Guatemala Earthquake Research Articles

Humanitarian Aid as Global Governance: The Architecture of the Red Cross’s Relief Operations after the 1976 Guatemala Earthquake

This paper analyses the disaster relief operation that was set in motion immediately after the 1976 Guatemala earthquake through the activities of the Guatemalan Red Cross Society. Already in the emergency phase, “Guatecruz” built field offices in the disaster region to manage both the relief efforts as well as to set up forty-three tent cities. By taking a seat in the disaster zone, as opposed to coordinating from a distant headquarters, Guatecruz became the main governing body in a large network of different national and international state and non-state actors. This paper sets out to elucidate the non-governmental logic behind the making, financing, building, and operation of Guatecruz’s field offices and introduce it as an early example of contracting out services that were previously controlled by the government to non-governmental organisations. Untangling a large-scale humanitarian crisis from these field offices, this paper introduces a specific institutional history that uses decentralisation as a method to plan, develop, and build an “architecture of global governance.”

Seismoacoustic observations at deep ocean observatories: ACO, H2O, and OSN1

Deep seafloor (~5000 m) observations of seismoacoustic arrivals are presented from the Aloha Cabled Observatory (ACO), Hawaii-2 Observatory (H20), and Ocean Seismic Network-1 (OSN1). Acoustic observations of local earthquakes from the ACO 24 kHz hydrophone show extraordinarily high frequencies up to 165 Hz at distances to ~200 km—a unique window into earthquake source dynamics. Combined seismic and hydrophone observations show that the traditional T wave propagates as a seismoacoustic polarized interface wave “Ti” coupled to the seafloor. Seismoacoustic Ti waves propagating at the sound speed of water are routinely observed over megameter distances at H2O between Hawaii and California, even though the seafloor site is within the shadow zone for acoustic wave propagation. Internal waves or thermohaline covariation may or may not be sufficient to scatter acoustic energy from the SOFAR channel to the deep seafloor. Ti has also been observed on seismometers SSW of Oahu at the OSN1 site at the seafloor and within an ODP borehole 242 m into the basalt basement. The observation of Ti from an earthquake in Guatemala at OSN1, whose path is blocked by the Island of Hawaii, is consistent with scattering from the vicinity of the Cross Seamount.

The 1976 Guatemala Earthquake: ESI Scale and Probabilistic/Deterministic Seismic Hazard Analysis Approaches

A hazard assessment of the 1976 Guatemala earthquake (M = 7.5) was conducted to achieve a better definition of the seismic hazard. The assessment was based on the environmental effects that had effectively contributed to the high destructive impact of that event. An interdisciplinary approach was adopted by integrating: (1) historical data; (2) co-seismic geological effects in terms of Environmental Seismic Intensity (ESI) scale intensity values; and (3) ground shaking data estimated by a probabilistic/deterministic approach. A detailed analysis of primary and secondary effects was conducted for a set of 24 localities, to obtain a better evaluation of seismic intensity. The new intensity values were compared with the Modified Mercalli Intensity (MMI) and Peak Ground Acceleration (PGA) distribution estimated using a probabilistic/deterministic hazard analysis approach for the target area. Our results are evidence that the probabilistic/deterministic hazard analysis procedures may result in very different indications on the PGA distributions. Moreover, PGA values often display significant discrepancy from the macroseismic intensity values calculated with the ESI scale. Therefore, the incorporation of the environmental earth effects into the probabilistic/deterministic hazard analysis appears to be mandatory in order to achieve a more accurate seismic estimation.

