a mass failure model for the initial degradation of fault scarps with application to the 1959 scarps at hebgen lake montana abstract calculation of earthquake scarp ages from scarp morphology usually assumes that scarp materials reach their angle of repose immediately after a rupture however observations of the 1959 hebgen lake montana earthquake scarp and similar features worldwide confirm that scarps require a finite period of mass failure to reach the initial conditions for hillslope diffusion so the age of features less than 1000 yr old cannot be accurately estimated with methods based only on the linear diffusion equation we apply a numerical model of this interval of mass failure degradation to vertical initial angle scarps from the 1959 rupture at hebgen lake montana the mass failure rate coefficient r m ranges from 1 0 10 2 to 1 2 10 1 m yr 1 for young scarps at hebgen lake and nine other locations and has little or no dependence on climate conditions such as annual temperature range or average rainfall including an interval of mass failure gives more accurate age estimates where a scarp age is of the same order as the characteristic mass failure relaxation time of 10 1000 yr

derivation of the 1 0 0 9 ga ferro potassic a type granitoids of southern norway by extreme differentiation from basic magmas major and trace elements sr and nd isotopic data as well as mineral compositions are presented for a selection of the 1 0 0 9 ferro potassic a type granitoids bessefjellet rustfjellet verhuskjerringi valle holum svofjell handeland tveit aseral lyngdal gabbronorites that occur close to the mandal ustaoset line mul of southern norway these hornblende biotite granitoids hbg define an extensive differentiation trend ranging from gabbronorites 50 wt sio 2 to granites 77 wt sio2 this trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions at 930 ma the hbg suite displays a narrower range in isr 0 7027 0 7056 than in end t 1 97 down to 4 90 suggesting some assimilation of a rb depleted lower crust afc process or and source variability an age of 929 47 ma is given by a rb sr isochron on the holum granite sri 0 7046 0 0006 mswd 1 7 geothermobarometers indicate a low pressure of emplacement 1 3 2 7 kbar and an oxygen fugacity close to nno high liquidus temperatures are given by the apatite saturation thermometer 1005 1054 c and are in agreement with results from other studies the basaltic parent magmas of the hbg suite are partial melts of an hydrous mafic potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it this contrasts with the 930 ma anorthosite mangerite charnockite suite amc suite of the rogaland province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated the present data imply the penecontemporaneous melting of two contrasting sources in southern norway the source duality could result from an increasing degree of metamorphism amphibolite to granulite from east to west an horizontal stratification of the lower crust or from the stratification of the lithosphere melting of the lower crust or upper mantle it may also indicate that the amc and hbg suites formed in two distinct crustal segments the linear alignment of the hbg suite along the mandal ustaoset shear zone suggests that a linear uprise of the asthenosphere following a lithospheric delamination under this structure could be the vector of the mantle heat 2003 elsevier science b v all rights reserved

seismic attenuation in northeastern sonora mexico we studied the attenuation of local and regional earthquake body waves recorded by the seismic network of northeastern sonora mexico red sismica del noreste de sonora resnes the source station paths of the selected data set cover the epicentral region of the may 1887 m w 7 5 sonora earthquake we used the best located earthquakes recorded by resnes between 2003 and 2004 to analyze the spectral amplitude decay of body waves with hypocentral distance the database consists of 44 earthquakes with magnitudes between 1 8 and 3 2 and hypocentral distances between 25 and 150 km we calculated fourier spectral amplitudes from more than 400 records and defined attenuation functions with a nonparametric model by inverting the observed spectral amplitudes at 23 individual frequencies between 0 4 and 63 1 hz we determined regional attenuation curves based on events located more than 100 km northwest of the center of the network and local curves based on earthquakes located near the fault zone that ruptured in the 1887 earthquake the obtained attenuation curves indicate that the spectral amplitudes decay faster in the area of the 1887 rupture than in the surrounding region to analyze the possible sources of attenuation we parameterized the attenuation functions considering three terms one that accounts for geometrical spreading another one for anelastic attenuation and a third for near surface attenuation we found the s wave quality factor q to be similar in both the general region and near the 1887 rupture however in the region of the 1887 rupture the near surface attenuation exceeds the regional average finally in the region of the 1887 rupture the nonparametric attenuation functions and the estimates of q indicate that at between 0 6 and 63 1 hz p waves attenuate more than s waves q p 20 8 f 1 1 whereas q s 83 8 f 0 9

