the denver earthquakes of 1967 1968 abstract a detailed study of the earthquakes associated with the rocky mountain arsenal disposal well is presented long period surface wave studies are used together with p wave first motions to show that the 10 april 9 august and 27 november 1967 earthquakes occurred at depths of 3 to 5 km and were characterized by normal faulting along a northwest striking fault plane a joint hypocenter relocation of 103 microearthquakes of a data set of 279 recorded between 1967 and 1968 shows a hypocenter pattern striking n50 w with most of the events located about 5 km northwest of the disposal well at depths between 3 to 8 km a fault plane dipping southwest is tenuously suggested by those earthquakes with depths less than 5 km modeling of near field seismoscope observations lend support to the focal mechanisms derived

broad band deconvolution of gdsn data summary advances in the theory of wave propagation and source mechanism have coincided with the recent deployment of a global network of digitally recording seismographs the gdsn global digital seismograph network we demonstrate that broad band records of body wave ground displacement and velocity with spectral information from several hertz to tens of seconds can be obtained by the simultaneous deconvolution of the long and short period components of the gdsn seismographs frequency dependent effects in body waves that may arise from source complexity or from propagation through the earth are much more apparent and measurable in the broad band records than in the original seismograms

implications of felt area magnitude relations for earthquake scaling and the average frequency of perceptible ground motion abstract observed relations between felt area and moment magnitude m for stable continental regions scr and california earthquakes can be explained by a simple model with geometrical spreading attenuation q 1 and earthquake stress drops that remain constant from m 4 to 8 differences in the slopes of the observed felt area m relations between scr and california earthquakes are consistent with differences in q between these tectonic settings fitting the felt area m data to the model yields an estimate of f q where f is the average frequency of minimum perceptible ground motion using fits to the data and instrumentally derived values of q i find that f is between about 2 and 4 hz i present an estimate of the spectral acceleration level associated with the lower limit of perceptibility

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

rupture process of a multiple main shock sequence analysis of teleseismic local and field observations of the tennant creek australia earthquakes of january 22 1988 on january 22 1988 three large intraplate earthquakes with ms 6 3 6 4 and 6 7 occurred within a 12 hour period near tennant creek australia these earthquakes which occurred over a small interval of time and within a small volume of space present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms depths and complexity of rupture of each of the three main shocks hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible the interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release as locations of the main shock hypocenters progressively moved from west to east we infer that the first and third main shocks denoted as ms1 and ms3 produced the southeast northwest trending scarps observed at the western end the kunayungku fault and at the eastern end the east end of the lake surprise fault respectively of the rupture zone the epicenter of the only immediate foreshock was located in the gap between these two fault scarps ms1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6 5 km ms3 ruptured predominantly to the se at a depth of 4 5 km the second main shock ms2 is inferred to have produced the deformation of the southwest trending central scarp segment the western end of the lake surprise fault from the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes ms1 and ms3 in contrast the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps the rupture process of ms2 turns out to be commensurately complex and sufficiently explains the geological complexity ms2 consisted of three subevents the southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface choice of the north dipping plane as the rupture plane of the third subevent consistent with the surface deformation and coplanar with a second plane delineated by aftershocks would imply conjugate faulting the majority of the aftershocks are concentrated near the edges of the fault planes and there is an absence of activity in the center of the planes the areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred the rupture process of each main shock is characterized by the rapid release of energy followed by a much slower release of moment and by aftershock zones whose dimensions exceed the inferred dimensions of the rupture these characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release the first main shock nucleated at the deepest part of its aftershock zone and ruptured upward in contrast ms2 and ms3 nucleated at depths that were one third to one half of the maximum depth of their aftershock zones this variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture

