estimation of magnitudes in central and eastern north america using intensity and felt area over 480 earthquakes from central and eastern north america having instrumental mb magnitudes and maximum modified mercalli intensities i were examined using exploratory data analysis techniques and modeled with robust estimation methods 91 of those earthquakes also have felt area data a previously undocumented distinct offset of magnitudes is observed between modified mercalli intensities vi and vii in both the central and eastern north america data set and a separate western north american catalog of earthquakes the offset is most probably a characteristic of the modified mercalli scale and brings into question the often assumed linear relationship between magnitude and intensity in particular linear regression models could not accurately estimate magnitudes for larger i vii or viii events instead robust estimates of center and spread for individual intensity interval distributions are recommended in studies where conversion of intensity to magnitude for groups of earthquakes is required e g studies involving the gutenberg richter recurrence relation the underlying distribution of magnitudes or its approximation for each intensity should be used linear regression models using i and or felt area and several transformations were tested the robust linear regression models m b 2 48 0 0769 log 2 f a and m b 2 16 0 0219 l 0 2 0 0596 log 2 f a where fa is felt area in square kilometers proved to be the most accurate magnitude estimation models for central and eastern north america earthquakes a comparison of regional felt area models indicates that regional differences in attenuation of seismic waves may exist between central and eastern north america that becomes apparent only for events of sufficient magnitude those differences appear as larger felt areas for earthquakes of central north america as compared to earthquakes of eastern north america

active faults and state of stress in the central part of the trans mexican volcanic belt mexico 1 the venta de bravo fault the central part of the trans mexican volcanic belt is being deformed by seismically active west east striking normal faults that cause pronounced scarps structural and seismic along strike discontinuities allow the following segmentation 1 the western segment the area between morelia and the los azufres volcanic center is characterized by 25 km long fault scarps that define the cuitzeo graben and the santa ana maya horst which has a relief 850 m with respect to the cuitzeo graben the faults cut several quaternary scoria cones as well as alluvium 2 the central neotectonic segment extends between maravatio in the west and altamirano volcano in the east the most prominent structure of this segment is the 45 km long venta de bravo fault with a relief 300 m the february 22 1979 mb 5 3 maravatio earthquake occurred on this fault the late quaternary vertical slip rate along the easternmost part of the venta de bravo fault is estimated at 2 mm yr the lateral extension of the segment is defined by the limits of the venta de bravo fault and the eastern termination of the cuitzeo graben 3 the eastern segment is made up of the acambay graben the limit between the central and eastern segments is defined by an extensional jog between the venta de bravo and pastores faults which indicates a left lateral strike slip component the most recent rupture of this segment occurred in the november 19 1912 ms 6 9 acambay earthquake that caused vertical displacements 50 cm along the faults flanking the graben stress determinations based on quaternary scoria cone alignments the focal mechanism of the 1979 earthquake and the inversion of striation measurements indicate a stress field with sv sene snnw the fault striations as well as the focal mechanism indicate a minor but consistent left lateral strike slip component the observed stress and deformation can be explained by the superposition of two sources of stress isostatically compensated surface loads related to the high elevation of the trans mexican volcanic belt probably cause horizontal deviatoric tension and the observed crestal normal faults which strike parallel to the axis of the volcanic belt the observed left lateral strike slip component on the other hand can be explained by compressional far field stresses caused by loads at the boundary between the cocos and north american plates the orientation of the faults with respect to the direction of plate convergence is a prefered orientation for left lateral slip along the observed faults alternatively the observed left lateral strike slip component may accommodate trench parallel motion of crustal blocks in southwestern mexico

the real southern basin and range mid to late cenozoic extension in mexico much of northern and central mexico underwent east northeast extension in the mid to late cenozoic this area constitutes a major but little recognized part of the basin and range province the extended region is bounded on the east by the laramide thrust front of the sierra madre oriental on the west the relatively unfaulted sierra madre occidental separates the extended area in central mexico from that around the gulf of california extension occurred as far south as what is now the trans mexican volcanic belt and in oaxaca south of the belt the basin and range province in mexico constitutes approximately half of the 19 x 10 5 km 2 of western north america that underwent mid to late cenozoic extension north northwest orientations of numerous epithermal vein systems indicate that east northeast extension began as early as 30 ma in areas north of the volcanic belt major episodes of faulting began at about 23 to 24 ma and 12 to 13 ma both in mexico and in the southwestern united states faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting widespread quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the trans mexican volcanic belt in contrast the tectonics in and south of the belt are now probably related to subduction of the rivera plate contemporaneity of the 12 ma episode with early extension around the gulf of california attributed to pacific north american plate boundary reorganization suggests that extension is related predominantly to plate boundary effects the beginning of extension at 30 ma may be related to initial encroachment of the east pacific rise upon the trench that lay off western north america

