western negev israel a thin microbial crust covers the lower part of longitudinal dunes in the western negev where average annual rainfall is 95 mm in order to study the effect of the microbial crust on rainfall runoff relationships under natural rainfall conditions runoff plots equipped with pressure gauges were established on opposite north and south facing slopes that differ in their vegetal cover and crust properties the study covered four years 1990 94 the first two years were wet and the following two years relatively dry one to five flow events were recorded per year no correlation was found between runoff yield rain amount and rain intensity unlike many microbial crusts reported in the literature the microbial crust in the western negev is not hydrophobic infiltration rate is high under dry surface conditions and of the order of 9 12 mm h 1 when the crust is saturated the high final infiltration rate is explained by the occurrence of large pores that do not seal when the crust is saturated typical hydrographs have very steep rising and falling limbs pointing at a limited contributing area most flows last less than 10 min and runoff volumes collected are on the whole very small owing to differences in crust properties runoff is higher on north than on south facing slopes 1997 john wiley sons ltd

evidence for great tsunamigenic earthquakes m 8 6 along the mexican subduction zone abstract a very large earthquake took place on 28 march 1787 along the mexican subduction zone a unique characteristic of the 1787 event is the large tsunami reported at various coastal locations in southern mexico the segment of the coast affected by the strong tsunami coincides with locations where high felt intensities modified mercalli intensity mmi viii were reported assuming as is generally the case for large and great earthquakes in mexico that the fault rupture of the 1787 earthquake encompasses the areas where intensities greater than viii were reported and where a strong tsunami invaded land the estimated length of the rupture is approximately 450 km long this fault length would correspond to an earthquake of approximately m w 8 6 in the instrumental record there is no evidence of earthquakes of this magnitude in the middle american subduction zone this evidence indicates that the rupture area of the 1787 earthquake was at least three times longer that those normally observed for earthquakes that take place in the mexican subduction zone the 1787 earthquake appears to have ruptured a long fault segment that more recently broke in earthquakes of more moderate magnitude m w 7 8 and relatively short recurrence times of about 30 to 40 yr examples of this mode of variable rupture length where great but infrequent earthquakes rupture the fault areas of relatively smaller and more frequent earthquakes have been observed in the sumatra andaman and colombia ecuador plate boundaries among others

a study of ground motion excitation based on the earthquake of september 8 2017 evidence that normal faults influence the stress parameter we performed a study of ground motion modeling based on multiple linear regression and random vibration theory using the earthquake sequence of september 8 2017 mw 8 2 our results show that there is high attenuation in the region of oaxaca chiapas and central mexico however the peak amplitude excitations of normal fault earthquakes are higher than those of thrust fault earthquakes the earthquake sequence involved a large number of events but not all the seismic events were aftershocks because many earthquakes had different rupture styles the thrust events follow a simple scaling relation with a constant stress parameter but the normal fault events are difficult to represent with a constant self similar model our best result consists of a q 0 240 pm 032 with a frequency dependent factor of 0 63 pm 0 05 the excitation term is well determined with a stress parameter of 80 bars for the thrust faults but for the normal faults the stress parameter is at least 180 bars with a non self similar model our results show that it is necessary to include new ground motion prediction equations in seismic hazard analysis for meganormal faults which have not previously been considered in subduction zones

geometry and seismic properties of the subducting cocos plate in central mexico the geometry and properties of the interface of the cocos plate beneath central mexico are determined from the receiver functions rfs utilizing data from the meso america subduction experiment mase the rf image shows that the subducting oceanic crust is shallowly dipping to the north at 15 for 80 km from acapulco and then horizontally underplates the continental crust for approximately 200 km to the trans mexican volcanic belt tmvb the crustal image also shows that there is no continental root associated with the tmvb the migrated image of the rfs shows that the slab is steeply dipping into the mantle at about 75 beneath the tmvb both the continental and oceanic moho are clearly seen in both images and modeling of the rf conversion amplitudes and timings of the underplated features reveals a thin low velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab by inverting rf amplitudes of the converted phases and their time separations we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting cocos plate and its thickness high poisson s and vp vs ratios due to anomalously low s wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure the evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates this may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust

