Body-wave Magnitude Research Articles

Lithospheric seismic structure of the eastern region of the Arabian Peninsula

The lithospheric structure of the eastern region of the Arabian Peninsula has been derived using the spectral analysis of intermediate period P-wave amplitude ratios. Sixteen earthquakes recorded at the intermediate 3-component DHR station during the period from 1986 to 1995 were selected for analysis based on the following criteria: focal depths with a range between 15 and 300 km, body-wave magnitudes greater than 5.0, epicentral distances with a range from 13° to 82°. By comparing the spectral peak positions of the observed and theoretical values, the thickness and velocity can be resolved within 3 km and 0.3 km s −1, respectively of the observed values. Earthquakes from the Eurasian plate, Japan and China show high cross-correlation ratios (>85%) and events from the Arabian plate, gulf of Aqabah, Red and Mediterranean Seas indicate cross-correlation ratios between 60–84%. The derived crustal model is not unique due to the theoretical assumptions (horizontal layering, constant densities and velocities in each layer), quality of the data and complexities of the crustal structure. The model suggests that the crust consists of five distinct layers with a strong velocity gradient of about 0.05 km s −1 km −1 in the upper crust and 0.03 km s −1 km −1 in the lower crust. The average results for several observations give a crustal thickness of 51 km and, a mean P-wave velocity of 6.2 km s −1. Depth to the crystalline basement is approximately 8 km which is in good agreement with values obtained by some oil wells in the eastern region. The Mohorovicic discontinuity indicates a velocity of 8.3 km s −1 of the upper mantle and 51 km depth.

P wave detection thresholds, Pn velocity estimates, and T wave location uncertainty from oceanic hydrophones

P wave arrivals recorded by the U.S. Navy's SOund SUrveillance System (SOSUS) hydrophone arrays were used to estimate earthquake detection thresholds and Pn velocities in the northeast Pacific Ocean. The Navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary (T) waves; however, use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment. The P wave detection threshold of the SOSUS hydrophones was quantified using the epicentral distance and magnitude of 250 northeast Pacific Ocean earthquakes. Earthquakes with body wave magnitudes as low as 2 have detectable P wave arrivals at epicentral distances of ≤500 km. Earthquakes with mb between 3.5 and 5 were detected ∼50% of the time at distances of 100–1500 km, while events with mb > 5 were all detected, even out to distances of 1000–1500 km. Both P and T wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes. The peak amplitude of the T wave coda and the onset of the P wave were used as the earthquake arrival times to estimate event locations. T wave arrival time residuals have a Gaussian distribution with zero mean, which implies that using T wave peak amplitude is consistent with using the P wave onset as the arrival time. There are typically ≤6 stations used to derive a T wave based location, hence location error ellipses are not well constrained. A Monte Carlo technique was employed to estimate T wave event location uncertainty. T wave locations have error bars of ∼1 km in latitude and longitude when >3 hydrophones are used for a location estimate. The detected P wave arrivals and earthquake locations were used to measure Pn velocities. Pn velocity values of 7.9 ± 0.1 and 8.0 ± 0.1 km/s were found for the Pacific and Juan de Fuca plates, respectively. A Pn velocity of 7.5 ± 0.1 km/s was measured for rays traveling northward from the Mendocino Triple Junction along the Cascadia subduction zone. A Pn velocity of 7.7 ± 0.3 km/s was estimated for ray paths originating onshore western North America and traveling to the offshore hydrophones.