recent left oblique slip faulting in the central eastern trans mexican volcanic belt seismic hazard and geodynamic implications 1 detailed structural analysis was conducted on the e w trending tenango fault system tfs at the western end of the la pera fault system a major structure within the trans mexican volcanic belt tmvb field mapping which included trenching the fault trace revealed the displacement of recent volcanic deposits the study indicates that the tfs is an active subvertical fault with transtensive left lateral movement consistent with the contemporary stress regime in the region the estimated holocene slip rate is 0 3 0 5 mm yr the tfs activity constitutes an important seismic hazard in the intra arc lerma basin several towns are located within this basin and are particularly vulnerable to seismic risk because of its underlying strata consisting of lacustrine and volcaniclastic deposits estimated maximum mw 6 8 7 the tfs is an important structural element in the complex tectonic deformation history of central eastern tmvb and influenced the behavior of recent volcanism within the lerma basin

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

characteristics of the summer 1987 flow field in the ionian sea we present an extensive analysis of the first complete data set in the northern ionian sea collected during the physical oceanography of the eastern mediterranean poem general circulation survey of september 1987 a four water mass structure of the basin is found to be represented by a first internal baroclinic rossby radius of deformation of 11 8 km the horizontal correlation scales decrease with depth and the subsurface flow is dominated by anticyclonic gyres large scale circulation trends of the temperature and salinity covariance matrices are compensated below 200 m and only the gyre scales 100 km persist at intermediate and deep levels the empirical orthogonal functions of the data set show that an horizontal scale separation exists between the first and higher vertical modes of the dynamic height field

the aljibes half graben active extension at the boundary between the trans mexican volcanic belt and the basin and range province mexico the aljibes half graben is located 140 km west northwest of mexico city and is characterized by several parallel 10 km long south dipping and east striking normal faults late miocene basalt flows with 40 ar 39 ar ages of 7 7 0 1 ma and 7 1 0 5 ma are displaced along these faults with reference to this basalt the composite throw and heave of the half graben are 480 m and 270 m respectively and the relative extension is 5 whereas the vertical slip rate along the faults of the aljibes half graben after the deposition of the basalt is 0 07 mm yr the aljibes half graben is located at the northern margin of the trans mexican volcanic belt and perpendicularly intersects northstriking normal faults of the basin and range province the relative sequence of activity of the two stress fields and fault systems cannot be resolved in the study area and the two systems may have been active simultaneously we explain this structural configuration by horizontal migration of the boundary between the two stress provinces with time which requires intermittent permutations between the intermediate and least principal stresses 2 and 3 this is supported by the low stress ratio 2 3 1 3 obtained from modeling the stress tensor associated with the slip along the faults of the aljibes half graben the orientations of the fault planes and slip vectors of the aljibes half graben are practically identical to those of the nearby acambay graben and the venta de bravo fault which caused the acambay earthquake of november 19 1912 m s 6 9 and the maravatio earthquake of february 22 1979 m b 5 3 furthermore the aljibes half graben forms the extrapolated western continuation of the northern master fault of the mezquital graben which ruptured partly in the cardonal earthquake of march 26 1976 m b 5 3 this suggests the aljibes half graben may also be seismically active although no historical earthquake is known to have occurred along this structure further evidence for the likely seismic activity of the aljibes half graben are two east to eastnortheast trending epicenter alignments between the aljibes halfgraben and the mezquital graben and structural indication for stickslip behavior based on the structure of the aljibes half graben we believe the maximum credible earthquake that may occur along the faults of the aljibes half graben is on the order of 2 10 27 dyne cm m 0 6 10 25 dyne cm and 7 5 m w 6 5