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

an assessment of source parameters of earthquakes in the cordillera of the western united states abstract source parameters for 50 earthquakes occurring between 1915 and 1988 have been used to examine the faulting characteristics of magnitude 5 5 to 7 8 earthquakes within the western u s cordillera an area extending from the eastern sierra nevada to the great plains and from northwestern montana to trans pecos texas source parameters used in this study include seismic information from first motion analyses body and surface waveform modeling geodetic studies and geological studies of surface faulting the principal results of this analysis include 1 all earthquakes occurred on faults dipping 38 or more and no evidence for listric of low angle planar faulting was found 2 all m 7 0 earthquakes occurred at depths 12 km and were composed of multiple subevents 3 most earthquakes 70 per cent had unilateral ruptures and 4 no individual subevent had a rupture length 21 km

the a d 1567 mw 7 2 ameca jalisco earthquake western trans mexican volcanic belt surface rupture parameters seismogeological effects and macroseismic intensities from historical sources the a d 1567 ameca jalisco earthquake west central mexico is the strongest known historical crustal earthquake in the trans mexican volcanic belt an active volcanic arc being deformed by an intra arc extensional fault network the location of the surface rupture at the base of the sierra de ameca mountain range its length 13 14 leagues 54 59 km and the scarp height 1 2 estados 1 67 3 34 m are inferred from the description of this earthquake in the relacion de ameca of 1579 which contains what is probably the earliest documentation of an earthquake surface rupture in the americas from scaling relations the moment magnitude of this earthquake can be estimated as 7 2 0 3 based on the rupture length the a d 1567 ameca earthquake is likely to have ruptured to the surface along two or all three major segments of the ameca san marcos normal fault system online material excerpts from five historical documents related to the 1567 ameca earthquake

attenuation of intensity for the 1887 northern sonora mexico earthquake abstract contouring the intensity data for the northern sonora earthquake of 1887 yields an area of perceptibility defined by the mm iii isoseismal that is greatly elongated to the southeast of the epicenter along the sierra madre occidental north of the epicenter the isoseismals are compressed due to the change in crustal structure between the colorado plateau and the southern basin and range province the higher intensity isoseismals are more symmetric although local structure strongly influences individual intensity values to evaluate the attenuation of intensity with distance three regression curves are calculated as below all data i 12 0 3 2 0 0015 r 1 5 in r 25 500 km i 12 0 0 69 0 0069 r 0 82 in r isoseismal i 12 0 8 1 0 0031 r 2 3 in r where r is the epicentral distance in kilometers the all data curve is derived from a regression computed using all 171 intensity values the 25 500 km curve eliminates the near and far points and the isoseismal curve is a regression on the average radii of the area enclosed by each isoseismal contour the latter two curves are similar to curves for the western united states as a whole anderson 1978 or the cordilleran curve of howell and schultz 1975 for intensities less than or equal to mm x the curve for all data indicates less attenuation at lower isoseismals than the other two 1887 regressions and falls between the cordilleran and eastern united states models of howell and schultz

drainage systems and the development of normal faults an example from pleasant valley nevada abstract drainage systems in regions of active extension contain information about fault zone structure and development and particularly about the lateral growth of individual fault segments that is difficult to obtain by more conventional means such as the dating of sedimentary material we investigate drainage fault interaction in pleasant valley nevada where a large normal faulting earthquake occurred in 1915 we find geomorphological evidence for a propagation increase in length of several km at both ends of one of the 1915 fault segments at one end we estimate the propagation rate to be about 50 m per earthquake this estimate is uncertain but is within the range predicted by theoretical models of fault growth drainage along the axes of half graben in this region may be influenced by the spacing of tilted fault blocks the highest valleys are also the narrowest whether the axial streams succeed in reaching lower neighbouring sinks depends on whether sedimentation rates are able to keep basins filled to the levels of the basement highs that separate half graben of opposite polarity a knowledge of modern drainage fault interactions may help predict drainage systems and the position of potential syn rift reservoir sediments in older extensional terrains where fault configurations but not facies distributions are known from seismic reflection surveys