rupture histories of eastern north american earthquakes abstract digital worldwide records of p and sh waves are used to invert for rupture histories of the recent larger eastern north american earthquakes the data include a broad bandwidth to facilitate recovery of both source details and total moment the following events are studied 9 january 1982 new brunswick 5 october and 23 december 1985 nahanni 25 november 1988 saguenay and 25 december 1989 ungava earthquakes we employ a finite fault waveform inversion scheme that discretizes the slip history as a function of position on the fault this formulation allows estimation of rise time and slip velocity in addition to the spatial distribution of slip static stress drops averaged over the entire rupture surface range from a few tens of bars to just over 100 bars and are similar to values estimated for western united states earthquakes estimates of stress drop for spatially limited asperities are in the range of a few hundred bars with the value for the saguenay earthquake asperity being the largest approaching 1 kbar the variation in stress drop is considerable but no evidence is seen for a scaling relation in which stress drop increases with moment maximum slip velocities on the fault also have a wide range from 50 cm sec to perhaps greater then 200 cm sec with the saguenay and 5 october nahanni earthquakes lying at the upper end of this range of the events studied the saguenay earthquake is unique in terms of its greater depth spatially concentrated source and large asperity stress drop

theoretical body wave interactions with upper mantle structure long period seismograms are generated by a spectral method for body waves interacting with the upper mantly discontinuities that define the low velocity and transition zones of the spherically symmetric but radially inhomogeneous earth models pem c and t7 a complex velocity profile incorporates the sl8 q model models pem c and t7 predict distinct differences in displacement waveforms for p waves that interact with the 400 and 670 km discontinuities the waveforms demonstrate that later arrivals such as interference head waves can be important constraints upon earth models theoretical seismograms were computed by a spectral method that modeled the velocity profile with arbitrarily thick inhomogeneous layer full wave theory the seismograms agreed well with those generated by methods that approximate the velocity profile with thin homogeneous layers the cagniard and reflectivity methods provided the displacements were convolved with source time functions and a long period instrument response minor differences among the results of the methods do not affect the interpretation of observed waveform data but do help define the limitations of the methods for example the thin layer methods suffer from numerical limitations on layer thickness and the umber of layers needed to approximate the earth model the full wave theory may occasionally also require the usemore of thin layers when the zero order asymptotic theory it uses fails to describe wave propagation properly in layers having rapid velocity variation with depth less

seismicity of the rocky mountains and rio grande rift from the earthscope transportable array and crest temporary seismic networks 2008 2010 we developed a catalog of small magnitude ml 0 1 to 4 7 seismicity across colorado and new mexico from the earthscope usarray transportable array and crest colorado rocky mountains experiment and seismic transects seismic networks from 2008 to 2010 to characterize active deformation in the rio grande rift we recorded over 900 earthquakes in the rio grande rift region not including induced earthquakes and mine blasts and find that the rift is actively deforming both broadly and in distinct regions seismic events that are likely induced mostly in the raton basin make up 66 of the catalog 1837 earthquakes neogene faults in the northern rift in north central colorado are seismically active in the north park basin and northwestern colorado the central rift from the san luis basin southern colorado to south of the socorro magma body is the most seismically active rift region and seismicity delineates the deformation in the colorado plateau transition zone which is spatially correlated with volcanic vents dikes and faults within the western jemez lineament the eastern jemez lineament is nearly aseismic and surrounded by a halo of seismicity culminating in boundaries defined by recent moderate mw 3 9 and mw 3 3 earthquakes the southern rift is characterized by diffuse seismicity in texas and mexico this study provides an updated seismic catalog built with uniformity in seismometer coverage and low epicentral uncertainties 2 km that allows for regional evaluation of seismicity during this time period clusters of seismicity and moderate magnitude earthquakes characterize deformation in a low strain rate extensional environment

seismic cycles along the aleutian arc analysis of seismicity from 1957 through 1991 we catalog and relocate aleutian arc seismicity between 1957 and 1991 two great earthquakes ruptured the same 250 km long portion of the central aleutian arc the 1957 aleutian islands earthquake and the 1986 andreanof islands earthquake because accurate estimates of the moment distribution of the 1957 earthquake are not available the spatial distribution of aftershocks for each of these events is compared and tested against models describing the modes of occurrence of great subduction zone earthquakes earthquake relocations are based on p wave arrival times published in the international seismological summary the bureau central international seismologique and the international seismological centre bulletins and include corrections for the near source velocity structure associated with the down going slab magnitude estimates are extracted from bulletins and prior to 1964 are estimated by us from microfilmed records our catalog is complete above magnitude 5 5 aftershocks associated with the 1957 and 1986 earthquakes appear to occur in different areas east of the main shock epicenters aftershock locations are anticorrelated west of the main shock epicenter aftershocks of the 1986 earthquake tended to concentrate along the updip edge of aftershock clusters associated with the 1957 earthquake if we assume aftershocks rim the distribution of seismic moment release associated with each event these observations imply that the moment distribution of the 1986 earthquake was different from that of the 1957 earthquake this suggests that we should use caution in identifying mechanically strong portions of a fault asperities by simply mapping the moment distribution of a single great earthquake a fundamental tenet of the asperity model that rupture always occurs on the strongest portions of the fault with weaker portions rupturing either aseismically or dynamically as a result of rupture on a strong fault patch may in the case of the central aleutian arc not be correct thus observing the moment distribution from a single great earthquake may tell us little about what the distribution of moment release will look like during the next earthquake