quaternary intra arc extension in the central trans mexican volcanic belt the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc parallel faulting major intra arc basins within the central part of the belt between long 998w and 1028w are from west to east the cuitzeo and acambay grabens the aljibes half graben and the mezquital graben in this region 100 eastwest2striking 2 km long steeply dipping normal faults expressed by pronounced multi event fault scarps have been mapped of the 100 faults 65 displace rocks of known quaternary age younger than 1 6 ma and 22 cut rocks with a documented age younger than 750 ka known historical surface ruptures are limited to faults of the acambay graben overall the faults cause north south to north northwest2south southeast2oriented extension of 1 km which is distributed over a 30250 km wide zone that has 5210 faults in cross section and a relative extension of 3 quaternary vertical slip rates estimated for 13 of the faults have a mean of 0 07 mm yr they are highest at the southern margin of the cuitzeo graben the northern margin of the acambay graben and in the aljibes half graben where they measure between 0 16 and 0 18 mm yr the north south to north northwest2south southeast2oriented quaternary bulk extension rate of the system is likely to be 0 2 6 0 05 mm yr this fault system is in an initial stage of coalescence its western and eastern parts consist mostly of isolated fault segments whereas in the central part deformation is localized onto a few through going faults the longest of these are the venta de bravo fault 45 km and the acambay tixmadeje fault 34 km the short fault traces have a simple structure whereas the longer ones are commonly composed of two or more segments there is no obvious migratory pattern of quaternary fault activity which suggests that the entire region is tectonically active

lherzolitic versus harzburgitic garnet trends sampling of extended depth versus extended composition reply to the comment by ivanic et al 2015 using the hill et al contrib mineral petrol 169 13 2015 doi 10 1007 s00410 014 1102 7 modeling technique we have tested the idea of ivanic et al contrib mineral petrol 164 505 520 2012 that decompression and metamorphic re equilibration of garnet with spinel causes garnet zoning perpendicular to the cr ca harzburgitic trend in garnet composition the modeling confirms that garnet zoning across the harzburgitic trend cannot form without spinel buffering the harzburgitic trend is very rare because it results from extreme compositional heterogeneity of the mantle at the same depth in contrast the common lherzolitic trend requires less diversity in the bulk composition of the mantle as it can be established with only a few samples of metamorphically re equilibrated mantle peridotite deriving from a variety of depths

stress distribution and subduction of aseismic ridges in the middle america subduction zone the regional distribution of stresses associated with the subduction of the cocos plate is inferred from a synthesis of 190 earthquake focal mechanisms body and surface wave analyses of large earthquakes and seismicity distributions broad patterns of consistent behavior are found across the region from the rivera plate boundary in the northwest to the guatemala el salvador border in the southeast and are used as a framework to evaluate evidence for variations in local stresses due to the subduction of two aseismic ridges the tehuantepec ridge and the orozco fracture zone information which bears on the seismic potential at locations of aseismic ridge subduction is particularly important in that no large ms 7 5 earthquakes have occurred historically we identify three major zones with consistent patterns in focal mechanisms and hypocentral distributions of seismicity the first closest to the trench and reflecting the mechanical interaction of the converging plates is a zone of shallow thrust earthquakes extending 100 150 km inland from the trench the second is a zone of normal faulting beginning at about 200 km inland from the trench h 60 km which extends continuously along the entire length of the descending plate throughout the region the third distinct zone exhibits a relatively low level of activity and separates the zones of thrust and normal faulting at about 150 200 km inland from the trench this zone extends from the rivera plate boundary in the northwest to the guatamala region in the southeast at this point the quiet region pinches out and the thrust and normal faulting zones abut and overlap superimposed on this overall pattern we find locally only minor changes in areas of aseismic ridge subduction aside from the prominent seismic slip gaps furthermore on october 25 1981 the playa azul earthquake ms 7 3 occurred in the midregion of the orozco fracture zone body and surface wave analyses of this event show a simple source rupture and shallow thrust fault mechanism as are found elsewhere in the region the seismic moment is mo 1 3 1020 n m the calculated stress drop is 4 5 mpa not extraordinarily high as might be expected in these pervasive seismic gaps an event in the tehuantepec ridge region on january 24 1983 ms 6 7 was a large normal faulting event but further interpretation is ambiguous due to the proximity of other normal faulting we conclude that while aseismic slip may be occurring in the areas of ridge subduction the possibility of large thrust earthquakes cannot be ruled out due to the overall similarities with adjacent regions