on the nature of regional seismic phases iii the influence of crustal heterogeneity on the wavefield for subduction earthquakes the 1985 michoacan and 1995 copala guerrero mexico earthquakes summary the most prominent feature of the regional seismic wavefield from about 150 to over 1000 km is usually the lg phase this arrival represents trapped s wave propagation within the crust as a superposition of multiple reflections and its amplitude is quite sensitive to the lateral variation in the crust along a propagation path in an environment where the events occur in a subduction zone such as the western coast of mexico quite complex influences on the character of the regional wavefield arise from the presence of the subduction zone the great 1985 michoacan earthquake mw 8 1 which occurred in the mexican subduction zone was one of the most destructive earthquakes in modern history and its notable character was that at mexico city located over 350 km from the epicentre there was strong ground shaking almost comparable to that in the epicentral region that lasted for several minutes considerable effort has been expended to explain the origin of the unusual observed waves that caused the severe damage in the capital city during the destructive earthquake the nature of the propagation process in this region can be understood in part by using the detailed strong motion records from the 1995 copala guerrero mw 7 4 earthquake near the coast to the south of mexico city which also had an enhanced amplitude in the valley of mexico numerical modelling of both p and s seismic waves in 2 d and 3 d heterogeneous crustal models for western mexico using the pseudospectral method provides direct insight into the nature of the propagation processes through the use of sequences of snapshots of the wavefield and synthetic seismograms at the surface a comparison of different models allows the influences of different aspects of the structure to be isolated 2 d and 3 d modelling of the 1985 michoacan and 1995 copala earthquakes clearly demonstrates that the origin of the long duration of strong ground shaking comes from the sn and lg wave trains these s wave arrivals are produced efficiently from shallow subduction earthquakes and are strongly enhanced during their propagation within the laterally heterogeneous waveguide produced by the subduction of the cocos plate beneath the mexican mainland the amplitude and duration of the lg coda is also strongly reinforced by transmission through the mexican volcanic belt from the amplification of s waves in the low velocity surficial layer associated withs to p conversions in the volcanic zone the further amplification of the large and long lg wave train impinging on the shallow structure in the basin of mexico city with very soft soil underlain by nearly rigid bedrock with a strong impedance contrast gives rise to the destructive strong ground shaking from the mexican subduction earthquakes

soil degradation and overland flow as causes of gully erosion on mountain pastures and in forests abstract in the european limestone alps lithic borofolists i e soils in a thick peat like organic surface layer occur widespread in the subalpine vegetation belt in northern italy the most important properties of the lithic borofolists are their field capacity of more than 600 of the dry weight and an extremely high infiltration capacity these soils prevent any hydraulic erosion and equilibrate the water budget of forests and the discharge of mountain torrents forest fires clear cutting or overgrazing by cattle and horses could result in the collapse of the ecosystem within a short period of time in the study area overgrazing and trampling by cattle lead to a rapid degradation of lithic borofolists to histic rendzinas on the summer pastures this causes a serious decrease in both field and infiltration capacity on such degraded soils the rainfall of an average thunderstorm may cause horton overland flow and erosional features on the slopes as well as on the valley bottoms overland flow from the valley heads carries enormous quantities of silt resulting in an extensive siltation of the summer pastures on the valley bottom and thereby lead to further reduction of the infiltration capacity on the valley floor the overland flow will be concentrated in shallow depressions where it causes the erosion of steep sided gullies even a beaten hunter path in a wooded area led to the formation of a deep gully during a thunderstorm

geometry and state of stress of the wadati benioff zone in the gulf of tehuantepec mexico 1 data recorded in the gulf of tehuantepec and published moment tensor solutions were used to infer the geometry of the wadati benioff zone w b zone and the stress distribution of the subducted cocos plate from northwest to southeast the subducted slab gently increases its dip bends up at depths of the order of 100 km and about 240 km inland from the trench then unbends and increases its dip toward the southeast below chiapas contour lines of equal depth at 50 and 100 km show a significant contortion of the w b zone west of the isthmus of tehuantepec and coincide with significant changes in the topographic features of the crust we calculated a vp vs ratio of 1 75 for the whole area and a moho depth in the gulf of tehuantepec of 28 5 3 5 km near the trench along the coupling zone t axes of interplate and intraplate earthquakes are oblique to the interface and parallel to the dip direction of the subducted plate respectively at depths greater than 50 km the t axes are roughly horizontal best fit stresses using the gephart and forsyth 1984 code resulted in the following variation of the minimum stresses t axes from northwest to southeast as plunge azimuth 3 40 37 to 3 28 72 then to 3 11 54 and finally to 3 27 35 we also analyzed recordings from the 30 september 1999 oaxaca earthquake mw 7 5 and its aftershocks we inferred an approximate fault area of 80 by 30 km inside the subducted cocos plate