interrelationship of sedimentary and volcanic deposits associated with tertiary extension in sonora mexico clastic sedimentary deposits and associated volcanic rocks record the progress of tertiary extension in the mexican state of sonora these deposits accumulated within basins located throughout the eastern two thirds of the state the rocks are slightly indurated conglomerates and sandstones clast types reflect the local highland exposures volcanic units that bound and are interlayered with the sedimentary rocks provide stratigraphic and time markers of basin evolution and by inference of marginal fault development this record has been examined in an east west belt across south central sonora the volcanic rocks occur in three distinct associations at the base and interbedded within the lower portions of the sedimentary sections are lava flows of dominantly basaltic andesite composition in most localities the overlying clastic sedimentary strata are conformable with these lava flows and we conclude that the magmatism was triggered by early faulting along the basin margins a second association present near the base of the sections comprises massive lava domes and flows of intermediate composition that contain distinctive phenocrysts of dark brown amphibole the domes apparently formed when viscous lava welled up along developing marginal basin faults in one case a transition was observed over a short distance from a structureless dome to a thick lava flow that is conformably interbedded with and sheds clasts laterally into the sedimentary section a third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the coarsest and thickest lower portions of the sedimentary sections k ar ages for volcanic rocks in the two oldest of these settings indicate that each tertiary basin had a distinct history with no apparent regional geographic pattern in the rio yaqui basin where exhumation by the major river of sonora has exposed a full section of the volcanic and clastic sequences extension was possibly as old as 27 ma elsewhere deposition of sediments was underway in most basins by 24 ma and in all basins by 20 ma k ar ages of the younger rhyolitic volcanic rocks fall within a narrow range from 12 8 to 10 5 ma these ages provide a younger time limit to deposition of the coarser and thicker portions of the clastic sedimentary sections the mafic lava flows dominantly basaltic andesite are similar in major element composition to mafic lava flows that cap felsic sections within the sierra madre occidental volcanic field of western mexico and the mid tertiary datil mogollon volcanic field in southwestern new mexico they are higher in silica and lower in total alkalies than younger capping mafic lava flows within the gulf of california extensional province of western sonora and neogene basaltic lava flows in the datil mogollon field they are similarly distinct from basaltic dikes and lava flows that are clearly associated with basin and range faulting in trans pecos texas and in northern durango state a convergent margin magmatic arc was active in the present day gulf of california region between 24 and 11 5 ma the earliest extension in south central sonora was therefore occurring in a back arc setting continued sedimentation and rotation of volcanic strata indicate that extension continued throughout the region after 10 ma with little coeval volcanism

structural configuration of the otates fault southern basin and range province and its rupture in the 3 may 1887 mw 7 5 sonora mexico earthquake abstract during the great 3 may 1887 sonoran earthquake surface rupture end to end length 101 8 km m w 7 5 0 3 an array of three north south striking basin and range province faults from north to south pitaycachi teras and otates slipped along the western margin of the sierra madre occidental plateau of northern mexico this detailed field survey of the previously undocumented 1887 earthquake rupture zone along the otates fault includes mapping the rupture scarp and measurements of surface deformation at 25 sites the surface rupture coincides with the mapped trace of the otates fault dips 68 w and has an end point to end point length of 18 2 km the lateral rupture terminations are controlled by cross faults striation measurements and the style of rupture indicate extensional dip slip without significant lateral displacement the average shortening p and extension t kinematic axes inferred from the striations are similar to published composite focal mechanisms of the microseismicity recorded in the epicentral region of the 1887 earthquake based on surface deformation maximum slip along the otates fault in the 1887 earthquake was 270 cm and the average slip was 189 cm a quadratic best fit to the along rupture surface offsets indicates an asymmetric distribution with the maximum offset near the northern end of the rupture segment which can be explained by the mechanical interaction of the teras and otates surface rupture segments through their stress fields the average recurrence interval of 1887 sized earthquakes on the otates fault is 30 42 kyr based on the long term geologic dip slip rate of the fault 0 06 mm yr and its amount of slip in the 1887 earthquake

hinge migration and hinge replacement abstract the axial migration angle is defined as the angle between the current hinge line and a marker line which was parallel to the initial hinge line it is generally assumed that the marker line is deformed in an entirely passive manner whereas the fold axis traces out the orientations of the long axis of the sectional strain ellipse this concept is modified when the competence contrast between a layer and its embedding medium is taken into consideration large values of may be obtained in constrictional deformations however because of strong hinge line arcuations of the nonplane noncylindrical folds the concept of hinge migration will not be meaningful hinge migration is greatly influenced by the competence of layers in inextensible layers the magnitude of hinge migration may become very large even in flattening and plane strain on the other hand hinge migration will be very small or negligible if buckle folding is associated with large layer parallel homogeneous strain the concept of hinge migration and hinge replacement have been critically analysed for development of types 0 1 and 2 fold interference patterns

mmi attenuation and historical earthquakes in the basin and range province of western north america earthquakes in central nevada 1932 1959 were used to develop a modified mercalli intensity mmi attenuation model for estimating moment mag nitude m for earthquakes in the basin and range province of interior western north america m is 7 4 7 5 for the 26 march 1872 owens valley california earthquake in agreement with beanland and clark s 1994 m 7 6 that was estimated from geologic field observations m is 7 5 for the 3 may 1887 sonora mexico earthquake in agreement with natali and sbar s 1982 m 7 4 and suter s 2006 m 7 5 both estimated from geologic field observations mmi at sites in california for earthquakes in the nevada basin and range appar ently are not much affected by the sierra nevada except at sites near the sierra nevada where mmi is reduced this reduction in mmi is consistent with a shadow zone produced by the root of the sierra nevada in contrast mmi assignments for earthquakes located in the eastern sierra nevada near the west margin of the basin and range are greater than predicted at sites in california these higher mmi values may result from critical reflections due to layering near the base of the sierra nevada