Lithospheric radial anisotropy beneath the Gulf of Mexico

The Lithosphere–Asthenosphere Boundary (LAB), where a layer of low viscosity asthenosphere decouples with the upper plate motion, plays an essential role in plate tectonics. Most dynamic modeling assumes that the shear velocity can be used as a surrogate for viscosity which provides key information about mantle flow. Here, we derive a shear velocity model for the LAB structure beneath the Gulf of Mexico allowing a detailed comparison with that beneath the Pacific (PAC) and Atlantic (ATL). Our study takes advantage of the USArray data from the March 25th, 2013 Guatemala earthquake at a depth of 200 km. Such data is unique in that we can observe a direct upward traveling lid arrival which remains the first arrival ahead of the triplications beyond 18°. This extra feature in conjunction with upper-mantle triplication sampling allows good depth control of the LAB and a new upper-mantle seismic model ATM, a modification of ATL, to be developed. ATM has a prominent low velocity zone similar to the structure beneath the western Atlantic. The model contains strong radial anisotropy in the lid where VSH is about 6% faster than VSV. This anisotropic feature ends at the bottom of the lithosphere at about the depth of 175 km in contrast to the Pacific where it extends to over 300 km. Another important feature of ATM is the weaker velocity gradient from the depth of 175 to 350 km compared to Pacific models, which may be related to differences in mantle flow.

GPS and seismic constraints on the M = 7.3 2009 Swan Islands earthquake: implications for stress changes along the Motagua fault and other nearby faults

We use measurements at 35 GPS stations in northern Central America and 25 seismometers at teleseismic distances to estimate the distribution of slip, source time function and Coulomb stress changes of the Mw = 7.3 2009 May 28, Swan Islands fault earthquake. This event, the largest in the region for several decades, ruptured the offshore continuation of the seismically hazardous Motagua fault of Guatemala, the site of the destructive Ms = 7.5 earthquake in 1976. Measured GPS offsets range from 308 millimetres at a campaign site in northern Honduras to 6 millimetres at five continuous sites in El Salvador. Separate inversions of geodetic and seismic data both indicate that up to ∼1 m of coseismic slip occurred along a ∼250-km-long rupture zone between the island of Roatan and the eastern limit of the 1976 M = 7.5 Motagua fault earthquake in Guatemala. Evidence for slip ∼250 km west of the epicentre is corroborated independently by aftershocks recorded by a local seismic network and by the high concentration of damage to structures in areas of northern Honduras adjacent to the western limit of the rupture zone. Coulomb stresses determined from the coseismic slip distribution resolve a maximum of 1 bar of stress transferred to the seismically hazardous Motagua fault and further indicate unclamping of normal faults along the northern shore of Honduras, where two M > 5 normal-faulting earthquakes and numerous small earthquakes were triggered by the main shock.

Seismicity and Crustal Structure of the Polochic-Motagua Fault System Area (Guatemala)

Research Article| November 01, 2009 Seismicity and Crustal Structure of the Polochic-Motagua Fault System Area (Guatemala) A. Franco; A. Franco Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 1Laboratoire de Géologie, Ecole Normale Supérieure, Centre National de la Recherche Scientifique (CNRS), Paris, France Search for other works by this author on: GSW Google Scholar E. Molina; E. Molina Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 2Instituto Nacional de Sismología, Vulcanología, Meterología e hidrología, Guatemala Ciudad, Guatemala Search for other works by this author on: GSW Google Scholar H. Lyon-Caen; H. Lyon-Caen Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 1Laboratoire de Géologie, Ecole Normale Supérieure, Centre National de la Recherche Scientifique (CNRS), Paris, France Search for other works by this author on: GSW Google Scholar J. Vergne; J. Vergne Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 1Laboratoire de Géologie, Ecole Normale Supérieure, Centre National de la Recherche Scientifique (CNRS), Paris, France 3Institut de Physique du Globe de Strasbourg, CNRS, Strasbourg, France Search for other works by this author on: GSW Google Scholar T. Monfret; T. Monfret Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 4Geosciences Azur, University of Nice, Sophia Antipolis, CNRS, Institut de Recherche pour le Developpement, Nice, France Search for other works by this author on: GSW Google Scholar A. Nercessian; A. Nercessian Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 5Institut de Physique du Globe de Paris, CNRS, France Search for other works by this author on: GSW Google Scholar S. Cortez; S. Cortez Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 6Universidad de Coban, Coban, Guatemala Search for other works by this author on: GSW Google Scholar O. Flores; O. Flores Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 7Centro de Estudios Superiores de Energí a y Minas-Universidad de San Carlos de Guatemala, Guatemala Ciudad, Guatemala Search for other works by this author on: GSW Google Scholar D. Monterosso; D. Monterosso Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 8Coordinadora Nacional para la Reducción de Desastres, Guatemala Ciudad, Guatemala Search for other works by this author on: GSW Google Scholar J. Requenna J. Requenna Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 6Universidad de Coban, Coban, Guatemala Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2009) 80 (6): 977–984. https://doi.org/10.1785/gssrl.80.6.977 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. Franco, E. Molina, H. Lyon-Caen, J. Vergne, T. Monfret, A. Nercessian, S. Cortez, O. Flores, D. Monterosso, J. Requenna; Seismicity and Crustal Structure of the Polochic-Motagua Fault System Area (Guatemala). Seismological Research Letters 2009;; 80 (6): 977–984. doi: https://doi.org/10.1785/gssrl.80.6.977 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search We report results from a six-month seismological experiment in the area of the eastern Polochic-Motagua fault system (Guatemala) designed to both characterize the present seismicity and bring some constraints on the lithospheric structure. The seismic activity occurs in the upper 15 km of the crust, on the Polochic and the Motagua faults as well as in a NS-trending graben south of the Motagua fault and within the active folds north of the Polochic fault. From receiver function analysis the Moho discontinuity is found at about 35 km depth north of the Polochic fault and south of the Motagua fault, while... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Observations of polarized seismoacoustic T waves at and beneath the seafloor in the abyssal Pacific ocean