synchronous low frequency earthquakes and implications for deep san andreas fault slip a b s t r a c t low frequency earthquakes lfes are slip events that occur repeatedly at source locations within the lower crust lfes and the associated seismic broadcast known as tremor have been observed in a diverse array of tectonic environments here we develop a suite of statistical tools to conduct a systematic study of the spatial and temporal correlations of the event occurrence patterns of the 88 lfe sources beneath the greater parkfield section of the san andreas fault we first examine correlations in the occurrence patterns on long time scales to show that the regions to the north and south of parkfield behave independently we next use the cumulative event signatures of each source to characterize the individual occurrence patterns on shorter time scales through application of a statistical clustering algorithm we demonstrate that individual lfe sources form spatially coherent clusters that may represent localized elastic structures or asperities on the deep fault interface we conclude by examining the fine scale features of the event rates within the lfe occurrence patterns through quantitative comparison to analogous laboratory shear experiments on granular fault gouge like materials we infer that the distinctive features of lfe occurrence patterns reflect variations in the in situ stress and frictional conditions at the individual lfe source locations these observations provide a framework to understand the spatial and temporal diversity of fault slip that occurs within the lower crust beneath parkfield and that may influence seismic hazard in the region 2015 elsevier b v all rights reserved

rare large earthquakes at the laramide deformation front colorado 1882 and wyoming 1984 the largest historical earthquake known in colorado occurred on 7 november 1882 knowledge of its size location and specific tectonic environment is important for the design of critical structures in the rapidly growing region of the southern rocky mountains more than one century later on 18 october 1984 an mb 5 3 earthquake occurred in the laramie mountains wyoming by studying the 1984 earthquake we are able to provide constraints on the location and size of the 1882 earthquake analysis of broadband seismic data shows the 1984 mainshock to have nucleated at a depth of 27 5 1 0 km and to have ruptured 2 7 km updip with a corresponding average displacement of about 48 cm and average stress drop of about 180 bars this high stress drop may explain why the earthquake was felt over an area about 3 5 times that expected for a shallow earthquake of the same magnitude in this region a microearthquake survey shows aftershocks to be just above the mainshock s rupture mostly in a volume measuring 3 to 4 km across focal mechanisms for the mainshock and aftershocks have ne sw trending t axes a feature shared by most earthquakes in western colorado and by the induced denver earthquakes of 1967 the only data for the 1882 earthquake were intensity reports from a heterogeneously distributed population interpretation of these reports also might be affected by ground motion amplification from fluvial deposits and possible significant focal depth for the mainshock the primary aftershock of the 1882 earthquake was felt most strongly in the northern front range leading kirkham and rogers 1985 to locate the epicenters of the aftershock and mainshock there the front range is a geomorphic extension of the laramie mountains both features are part of the eastern deformation front of the laramide orogeny based on knowledge of regional tectonics and using intensity maps for the 1984 and the 1967 denver earthquakes we reinterpret prior intensity maps for the 1882 earthquake as reflecting low population to the north and east of available intensity data we estimate that had there been a more evenly distributed population the 1882 earthquake would have been felt over an area of about 850 000 km2 with a corresponding moment magnitude mw of 6 6 0 6 our study in the context of regional tectonics implies that rare earthquakes of magnitude up to mw 6 6 0 6 at depths from shallow to mid crustal are possible throughout the laramie mountains and the front range approximately from casper wyoming to pueblo colorado