crustal velocity field of mexico from continuous gps measurements 1993 to june 2001 implications for the neotectonics of mexico 1 we combine velocities for 14 continuous gps stations spanning mexico and 173 additional continuous gps sites on the north american and pacific plates to study the large scale deformation of mexico the new station velocities which are derived from more than 6000 days of previously unused gps data provide the first ever view of the crustal velocity field of mexico key results are as follows 1 areas north of the mexican volcanic belt not including baja california move with the north american plate interior within the 1 2 mm yr 1 station velocity uncertainties station velocities for the mexican basin and range are consistent with no present day extension and yield an upper 95 limit of 1 3 mm yr 1 for any regional extension 2 south of the mexican volcanic belt five of the six sites move significantly relative to the north american plate all sites in the yucatan peninsula move toward the east at 3 4 mm yr 1 possibly defining an independent yucatan block 3 site velocities are consistent with limits of 0 4 mm yr 1 for present slip across the mexican volcanic belt 4 tampico on the gulf coast exhibits eastward motion consistent with gravity sliding known to occur in the adjacent mexican ridges fold belt 5 southeastward motion of la paz relative to the pacific plate is consistent with the hypothesis that the baja peninsula is not fully attached to the pacific plate 6 residual velocities for 160 north american plate gps stations outside of mexico exhibit no coherent regional patterns indicative of internal plate deformation

cretaceous and tertiary sedimentary magmatic and tectonic evolution of north central sonora arizpe and bacanuchi quadrangles northwest mexico the arizpe and bacanuchi quadrangles provide a geologic history representative of the north central part of sonora where lithologies are dominated by late mesozoic and cenozoic igneous rocks in this study new geologic mapping 40ar 39ar dating and geochemical analyses have been combined to provide a stratigraphic framework for this area ten lithostratigraphic units and several igneous and tectonic events can be recognized the oldest outcropping rocks are lower cretaceous strata of the bisbee group which along with the picacho conglomerate record a middle cretaceous compressive tectonic event and associated sedimentation laramide igneous activity is widespread and represented by 1 highly altered andesitic flows and volcaniclastic rocks arroyo alcaparros andesitic rocks of late campanian to maastrichtian age 2 less altered andesitic and dacitic flows cerro las jarillas volcanic rocks of late paleocene age and the intrusive bodies of 3 sierra el manzanal granodiorite and 4 rancho vaqueria quartz monzonite the sierra el manzanal granodiorite was emplaced at ca 68 ma on the basis of a 40ar 39ar biotite age 67 97 0 19 ma and cooled relatively rapidly according to less precise 40ar 39ar hornblende and k feldspar ages from the same sample 64 8 1 0 ma and 62 8 0 3 ma respectively the cerro las jarillas volcanic rocks are slightly younger 40ar 39ar biotite age of 58 67 0 17 ma the rancho vaqueria quartz monzonite was emplaced at ca 57 ma 40ar 39ar biotite age of 56 73 0 14 ma and a less precise 40ar 39ar hornblende age of 55 0 0 7 ma a protracted cooling history of this pluton is indicated by the age spectrum of k feldspar from the same sample a probable magmatic lull and denudation seem to have occurred between middle and late eocene time and probably until the early oligocene subsequently rhyolitic to mafic volcanism began close to late oligocene time and lasted until the early miocene felsic volcanism is represented by the cerro cebadehuachi volcanic rocks from which 40ar 39ar hornblende ages of 27 25 0 09 and 27 32 0 06 ma and a biotite age of 26 97 0 06 ma were obtained at three different localities the mesa pedregosa volcanic rocks represent the transition to younger mafic volcanic activity that occurred during the late oligocene as indicated by a sanidine 40ar 39ar age of 25 48 0 05 ma this late oligocene and early miocene magmatism was paired by two episodes of extensional deformation the first phase is characterized by northwest striking normal faults and folds which expose the deepest structural levels of the area and by the related basin fill the bacanuchi conglomerate the second phase is represented by north striking normal faults and by the syntectonic basin fill the arizpe conglomerate basaltic andesite volcanic flows at the base of the arizpe conglomerate yielded 40ar 39ar whole rock ages of 23 52 0 17 and 21 0 20 ma the extensional deformation 27 to 23 ma in the study area is coeval with the development of metamorphic core complexes in neighboring areas of sonora and with the onset of extension in southern sonora the mafic volcanic rocks and clastic sedimentary units associated with this extension resemble the basin fills that in other parts of sonora are assigned to the baucarit formation geochemical information from samples representing each of the igneous events displayed high k calc alkalic and mostly metaluminous compositions the older units including the arroyo alcaparros andesitic rocks the cerro las jarillas volcanic rocks the sierra el manzanal granodiorite and the rancho vaqueria quartz monzonite are characterized by steep chondrite normalized ree rare earth element slopes and generally well developed negative eu anomalies suggesting garnet and plagioclase removal in the source the younger igneous events including the cerro cebadehuachi and mesa pedregosa volcanic rocks and the basaltic flows associated with the arizpe conglomerate showed basin shaped ree slopes with no eu anomalies suggesting clinopyroxene or amphibole fractionation