Combined seismic and hydrophone observations show that the traditional T wave propagates as a seismoacoustic polarized interface wave (Ti) coupled to the seafloor. Seismoacoustic Ti waves propagating at the sound speed of water are routinely observed over megameter distances at the deep (4979 m) seafloor Hawaii-2 Observatory (H2O) between Hawaii and California, even though the seafloor site is within a shadow zone for acoustic wave propagation. Ti has also been observed on seismometers 225 km SSW of Oahu at the OSN1 site at the seafloor and within an ODP borehole into the basalt basement. Analyses of timing, apparent velocity, energy, and polarization of these interface waves are presented. At low frequency (< approximately 5 Hz) Ti propagates dominantly in the sediments and is consistent with higher-mode Rayleigh waves. At higher frequencies the observed Ti waves dominantly propagate acoustically with characteristics suggesting local scattering. The observation of Ti from an earthquake in Guatemala at OSN1, whose path is blocked by the Island of Hawaii, is consistent with scattering from the vicinity of the Cross Seamount.

Reactions to Trauma: The 1976 Earthquake in Guatemala

Guatemala was already a disaster when a 7.5 Richter scale earthquake hit the country on February 4, 1976. Economic development based on low-wage large-scale capitalist agriculture for export and low wage industrial growth had not improved life for the majority of Guatemalans, who were and are among the poorest people in Latin America. To make matters worse and to protect wealth from the growing protests of the poor, national political power rested on legal and extra-legal state violence. Between the time when the Unites States engineered a coup against the reformist government in Guatemala in 1954 and February 4, 1976, state violence had taken the lives of 25,000 Guatemalans, a figure slightly larger than the approximately 21,000 killed in the 39 seconds of the 3 AM earthquake.

Researchers investigate submarine faults north of Puerto Rico

A 23‐day marine geophysical expedition in the summer of 1996 revealed a previously unidentified strike‐slip fault zone within 60–100 km of the densely populated northern coast of Puerto Rico. The purpose of the expedition, which was held aboard the U.S. research vessel Maurice Ewing, was to map the northeastern portion of the North America Caribbean plate boundary.The 3000‐km‐long North America‐Caribbean plate boundary stretches from Guatemala to the Lesser Antilles arc. The dominantly left‐lateral strike‐slip boundary accommodates slow (1–2.6 cm/yr) eastward motion of the Caribbean plate relative to North America. Distributed interplate motion in the region generates large earthquakes (Figure 1, inset). In 1976, an M 7.5 left‐lateral, strike‐slip earthquake in Guatemala killed approximately 23,000 people and left a quarter of the nation's population homeless.