The crustal and upper-mantle structure of the interior Arabian platform

The crustal and upper-mantle velocity structure of the interior Arabian platform is derived using the spectral analysis of long-period P-wave amplitude ratios. The ratio of the vertical to the horizontal component is utilized to obtain crustal transfer functions using the Thomson–Haskell matrix formulation for horizontally layered crustal models. 20 earthquakes recorded at the long-period station RYD between azimuths N20°W and N150°E were selected for the analysis based on the following criteria: focal depths in the range 5 to 215 km, body-wave magnitudes greater than 5.0, and epicentral distances in the range 7° to 97°. A careful quality check of the data left us with six events, out of 29, that had short epicentral distances (<20°) to be analysed. The selection criterion for the final model in the forward modelling process was based on the correlation coefficient between observed and theoretical transfer function. The model suggested that the crust consists of five distinct layers. The upper crustal layer has a P-wave velocity of about 5.6 km s−1 and is about 3 km thick. The second layer has a velocity of about 6.3 km s−1 and is 10 km thick. The third layer has a velocity of 6.6 km s−1 and is 8 km thick. The fourth layer has a velocity of 6.9 km s−1 and is 15 km thick. The lower layer has a velocity of about 7.6 km s−1 and is 10 km thick. For the Mohorovicic discontinuity, a velocity of 8.3 km s−1 for the upper mantle and 46 km depth are indicated.

Average attenuation of 0.7-5.0 Hz Lg waves and magnitude scale determination for the region bounding the western branch of the East African Rift

SUMMARY The investigation of Lg attenuation characteristics in the region bounding the western branch of the East African rift system using digital recordings from a seismic network located along the rift between Lake Rukwa and Lake Malawi is reported. A set of 24 recordings of Lg waves from 12 regional earthquakes has been used for the determination of anelastic attenuation, Q Lg , and regional body-wave magnitude, m bLg , scale. The events used have body-wave magnitudes, m b , between 4.6 and 5.5, which have been determined teleseismically and listed in ISC bulletins. The data were timedomain displacement amplitudes measured at 10 diVerent frequencies (0.7‐5.0 Hz). Q Lg and its frequency dependence, g, in the region can be represented in the form Q Lg =(186.2±6.5) f(0.78±0.05). This model is in agreement with models established in other active tectonic regions. The Lg -wave-based magnitude formula for the region is given by m bLg =log A+(3.76±0.38) log D’(5.72±1.06), where A is a half-peak-topeak maximum amplitude of the 1 s Lg wave amplitude in microns and D is the epicentral distance in kilometres. Magnitude results for the 12 regional earthquakes tested are in good agreement with the ISC body-wave magnitude scale.

The empirical relationships between M s, m b and M L for China and vicinity

Data from 753 earthquakes are used to determine a relationship between surface-wave magnitude (Ms) and bodywave magnitude (mb), and from 541 earthquakes to determine a relationship between surface-wave magnitude (Ms) and local magnitude (ML) for China and vicinity: Ms=0.9883 mb-0.0420, Ms=0.9919 ML-0.1773. The relationship of Msversus mb is obtained for 292 events occurred in the Chinese mainland in the time period from 1964 to 1996, 291 events occurred in Taiwan in the time period from 1964 to 1995 and 170 events occurred in the surrounding area. Standard deviation of the fitting is 0.445. Relationship of Msversus ML is obtained for 36 events occurred in the Chinese mainland, 293 events occurred in Taiwan, China and 212 events occurred in the surrounding area. The total amount is 541 events. Standard deviation of the fitting is 0.4673.

Source mechanism of an explosively induced mine collapse

Abstract Mining explosions and collapses, in addition to earthquakes, may trigger the future Comprehensive Test Ban Treaty (CTBT) monitoring system. Many shallow, spontaneously occurring mine collapses have implosional source mechanisms that might provide a physical basis to discriminate them from explosions. In this study, an explosively induced mine collapse was investigated. The collapse occurred immediately after the support pillars of a 320-m-deep underground mine opening were destroyed by explosives. It had an Lg body-wave magnitude (mbLg) of 2.8. We analyzed free-surface ground-motion data (within 1200 m) from the collapse by waveform forward modeling and time-dependent source moment-tensor inversion. The results indicate that the source mechanism of the collapse can be represented by a horizontal opening and closing crack. The time functions of the diagonal source moment-tensor components are similar to that of a spall source accompanying an underground explosion. A unique source characteristic of the induced collapse is that, unlike spontaneous collapses, the induced collapse initiated as a tensile crack. Because of the initially expansive source characteristic, this kind of induced mine collapses may pose some difficulties to the seismic discrimination problem. Despite the similarities between the induced mine collapse and underground explosions, the collapse has a more band-limited source spectrum and seems to be more efficient in shear and surface-wave generation.