frequency and size of quaternary surface ruptures of the pitaycachi fault northeastern sonora mexico movements along the pitaycachi fault since the miocene juxtaposed different alluvial units and created 2 to 45 m high fault scarps downslope from a pedimented mountain front prior to 1887 in 1887 a major earthquake formed a 75 km long 1 2 to 4 m high scarp along the trace of prehistoric surface ruptures diverse evidence from many study sites indicates that about 200 000 yr elapsed between the prior youngest pleistocene event and the 1887 surface rupture cumulative displacements of pliocene to mid pleistocene alluvial fans and stream terraces decrease with decreasing age the trace of the prior rupture was largely buried by sheets of late pleistocene and holocene piedmont alluvium late pleistocene soils are offset about the same amount as the height of the 1887 scarp valleys that are as much as 40 m deep and 0 5 to 0 9 km wide have been eroded since the prior event they contain multiple late pleistocene and holocene stream terraces that were not faulted until 1887 pre 1887 alluvial fault scarps were degraded to 2 to 9 slopes before the 1887 event even in resistant materials such as clay rich soil horizons with unweathered rhyolite cobbles and calcrete scarp height maximum slope regressions and diffusion equation analyses for reconstructed pre 1887 scarp profiles indicate that the prior event occurred more than 100 000 yr ago acceleration of scarp degradation rates during the holocene and or relatively resistant materials exposed in the scarps would increase the age estimates to 200 000 yr or more very long recurrence intervals are the characteristic style of movement on the pitaycachi fault at one site six ages of diverse valley fills were inset into pedimented granodiorite upslope from the fault between the prior and 1887 events only 3 m of relief remained before the 1887 rupture increased the scarp height from 3 to 6 m some hillslopes have triangular talus facets of carbonatecemented colluvium that resulted from infrequent fault movements and intervening periods of erosion smooth hillsides of resistant volcanic rocks between the facets show that virtually all of the prior surface rupture event scarps had been removed by prolonged slope degradation

plate interactions control middle late miocene proto gulf and basin and range extension in the southern basin and range middle late miocene proto gulf 12 6 ma extension around the gulf of california gulf extensional province is commonly interpreted as resulting from partitioning of oblique pacific north american plate motion into strike slip displacement along the margin and east northeast extension perpendicular to the margin within the north american plate we propose that this mechanism also applies to kinematically similar predominantly east northeast extension that occurred at the same time throughout the southern basin and range province from southern arizona and new mexico to the trans mexican volcanic belt new field and 40ar 39ar data in sinaloa and durango confirm that this episode of extension occurred on the mainland side of the gulf and in the basin and range east of the sierra madre occidental which is generally considered the eastern margin of the gulf extensional province published data indicate the middle late miocene episode also occurred across the northern and southern ends of the sierra madre where the gulf extensional province connects with the basin and range 1 from central sonora into southern arizona and new mexico and 2 from nayarit into central mexico north of the trans mexican volcanic belt this episode appears to have avected an area that continues to the eastern edge of the basin and range province in texas and san luis potosi recognition that this episode of extension avected the entire southern basin and range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the gulf extensional province alone published plate reconstructions require 160 to 110 km of east northeast extension between 12 and 6 ma if taken up solely within the gulf extensional province this would have generated 66 to 78 extension which is much greater than observed spread across the entire southern basin and range it requires only 20 total extension which is more consistent with observations of cumulative extension between 12 and 6 ma extension was partitioned into the gulf extensional province because 1 it lies between two stable batholith belts mesozoic peninsular ranges on the west and mid tertiary sierra madre occidental on the east that resisted extension and 2 the gulf was thermally weakened by immediately preceding arc magmatism extension in the main basin and range province in part probably avoided the relatively strong batholithic crust of the sierra madre occidental 2000 elsevier science b v all rights reserved

the seismicity of chihuahua mexico and the 1928 parral earthquake abstract studies of the seismicity of chihuahua mexico and determination of source parameters for the 1928 parral earthquake suggest that the eastern portion of chihuahua is currently undergoing extension the parral earthquake has similar source parameters to other southern basin and range rio grande rift events the focal mechanism for the parral event however indicates northeast southwest oriented extension a direction that is parallel to neither the east west oriented extension seen near the us mexican bordern nor to the physiographic province boundary between the sierra madre occidental and the southern basin and range where the earthquake occurred intensity information for the parral event and the 1931 valentine texas earthquake suggests that attenuation within the region lies somewhere between relationships developed for the eastern and cordilleran portions of the usa