Destructive upper-crustal earthquakes of Central America since 1900

Abstract We have compiled a catalog of 51 destructive upper-crustal earthquakes in Central America since 1900. An event is included if it caused casualties or heavy damage of Modified Mercalli (MM) intensity ≧ VII. All events larger than magnitude Ms 5.7 are found to be destructive. We believe the catalog is complete for events of Ms ≧ 6 along the volcanic front. The catalog includes estimates of epicentral coordinates, depth, magnitude, and casualties for all events and presents MM intensity VII contours for most events. Data in this catalog place severe constraints on the spatial, temporal, and magnitude distribution of destructive upper-crustal earthquakes in Central America. The catalog contains 30 events of Ms ≧ 6. Depths of well-constrained mainshock hypocenters range from 5 to 15 km. The most striking feature of the catalog is the spatial alignment along the volcanic front of 23 of the 30 events; volcanic-front earthquake-source mechanisms for such events show east-west tension and are compatible with a model of the volcanic front as a zone of right-lateral strike-slip faulting. Volcanic-front earthquake magnitudes do not exceed Ms 6.5, except in southeastern Guatemala where they reach Ms 6.9; maximum earthquake magnitudes are apparently related to the distance between adjacent Holocene volcanic centers. Destructive events occur at offsets in the volcanic chain as isolated mainshock-aftershock sequences. However, within linear segments, destructive earthquakes have a strong tendency to cluster and are often preceded by significant foreshock activity. Although the volcanic front is the locus of both destructive earthquakes and volcanism, we find no temporal correlation between the two. Of the seven events of Ms ≧ 6 that did not originate near the volcanic front, six originated near the transcurrent Caribbean-North American plate boundary, including the Ms 7.5 Guatemala earthquake of 1976. Along the volcanic front, destructive upper-crustal earthquakes have occurred at a rate of about one event every 212 years during this century, much more frequently than along either the Caribbean-North American plate boundary or the subduction zone. In Central America since 1900, about 17,000 people have died from volcanic-front earthquakes, but only about 2000 people are known to have died from subduction-zone earthquakes. Although subduction zone earthquakes can have larger magnitudes and produce more widespread damage, destructive volcanic-front earthquakes pose the greater hazard because they are more frequent and originate much closer to the bulk of Central America's population, which is concentrated along the volcanic front.

Inversion of complex body waves—III

Abstract We have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes. The rupture pattern is represented as a sequence of subevents distributed on the fault plane. This method is an extension of our earlier method in which the subevent mechanisms were fixed. In the new method, the subevent mechanisms are determined from the data and are allowed to vary during the sequence. When subevent mechanisms are allowed to vary, however, the inversion often becomes unstable because of the complex trade-offs between the mechanism, the timing, and the location of the subevents. Many different subevent sequences can explain the same data equally well, and it is important to determine the range of allowable solutions. Some constraints must be imposed on the solution to stabilize the inversion. We have developed a procedure to explore the range of allowable solutions and appropriate constraints. In this procedure, a network of grid points is constructed on the τ - I plane, where τ and I are, respectively, the onset time and the distance from the epicenter of a subevent; the best-fit subevent is determined at all grid points. Then the correlation is computed between the synthetic waveform for each subevent and the observed waveform. The correlation as a function of τ and I and the best-fit mechanisms computed at each τ - I grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution. The method is illustrated using the data for the 1976 Guatemala earthquake.

The Guatemala earthquake of 1816 on the Chixoy-Polochic fault

Abstract Forty historical documents have been found which describe a previously unknown very large earthquake on 22 July 1816. This event occurred along the strike-slip boundary between the Caribbean and North American plates in Guatemala and southern Mexico. Modified Mercalli intensities are estimated from these accounts, and a rough isoseismal map is constructed. The damage pattern indicates that the causative fault was the left-lateral Chixoy-Polochic fault for which no damaging earthquake has previously been reported. Damage of Modified Mercalli intensity VII or greater covered an area of at least 13,000 km 2 , extending over 340 km from Alta Verapaz province in Guatemala westward to San Cristobal Ias Casas, in Chiapas, Mexico. The area of intensity VII indicates a moment of 1 × 10 28 dyne-cm, while the estimated length of the intensity VII isoseismal indicates a more likely figure of 3.5 × 10 27 dyne-cm, or an equivalent magnitude ( M w ) of 7 1 2 to 7 3 4 . The reported aftershock sequence is compatible with an earthquake in this magnitude range. These data demonstrate that the Chixoy-Polochic fault is currently an active part of the Caribbean-North American plate boundary and is capable of producing very large earthquakes.