Empirical determination of depth‐distance corrections for mb and Mw from global seismograph network stations

Using P wave amplitudes from 52 deep earthquakes, recorded by the Global Seismograph Network (GSN) between 1991 and 1996, we empirically determined depth‐distance corrections for body wave magnitude mb and moment magnitude Mw. After correcting vertical, broadband records from stations of the GSN for instrument response and source radiation factors, P wave amplitudes from velocity records were read for the case of mb and the surface area under the displacement pulse of the P wave for Mw. Amplitudes were then normalized to a scalar moment M0=1018 Nm, or mb=5.7, Mw=5.9. Using the generic logarithmic relationship for each magnitude, correction factors Q(Δ, h) were determined.A total of 973 observed values of Q in the plane of distance Δ vs. depth h were smoothed by local averaging, as well as fitted by a parameterized smooth surface. These empirical Q(Δ, h) surfaces differ in important detail from those historically established by Gutenberg and Richter, and markedly from the theoretical curves predicted by Veith and Clawson (1971). We present a first empirical correction factor for the determination of Mw from P wave displacements.

The crustal structure of the western Arabian platform from the spectral analysis of long-period P-wave amplitude ratios

The crustal structure of the western Arabian platform is derived using the spectral analysis of long-period P-wave amplitude ratios. The ratio of the vertical to the horizontal component is used to obtain the crustal transfer function based on thickness variations, crustal velocities, densities and the angle of emergence at the lower crust and upper mantle interface. Eleven well-defined earthquakes recorded at the long-period RYD station during the period from 1985 to 1994 were selected for analysis based on the following criteria: focal depths with a range between 7 and 89 km, body-wave magnitudes greater than 4.7, epicentral distances with a range from 8.8° to 26.5°, and back azimuthal coverage from 196° to 340°. Spectral analysis calculations were based on the comparison of the observed spectral ratios with those computed from theoretical P-wave motion obtained using the Thomson–Haskell matrix formulation for horizontally layered crustal models. The selection of the most suitable model was based on the identification of the theoretical model which exhibits the highest cross-correlation coefficient with the observed transfer function ratio. By comparing the spectral peak positions of the observed and theoretical values, the thickness and velocity can be resolved within 3 km and 1 km/s, respectively, of the observed values. The spectral analysis of long-period P-waves can detect a thin layer near the surface of about 1.6 km thick and a velocity contrast of about 10% with that of the underlying layer. A strong velocity gradient of about 0.05 km/s per km was found in the upper crust and 0.02 km/s per km in the lower crust. The derived crustal model is not unique due to the theoretical assumptions (horizontal layering, constant densities and velocities in each layer), quality of the data and complexities of the crustal structure. The crustal model suggests that the crust consists of five distinct layers. The upper crustal layer has a P-wave velocity of about 5.6 km/s and is about 1.6 km thick. The second layer has a velocity of about 6.2 km/s and is 10.2 km thick. The third layer shows a velocity of 6.6 km/s and is 6.8 km thick. The fourth layer has a velocity of about 6.8 km/s and is 12.3 km thick. The lower crustal layer has a velocity of about 7.5 km/s and is 9.3 km thick. The Mohorovicic discontinuity beneath the western Arabian platform indicates a velocity of 8.2 km/s of the upper mantle and 42 km depth.