coseismic throw variation across along strike bends on active normal faults implications for displacement versus length scaling of earthquake ruptures fault bends and associated changes in fault dip play a key role in explaining the scatter in maximum offset versus surface rupture length fault scaling relationships detailed field measurements of the fault geometry and magnitude of slip in the 2016 2017 central italy earthquake sequence alongside three examples from large historical normal faulting earthquakes in different tectonic settings provide multiple examples in which coseismic throw increases across bends in fault strike where dip also increases beyond what is necessary to accommodate a uniform slip vector coseismic surface ruptures produced by two mainshocks of the 2016 2017 central italy earthquake sequence 24 august 2016 mw 6 0 and 30 october 2016 mw 6 5 cross a 0 83 km amplitude along strike bend and the coseismic throws for both earthquakes increase by a factor of 2 3 where the strike of the fault changes by 28o and the dip increases by 20 25o we present similar examples from historical normal faulting earthquakes 1887 sonora earthquake mw 7 5 1981 corinth earthquakes mw 6 7 6 4 and 1983 borah peak earthquake mw 7 3 we demonstrate that it is possible to estimate the expected change in throw across a bend by applying equations that relate strike dip and slip vector to horizontal strain conservation along a nonplanar fault for a single earthquake rupture the calculated slip enhancement in bends can explain much of the scatter in maximum displacement dmax versus surface rupture length scaling relationships if fault bends are unrecognized they can introduce variation in dmax that may lead to erroneous inferences of stress drop variability for earthquakes and exaggerate maximum earthquake magnitudes derived from vertical offsets in paleoseismic data sets

evolution of stratigraphic sequences in multisegmented continental rift basins comparison of computer models with the basins of the east african rift system this article presents a series of numerical simulations of the stratigraphic evolution of continental rift basins we model the geomorphic and tectonic processes acting in this depositional environment which contrasts with the traditional approach of modeling the settling of sediments especially in marine clastic basins what is new in our model is that it can simulate the formation of basins with linked half grabens which is a fundamental feature of rift systems the attributes of the numerical simulations such as the overall morphology basin architecture drainage systems and stratigraphy are in good agreement with digital elevation models and reflection seismic lines of the basins of the east african rift system specifically the tanganyika and malawi rifts

the lanthanide tetrad effect and its correlation with k rb eu eu sr eu y ho and zr hf of evolving peraluminous granite suites lanthanide tetrad effects are often observed in ree patterns of more highly evolved variscan peraluminous granites of mid eastern germany central erzgebirge western erzgebirge fichtelgebirge and northern oberpfalz the degree of the tetrad effect te1 3 is estimated and plotted vs k rb sr eu eu eu y ho and zr hf the diagrams reveal that the tetrad effect develops parallel to granite evolution and significant tetrad effects are strictly confined to highly differentiated samples mineral fractionation as a cause for the tetrad effect is not supported by a calculated rayleigh fractionation which also could not explain the fractionation trends of sr eu and eu eu the strong decrease of eu concentrations in highly evolved rocks suggests that eu fractionates between the residual melt and a coexisting aqueous high temperature fluid mineral fractionation as a reason for the tetrad effect is even more unlikely as ree patterns of accessory minerals display similar tetrad effects as the respective host rocks the accessory minerals inherit the ree signature of the melt and do not contribute to the bulk rock tetrad effect via mineral fractionation these results point in summary to significant changes of element fractionation behavior in highly evolved granitic melts ionic radius and charge which commonly control the element distribution between mineral and melt are no longer the exclusive control the tetrad effect and the highly fractionated trace element ratios of y ho and zr hf indicate a trace element behavior that is similar to that in aqueous systems in which chemical complexation is of significant influence this distinct trace element behavior and the common features of magmatic hydrothermal alteration suggest the increasing importance of an aqueous like fluid system during the final stages of granite crystallization the positive correlation of te1 3 with bulk rock fluorine contents hints at the importance of ree fluorine complexation in generating the tetrad effect as the evolution of a ree pattern with tetrad effect m type implies the removal of a respective mirroring ree pattern w type the tetrad effect identifies open system conditions during granite crystallization

rupture terminations and size of segment boundaries from historical earthquake ruptures in the basin and range province abstract the fault segmentation method is commonly used to estimate the potential earthquake size segment boundaries play an important role in arresting earthquake ruptures from event to event in the basin and range province earthquake rupture terminations are commonly associated with structural discontinuities but not all structural discontinuities have the capability to terminate an earthquake rupture the size of structural discontinuities with respect to the rupture length and displacement may play an important role in controlling rupture termination studies of the geometric pattern of seven well documented historical earthquake ruptures in the basin and range province reveal three important relationships between rupture termination and the size of structural discontinuities firstly terminations of normal faulting earthquakes are often associated with structural discontinuities at least for the five historical ruptures studied in this paper secondly the sizes of structural discontinuities at the end of earthquake rupture zones are generally the largest among the structural discontinuities within the rupture zone thirdly there appears to be a trend that larger earthquake ruptures are stopped by larger structural discontinuities the size of structural discontinuities that stopped the earthquake seems to scale with the length and displacement of earthquake surface rupture