a study of the local magnitude scale in the basin of mexico mutually consistent estimates of log a0 and ground motion scaling a study of the local magnitude scale in the basin of mexico is presented using about 1810 synthetic wood anderson peak amplitudes at 13 stations from the valley of mexico seismic network rsvm data from all available rsvm earthquakes located in central mexico were used and the events within the basin of mexico were relocated parametric and nonparametric expressions for the log a r curve were determined and corrected to richter s reference level which was obtained after the linear separation of source site and distance terms the parametric attenuation function is best expressed as log a r 0 48 log r 100 0 0018 r 100 3 the results of the attenuation curve are consistent with ground motion scaling studies geometrical spreading qs and stochastic simulations the basin of mexico is characterized by a complex attenuation model at short distances but the general trend is that of a highly attenuative region we observe that the coda magnitude has been underestimating the excitation levels of local events in the basin of mexico and may require station corrections to improve its accuracy our study will provide local magnitudes to the seismicity catalog in the basin of mexico thus contributing to more successful earthquake hazard analyses in the most important and heavily populated region of mexico

lateral variation of lg wave propagation in southern mexico 1 in this study we investigated lateral variation of lg wave propagation in southern mexico from recordings of 92 crustal earthquakes along 591 travel paths the efficiency of lg propagation was measured in terms of lgto pn spectral ratio it was found that lgpropagation is inefficient for travel paths through the gulf of mexico coastal plains and the gulf of tehuantepec areas with thick layers of sediments an average lg quality factor qlg as a function of frequency for southern mexico was estimated for the efficient lg travel paths the relation obtained for qlg in the frequency range 1 6 8 hz is qlg f 204 f0 85 the lateral variation of qlg 1was solved as a mixed determined inverse tomography problem separately for each frequency in which a spatial smoothness constraint was imposed and a priori information was added in poorly covered regions the spatial resolution obtained was about 200 km it was found that the trans mexican volcanic belt the gulf of mexico coastal plains and the area east of 94 w are characterized by lower than average qlgvalues i e higher attenuation high qlg values were obtained for the mixteco oaxaca terranes while for the guerrero terrane values similar to the average were obtained the results show a correlation between qlg and crustal structure and provide valuable information on lateral variation of qlg which is needed for reliable prediction of ground motion during future earthquakes

attenuation relations of strong ground motion in japan using site classification based on predominant period a spectral acceleration attenuation model for japan is presented in the present study the data set includes a very large number of strong ground motion records up to the 2003 off tokach main and aftershocks site class terms instead of individual site correction terms are used the site classes of recording stations are from a recent study on site classification for strong motion recording stations in japan according to a classification scheme that has been used in japanese engineering design the use of site class terms enables tectonic source type effects to be identified and accounted for in the present model the effects of a faulting mechanism for crustal earthquakes also are accounted for for crustal and interface earthquakes a simple form of an attenuation model with respect to distance is able to capture the main strong motion characteristics and achieves unbiased estimates for subduction slab events a simple distance modification factor is employed to achieve plausible and unbiased predictions the effects of source depth tectonic source type and faulting mechanism of crustal earthquakes are significant the need for magnitude squared terms is evaluated and the use of magnitude squared terms reduces the interevent error further

seismic attenuation and mantle wedge temperatures in the alaska subduction zone 1 anelastic loss of seismic wave energy or seismic attenuation 1 q provides a proxy for temperature under certain conditions the q structure of the upper mantle beneath central alaska is imaged here at high resolution an active subduction zone where arc volcanism is absent to investigate mantle thermal structure the recent broadband experiment across the alaska range beaar provides the first dense broadband seismic coverage of this region the spectra of p and sh waves for regional earthquakes are inverted for path averaged attenuation operators between 0 5 and 20 hz along with earthquake source parameters these measurements fit waveforms significantly better when the frequency dependence of q is taken into account and in the mantle frequency dependence lies close to laboratory values inverting these measurements for spatial variations in q reveals a highly attenuating wedge with q 500 assuming frequency dependence comparison with p results shows that attenuation in bulk modulus is negligible within the low q wedge as expected for thermally activated attenuation mechanisms bulk attenuation is significant in the overlying crust and subducting plate indicating that q must be controlled by other processes the shallowest part of the wedge shows little attenuation as expected for a cold viscous nose that is not involved in wedge corner flow overall the spatial pattern of q beneath alaska is qualitatively similar to other subduction zones although the highest wedge attenuation is about a factor of 2 lower the q values imply that temperatures exceed 1200 c in the wedge on the basis of recent laboratory based calibrations for dry peridotite these temperatures are 100 150 c colder than we infer beneath japan or the andes possibly explaining the absence of arc volcanism in central alaska