the 1974 peru earthquake series the aftershocks of the 3 october 1974 mw 8 1 earthquake have been relocated using data from a 10 station temporary seismic network these aftershocks are among the best observed of great circum pacific subduction earthquakes recent modelings of the rupture history of the mainshock done in other studies indicate a bilateral rupture on the thrust interface updip to the peru chile trench preseismicity was predominately downdip of the mainshock rupture consistent with the mainshock s asperities being loaded by sinking of the nazca plate the aftershocks including an ms 7 1 event are located primarily near the downdip periphery of the mainshock s shallow dipping rupture surface for a concentration of aftershocks furthest downdip a composite focal mechanism shows east trending right lateral strike slip displacement corresponding to the dominant moment release of the mainshock being north of the strike slip deformation at a depth of 20 to 25 km the 11 ne dipping nazca plate increases in dip to 30 ne although a postmainshock episode of plate motion must have bridged both this hinge and the bend at the peru chile trench the associated bending moments were aseismic the lima basin is directly over the downdip edge of the shallow dipping strongly coupled interface and the aftershocks there similar in location to basins in outer rises of other regions the space time history of the aftershocks shows an alternation between heightened activity in the updip of hinge and downdip of hinge zones with many earthquakes near the pending rupture of the 9 november 1974 ms 7 1 aftershock and a regional seismic quiescence following this large aftershock

source spectra for the 1988 saguenay quebec earthquakes abstract source spectra are obtained for the 1988 saguenay quebec earthquake m n 6 5 its foreshock m n 4 8 and largest aftershock m n 4 1 using recordings from analog and digital seismographs on rock sites at epicentral distances from 40 to 700 km the source spectra of the foreshock and the aftershock are well matched by a single corner frequency 2 model with seismic moments of 2 2 10 22 and 2 2 10 21 dyne cm respectively and a stress parameter of 65 bars for both events by contrast the spectrum of the mainshock differs dramatically from this simple shape a stress parameter of 500 bars is required to match the observed high frequency amplitudes using the seismic moment determined from teleseismic observations as a constraint this single corner frequency model however overestimates amplitudes at frequencies near 1 hz by a factor of about 2 the relative radiation of short and long period energy for the mainshock is inconsistent with that from other intraplate earthquakes the fore and aftershocks appear typical in this regard the extraction of source spectra from the regional data required a careful examination of attenuation azimuthal variations were observed with the attenuation being most severe to the southeast normal to the st lawrence valley and the dominant structural grain of the region the st lawrence river coincides in part with the boundary between the grenville and appalachian tectonic provinces and the difference in attenuation may be related to the difference in tectonic province the distribution of data however makes it difficult to obtain good estimates of the attenuation in each province lumping the data from both provinces and the three earthquakes together results in an anelastic attenuation factor given by q 755 f 0 5 for frequencies f from 0 6 to 18 hz using data at distances r beyond 100 km from the sources and assuming 1 r geometrical spreading this attenuation is similar to that obtained in a number of other studies in the general area three component recordings were available at a few stations and from these the following frequency dependent ratio of horizontal to vertical motion was obtained hiv 1 14 0 118 f 0 00638 f 2 for frequencies from 0 2 to 15 hz different methods of correcting observations for attenuation in order to derive source spectra were examined source spectra derived by simple regression analyses of observed data assuming a uniform medium are very similar to those based on a more complicated synthetic seismogram correction which accounts for wave propagation in a layered earth