the long lasting aftershock series of the 3 may 1887 mw 7 5 sonora earthquake in the mexican basin and range province we study local and regional body wave arrival times from several seis mic networks to better define the active regional fault pattern in the epicentral region of the 3 may 1887 mw 7 5 sonora mexico southern basin and range province earthquake we determine hypocenter coordinates of earthquakes that originated between 2003 and 2007 from arrival times recorded by the local network resnes red sismica del noreste de sonora and stations of the network of autonomously recording seismographs nars baja array for events between april and december 2007 we also incorporated arrival times from usarray stations located within 150 km of the united states mexico border we first obtained preliminary earthquake loca tions with the hypoinverse program klein 2002 and then relocated these initial hypocenter coordinates with the source specific station term ssst method lin and shearer 2005 most relocated epicenters cluster in the upper crust near the faults that ruptured during the 1887 earthquake and can be interpreted to be part of its long lasting series of aftershocks the region of aftershock activity extends along the same fault zone 40 50 km south of the documented southern tip of the 1887 rupture and includes faults in the epicentral region of the 17 may 1913 imax viii mi 5 0 0 4 and 18 december 1923 imax ix mi 5 7 0 4 granados huasabas sonora earthquakes which themselves are likely to be aftershocks of the 1887 event the long aftershock duration can be explained by the unusually large magnitude of the mainshock and by the low slip rates and long mainshock recurrence times of the faults that ruptured in 1887

macroseismic study of shallow earthquakes in the central and eastern parts of the trans mexican volcanic belt mexico we present isoseismal maps for eight shallow earthquakes 1887 pinal de amoles 1912 acambay 1920 jalapa 1950 ixmiquilpan 1976 cardonal 1979 maravatio 1987 actopan and 1989 landa 4 1 mb 6 9 that occurred in the central and eastern parts of the trans mexican volcanic belt based on their intensity distributions we can relate several of these earthquakes to east west striking quaternary normal faults known from geological observations furthermore we derive for these events by empirical curve fitting linear relations between the magnitude calculated from the instrumental record the intensity distribution and the maximum intensity the comparison of these relations with equations from the literature suggests that the area shaken with a certain intensity by an earthquake in the trans mexican volcanic belt is approximately 10 times smaller than the area shaken by a shallow earthquake of the same magnitude in the intracratonic part of north america moreover we use the derived relationships between magnitude isoseismal area and maximum intensity to estimate the intensity magnitude mi of the 1887 pinal de amoles earthquake as 5 3 0 5

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

synextensional pliocene pleistocene eruptive activity in the camargo volcanic field chihuahua mexico the camargo volcanic field is the largest mafic alkalic volcanic field in the mexican basin and range province and the relationship between volcanism and normal faulting is especially strong the camargo volcanic field lies in the northern part of the province midway between the sierra madre occidental and trans pecos texas it is formed by pliocene pleistocene 4 7 0 09 ma intraplate mafic alkalic volcanic rocks some of which contain peridotite pyroxenite and granulite xenoliths the volcanic field covers 3000 km2 and has an estimated volume of 120 km3 erupted from 300 recognized vents twenty six new 40ar 39ar age determinations for the camargo volcanic field and its environs show that volcanic activity began in the southwest part of the field and shifted toward the northeast at 15 mm yr the average magmatic eruption rate during growth of the field was 0 026 km3 k y the camargo volcanic field lies within an accommodation zone with west dipping faults and east tilted blocks to the north and east dipping faults and west tilted blocks to the south these faults are expressed in the volcanic field by a n30 w trending graben with scarps up to 100 m high through its central part volcanism and faulting were at least in part coeval and younger volcanic products commonly drape fault scarps that cut earlier lavas normal faulting is bracketed between 4 7 and 2 1 ma and may have also migrated northeastward estimated vertical slip rates on four pliocene faults range from 0 03 mm yr a likely long term rate to 1 67 mm yr interpreted as a short term rate operative during periods of active faulting northwest striking normal faults that cut alluvial fan deposits and pleistocene lavas in the northern camargo volcanic field and geomorphic evidence for recent uplift to the south of the volcanic field suggest that the region is still extending

the sanfandila earthquake sequence of 1998 queretaro mexico activation of an undocumented fault in the northern edge of central trans mexican volcanic belt abstract a sequence of small earthquakes occurred in central mexico at the northern edge of the trans mexican volcanic belt tmvb in the state of queretaro during the first 3 months of 1998 medium to large events in the continental regime of central mexico are not common but the seismic history of the region demonstrates that faults there are capable of generating destructive events the sequence was analyzed using data from a temporary network with the goals of identifying the causative fault and its relation to regional tectonics employing a waveform inversion scheme adapted from a method used for regional studies we found that the source mechanisms conform to the style of faulting i e extension in the e w direction representative of the taxco san miguel allende fault system this system has been proposed as the southernmost extension of the basin and range br province the spatial distribution of hypocenters and source mechanisms indicate that the seismogenic segment was a fault with an azimuth of approximately 334 with almost pure dip slip since events which occurred just south from this region show features which are consistent with tmvb tectonics i e extension in an n s direction the sequence may mark the boundary between the tmvb and br stress domains