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

moment energy stress drop and source spectra of western united states earthquakes from regional coda envelopes we present a new method to estimate stable seismic source parameters such as energy moment and orowan stress drop using regional coda envelopes from as few as one broadband station we use the method to compute path and site corrected seismic moment rate spectra for 117 recent western united states earthquakes empirical green s function corrections were applied to our surface and body wave coda envelope measurements to generate s wave source spectra these source spectra provide stable single station estimates of radiated seismic energy es and seismic moment mo that for common events are in excellent agreement with network averaged estimates obtained using local and regional data teleseismic moment estimates are compatible with our regional results but teleseismic energy estimates appear to be nearly an order of magnitude low we estimated the seismic moment of events ranging between mw 2 2 and 7 3 and energy estimates for which we had measured at least 70 of the total energy generally events above mw 3 3 we use these estimates to examine the behavior of derived parameters such as the orowan stress drop 2 es mo while the earthquakes we studied have a small range in orowan stress drop generally between 0 1 and 20 mpa they show a strong tendency for orowan stress drop to increase with moment approximately as mo0 25 we believe this is a source effect and is not due to inadequate bandwidth or attenuation correction and note that this trend appears to continue for microearthquakes as described in a recent deep borehole study in southern california many of the large high stress drop earthquakes show complexity in their moment rate spectra near the corner frequency and cannot be fit by a simple square model instead above the first corner frequency the spectral decay ranges between f 1 0 and f 1 5 this leads to larger estimates of radiated energy than predicted with a simple square model and has implications for seismic hazard estimation coda envelopes have three main advantages over direct arrivals for estimating seismic moment and energy 1 coda amplitudes vary little with geology and source radiation anisotropy and allow accurate single station applications 2 path corrected coda amplitude measurements can be applied to very large regions allowing a comparison of source parameters throughout the western united states using a common methodology and stations 3 because long period coda can last for hours for large local and regional events it allows the analysis of seismograms with clipped early arrivals

regional wave propagation from mexican subduction zone earthquakes the attenuation functions for interplate and inslab events the seismic waves from subduction zone earthquakes are significantly affected by the presence of 3d variation in crust and upper mantle structure around the source area these heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances this is illustrated by studying shallow interplate earthquakes along the mexican subduction zone and deeper inslab normal faulting earthquakes in the subducted cocos plate beneath mexican mainland the strong motion recordings of these earthquakes are used to evaluate the character of wave propagation along the path between the source region and mexico city we compare the wavefield from two large earthquakes of different source type during the shallow h 17 km interplate 1995 guerrero earthquake m w 7 3 the lg phase is the most prominent feature at regional distances of about 150 to a few hundred kilometers from the source the presence of a lateral velocity gradient in the crust caused by the subduction of the cocos plate enhances the lg wave amplitude which is then amplified further in the mexican volcanic belt by amplification in the low velocity volcanic rocks both effects lead to very large ground motions along the path from the coast to the mexican inland in the frequency band from 0 2 to 4 hz however for the deeper h 40 km inslab normal faulting 1999 oaxaca earthquake m w 7 5 the amplitude of the lg phase is too small to produce the abnormal wave propagation and the direct s wave and its multiple sms reflections between the free surface and moho show a simple attenuation with increasing distance we compare these observations with numerical simulations of seismic wave propagation using the fourier spectral method the results provide a key to the understanding of seismic wave field generated by shallow interplate and deeper inslab earthquakes in a realistic 3d heterogeneous structure