Specral magnitudes and seismograms measure radiation from seismic sources

For over 100 years, efforts have been made to measure the strength of earthquakes. In 1935, Richter established the local magnitude scale, ML, for earthquakes in Southern California using Wood Anderson Seismographs. The seismic waves used had periods ranging from 0.1 s to 2 s. In 1945, Gutenberg [1945a] developed the surface wave magnitude scale (Ms) based on the amplitudes of horizontal surface waves with periods of (20±3) s, recorded at large distances. Gutenberg [1945b] also used seismic body P and S waves, with average periods of 5 s and 10 s, respectively, to introduce the bodywave magnitude scale (mb).

Seismic Evaluation of Brent-Spence Bridge

This is the second part of a two-part paper on field testing and analytical studies related to seismic behavior of the Brent-Spence bridge. This part of the paper deals with seismic evaluation of the bridge, while the first part dealt with free and ambient vibration studies of the bridge by field testing and finite-element analysis. Site-specific ground motion scenarios are developed for the bridge to represent probable earthquakes that may occur in these seismic zones. The time histories of these events are then used in the seismic analysis of the main bridge and response spectra are used for analyzing the approach spans. A three-dimensional finite-element model of the main bridge was subjected to the time histories to determine maximum responses (displacements, forces, and stresses). The seismic analysis of approach spans dealt only with the potential for loss of span due to excessive longitudinal displacement along the highway main line. For the maximum credible earthquake of magnitude 8.5 Ms (7.3 mb,Lg) on the Richter scale at New Madrid, the seismic evaluation indicates that the main bridge will survive the earthquake in the elastic range without significant damage and no loss of span. The approach spans are found to be vulnerable to loss of span failure in the 8.5 Ms event.

Microseismicity evidence for subduction of the Caribbean plate beneath the South American Plate in northwestern Venezuela

Over 1100 microearthquakes with body wave magnitude mb&lt;4 have been located in western Venezuela and the southwestern Caribbean region since the installation in 1980 of the Venezuelan Seismological Array, together with 120 events of mb≥4, one of them with surface wave magnitude Ms∼6. This tectonically complex region is part of the boundary between the Caribbean and the South American plates. The main seismically active feature inland in western Venezuela is the northeast striking, 600‐km long, 100‐km wide, right‐lateral strike‐slip Boconó fault zone along the Venezuelan Andes. About 80% of the earthquakes located in the entire region in the period 1980‐mid‐1995 have occurred on this fault zone, at focal depths &lt;20 km. Microearthquake activity at lower rates also occurs northwest of the Venezuelan Andes, both in the continental and Caribbean sea regions. Part of this activity takes place at depths down to ∼150 km. Northwest oriented seismicity depth profiles show the existence of a Benioff zone dipping to the southeast beneath northwestern Venezuela and northern Colombia. This indicates the presence of a northeast striking, southeast dipping subducted slab of the Caribbean plate beneath the South American plate. Hypocentral locations show that the northeastern end of this subduction occurs northwest of the Curaçao‐Aruba region, in the vicinity of a northwest trending, right‐lateral strike‐slip fault zone that joins up with the northeastern end of the Boconó fault zone. This latter place turns out to be the western end of the east‐west striking San Sebastián fault along the Venezuelan coast.

