Local Seismic Stations Research Articles

Implementation of Filter Picker Algorithm For Aftershock Identification of Lombok Earthquake 2018

The ability to identify earthquake events that are consistent, efficient and accurate is increasingly needed along with the increase in the amount of data analyzed. In this paper a filter picker algorithm is implemented to identify aftershock events and determination of arrival time automatically, especially for the P wave phase. Here modifications are made in determining the uncertainty of arrival time and there are additional criteria in determining the time of arrival used. The additional criteria are that in a certain time span, there are at least 5 stations determined by the time the filter picker arrives. This is done to minimize identification errors due to local noise and other practical reasons, namely the minimum number of stations to determine the location and other seismological analysis. To test the filter picker algorithm, aftershock data from the Lombok earthquake occurred on July 29 (M 6.4), August 5 (M 7), and August 19 (M 6.3 and M 6.9) 2018. The aftershock data were used for 30 days, from August 4, 2018 to September 4, 2018 using local seismic station in Lombok Island. The results of the filter picker algorithm were evaluated by comparing the number of earthquake events detected and the accuracy of determining the P wave arrival time automatically to the results of manually arriving time. In addition, a comparison of the results obtained from a broadband type seismometer with a short period is used to find out how much influence the type of tool has on its performance results. The results of the comparison with the manual arrival time show that more than 85 percent of the results of the automatic arrival time have a difference below 0.2 seconds. Therefore, it can be said that the filter picker algorithm is quite effective for identifying events and determining the arrival time of P waves. In this paper it is also shown that this algorithm can be used for broad band and short period seismometer sensor, even without the prior correction of instruments.

ASSESSMENT OF A POTENTIAL HYDROCARBON RESERVOIR IN A YOGYAKARTA BASIN USING BROADBAND-NOISE ANALYSIS

The search for hydrocarbon reservoirs through geophysical exploration in a particular basin has been in common recently. In this study, a potential hydrocarbon reservoir situated in a Yogyakarta Basin in Central Java, Indonesia was assessed using broadband-noise spectral analysis of spectrograms recorded by 2 local seismic stations: UGM and YOGI, where the broadband data included 3 components of waveforms.The aims of this study are thus twofold; firstly, we report the characteristics of micro-tremor seismic signalsgenerated by a hydrocarbon reservoir in the region of interest and then secondly, we examine these findings in the context of determining which seismic station is likely to be positioned above the potential reserve for hydrocarbons and estimating the depth of a hydrocarbon reservoir. The results of broadband-noise seismic experiments revealed that the peak of micro-tremor anomalies associated with a hydrocarbon reserve wasonly detected by YOGI station. This primary finding suggests that, instead of UGM station, the YOGI station is arguably lying above a potential hydrocarbon reservoir with the layer depth is estimated to be 12-35 m. Discussions on these results include the practical use of broadband-noise experiments with a low pass-filtered frequency in geotechnical and industrial applications.

Seismicity Study of Botswana from 1966 to 2012

The history of Botswana’s major seismic events has been initiated by two earthquakes that occurred on the 11th of September and 11th of October 1952 with ML Richter magnitudes of 6.1 and 6.7, respectively, in the Okavango Delta Region (ODR). Not much is known and well documented about the seismicity of a larger coverage of Botswana primarily because over the years, local seismic stations were biased to the north-western part of Botswana, to monitor the more seismogenic ODR. The objective of this study is to use data from the International Seismological Centre (ISC) bulletin for the period 1966-2012 to estimate the relative size distribution of seismic events (b-value), the rate of seismic activity (a-value), and associated stress condition prevailing in Botswana to assist in the quest of hazard mitigation. This study shows that micro-seismic activities of magnitude ranging from 1.3 to 5.7 are distributed not only on the northern part of Botswana, but also in other parts of the country such as the Southern (including Kweneng East and West), Central and Eastern Botswana. Based on 327 events extracted from data compiled by the ISC, the frequency-magnitude distribution (FMD) of earthquakes in Botswana from 1966 to 2012 was estimated with b-value and a-value for the entire catalogue found to be 1.2 and 6.3, respectively, implying a region of low stress dominated by small to moderate events. The minimum completeness magnitude (MC), a value that indicates the lowest magnitude above which all seismic events are reliably recorded was found to be 3.8. From this analysis, annual probabilities of occurrence for M4 and M5 events were found to be 67.2% and 4.3%, respectively, while M6 or larger event has an annual probability of 0.3%.