Use of seismoscope records to determineMLand peak velocities

AbstractMore information about ground motion can be extracted from seismoscope records than a single point on a response spectrum. To demonstrate this, the relation between seismoscope response and Wood-Anderson instrument output and peak horizontal ground velocity has been studied by simulating the various responses for a range of distances and magnitudes. The simulations show that the relation used by Jennings and Kanamori (1979) to convert from peak seismoscope readings to the peak response of a Wood-Anderson instrument has a distance- and magnitude-dependent systematic error. The error is negligible, however, for modern seismoscopes at distances of a few tens of kilometers. At several hundred kilometers, the relation underestimates the Wood-Anderson response by as much as a factor of two. The spread in Jennings and Kanamori's estimate of ML for the 1906 San Francisco earthquake, recorded on seismoscopes having relatively low natural frequencies (0.26 and 0.5 Hz), is reduced by the results in this paper—the upper value, from a seismoscope in Carson City, Nevada, at 290 km from the fault, going from ML = 7.2 to ML = 7.0 and the lower value, from Yountville, California (R ≈ 60 km), going from about 6.3 to 6.4. About 0.3 units of the remaining spread may be due to local geologic site conditions. If the 0.3 units is distributed equally between the Yountville and Carson City recordings, the estimates of ML for the San Francisco earthquake then become 6.5 and 6.8, somewhat lower than Jennings and Kanamori's final estimates of 634 to 7. Although the error in using the relation of Jennings and Kanamori to estimate Wood-Anderson response was at most a factor of 1.6 for the 1906 earthquake, the error can be substantially larger for smaller earthquakes recorded on similar low frequency seismoscopes.The relation between Wood-Anderson and seismoscope response used by Jennings and Kanamori can be combined with an empirical relation between peak horizontal velocity and Wood-Anderson response to predict peak velocity from seismocope recordings. The simulations show that this relation (vmax = 8.1Awa, where vmax is the peak horizontal velocity in centimeters/second and Awa is one-half the range of the Wood-Anderson motion in meters) forms a lower bound for estimates of peak velocity from seismoscope recordings. The relation is good for stations within about 100 km of earthquakes with moment magnitudes of about 4.5 to 6.5, and it underestimates peak velocity by factors up to 2 or 3 for larger earthquakes at distances within 100 km. An application of the simulation method to the 1976 Guatemala earthquake (moment magnitude = 7.6) results in 37 cm/sec as a lower bound to vmax, with 66 cm/sec as a more likely value, from the seismocope recording in Guatemala City (approximately 25 km from the Motagua fault).

Complexity of rupture in large strike-slip earthquakes in Turkey

Complexity of rupture propagation has an important bearing on the state of stress along the earthquake fault plane and on the prediction of strong ground motion in the near-field. By studying far-field body waveforms recorded by WWSSN long-period seismograms it has been possible to investigate the degree of complexity of several Turkish earthquakes. The results, which are obtained by matching synthetic P waveforms to observed data indicate that the July 22, 1967 Mudurnu Valley earthquake ( M s = 7.1) is a complex event which can be explained by the superposition of elementary sources with variable amplitudes and source time sequence history. In this regard, it is very similar to the February 4, 1976 Guatemala earthquake ( M s = 7.5). A comparison of these two events indicates that their source-time series ranges from 5 to ca. 20 s and, regardless of the total moment of the earthquake, the moment of the individual events is bounded at around 5 × 10 26 dyn cm. The November 24, 1976 E. Turkey earthquake ( M s = 7.3), on the other hand, has a complexity which cannot be explained by such a simple model; in this respect, it may be more similar to the Tangshan, China, earthquake and as such, may involve significant thrust, normal or other complications to its faulting mechanism than the strike-slip mechanism of the P-wave first-motion data. The source time history for the 1967 Mudurnu Valley event is used to illustrate its significance in modeling strong ground motion in the near field. The complex source-time series of the 1967 event predicts greater amplitudes (2.5 larger) in strong ground motion than a uniform model scaled to the same size for a station 20 km from the fault. Such complexity is clearly important in understanding what strong ground motion to expect in the near-field of these and other continental strike-slip faults such as the San Andreas.