Regional seismic discriminants using wave-train energy ratios

AbstractWe have examined broadband regional waveforms of recent (since 1988) Nevada Test Site (NTS) underground explosions and earthquakes throughout the southwestern United States and Baja, Mexico, recorded by TERRAscope and other IRIS stations in order to characterize seismic sources for the purposes of event identification. As expected, earthquakes tended to be richer in long-period surface-wave and short-period shear-wave energy relative to explosions of comparable P-wave strength. Also, explosions, in general, were found to be richer in 1- to 6-sec surface-wave (Rg) energy and other late-arriving coda energy than were earthquakes. Most earthquakes show relatively little long-period (T &amp;gt; 6 sec) Rg and surface-wave coda energy, which we attribute to their deeper source depths, whereas known shallow earthquakes do exhibit these phases. We have developed several seismic discriminants based on our observations. The most promising discriminant is the ratio of short-period (f ≧ 1.0 Hz), vertical component, Pnl wave-train energy (EspPz) to long-period (0.05 to 0.167 Hz), three-component, surface-wave energy (Elp−3). For this ratio, explosions tend to have a higher value than do earthquakes. This discriminant works on the same premise as the teleseismic mb:MS ratio, for which earthquakes are richer in long-period surface-wave energy relative to explosions with the same body-wave magnitude. The long-period passband was chosen to limit the effect of longer-period noise and to remove the effect of the coda surface waves. Another potential discriminant examined is the ratio of short-period (f ≧ 1.0 Hz), vertical-component, P-wave to S-wave energy (EspPz:EspSz). We find that this criterion only yields marginal separation of the source populations but becomes more effective at higher frequency bands (f ≧ 4.0 Hz) or when looking at single-station observations. It does, however, help to quantify significant short-period waveform differences between the three test subsites, with Pahute Mesa shots generating relatively little S-wave energy compared to those of Yucca Flat for which the S wave (or Lg) is often the largest phase, while Rainier's shots are intermediate in character with distinct but less prominent S waves. This S-wave generation is thought to be caused by near-source scattering to converted phases and appears to be highly dependent on the near-source geology. These two discriminants are useful in that they are simple and fast to calculate. Using regional stations for sources 200 to 1300 km away, the magnitude threshold for the EspPz:Elp−3 discriminant is roughly ML ≧ 4.0, the limiting factor being the signal level of the Airy phase, while that for the EspPz:EspSz discriminant is roughly ML ≧ 3.0 for the same distance ranges.

A note on the depth recurrence and strain release of large Vrancea earthquakes

The large 1940 Vrancea event with body wave magnitude m B = 7.3 was investigated in terms of the depth of rupture initiation and extent. It was found that the rupture started at 150 km depth and propagated downwards. The ruptured zones of the 1977, 1986 and 1990 Vrancea events were estimated from the distribution of aftershocks located with the joint hypocentre determination method. Their depth ranges are 90–110 km, 130–150 km and 70–90 km, respectively. Because the body wave magnitudes m b were saturated for the last three events and the surface wave magnitudes M S were underestimated (not being corrected for depth) we chose the moment magnitudes to quantify the four events: 7.7 (1940), 7.4 (1977), 7.1 (1986) and 6.9 (1990), respectively. The depth interval 110–130 km is found to be unbroken since at least 1838 or 1802 and therefore is expected to accommodate the next large moment release ( M w = 7.0 to 7.4). The strain in the vertical and in the SE-NW directions is about 0.0003, giving a moderate strain rate of about 10 −13 s −1. A correlation between the depth interval of the large events and the year of occurrence in a century is suggested, offering a physical basis to the observed quasi-cyclicity.

Investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the Southern East Pacific Rise

Between February 1991 and May 1992, 33 intraplate earthquakes having body wave magnitudes between 4.3 and 6.0 were located on the west flank of the Southern East Pacific Rise by the International Seismological Center. Seven months after the last teleseismic event, we deployed four ocean bottom seismometers at the site of the teleseismic swarm. One hundred and ninety‐two microearthquakes were located using P and S travel times of events recorded by three or more instruments during the 16‐day deployment. Most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains. In addition, several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis. The focal depths of the microearthquakes range from 1 to 15 km, and most are between 5 and 12 km, similar to the depth range of the teleseismic events [Hung and Forsyth, 1996]. The composite P wave polarities indicate that the microearthquakes had a variety of focal mechanisms. We developed a new grid‐search, inversion technique that utilizes the P wave polarities and the empirically corrected ratios of P and S wave amplitudes to find the focal mechanisms of individual events. Within the acceptable travel time and amplitude misfits, focal solutions are fairly stable. Normal faulting is found in the ridge‐parallel seismicity line. The thrust and strike‐slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events. There is no apparent depth dependence of fault types. None of the existing models of the sources of stress (ridge push, thermoelastic stresses, loading by local topographic features, caldera collapse, and north‐south extension of the Pacific Plate) provides a satisfactory explanation for both the teleseismic swarm and microearthquakes. We propose a new tectonic scenario. In this scenario, the lithosphere is prestressed by the cooling of the plate. Magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere, which is already under extensional, thermal stress, producing the larger, teleseismically detected events. Once the dikes propagate into the lithosphere, the region surrounding the dikes behind the tips is compressed by the overpressure of magma. Depending on the geometry of the dikes, the local orientations of the minimum principal stress, and the local weaknesses in the lithosphere, thrust or strike‐slip faulting (the microearthquakes) may occur.