Influence of wind turbines on seismic stations in the upper rhine graben, SW Germany

By analysing long- and short-term seismological measurements at wind farms close to the town of Landau, SW Germany, we present new insights into ground motion signals from wind turbines (WTs) at local seismic stations. Because of their need to be located in similar regions with sparsely anthropogenic activities, wind turbines impact seismic stations and their recordings in a way that is not yet fully understood by researchers. To ensure the undisturbed recording tasks of a regional seismic array or a single station by a protected area around those endangered stations, it is very important to investigate the behavior of WTs as a seismic source. For that reason, we calculate averaged one-hour long spectra of the power spectral density (PSD) before and after the installation of a new wind farm within the investigated area. These PSD are ordered according to the rotation speed. We observe a clear increase of the PSD level after the WT installation in a frequency range of 0.5 to 10 Hz up to a distance of 5.5 km away from the WT. By analysing seismic borehole data, we also observe a decrease of the PSD of wind dependent signals with depth. The impact of wind-dependent signals is found to be much more pronounced for the shallower station (150 m depth) than for the deeper one (305 m depth). Using short-term profile measurements, we fit a power-law decay proportional to 1/r b to the main WT-induced PSD peaks and differentiate between near-field and far-field effects of ground motions. For low frequencies in the range from 1 to 4 Hz, we determine a b value of 0.78 to 0.85 for the far field, which is consistent with surface waves. The b value increases (up to 1.59) with increasing frequencies (up to 5.5 Hz), which is obviously due to attenuating effects like scattering or anelasticity. These results give a better understanding of the seismic wavefield interactions between wind turbines (or wind farms) with nearby seismic stations, including borehole installations, in a sedimentary setting.

LOCAL EARTHQUAKE TOMOGRAPHY IN THE BROADER AREA OF WESTERN CORINTH GULF

In this study, we applied Local Earthquake Tomography in order to investigate the detailed 3-D structure within and around the broader region of Western Corinth Gulf which is one of the most seismically active regions in the world. We use data from the 2012-2014 time-period, selecting about 2,000 seismic events recorded by the local seismic stations of Hellenic Unified Seismological Network (HUSN) and the Corinth Rift Laboratory Network (CRLN). Applying Tomographic Inversion, we produce and interpret 3-D models of Vp, Vs and Vp/Vs ratio in the study area. The obtained results shows several distinct structures, namely areas of high and low Vp/Vs ratio correlating the hypocenter distribution with changes in lithology or fluid concentration. In the area of Patraikos Gulf, an ascending velocity volume was traced that could possibly be connected to salt tectonics in the alpine basement, effecting the fluid circulation as well as the behavior of local faults.

Calving localization at Helheim Glacier using multiple local seismic stations

Abstract. A multiple-station technique for localizing glacier calving events is applied to Helheim Glacier in southeastern Greenland. The difference in seismic-wave arrival times between each pairing of four local seismometers is used to generate a locus of possible event origins in the shape of a hyperbola. The intersection of the hyperbolas provides an estimate of the calving location. This method is used as the P and S waves are not distinguishable due to the proximity of the local seismometers to the event and the emergent nature of calving signals. We find that the seismic waves that arrive at the seismometers are dominated by surface (Rayleigh) waves. The surface-wave velocity for Helheim Glacier is estimated using a grid search with 11 calving events identified at Helheim from August 2014 to August 2015. From this, a catalogue of 11 calving locations is generated, showing that calving preferentially happens at the northern end of Helheim Glacier.