seismicity in the epicentral region of the 1887 northeastern sonoran earthquake mexico the 1887 sonoran earthquake in northeastern sonora mexico has an 80 km rupture length with an average 3 m displacement and an assumed 16 km depth of rupture this corresponds to a seismic moment of 1 27 x 1027 dyne cm and an ms of 7 4 seven to ten portable seismographs with an average spacing of 10 km were operated in a 2800 km 2 region surrounding the 1887 fault scarp thirtythree earthquakes were detected during 30 days of usable recording for all events 1 3 m 2 the most accurate locations horizontal error 5 km define a west dipping normal range bounding fault all events occurred within 15 km of the surface the name pitaycachi fault after the most prominent peak of the horst block is proposed for this zone of active faulting seismicity along the zone terminates abruptly 13 km north of the fault scarp and extends to the south for at least 32 km beyond the southern a composite focal mechanism from events at the southern end of the 1887 scarp indicates a normal fault striking n35 e and dipping 72 to the northwest

active tectonics of northeastern sonora mexico southern basin and range province and the 3 may 1887 mw 7 4 earthquake north south striking and west dipping basin and range province normal faults form the western edge of the sierra madre occidental plateau in northeastern sonora these faults and associated half grabens extend over a distance of more than 300 km between the san bernardino basin in the north and the sahuaripa basin in the south an earthquake in 1887 ruptured three neighboring segments of this major fault zone our field mapping in this region indicates that the surface rupture of the 1887 earthquake extends farther to the south and is considerably longer 101 4 km end to end length than previously reported a compilation of the seismicity in the epicentral region of the 1887 earthquake shows the epicenters to be distributed in well defined clusters at the northern end of the 1887 surface rupture in the step overs between the three rupture segments on a neighboring fault in the west fronteras fault and on fault segments farther south along the same fault zone granados region the distribution of seismicity correlates well with calculated changes in coulomb failure stress resulting from the 1887 earthquake

geomorphology and paleoseismology of the weinan fault shaanxi central china and the source of the 1556 huaxian earthquake the 1556 ce huaxian earthquake resulted in an estimated 830 000 deaths and caused widespread devastation in the weihe basin china seismic intensities from historical accounts yield via magnitude intensity relations a commonly quoted magnitude of 8 to 8 the maximum recorded shaking was confined to a zone close to the huashan and weinan faults which exhibit fresh scarps up to 7 8 m high recent palaeoseismic studies have suggested however that the weinan fault has not ruptured at the surface for several thousand years furthermore the 90 km combined length of the huashan and weinan faults is short for an earthquake of magnitude 8 to 8 we present a detailed analysis of the weinan fault at one well preserved site combining field observations and age constraints from fluvial terraces displaced by faulting analysis of a high resolution digital elevation model dem interpretation of the walls of a quarry that cuts through the fault zone and from a profile of borehole cores across the fault we find that the fault ruptured within the last 900 years and is likely along with the huashan segment to be the causative fault for the 1556 earthquake the magnitude remains uncertain with m w 7 5 being a plausible estimate given the fault length and no more than m w 8 0 if we use the maximum estimates of slip these estimates are considerably smaller than magnitudes estimated from intensities with importance in estimating the recurrence intervals between destructive earthquakes and hazard across central china

rupture of the pitaycachi fault in the 1887 mw 7 5 sonora mexico earthquake southern basin and range province rupture kinematics and epicenter inferred from rupture branching patterns during the 3 may 1887 mw 7 5 sonora earthquake surface rupture end to end length 101 8 km an array of three north south striking basin and range province faults from north to south pitaycachi teras and otates slipped sequentially along the western margin of the sierra madre occidental plateau this detailed field survey of the 1887 earthquake rupture zone along the pitaycachi fault includes mapping the rupture scarp and measurements of surface deformation the surface rupture has an endpoint to endpoint length of 41 0 km dips 70 w and is characterized by normal left lateral extension the maximum surface offset is 487 cm and the mean offset 260 cm the rupture trace shows a complex pattern of second order segmentation however this segmentation is not expressed in the 1887 along rupture surface offset profile which indicates that the secondary segments are linked at depth into a single coherent fault surface the pitaycachi surface rupture shows a well developed bipolar branching pattern suggesting that the rupture originated in its central part where the polarity of the rupture bifurcations changes most likely the rupture first propagated bilaterally along the pitaycachi fault the southern rupture front likely jumped across a step over to the teras fault and from there across a major relay zone to the otates fault branching probably resulted from the lateral propagation of the rupture after breaching the seismogenic part of the crust given that the much shorter ruptures of the otates and teras segments did not develop branches