seismic moment assessment of earthquakes in stable continental regions iii new madrid 1811 1812 charleston 1886 and lisbon 1755 summary the sizes of three major or great historical earthquakes are reassessed using the isoseismal area regression tools developed in parts i and ii of this study of stable continental region scr seismicity the earthquakes are 1811 new madrid central united states and its following sequence 1886 charleston coastal south carolina and 1755 lisbon oceanic intraplate off the continental shelf of portugal the analysis confirms the large size of these events and for the first time places constraints on the uncertainty of their seismic moment release because of the exceptionally low seismic wave attenuation of eastern north america ena a separate north american regression of seismic moment on isoseismal area was developed additionally the unknown western extents of the new madrid isoseismal areas were calibrated with the patterns of the m 6 3 6 6 1843 and 1895 earthquakes application of part ii analysis procedures with these corrections yields new madrid size estimates expressed as moment magnitude of m 8 1 0 31 for the 1811 december 16 m 7 8 0 33 for the 1812 january 23 and m 8 0 0 33 for the 1812 february 7 principal events the charleston earthquake s magnitude decreases from m 7 4 to m 7 3 0 26 after compensation for the effect of coastal plain sediments on its inner isoseismals intensity regressions for lisbon are calibrated against the isoseismal pattern of the nearly co located m 7 8 1969 st vincent earthquake which in this case increases the predicted size of lisbon from m 8 4 to m 8 7 0 39 these size estimates are supported by data from independent phenomena extent and severity of liquefaction the maximum distance of induced landslides and for lisbon tsunami wave amplitudes estimated source parameters are controlled by crustal or lithospheric temperature which governs the depth extent of brittle faulting using estimated continental and oceanic geotherms viable fault lengths are 30 80 km for charleston 120 180 km for 1811 new madrid and 180 280 km for lisbon for average displacements of 2 4 m 8 11 m and 10 14 m respectively and for average static stress and strain drops at the estimated seismic moments of this study the 1811 new madrid and the 1755 lisbon events are respectively the largest known scr and oceanic lithosphere earthquakes

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

a re assessment of focal depth distributions in southern iran the tien shan and northern india do earthquakes really occur in the continental mantle summary we investigate the depth distribution of earthquakes within the continental lithosphere of southern iran the tien shan and northern india by using synthetic seismograms to analyse p and sh body waveforms in the zagros mountains of southern iran earthquakes are apparently restricted to the upper crust depths of 20 km whereas in the tien shan and northern india they occur throughout the thickness of the continental crust to depths of 40 45 km we find no convincing evidence for earthquakes in the continental mantle of these regions in spite of previous suggestions to the contrary and question whether seismicity in the continental mantle is important in any part of the world in some regions such as iran the aegean tibet and california seismicity is virtually restricted to the upper continental crust whereas in others including parts of east africa the tien shan and northern india the lower crust is also seismically active although usually less so than the upper crust such variations cannot reliably be demonstrated from published catalogue or bulletin locations even from ones in which depth resolution is generally improved in contrast to the oceanic mantle lithosphere in which earthquakes certainly occur the continental mantle lithosphere is we suggest virtually aseismic and may not be significantly stronger than the lower continental crust these variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness suggesting that the strength of the continental lithosphere resides in the crust and require some modification to prevalent views of lithosphere rheology

common features of the excitation and propagation of strong ground motion for north american earthquakes the central matter of this investigation is the commonly held belief that eastern north america ena earthquakes are felt to much greater distances at all levels of intensity than their western u s wus counterparts of the same source strength by comparing the areas enclosed by modified mercalli intensities i to iii felt iv v vi and vii for ena and wus earthquakes at the same source strength we find that this proposition is mostly false at damaging levels of ground motion intensities vi to vii and greater for m 7 neither should such differences exist if strong ground motion is confined to r 150 km and arrives principally as body waves and if ena and wus earthquakes share the same average stress drop in fact observable differences in the areas enclosed by intensity vi a vi for ena and wus earthquakes at the same m point to slight but significant differences in stress drop the ena earthquakes having stress drops a factor of about 2 higher only for the largest earthquakes m 7 as the 1811 to 1812 new madrid and 1886 charleston earthquakes seem to be can strong ground motion propagate to much greater distances as low loss higher mode surface waves in ena relative to wus the strong dependence of high frequency strong ground motion on stress drop and its weak dependence on m admit the possibility that the 1811 to 1812 events and the 1886 charleston earthquakes are no larger than m 6 to 7 provided that their stress drops are higher than average by a factor of 2 or so

the role of gravitational potential energy in active deformation in the southwestern united states in orogenic belts intrinsic buoyancy and extrinsic boundary forces act on the continental lithosphere determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation in the western united states estimates of gravitational potential energy show that given available constraints on rheology buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation including absence of deformation this underscores the importance of gravitational potential energy for deformation in orogenic belts