Source modeling of the 2015 Mw 7.8 Nepal (Gorkha) earthquake sequence: Implications for geodynamics and earthquake hazards

The Gorkha earthquake on April 25th, 2015 was a long anticipated, low-angle thrust-faulting event on the shallow décollement between the India and Eurasia plates. We present a detailed multiple-event hypocenter relocation analysis of the Mw 7.8 Gorkha Nepal earthquake sequence, constrained by local seismic stations, and a geodetic rupture model based on InSAR and GPS data. We integrate these observations to place the Gorkha earthquake sequence into a seismotectonic context and evaluate potential earthquake hazard.Major results from this study include (1) a comprehensive catalog of calibrated hypocenters for the Gorkha earthquake sequence; (2) the Gorkha earthquake ruptured a ~150×60km patch of the Main Himalayan Thrust (MHT), the décollement defining the plate boundary at depth, over an area surrounding but predominantly north of the capital city of Kathmandu (3) the distribution of aftershock seismicity surrounds the mainshock maximum slip patch; (4) aftershocks occur at or below the mainshock rupture plane with depths generally increasing to the north beneath the higher Himalaya, possibly outlining a 10–15km thick subduction channel between the overriding Eurasian and subducting Indian plates; (5) the largest Mw 7.3 aftershock and the highest concentration of aftershocks occurred to the southeast the mainshock rupture, on a segment of the MHT décollement that was positively stressed towards failure; (6) the near surface portion of the MHT south of Kathmandu shows no aftershocks or slip during the mainshock. Results from this study characterize the details of the Gorkha earthquake sequence and provide constraints on where earthquake hazard remains high, and thus where future, damaging earthquakes may occur in this densely populated region. Up-dip segments of the MHT should be considered to be high hazard for future damaging earthquakes.

Seismic and infrasonic analysis of the 9 March 2014 fireball in South Korea

On the evening of 9 March 2014, a bright fireball was observed in many places in South Korea, the energy associated with the event was also recorded at local seismic and regional infrasound stations. Using impulsive seismic signals recorded at 19 stations, we calculated the possible extent of the sonic source of the fireball by assuming point- and line-source models, which might be physically linked to either a point-source explosion in the atmosphere or shock waves radiating from the fireball’s trail, respectively. By comparing the observed seismic arrival times with those predicted by both models, a line-source model resulted in better solution than a point-source model. It means that shock waves along the meteor’s high-speed passage through the atmosphere probably acted as the dominant source of the seismic signals. The parameters of the fireball’s sonic trajectory, after incorporating a realistic sound velocity model of the atmosphere, are estimated to be an azimuth arrival angle of 313.5° clockwise from the north and an elevation arrival angle of 44.5° above the Earth’s surface, with a time of 11:04:51 UTC at which the trajectory meets the Earth’s surface. The estimated ground impact point is about 2 km offset from locations where meteorites associated with the fireball were found. The line-source model also explained the large azimuthal deviation of infrasound waves observed at arrays in the vicinity of the trajectory. From the infrasound measurements, the kinetic local-source energy generating the sound was estimated to be approximately 0.8 tons TNT, based on semi-empirical relations for the point- and line-source models.

Velocity structure and the role of fluids in the West Bohemia Seismic Zone

Abstract. In this study, we apply the double-difference tomography to investigate the detailed 3-D structure within and around the Nový Kostel Seismic Zone, an area in the Czech Republic known for frequent occurrences of earthquake swarms. We use data from the 2008 swarm since it has already been analysed in terms of earthquake focal mechanisms, principal faults, tectonic stress and foci migration. We selected about 500 microearthquakes recorded at 22 local seismic stations of the West Bohemia seismic monitoring network (WEBNET). Applying double-difference tomography, combined with weighted average model (WAM) post-processing to correct for parameter dependence effects, we produce and interpret 3-D models of the Vp-to-Vs ratio (Vp/Vs) in and around the focal zone. The modelled Vp/Vs ratio shows several distinct structures, namely an area of high Vp/Vs ratio correlating with the foci of the microearthquakes, and a layer of low values directly above it. These structures may reflect changes in lithology and/or fluid concentration. The overlaying low Vp/Vs ratio layer coincides with the base of the Fichtelgebirge (Smrčiny) granitic intrusion. It is possible that the base of the layer acts as a fluid trap and an upper limit to the seismicity, resulting in observed periodic swarms.