Microseismicity, tectonics and seismic potential in southern Caribbean and northern Venezuela

Approximately one thousand microearthquakes with body-wave magnitude mb have been located in northern Venezuela and the southern Caribbean region (9–12° N; 64–70° W) since the installation in 1980 of the Venezuelan Seismological Array, together with forty events of mb ≤ 4, one of them with surface-wave magnitude Ms ∼ 6. Focal depths are in the range of 0 to <15 km. This geologically complex region is part of the boundary between the Caribbean and the South American Plates. Epicentral locations indicate that this E–W oriented portion of the boundary is formed by two ∼400 km long subparallel fault zones: San Sebastian fault zone (SSF), ∼20 km north of Caracas along the coast; and La Victoria fault zone (LVF), ∼25 km south of the city. They are clearly delineated by the microseismicity. New composite focal mechanism solutions (CFMS) along these faults show right-lateral strike-slip (RLSS) motion on nearly E–W oriented fault planes. NW-striking subsidiary active faults occur in the region and intercept the two main E–W fault zones. These interceptions show high levels of microearthquake activity and seismic moment release when compared to other portions of both, the main and subsidiary faults. New CFMS at those fault crossing sites show NW-striking RLSS motion and normal faulting, in an en-echelon-like structural behavior. Geological data and quantitative comparisons with other transcurrent plate boundaries in the world suggest that the rate of plate motion in this area is on the order of 20 mm/y. Several moderate and large shocks have occurred along the SSF and LVF since ∼1640, including an Ms ∼ 7.6 event in 1900 on SSF. Although the region may be relatively far from a repeat of this earthquake, seismicity data indicate that strong shocks could take place along segments of the seismically active faults identified in this study.

Seismic hazards in Uganda

A probability seismic hazard map of Uganda is presented, based on instrumentally-derived data. The Uganda catalogue, compiled by Twesigomwe for the period 1900 to 1991, has been used. The magnitudes in the catalogue have been homogenised to surface wave magnitude (Ms); the conversion of body wave magnitude (m b) and local magnitude (M L to Ms were carried out where necessary using formulae derived for the Ugandan earthquakes. Based on seismicity and tectonics, the region was divided into 13 seismic source areas, each of which contributes to the seismic hazard throughout Uganda according to its specific seismicity. The uncertainties in the input parameters were accounted for by using a logic tree approach. On the logic tree, each uncertain parameter is represented by a node and branches emanating from that node represent alternative values on that parameter value and their assigned likelihood of being correct. Due to a lack of strong motion data, a semi-theoretical approach was used to develop an attenuation relation for the region. The mean peak ground acceleration (PGA), which is exceeded on average once every 50 years, was calculated. The resulting hazard map suggests that the whole of Uganda except in or close to the rifts, can expect to experience a PGA of between 0.5 and 0.6 m s −2, equivalent to an earthquake of intensity V–VI (slight damage) on average once every 50 years. In or close to the Western Rift, the expected PGA is between 1.0 and 2.2 m s −2, equivalent to an earthquake of intensity VII–VIII (moderate to heavy damage) on average once every 50 years. The frequency of occurrence of a PGA=2.0 m s -2 (heavy damage earthquake) for various parts of Uganda was also calculated. The results indicate that northeast Uganda can expect a destructive earthquake on average once in more than 3000 years, while south of latitude 0.5°N, except in and close to the Western Rift, can expect a destructive earthquake on average once every ∼1000–1500 years. In or close to the Western Rift the return period for destructive earthquake is on average less than about 50 years.