Geodetic determination of horizontal deformation associated with the Guatemala earthquake of 4 February 1976

abstractA 15-station geodetic control network extending 10 km north and 40 km south of the Motagua fault in Guatemala was reobserved 2 yr after the 4 February 1976 MS = 7.5 earthquake. The initial first-order triangulation was observed in two parts: the Guatemala-Honduras border arc completed in 1935 and the Motagua valley arc completed in 1953. Both angles and distances were observed in the 1978 resurvey. The angle changes between the combined 1935 to 1953 survey and the 1978 resurvey were used to calculate earthquake-related fault slip on two dislocation models of the rupture surface. Left-lateral slip of 1.3 ± 0.1 m is calculated for a model fault with constant displacement from 0 to 15 km. For a model fault divided into three successively deeper sections, the calculated left-lateral slip is 0.8 ± 0.1 m near the surface, increasing to 2.1 ± 0.5 m in the intermediate 5- to 15-km section, and is undetermined at greater depths. Both models are consistent with the 0.9 to 1.1 m of accumulated slip and afterslip observed near the network by Bucknam et al. (1978). The second model is in better agreement with the 2 m of slip derived by Kanamori and Stewart (1978) from a long-period seismic surface wave estimate of the seismic moment.

Earthquake-Induced Liquefaction Near Lake Amatitlan, Guatemala

One of the effects of the February 4, 1976, Guatemala earthquake was the extensive liquefaction which occurred at the settlement of La Playa on the northeast shore of Lake Amatitlan. Ground accelerations in the area resulting from the magnitude 7.5 earthquake are estimated to be of the order of 0.12 g to 0.15 g. The characteristics of the sand are investigated, including its standard penetration resistance and its cyclic loading characteristics. Despite the fact that the sand is somewhat lighter in weight than sand deposits which have liquefied in other earthquakes, its liquefaction characteristics are apparently influenced by the same factors as other sand deposits; its overall behavior is consistent with that exhibited by other sands. The high degree of liquefaction at the La Playa site was probably due in large measure to the lightweight nature of the pumiceous sands.

Determination of local magnitude, ML, from seismoscope records

A method is presented for determining the local magnitude, M_L, from records from seismoscopes and similar instruments. The technique extrapolates the maximum response of the standard Wood-Anderson seismograph, which determines M_L, from the maximum response of the seismoscope. The standard deviation of the steady-state response of an oscillator subjected to white noise excitation is used to derive a relation correcting for the different periods, dampings, and gains of the two instruments. The accuracy of the method is verified by application to data from the San Fernando and Parkfield earthquakes wherein both accelerograph and seismoscope records are available from the same sites. The accelerograms are used to synthesize Wood-Anderson responses whose maxima are compared to those extrapolated from the seismoscope data. In both earthquakes, the average magnitudes and standard deviations determined by the two approaches are very nearly equal. The method is then applied to the strong-motion data from the Managua, Nicaragua earthquake of December 23, 1972 (M_S = 6.2, mb = 5.6). A value of M_L = 6.2 is indicated from the seismoscope and accelerograph data. The next application is to the Guatemala earthquake of February 4, 1976 (M_S = 7.5, mb = 5.8). The only seismic instrumentation available for determining M_L is a seismoscope record from Guatemala City, which indicates M_L = 6.9 when a representative distance of about 35 km is used. As a final example, the records obtained during the 1906 San Francisco earthquake Formula from the Ewing duplex pendulum seismograph at Carson City, Nevada and the simple pendulum at Yountville, California are analyzed. After restoring the Carson City instrument, its period and damping were determined experimentally as were the period and damping of a similar instrument in the London Science Museum. On the basis of the strong-motion records from Carson City and Yountville, it is estimated that the local magnitude of the 1906 earthquake lies in the range Formula to 7. The use of seismoscope data further extends the instrumental base from which M_L can be determined and allows the rapid determination of M_L in earthquakes where seismoscope data are available. The applications in this study provide further instrumental evidence for the saturation of M_L in the 7 to Formula range, with the value of 7.2 for the Kern County earthquake of 1952, the largest so far determined.