High-resolution fault image from accurate locations and focal mechanisms of the 2008 swarm earthquakes in West Bohemia, Czech Republic

We have analyzed 463 micro-earthquakes in the magnitude range from 0.5 to 3.8 that occurred during the 2008 earthquake swarm in West Bohemia, Czech Republic, in order to screen the detailed structure of the focal zone situated at depths between 7 and 11km. The double-difference location method was applied to records of 22 local seismic stations with an epicentral distance of less than 25km in order to retrieve highly accurate locations of hypocenters with an accuracy of less than 20m. The hypocenters are well-clustered and distinctly map the system of activated faults. The fault system has a complex geometry being composed of several fault segments with different orientations. Some of the segments intersect each other. The orientations of the segments coincide well with the focal mechanisms. We have introduced and evaluated the so-called fault instability of the individual fault segments. The fault instability ranges from 0 (most stable faults) to 1 (most unstable faults) and measures the susceptibility of the fault to be activated under specified stress. In the West Bohemia focal zone, two fault segments are optimally oriented with respect to the tectonic stress being characterized by an instability value higher than 0.9. Tractions on these fault segments are concentrated in the Mohr's diagram in the area of validity of the Mohr–Coulomb failure criterion and the associated micro-earthquakes are mainly shear. The other fault segments are slightly misoriented with instability values between 0.7 and 0.9, and the shear traction is significantly lower. These earthquakes are probably more tensile and activated most likely by the local redistribution of Coulomb stress during swarm activity.

Seismic Network Calibration for Retrieving Accurate Moment Tensors

We present a method for calibrating seismic networks in order to retrieve highly accurate seismic moment tensors. The method is based on a joint inversion of large datasets of earthquakes for moment tensors and for amplifications of the network stations, which encompass the instrumental amplifications of sensors and the gain factor of the acquisition system, as well as the local site effects neglected in modeling of Green’s functions. The method is capable of detecting the reverse polarity of sensors, incorrect orientation of sensors, or anomalous site effects caused by local geological conditions at individual stations. The robustness and accuracy of the method are tested on synthetic data with different noise levels, station configurations, and a variety of focal mechanisms. The numerical modeling confirms that the inversion code works well and yields robust results. The tests show that the moment tensors, calculated from data of properly calibrated seismic networks, are significantly more accurate. Finally, the method is applied to observations in West Bohemia, Czech Republic, in order to calibrate a network of 22 local seismic stations operated in this region and to calculate accurately the double‐couple (DC) and non‐double‐couple (non‐DC) components of moment tensors of 200 selected micro‐earthquakes. The results indicate that the method is efficient and can easily be used to calibrate other networks. For example, it can be used in inverting laboratory data, where the coupling effects between the sensor and a rock sample are difficult to quantify, or mining and borehole data, where the calibration and orientation of the sensors are frequently unknown. Moreover, the method can be applied to all studies, which deal with retrieving and interpreting highly accurate moment tensors and their DC and non‐DC components.