attenuation of modified mercalli intensity with distance in mexico abstract two relations are proposed to predict the attenuation of modified mercalli intensity i with distance d for mexican earthquakes i e ln i b 0 b 1 ln d d 9 b 2 d d 9 b 3 ln m s ln i b 0 b 1 d d 9 b 2 ln d d 9 b 3 ln m s m s is the earthquake surface wave magnitude d is a distance related to the maximum i mapped for an earthquake i or to m s the coefficients b i i 0 1 2 3 were obtained by fitting in a least square sense the information contained in the intensity maps of 32 events to the relations those events were classified in three groups according to their epicentral location focal mechanism and depth i e events related to the subduction zone intermediate depth earthquakes in south central mexico and to shallow crustal events along the trans mexican volcanic belt the i predicted by the proposed relations compare well with the i observed for historical earthquakes not included in the fitting results obtained from a parametrical study showed that the attenuation of i with d is different for each of the three types of earthquakes for distances of less than about 200 km the earthquakes associated with the subduction zone have a larger attenuation than the ones originating in the south central region of mexico for greater distances d 200 km the opposite behavior is observed the events located in the trans mexican volcanic belt have a larger attenuation with distance than that of events in the other two regions from these results it seems advisable in mexico to use several attenuation relations to estimate the seismic hazard at a site depending on the particular tectonic setting and the path of the events under consideration

magnitudes of large shallow earthquakes from 1904 to 1980 abstract in order to obtain a uniform magnitude catalogue surface wave magnitudes ms and broad band body wave magnitudes mb have been determined for large shallow earthquakes from 1904 to 1980 in making the catalogue homogeneous the author consistently adheres to the original definitions of ms and mb given by gutenberg 1945 and gutenberg and richter 1956 the determinations of ms and mb are all based on the amplitude and period data listed in gutenberg and richter s unpublished notes bulletins from stations worldwide and other basic information mb is measured on broad band instruments in periods of 8 s consistency of the magnitude determinations from these different sources is carefully checked in detail more than 900 shallow shocks of magnitude 7 and over are catalogued the meaning of the magnitude scales in various catalogues is examined in terms of ms and mb most of the magnitudes listed by gutenberg and richter 1954 in their seismicity of the earth are basically ms for large shocks shallower than 40 km but are basically mb for large shocks at depths of 40 60 km the surface wave magnitudes given by earthquake data reports are higher than ms by 0 2 unit whether the combined horizontal amplitude or the vertical amplitude is used mb and the currently used 1 s body wave magnitude are measured at different periods and should not be directly compared

volcano stratigraphy and physical characterization of the san mateo pomez unit cerritos member temascalcingo volcano central mexico abstract the temascalcingo volcano is a dacitic stratovolcano located at the eastern part of the acambay graben a tectonic basin in the central sector of the trans mexican volcanic belt tmvb which is an active mostly calc alkaline volcanic arc that crosses mexico from the pacific ocean to the gulf of mexico this study characterises the explosive activity of the san mateo pumice eruption that is the most recent and largest paroxysmal episode of this volcano and the last unit of the cerritos member indeed this eruption is associated with an undated debris avalanche event consisting of a major sector collapse developed on the western flank of the volcano stratigraphic data suggest a highly explosive eruption that generated a relatively thick up to 10 m and widespread pumice fallout deposit trending to the ne and an aerial distribution of at least 100 km2 this subplinian type eruption of vei 4 lasted about 9 h and produced a plume height up to 12 km above the vent i e 14 km above sea level this type of explosive eruption is relatively common in the tmvb so a comprehensive geological study with the corresponding numerical simulations like the one that was carried out contributes to the understanding of the explosive volcanism of modest size stratovolcanoes that are common in the tmvb

large scale landslides triggered by quaternary tectonics in the acambay graben mexico detailed analysis was conducted on large scale gravitational tectonic deformations and landslides in the acambay graben an intra arc basin in the trans mexican volcanic belt tmvb field mapping and remote sensing revealed the slope instability of the northern graben boundary induced by the acambay tixmadeje fault two major landslides of 0 1 km3 and 0 05 km3 in volume were identified and their characteristics were analyzed according to the role of tectonics mechanism of slope failure and possible triggering factors quaternary faulting played a major role in increasing the local relief and the activity of the acambay tixmadeje fault represents the main geomorphic factor conditioning the gravitational movements moreover displacements along this fault generated sliding surfaces and reduced the strength of the rock mass the two landslides are classified as large scale rotational slides involving volcanic rocks of late miocene pleistocene age since the acambay graben is a seismogenic area with a known maximum horizontal ground acceleration of 0 5 g a strong earthquake could be ascribed as the possible triggering mechanism of the landslides our work represents the first analysis of large gravitational slope movements in tectonically active regions in mexico a process that can be common in the intra arc basins of the tmvb where active tectonic seismicity weak altered volcanic rocks and heavy rains affect the slope stability copyright 2010 john wiley sons ltd