tectonic and climatic significance of a late eocene low relief high level geomorphic surface colorado new paleobotanical data suggest that in the late eocene the erosion surface which capped the front range colorado was 2 2 3 3 km in elevation which is similar to the 2 5 km present elevation of surface remnants this estimated elevation casts doubt on the conventional belief that the low relief geomorphic surface was formed by lateral planation of streams to a base level not much higher than sea level and that the present deeply incised canyons must represent neogene uplift of colorado description of the surface calculations of sediment volume and isostatic balance and fluvial landsculpting models demonstrate that while the high elevation of the erosion surface was due to tectonic forces its smoothness was mostly a result of climatic factors a sediment balance calculated for the front range suggests that from 2 to 4 km of material were eroded by the late eocene consistent with fission track ages this amount of erosion would remove a significant portion of the 7 km of laramide upper crustal thickening isostatic modeling implies that the 2 2 3 3 km elevation was most likely created by lower crustal thickening during the laramide a numerical model of fluvial erosion and deposition suggests a way that a late eocene surface could have formed at this high elevation without incision a humid climate with a preponderance of small storm events will diffusively smooth topography and is a possible mechanism for formation of low relief high level surfaces paleoclimate models suggest a lack of large storm events in the late eocene because of cool sea surface temperatures in the equatorial region return to a drier but stormier climate post eocene could have caused the incision of the surface by young canyons by this interpretation regional erosion surfaces may represent regional climatic rather than tectonic conditions

comparison of source scaling relations of eastern and western north american earthquakes source scaling relations have been obtained for earthquakes in eastern north america and other continental interiors and compared with a relation obtained for earthquakes in western north america the scaling relation for eastern north american earthquakes was constructed from measurements of seismic moment and source duration obtained by the waveform modeling of seismic body waves the events used include nine events of m blg magnitude 4 7 to 5 8 that occurred after 1960 and four earlier events with magnitudes between 5 5 and 6 6 the scaling relation for events in other continental interiors was used for comparative purposes and to provide constraints for large magnitudes detailed analysis of the uncertainties in the scaling relations has allowed the resolution of two important issues concerning the source scaling of earthquakes in eastern north america first the source characteristics of earthquakes in eastern north america and other continental interiors are consistent with constant stress drop scaling and are inconsistent with nonconstant scaling models such as that of nuttli 1983 second the stress drops of earthquakes in eastern north america and other continental interiors are not significantly different from those of earthquakes in western north america and have median values of approximately 100 bars the source parameters of earthquakes in eastern north america are consistent with a single constant stress drop scaling relation whereas the source parameters of earthquakes in western north america are much more variable and show significant departures from an average scaling relation in which stress drop decreases slightly with seismic moment

rupture complexity of a moderate sized mb 6 0 earthquake broadband body wave analysis of the north yemen earthquake of 13 december 1982 abstract the occurrence of the north yemen earthquake of 13 december 1982 provides an important opportunity to study source properties of earthquakes in the southern arabian peninsula although moderate in size m b 6 0 this earthquake is the largest to have occurred in the region since the deployment of the global digital seismograph network p waves recorded at teleseismic distances can be obtained from stations of this network and of the graefenberg array that are flat to displacement and velocity in the frequency range from 0 01 to 5 0 hz the broad bandwidth of the data is necessary for deriving a dynamic description of the earthquake as well as the static properties of the source in contrast inversions using only long period data provide much less insight into the complexity of rupture by fitting the displacement pulse shapes with synthetics we find that the yemen earthquake was a complex rupture consisting of two events that occurred about 3 0 sec apart the depth of both hypocenters is estimated to be 7 km the focal mechanisms of the events each have a nodal plane that strikes 340 and dips 60 to the east however the slip directions of the focal mechanisms are in slightly different directions 230 and 300 respectively the total moment of the earthquake is 3 0 10 25 dyne cm with the second event having three times the moment of the initial event by inverting p wave arrival time differences at each station we find the second hypocenter is located 7 5 km from the initial hypocenter in a direction that is nearly colinear with the nnw striking nodal plane of the focal mechanisms of each event the trend of extensional ground cracks in the epicentral region and the predominant strike of composite focal mechanisms of locally recorded aftershocks are also in a nnw direction from the consistency of these observations we infer that the east dipping nnw striking nodal planes are the fault planes the static stress drop of the first and second events are 44 and 31 bars respectively inverting the energy flux in the velocity waveforms we find the energy radiated by the earthquake is 4 8 10 20 dyne cm this implies an average apparent stress of 4 bars at different times during the aftershock sequence seismicity tended to cluster about one or another of two different planes which although having roughly the same strike had conjugate dips one of these planes delineated late in the aftershock sequence coincides with the inferred fault plane for the main shock the complexity of the main shock and of the aftershock sequence suggests that strain accumulated on a system of strongly interdependent faults as the region became critically loaded the effect of a major rupture was to critically stress faults adjacent to the initial nucleation the second event of the main shock represents the rapid release of stress from the fault system while the ensuing aftershock sequence represents the gradual release of stress