Relazioni fra sismicità e moti lenti nella crosta terrestre

At Somplago near Tolmezzo, two horizontal pendulums,reaching periods up to 20 minutes, are operanting (experimentally) in a130 m high, excavated mountain pit; the pit is connected with the subterraneanpower plant house where, in a room, is installed an electromagneticseismic station. The sensibility of the pendulums to the variations of theapparent vertical is extreme. At present, their period is on the order of5 minutes.In the first half of January 1969, in the region of the Ambiesta (aright-side confluent of the Tagliamento river, with which it unites nearTolmezzo) there occurred a small seismic period, characterized by two smallshocks, almost unnoticed by the local population; the first on the 7th andthe second, stronger one, on the 14th of January. After about 10 yearsof near immobility, at the end of November and along all of December,the clinographs functioning in the rocks, on the right of the Ambiesta atthe level of the dam barring te river, showed a perceptible, slow ondulationof the ground. This variation of the vertical can be put in relation withthe minor shock of January 7th 1969, that was perceived by the personnel of the Ambiesta-dam and recorded by the local seismic station as well asby the seismic stations of Tolmezzo and of Somplago. This first shock isto be considered definitely superficial.The second shock occurred on January 14th, in the region betweenthe Ambiesta-river and Somplago. Its depth has been estimated 011 theorder of 13 km. This shock was preceded by a very small displacementof pendulum A at Somplago on January 12th, and then followed —several hours after the earthquake — by a perceivable deviation of thevertical, recorded by both pendulums A and B The displacement(slightly above one second of arc) occurred more or less in direction ofthe source of the earthquake, with trend S 46° from Somplago. This isin accordance with the fact that the Somplago seisndc station recorded theearthquake as starting in the form of a compression (upward thrust). Themaximum displacement was reached in only a few hours, while it took about20 days to bring the pendulums back to their primitive position. If —as everything seems to indicate — the displacement showed by the longperiodpendulums is to be related to the consequences provoked by the shockin his ipocentral location, it can be deduced that the propagation in directionSomplago of the recorded ondulating movement has been very slow (witha velocity inferior to 1 km/h). In other words, the displacement of mattersubsequent to the breack in the elastic field occurred in the liypocenter,lias taken on the form of an extremely slow ondulation, evidently associatedwith the gradual proceeding of the deformed stratification to a new systemof equilibrium. There have been other occasions, in the Tolmezzo area,permitting to detect a clear relation between slow variations of the verticaland local earthquakes. In the contemplated case at Somplago, the ondulationsfollowing the earthquake of 14. January have been definitely greaterthan the ones preceding the quake.In my opinion, it is with this course of action — geodynamic, geodetic,geomagnetic and geoelectric observations — that one can hope to gatherthe information apt to confer a more realistic character to the forecastingof seismic phenomena. To insist — as it is yet done in Italy — with thestatistical methods, is plainly illusory work. Max Planck spoke ironicallyof the pretense of some of his colleagues to resolve fundamental problemsof physics basing on the probability table. Only the strictly dynamiclaws — the famous physicist pointed out — satisfy wholly our need ofknowledge, while no statistical law can actually satisfy, because it has noabsolute value but admits of exceptions in the single cases and thus leavesus confronting the problem to know which are the cases where such exceptionsmay occur . Therefore it appears convenient — it is still Planckspeaking — to claim among the postulates of physics not only the existenceof laws, but also their strictly causal character and to never consider closedthe investigation, until each and every statistic law is resolved into oneor more dynamic laws

Variable-Period Surface-Wave Magnitudes: A Rapid and Robust Estimator of Seismic Moments

Abstract We demonstrate that surface-wave magnitudes ( M s ), measured at local, regional, and teleseismic distances, can be used as a rapid and robust estimator of seismic moment magnitude ( M w ). We used the Russell (2006) variable-period surface-wave magnitude formula, henceforth called M s (VMAX), to estimate the M s for 165 North American events with 3.2 M w M w estimated from broadband waveform modeling (Herrmann et al. , 2008) were regressed against M s (VMAX). M w can be estimated from M s (VMAX) using the relationship: M w =1.91+0.66 * M s (VMAX) for 2 M s M w [ M s (VMAX)] with respect to M w [Waveform Modeling] was approximately ±0.2 magnitude units (m.u). The residuals between M w [ M s (VMAX)] and M w [Waveform Modeling] show a significant focal mechanism effect, especially when strike-slip events are compared with other mechanisms. Validation testing of this method suggests that M s (VMAX)-predicted M w ’s can be estimated within minutes after the origin of an event and are typically within ±0.2 m.u. of the final M w [Waveform Modeling]. While M w estimated from M s (VMAX) has a slightly higher variance than waveform modeling results, it can be measured on the first short-period surface-wave observed at a local or near-regional distance seismic station after a preliminary epicentral location has been formed. Therefore, it may be used to make rapid measurements of M w , which are needed by government agencies for early warning systems.