a comparison of earthquake damage areas as a function of magnitude across the united states this study investigates the relationship between earthquake magnitude and the size of damage areas in the eastern and western united states to quantify damage area as a function of moment magnitude m 149 mmi vi and vii areas for 109 earthquakes 88 in the western united states 21 in the eastern united states and canada were measured regression of isoseismal areas versus m indicated that areas in the east were larger than those in the west at both intensity levels by an average 5 in the m 4 5 to 7 5 range in terms of radii for circles of equivalent area these results indicate that damaging ground motion from shocks of the same magnitude extend 2 the epicentral distance in eastern north america compared to the west to determine source and site parameters consistent with the above results response spectral levels for eastern north america were stochastically simulated and compared with response spectral ordinates derived from recorded strong ground motion data in the western united states stress drop values of 200 bars combined with a surficial 2 km thick low velocity sedimentary layer over rock basement produced results that are compatible with the intensity observations i e similar response spectral levels in the east at approximately twice their epicentral distance in the western u s distance these results suggest that ground motion modeling in eastern north america may need to incorporate source and site parameters different from those presently in general use the results are also of importance to eastern u s hazard assessments as they require allowance for the larger damage areas in preparedness and mitigation programs

seismological constraints on source properties of the mb 4 0 1985 ardsley new york earthquake a characteristic rupture on october 19 1985 a magnitude 4 0 earthquake occurred in southern westchester county new york approximately 30 km north of central manhattan barstow et al 1988 concluded that this earthquake consisted of two sub events with the larger sub event occurring 0 6 s after the first one we present analysis of data for the main shock as well as aftershocks and foreshocks which were recorded on the new york state seismic network because this data set includes a range of source magnitudes from less than 2 to 4 recorded for virtually identical source receiver pairs an empirical green s function technique can be used to study the source characteristics of the larger events the corner frequency of the main shock and an attenuation parameter t are resolved from the initial portion of the seismogram at station pal in palisades new york 8 km northwest of the epicenter spectra of small foreshocks and aftershocks can then be corrected for attenuation using t following an iterative procedure it is then possible to make reliable determinations of seismic corner frequency for these events several conclusions are obtained 1 the spectrum of the first subevent in the main shock can be well modelled by a brune spectrum from 1 to 10 hz with a corner frequency of 7 1 hz at higher frequencies the source spectrum contains sharp peaks which do not fit an omega square model and may reflect complexities in the rupture process 2 the average stress drop of the sequence is 205 bars the stress drop of the main event is 540 bars but no stress drop dependency on moment is resolved over the magnitude range 1 8 4 0 3 attenuation parameters t are obtained for the 8 km northwestward path from ardsley to palisades and for a 52 km westward path to a station in the hudson highlands and their similarity suggests an increase of q with depth 4 assuming that the two main shock sub events did not rerupture the same area but were contiguous a maximum rupture dimension is 440 680 m the rupture diameter of the largest aftershock mb 3 is determined to be approximately 320 meters the combined rupture for the sequence inferred from spectral methods is consistent with the size of the rupture inferred from the area surrounded by the early aftershocks 700 m diameter the rupture dimension of the ardsley sequence may be characteristic of the dobbs ferry fault and other seismogenic faults in the manhattan prong which are segmented with characteristic dimensions of 0 5 2 km seeber and dawers 1989 assuming that the range of stress drops in the ardsley sequence is also characteristic of the manhattan prong this range of dimensions corresponds to a range of characteristic magnitudes between mb of 3 3 and 5 5 we note that this magnitude range encompasses the five largest known earthquakes in the new york city seismic zone the 1985 ardsley main shock and events in 1737 1783 1884 and 1895