rupture history of september 30 1999 intraplate earthquake of oaxaca mexico mw 7 5 from inversion of strong motion data near source strong motions are inverted to estimate the rupture history of intraslab normal faulting september 30 1999 oaxaca mexico earthquake two focal mechanisms harvard and neic cmt solutions are tested for the source geometry the inversion with the ne dipping fault plane of the harvard solution best matches the data strike 295 dip 50 rake 82 we estimated the slip distribution on the fault and the associated rupture front propagation as well as the rise time the inversion results show that the rupture mainly propagated from ese to wnw and slightly downdip with an average rupture velocity of about 3 km s the rise time ranges between 1 and 2 s the slip distribution on the fault is mainly concentrated in two interconnected patches with a maximum slip of 2 5 m located about 20 km and 40 km wnw of the hypocenter most of the slip is released at an average depth of 45 km a smaller area with a maximum slip of 1 5 m is also observed close to the hypocenter the total co seismic moment released is equal to 1 8 1020 nm

geometry and state of stress of the downgoing cocos plate in the isthmus of tehuantepec mexico the seismicity in the isthmus of tehuantepec mexico is anomalously active compared with other parts of the middle american subduction zone the results of a microearthquake study conducted in this region show that the slab changes dip from an almost subhorizontal geometry west of longitude 96 w to a dip of about 45 to 50 to the east of this longitude the change in dip occurs gradually in a broad flexure of the subducting slab that takes place over 150 km this geometry of the slab is reminiscent of that in southern peru the change in dip however takes place more gently in tehuantepec in addition to the change in the dip of the subducted cocos plate a rapid increase is observed in the maximum depth extent of the subducted slab in the region of subhorizontal subduction focal depths are consistently shallower than about 80 km whereas earthquakes as deep as 200 km are found where the subducted slab dips at a steeper angle both the change in the dip of the slab and of the maximum depth of intermediate depth earthquakes appear to correlate with the presence of the tehuantepec fracture zone which separates two distinct provinces of the oceanic cocos plate the age and the crustal thickness of the cocos plate increase to the southeast of the tehuantepec fracture zone relative to its northwestern counterpart in tehuantepec the dip and maximum depth of the subducted slab appears to be correlated with the age of the oceanic plate as has been observed in other subduction zones of the world

shallow depth of seismogenic coupling in southern mexico implications for the maximum size of earthquakes in the subduction zone abstract studies of locally recorded microearthquakes and the centroidal depths of the largest earthquakes analyzed using teleseismic data show that the maximum depth of thrust faulting along the mexican subduction zone is anomalously shallow this observed maximum depth of about 25 5 km is about half of that observed in most subduction zones of the world a leveling line that crosses the rupture zone of the 19 september 1985 michoacan event was revisited after the earthquake and it shows anomalously low deformation during the earthquake the comparison between the observed coseismic uplift and dislocation models of the seismogenic interplate contact that extend to depths ranging from 20 to 40 km shows that the maximum depth at which seismic slip took place is about 20 km this unusually shallow and narrow zone of seismogenic coupling apparently results in the occurrence of thrust events along the mexican subduction zone that are smaller than would be expected for a trench where a relatively young slab subducts at a rapid rate of relative motion a comparison with the chilean subduction zone shows that the plate interface in mexico is half that in chile not only in the down dip extent of the seismogenic zone of plate contact but also in the distance of the trench from the coast and in the thickness of the upper continental plate it appears that the narrow plate contact produced by this particular plate geometry in mexico is the controlling variable defining the size of the largest characteristic earthquakes in the mexican subduction zone

seismic wave attenuation and source excitation in la paz los cabos baja california sur mexico we present results from a regional study of seismic wave attenuation and source excitation from small magnitude earthquakes recorded at distances from 6 to 180 km in the la paz los cabos region at the south end of the baja california peninsula data were recorded using 32 strong motion seismic stations from the la paz network lap a least squares regression separating the excitation site and propagation effects was carried out we performed the analyses in the time and frequency domains and we compared these results with results from a coda normalization method the propagation term was parameterized to represent a geometrical spreading function and a frequency dependent q f at a reference distance of 40 km we estimated the regional attenuation by measuring the maximum amplitude of the s or lg waves as a function of frequency defining a continuous piecewise propagation term d r f after separating the excitation and site terms our results show that the attenuation is lower than that of central or northern mexico recorded data were of remarkably good quality despite the fact that the strong motion network recorded only small earthquakes our best model is that of a quality factor of q f 380 f 0 10 with a complex geometrical spreading function