contemporary seismicity faulting and the state of stress in the colorado plateau the contemporary seismicity of the colorado plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude and contrary to earlier views of a low to moderate rate of occurrence with earthquakes widely distributed concentrations of earthquakes have been observed in a few areas of the plateau the most seismically active area of the colorado plateau is the eastern wasatch plateau book cliffs where abundant small magnitude seismicity is induced by coal mining the largest earthquakes observed to date of estimated richter magnitude m l 5 6 have generally occurred in northern arizona although very few earthquakes can be associated with known geologic structures or tectonic features in the colorado plateau seismicity appears to be the result of the reactivation of pre existing faults lacking surficial expression but favorably oriented to the tectonic stress field the small to moderate size of the earthquakes and their widespread distribution are consistent with a highly faulted precambrian basement and upper crust and a moderate level of differential tectonic stress earthquakes in the plateau generally occur in the upper crust from the near surface to a depth of 15 20 km although events have been observed in both the lower crust and uppermost mantle in areas of low to normal heat flow the latter suggests that temperatures are sufficiently low at these depths that brittle failure and hence earthquakes are still possible the predominant mode of tectonic deformation within the plateau appears to be normal faulting on northwest to north northwest striking faults with some localized occurrences of strike slip faulting on north west or northeast striking planes at shallow depths the contemporary state of stress within the plateau is characterized by approximate northeast trending extension in contrast to the previous belief that the plateau was being subjected to east west tectonic compression one area of the plateau the eastern wasatch plateau and book cliffs may still be characterized by compressive stresses however the nature of these stresses is not well understood

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

aftershock patterns and main shock faulting we have compared aftershock patterns following several moderate to large earthquakes with the corresponding distributions of coseismic slip obtained from previous analyses of the recorded strong ground motion and teleseismic waveforms well located aftershock hypocenters are projected onto the main shock fault plane and their positions are examined relative to the zones of coseismic displacement indicated by the estimated distributions of main shock slip we also examine the aftershock focal mechanisms when these data are available in an attempt to identify possible patterns of secondary faulting within the aftershock zone our results are consistent with a hypothesis of aftershock occurrence that requires a secondary redistribution of stress following primary failure on the earthquake fault aftershocks following the earthquakes examined in this study occur mostly outside of or near the edges of the source areas indicated by the patterns of main shock slip the spatial distribution of aftershocks reflects either a continuation of slip in the outer regions of the areas of maximum coseismic displacement or the activation of subsidiary faults within the volume surrounding the boundaries of main shock rupture

redox dynamics in the active layer of an arctic headwater catchment examining the potential for transfer of dissolved methane from soils to stream water the linkages between methane production transport and release from terrestrial and aquatic systems are not well understood complicating the task of predicting methane emissions we present novel data examining the potential for the saturated zone of active layer soils to act as a source of dissolved methane to the aquatic system via soil water discharge within a headwater catchment of the continuous permafrost zone in northern canada we monitored redox conditions and soil methane concentrations across a transect of soil profiles from midstream to hillslope and compare temporal patterns in methane concentrations in soils to those in the stream we show that redox conditions in active layer soils become more negative as the thaw season progresses providing conditions suitable for net methanogenesis and that redox conditions are sensitive to increased precipitation during a storm event but only in shallower surface soil layers while we demonstrate that methane concentrations at depth in the hillslope soils increase over the course of the growing season as reducing conditions develop we find no evidence that this has an influence on stream water methane concentrations sediments directly beneath the stream bed however remain strongly reducing at depth throughout the thaw season and contain methane at concentrations 5 orders of magnitude greater than those in hillslope soils the extent of substreambed methane sources and the rates of methane transport from these zones may therefore be important factors determining headwater stream methane concentrations under changing arctic hydrologic regimes