paleoseismology of the 1912 acambay earthquake and the acambay tixmadeje fault trans mexican volcanic belt the acambay graben is a major intra arc basin of the trans mexican volcanic belt trenching studies at four sites along the acambay tixmadeje fault reveal evidence for the 1912 acambay earthquake and at least four late pleistocene and holocene ground rupturing earthquakes with similar displacements to the 1912 event evidence for multiple events was recognized by a combination of upward terminating faults fissure fills erosionally truncated fissures colluvial wedge fills and downward increasing separation of stratigraphic units sites along the central part of the ese wnw trending rangefront fault indicates large dip slip separations across a multi event scarp in holocene colluvium with a penultimate event that occurred since 5230 calibrated years before present cal yr b p on the huapango plain near the eastern end of the 1912 rupture where slip was oblique the penultimate earthquake occurred after 4700 cal yr b p and there is evidence for at least four more earthquakes in the last 34 000 cal yr b p three dimensional excavation of lacustrine and channel deposits at las lomas yields a left normal slip vector of s40 5 e with 105 10 cm of slip for the last three events there this slip vector is consistent with nne oriented extension across the graben the amount and style of slip documented for prehistoric ruptures are also consistent with the 1912 rupture with a mean of 60 cm event of dip slip on the rangefront and a mean of 35 cm event of oblique slip on the n48 w trending huapango plain assuming these values represent a characteristic behavior yields an average recurrence interval of 3600 years for the last four large earthquakes since 11 570 cal yr b p and a slip rate of 0 17 mm yr across the rangefront empirical earthquake parameters calculated from our paleoseismic data support instrumental measures indicating that the 1912 acambay earthquake was of m 6 8 7 0 the 1912 event is representative of ground rupturing earthquakes on the acambay tixmadeje fault and demonstrates that large seismic events m 7 can be generated by faults in the trans mexican volcanic belt and in other intra arc settings

orientational data on the state of stress in northeastern mexico as inferred from stress induced borehole elongations preferential elongation directions are presented from 46 subvertical wells in northeastern mexico from depth intervals ranging from 161 to 4 912 m the measurements document a remarkably consistent west east to northwest southeast direction of the contemporary least horizontal stress in the upper crust of this intraplate region in the regional neotectonic framework the data suggest that mexico north of the trans mexican volcanic belt is being stretched in west east to northwest southeast direction the measurements from the area of the laramide chihuahua and coahuila fold thrust belts define the southern extend of the basin and range rio grande rift stress province of the southwestern united states into northern mexico the least horizontal in situ stress directions recorded in the gulf coastal plain burgos and tampico misantla basins are oblique to the continental margin of the gulf of mexico but subparallel to the direction measured in the area of the basin and range rio grande rift stress province this suggests that the stress distribution in these basins is not mainly the result of gravitational loading as in the gulf coast stress province of the united states but is controlled by the same lithospheric stress system that characterizes the basin and range rio grande rift stress province this is also supported by the north south trending zone of oligocene quaternary alkaline volcanism that crosses the tampico misantla basin furthermore the data indicate that the active deformation of the mexican ridges fold belt east of the study area in the gulf of mexico cannot be caused by a compressional external tectonic load the direction of the least horizontal in situ stress measured in the area of the laramide sierra madre oriental fold thrust belt in hidalgo and san luis potosi states is consistent with that of the other zones however wellbore elongations were recorded less frequently and show a rather large circular variance which suggests that the minimum and maximum horizontal far field stresses are about equal in this area

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 acambay graben active intraarc extension in the trans mexican volcanic belt mexico the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench the central part of the belt is being deformed by the chapala tula fault zone an approximately 450 km long and 50 km wide zone of active extension the volcanic arc and the arc parallel chapala tula fault zone are superposed nearly perpendicularly on the preexisting stress and deformation province of the mexican basin and range the acambay graben about 40 km long and 15 km wide is located approximately 100 km northwest of mexico city and is one of the major troughs within the chapala tula fault zone the border faults of the acambay graben acambay tixmadeje in the north and pastores in the south are separated in the west by stepovers from range bounding faults of similar orientation epitacio huerta in the north and venta de bravo in the south the stepovers occur at the intersection of these faults with an older system of basin and range faults an early stage right lateral component of motion along the venta de bravo and pastores faults is inferred on a map scale from a left stepping en echelon array of normal fault segments the divergence of the en echelon segments from the general fault trend decreases gradually from west to east suggesting that the early extension was rotational the present relative displacement along the southern margin of the system on the other hand results in a left lateral strike slip component this is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left oblique slip the striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component this is supported on a map scale by the structure of the right stepover between the acambay tixmadeje and epitacio huerta faults which shows no evidence of local extension or shortening the divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation this could explain why no active extension has been observed to the east of the chapala tula fault zone in the eastern part of the trans mexican volcanic belt during the ms 6 9 acambay earthquake of november 19 1912 surface rupture occurred along both margins of the graben at the base of multiple event scarps along the acambay tixmadeje fault the coseismic rupture is 41 km long the vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum furthermore the change in the vertical surface offset along the fault is approximately proportional to the change in height of the acambay tixmadeje multiple event fault scarp the easternmost part of the ground rupture passes through a plain and not at the base of a multiple event scarp as farther west it may therefore correspond partly to an increase in length of the acambay tixmadeje fault during the 1912 earthquake the slip rate along the southern border of the acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a 40ar 39ar age of 0 4 0 1 ma this basalt may be displaced up to 15 m by the pastores fault which indicates a middle late quaternary slip rate of 0 04 mm yr furthermore based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake we estimate a recurrence interval of 5000 years for major earthquakes along the faults of the acambay graben