inslab earthquakes of central mexico peak ground motion parameters and response spectra we developed equations to predict pseudoacceleration response spectra 5 damping peak ground acceleration and peak ground velocity at free field rock sites for intermediate depth normal faulting inslab earthquakes of central mexico the data set comprises 16 earthquakes 5 2 m w 7 4 35 h 138 km recorded at local and regional distances r 400 km it represents a homogeneous catalog with respect to tectonic regime fault mechanism and soil class our results show larger amplitudes in the epicentral area from inslab events than from interplate events a consequence of higher stress drops during the former type of earthquakes peak ground accelerations from moderate to large m w 6 0 inslab events significantly exceed those from interplate events with similar magnitude reaching almost three times for the largest events the ground motion due to inslab events however decays faster than for the thrust events our results are in reasonable agreement with other studies based on japanese and the worldwide data

modeling geometrical spreading and the relative amplitudes of vertical and horizontal high frequency ground motions in eastern north america abstract horizontally layered velocity models were used with point source and finite fault sources to investigate geometrical spreading and the relative amplitudes of vertical and horizontal ground acceleration within 120 km of the source full wave field simulations were done for a range of focal depths and for strike slip and reverse focal mechanisms the attenuation of the geometric mean of randomly oriented horizontal component maximum acceleration amplitudes averaged over all azimuths significantly exceeds the theoretical geometrical spreading for far field body waves in a homogeneous whole space for hypocentral distances less than approximately 60 km the behavior of the vertical component is different from the horizontal vertical attenuation near the epicenter is greater and is more dependent on source mechanism and depth because of the rapid near source decay of the direct s wave reflections from the mid lower crust and moho control the maximum amplitude of the vertical component acceleration in the 60 120 km hypocenter distance range resulting in a flattening of the vertical amplitude distance relation near source vertical maximum amplitudes averaged over all source receiver azimuths tend to be less than the geometric mean horizontal amplitude for strike slip focal mechanisms but near the source for reverse faults the azimuthally averaged vertical component amplitude exceeds that of the geometric mean horizontal the modeling indicates that similar vertical and horizontal component geometrical spreading and approximately constant horizontal vertical amplitude ratios observed in connection with the lg phase at distances greater than approximately 100 km in eastern north america may not hold at smaller distances ground motion prediction models for the vertical component near the source may need to incorporate strong geometrical spreading and dependence on radiation pattern

ground motion attenuation relationships for subduction zone earthquakes in northeastern taiwan subduction zone earthquakes have not been taken into special consid eration in most previous probabilistic seismic hazard analyses psha in taiwan however they may be critical to properly analyze the earthquake hazard in metro politan taipei so they need to be studied strong motion data from subduction zone earthquakes of both interface and intraslab types obtained by the tsmip and smart1 arrays in northeastern taiwan are used to establish the attenuation equa tions for peak ground acceleration pga and response spectral acceleration sa the resultant pga and sa attenuation equations include two site classes and two earth quake source types the ground motion values predicted by these attenuation equa tions are higher than those obtained from the crustal earthquake attenuation equations previously used in taiwan but are lower than those predicted by the attenuation equa tions for worldwide subduction zone earthquakes

gully and sheet erosion on subtropical mountain slopes their respective roles and the scale effect abstract as most mountains in tropical and subtropical zones the western sierra madre suffers active present erosion which may create some constraints to the social and economic development in the area the objectives of this study of soil degradation in the western sierra madre are to determine the respective roles of gully and sheet erosion this research is based on field observations field measurements of runoff and soil losses at the plot as well as the watershed scales as an analysis of an exhaustive census of the few gullies located in an experimental area measured soil losses in the western sierra madre are high although there are few gullies most of the sediment yield seems to originate in widespread degraded areas where stoniness is the main evidence of a previous stage of erosion previously overgrazing and deforestation were determined as the factors of the appearance of new soil surface characteristics which explain the high runoff and sediment productions the soil compacted by cattle trampling reduces infiltration the decrease of the vegetation cover triggers a rise in the splash effect and thus a soil sealing these processes induce an increase in runoff and soil losses the main erosion type has been described as sheet erosion it is characterised by the removal of fine soil particles and the remains of gravels pebbles and blocks which constitute a pavement on the soil gullies generally appear on the bottom of wide valleys and depressions where soils are thick it is shown that sheet erosion is two orders of magnitude higher than gully erosion at the hillslope scale due to the spatial distribution of land use and the geological context such as the heavily degraded areas close to the main rivers the reduction of runoff and soil loss rates within the extension of a considered area commonly observed in hydrology only applies up to the elementary catchments scale 1 to 50 km 2 above this area runoff coefficient and soil loss rates increase