Secondary faulting near the terminus of a seismogenic strike-slip fault: Aftershocks of the 1976 Guatemala earthquake

abstract Aftershocks of the February 4, 1976 Guatemalan earthquake (Ms = 7.5) were monitored by a network of portable seismographs from February 9 to February 27. Although seismic data were obtained all along the 230-km surface rupture of the causal Motagua fault, the field program was designed to concentrate on the aftershock activity at the western terminus of the fault. Data from that locale revealed several linear or near-linear trends of aftershock epicenters that splay to the southwest away from the western end of the main fault. These trends correlate spatially with mapped surface lineaments and, to some degree, with ground breakage patterns near Guatemala City. The observed splay pattern of aftershocks and the normal-faulting mode of the splay earthquakes determined from composite focal mechanism solutions, may be explained by a theoretical pattern of stress trajectories at the terminus of a strike-slip fault. Composite focal mechanism solutions for aftershocks located on or near the surface break of the Motagua fault, to the north and east of the linear trend splay area, agree with the mapped surface movements, i.e., left-lateral strike-slip.

Quaternary faulting along the Caribbean-North American plate boundary in Central America

Recent detailed mapping along the Motagua fault zone and reconnaissance along the Chixoy—Polochic and Jocotán—Chamelecón fault zones provide new information regarding the nature of Quaternary deformation along the Caribbean—North American plate boundary in Central America. The southern boundary of the Motagua fault zone is defined by a major active left-slip fault that ruptured during the February 4, 1976 Guatemala earthquake. The recurrent nature of slip along the fault is dramatically demonstrated where stream terraces of the Río El Tambor show progressive left-slip and vertical (up-to-the-north) slip. Left-slip increases from 23.7 m (youngest mappable terrace) to 58.3 m (oldest mappable terrace) and vertical slip increases from 0.6 m to 2.5 m. The oldest mappable terrace crossed by the fault appears to be younger than 40,000 years and older than 10,000 years. Reconnaissance along the Chixoy—Polochic fault zone between Chiantla and Lago de Izabal has located the traces of a previously unmapped major active left-slip fault. Geomorphic features along this fault are similar to those observed along the active trace of the Motagua fault zone. Consistent and significant features suggestive of left-slip have so far not been observed along the Guatemala section of the Jocotán—Chamelecón fault zone. In Central America, the active Caribbean—North American plate boundary is comprised of the Motagua, Chixoy—Polochic, and probably the Jocotán—Chamelecón fault zones, with each accommodating part of the slip produced at the mid-Cayman spreading center. Similarities in geomorphic expression, apparent amount of left-slip, and frequency and magnitude of historical and instrumentally recorded earthquakes between the active traces of the Motagua and Chixoy—Polochic fault zones suggest a comparable degree of activity during Quaternary time; the sense and amount of Quaternary slip on the Jocotán—Chamelecón fault zone remain uncertain, although it appears to be an active earthquake source. Uplift of major mountain ranges on the north side of each fault zone reflects the small but consistent up-to-the-north vertical component (up to 5% of the lateral component) of slip along the plate boundary. Preliminary findings, based on offset stream terraces, indicate a late Quaternary slip rate along the Caribbean—North American plate boundary of between 0.45 and 1.8 cm/yr. Age dating of offset Quaternary terraces in Guatemala will allow refinement of this rate.

Earthquake hazards

In this century, apart from Alaska, there has been only one great earthquake in the contiguous United States: the April 18, 1906, California earthquake caused 600 deaths and considerable damage in a number of towns and cities from shaking and subsequent conflagration of much of downtown San Francisco. Yet around the world, earthquakes continue as great killers and inflict heavy economic loss; on the average, about $7 billion a year (see Figure 1).In 1976, for example, people grew alarmed at the dreadful toll of life in earthquake catastrophes. As many as 700,000 people may have died. The Guatemala earthquake on February 4 killed 23,000; the Italian earthquake on May 6 in the Friuli region caused about 1000 deaths; near Tangshan, China, an earthquake on July 28 (local time) killed more than 600,000 people; in West Irian, Indonesia, on June 25, a magnitude 7.1 shock resulted in 6000 casualties; in the following August an earthquake and accompanying tsunami in the Philippines caused 2000 fatalities; and over 4000 persons died in Turkey in an earthquake on November 24. Most of the deaths were caused by the collapse of weak houses and buildings.