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

comment on active crustal deformation in the trans mexican volcanic belt as evidenced by historical earthquakes during the last 450 years by g suarez et al

scaling differences between large interplate and intraplate earthquakes a study of large intraplate earthquakes with well determined source parameters shows that these earthquakes obey a scaling law similar to large interplate earthquakes in which m 0 l 2 or u l where l is rupture length and u is slip in contrast to interplate earthquakes for which 1 10 5 for for the intraplate events 6 10 5 which implies that these earthquakes have stress drops about 6 times higher than interplate events this result is independent of focal mechanism type this implies that intraplate faults have a higher frictional strength than do plate boundaries and hence that faults are velocity or slip weakening in their behavior this factor may be important in producing the concentrated deformation that creates and maintains plate boundaries

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

heterogeneities of the instrumental seismicity catalog 1904 1980 for strong shallow earthquakes we analyze the world wide consistency of teleseismic reporting completeness of the seismicity record and homogeneity of magnitude determination for strong shallow earthquakes magnitude m 6 depth h 70 km in the period 1904 to 1980 for earthquakes with magnitude m 7 we used the catalog given by abe 1981 which lists the surface wave magnitude ms of every reported large event that occurred between 1904 and 1980 according to the original formulation of gutenberg 1945 under the postulate that the rate of earthquake occurrence for the entire world as a whole is consistent on a time scale of decades and that since the installation of the world wide standardized seismograph network in the early 1960s the earthquake catalog for strong m 6 shocks is complete and the seismicity rates are typical of all periods in the century we find that for instrumental related reasons the ms of large events prior to 1908 have been systemically overestimated by at least 0 5 magnitude unit and the ms of events in the period 1908 to 1948 have been consistently overestimated by 0 2 unit relative to the ms assigned to shocks occurring after 1948 when these corrections are taken into account the catalog of events with ms 7 0 is shown to be largely complete i e nearly all the large shallow shocks which occurred in the earth are listed for most of the world since early in the century for earthquakes as the magnitude 6 level we used the international seismological center isc tape and regional catalogues of earthquakes this file compiles the earthquake data given by gutenberg and richter 1954 for the first half of the century and data from various international agencies after 1945 we find a highly incomplete seismicity catalog at this magnitude level for the first half of the century mainly due to numerous increases and decreased in the earthquake detection and reporting capabilities a more complete but extremely inhomogeneous catalog for the period 1950 to 1963 due to the use of different formulations and criteria to calculate and report the parameter magnitude by the various seismological agencies and a largely complete and homogeneous in ms record since 1964

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

a catalog of shallow earthquakes of mexico from 1900 to 1981 a catalog of shallow earthquakes depth 65 km of mexico 15 to 20 n 94 5 to 105 5 w covering the period 1900 to 1981 is presented the catalog includes only those events whose magnitudes m ms or magnitude from pasadena are 5 9 it is most likely complete for shocks with ms 6 5 from july 1906 to december 1981 for many shocks ms has been determined from uppsala bulletins and or uppsala and gottingen wiechert seismograms the statistics of incremental frequency n ms shows a peak at ms 7 7 reinforcing a previous observation by singh et al 1981 in the range 6 4 ms 7 5 the observed n ms deviates significantly from an exponential distribution the catalog provides a basis for further statistical analysis of earthquakes in mexico