Damage Accumulation Model for Solids and the Catastrophy Prediction FOR Large-Scale Objects

The failure concentration criterion has been approved in the laboratory tests of polymeric, composite and natural materials. It is of interest to check its applicability to larger scale objects, in particular, rocks, where prediction of catastrophic situation is a topical problem. In terms of a rockburst-hazardous mine in the area of Severouralsk town, based on the seismic event database collected by the local seismic service station for more than 30 years, the authors have analyzed the available information from the viewpoint of the concentration criterion of a destruction source formation. The paper illustrates workability of the criterion to revealing a rockburst source and predicting a strong rockburst in complex underground conditions

Infrasound observations of the 2008 explosive eruptions of Okmok and Kasatochi volcanoes, Alaska

The recent explosive eruptions of Okmok and Kasatochi volcanoes provide an opportunity to use seismic, local infrasound, distant infrasound array, and remote sensing data in concert to better monitor volcanoes in the Aleutian Arc and to better understand the source processes. The eruption of Okmok Volcano began on 12 July 2008 and included a seismically active phase that lasted continuously for about 10 h. In contrast, the eruption of Kasatochi which began on 7 August 2008 consisted of five explosive events that lasted from 26 to 68 min each and had a cumulative duration of 3.4 h. Given the event times by local seismic stations, the corresponding infrasound signals were found in the data recorded by local infrasound sensors and by distant infrasound arrays. Signals from the Okmok eruption were detected by three International Monitoring System (IMS) arrays as far away as 4400 km; signals from the Kasatochi eruption were detected at greater distances up to 5200 km away by seven infrasound arrays including the ones that detected the event at Okmok Volcano. Back azimuth propagation and a simple acoustic wave propagation model in unison with known event times were used to confirm that the planar, acoustic signals recorded at the arrays had originated from the eruptions. The infrasound array data reflected the differences in eruption styles between Okmok and Kasatochi as the signals from Kasatochi were of shorter duration, of greater amplitude, and detected over greater distances. The infrasound array data were also able to distinguish between two types of tremor episodes that occurred at Kasatochi Volcano based on atmospheric disturbance.

The intraplate Porto dos Gaúchos seismic zone in the Amazon craton — Brazil

The largest earthquake observed in the stable continental interior of the South American plate occurred in Serra do Tombador, Mato Grosso state — Brazil, on January 31, 1955 with a magnitude of 6.2 m b. Since then no other earthquake has been located near the 1955 epicentre. However, in Porto dos Gaúchos, 100 km northeast of Serra do Tombador, a recurrent seismicity has been observed since 1959. Both Serra do Tombador and Porto dos Gaúchos are located in the Phanerozoic Parecis basin. Two magnitude 5 earthquakes occurred in Porto dos Gaúchos, in 1998 and 2005, with intensities up to VI and V, respectively. These two main shocks were followed by aftershock sequences lasting more than three years each. Local seismic stations have been deployed by the Seismological Observatory of the University of Brasilia since 1998 to study the “Porto dos Gaúchos” seismic zone (PGSZ). A local seismic refraction survey was carried out with two explosions to help define the seismic velocity model. Both the 1998 and 2005 earthquake sequences occurred in the same WSW–ENE oriented fault zone with right-lateral strike-slip mechanisms. The epicentral zone is in the Parecis basin, near its northern border where there are buried grabens, generally trending WNW–ESE, such as the deep Mesoproterozoic Caiabis graben which lies partly beneath the Parecis basin. However, the epicentral distribution indicates that the 1998 and 2005 sequences are related to a N60°E fault which probably crosses the entire Caiabis graben. The 1955 earthquake, despite the uncertainty in its epicentre, does not seem to be directly related to any buried graben either. The seismicity in the Porto dos Gaúchos seismic zone, therefore, is not directly related to rifted crust. The probable direction of the maximum horizontal stress near Porto dos Gaúchos is roughly E–W, consistent with other focal mechanisms further south in the Pantanal basin and Paraguay, but seems to be different from the NW–SE direction observed further north in the Amazon basin. The recurrent seismicity observed in Porto dos Gaúchos, and the large 1955 earthquake nearby, make this area of the Parecis basin one of the most important seismic zones of Brazil.