Spatial Earthquake Hazard Assessment of Evansville, Indiana

The earthquake hazard has been evaluated for a 150-square-kilometer area around Evansville, Indiana. GIS-QUAKE, a system that combines liquefaction and ground motion analysis routines with site-specific geological, geotechnical, and seismological information, was used for the analysis. The hazard potential was determined by using 586 SPT borings, 27 CPT sounding, 39 shear-wave velocity profiles and synthesized acceleration records for body-wave magnitude 6.5 and 7.3 mid-continental earthquakes, occurring at distances of 50 km and 250 km, respectively. The results of the GIS-QUAKE hazard analyses for Evansville identify areas with a high hazard potential that had not previously been identified in earthquake zonation studies. The Pigeon Creek area specifically is identified as having significant potential for liquefaction-induced damage. Damage as a result of ground motion amplification is determined to be a moderate concern throughout the area. Differences in the findings of this zonation study and previous work are attributed to the size and range of the database, the hazard evaluation methodologies, and the geostatistical interpolation techniques used to estimate the hazard potential. Further, assumptions regarding the groundwater elevations made in previous studies are also considered to have had a significant effect on the results.

Attenuation relations for strong seismic ground motion in the Himalayan region

Strong motion data from various regions of India have been used to study attenuation characteristics of horizontal peak acceleration and velocity. The strong ground motion data base considered in the present work consists of various earthquakes recorded in the northern part of India since 1986 with magnitudes 5.7 to 7.2. Using these data, relations for horizontal peak acceleration and velocity, which are $$\begin{gathered} log_{10} a = 1.14 + 0.31M + 0.65log_{10} R \hfill \\ log_{10} v = 0.571 + 0.41M + 0.768log_{10} R \hfill \\ \end{gathered} $$ have been proposed wherea is the peak horizontal acceleration in cm/sec2,v is the peak horizontal velocity in mm/sec,M is body wave magnitude, andR is the hypocentral distance in km. The proposed relations are in reasonable agreement with the small amount of strong ground motion data available for the northern part of India. The present results will be useful in estimating strong ground motion parameters and in the earthquake resistant design in the Himalayan region.

Moderate-magnitude seismicity remotely triggered in the Taiwan region by large earthquakes around the Philippine Sea plate

Abstract Seismicity in the Taiwan region was studied before and after the 12 largest seismic events with body-wave magnitudes exceeding 6.5, occurring around the Philippine Sea plate in the 1973 to 1994 period. The total local seismicity rate involving events with magnitudes as small as 2.0 was not affected by remote large events. However, the number of local earthquakes with magnitude ML ≧ 4.5 in the 15 days following a large event exceeded the number before it in 9 out of 12 cases studied. When earthquakes with ML ≧ 4.0 were considered, then 10 cases showed an increase in seismic activity after the big event. This suggests a 75 to 83% probability of remote triggering.

Connection of the earth's rotation with the atmospheric angular momentum and the strongest earthquakes

Abstract Quick changes of the effective atmospheric angular momentum (AAM) functions are compared with the occurence of the strong earthquakes (with a body-wave magnitude m b ≥ 6.0) and quick changes of the Earth's rotation parameters (ERP). A strong correlation exists between these changes of the AAM functions and the times of earthquakes. The quick variations of the length-of-day (LOD) (up to 0.2 msec) are induced by the variations of the axial component of the AAM functions and strongly correlate with the times of earthquakes too. Delta-like changes of the equatorial components of the AAM functions correlate with the times of earthquakes and do not correlate with quick variations of polar motion. Common periodicities exist both in the AAM functions and earthquake series.