Characteristics of the October 2005 Microearthquake Swarm and Reactivation of Similar Event Seismic Swarms over Decadal Time Periods near Socorro, New Mexico

Seismicity in the Rio Grande rift in central New Mexico, southwestern United States, has been dominated by microearthquakes occurring above the midcrustal Socorro magma body (SMB) for at least the past century. The SMB is a sill-like feature ≥3400 km2 in area, with a top surface at 19-km depth and centered below the inner rift half-graben system. A recent swarm of microearthquakes above the magma body began on 20 October 2005. The activity intensified toward 30 October 2005, with the largest event being a shallow felt local magnitude ( M L) 2.4 earthquake at 02:57:35 (UTC). Seismicity increased in the region following this event, with over 1600 earthquakes detectable at a minimum of one local seismic station during the subsequent month. Seismic waveforms for the earthquakes in this sequence are remarkably similar, implying consistently similar location and faulting geometry. The time–space distribution of earthquakes indicates that this sequence is highly swarmlike, in contrast to tectonic mainshock–aftershock sequences. Given the local tectonic setting and shallow (<8 km) depth of individual events above the SMB, a likely explanation for the Socorro earthquake sequence is fluid flow- or pressure-induced triggering associated with the evolution of the SMB. Swarms such as the 2005 sequence are not uncommon in this area (e.g., prominent previous sequences occurred in May and July of 1983). Here we characterize the 2005 earthquake sequence by presenting composite focal mechanisms, magnitude estimates, and high-precision relative locations based on waveform cross-correlation techniques that capitalize on the similar waveforms. A new waveform scanning technique identifies over 1600 small similar earthquakes during this period from continuous seismic records. Waveforms and locations are also observed to be similar between the 2005 and 1983 sequences, suggesting consistency in source location, geometry, and thus mechanism over decadal time scales.

3-D seismic structure of the northeast India region and its implications for local and regional tectonics

In this study we attempted to estimate 3-D P-wave velocity structure of northeast India region using the first arrival data of local earthquakes that were recorded by about 77 temporary/permanent local seismic stations. The data set, the published bulletins, include 3494 P-wave travel times and 3064 S–P travel times from 980 local earthquakes that were located with a minimum of six observations. The located earthquakes having a travel-time root mean square (RMS) residual ⩽0.49 s and azimuthal gap ⩽180° are selected to compute a 1-D velocity model for the region, which is used as initial model for the subsequent 3-D inversions. Our results demonstrate that the computed 3-D velocity model has significantly improved hypocenter locations of the selected 980 earthquakes by reducing the RMS error (⩽0.06) by about 88% with respect to that by the 1-D velocity model. The reconstructed P-wave velocity ( V p) structure, with relocated events, reveal strong heterogeneity in lateral as well as in vertical direction corresponding to the local and regional geology/tectonics of the region. High V p is mapped beneath the Shillong Plateau–Mikir hills and in the vicinity of Indo-Burma ranges at shallower crust (<10 km) suggesting dense crystalline rocks under compressional stress. A prominent NW–SE trending low V p structure is imaged between the Shillong Plateau and Mikir hills at 20 km depth, which corresponds to the Kopili fault. The Kopili fault system extends down to 30 km depth as evidenced by the low V p. A high V p is imaged below the Mikir hills at 40 km depth, which is possibly the stress concentrator for high seismic activity along the Kopili fault, particularly at the fault end. The Bengal basin, south of the Shillong Plateau, is identified as a low V p zone extending down to a depth of about 20 km, that indicates the thick alluvium sediments.

Sediment Thicknesses and Qs vs. Qp Relations in the Kachchh Rift Basin, Gujarat, India Using Sp Converted Phases

Delineation of the top sedimentary structure and its Qs vs. Qp relationship using the travel-time difference of direct S and converted Sp phase is key to understanding the seismic hazard of any sedimentary basin area. We constructed filtered displacement waveforms from local ETNA Episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 Bhuj earthquake recorded by the Kachchh seismological network of the National Geophysical Research Institute (NGRI), Hyderabad, India during 2001–2004. Stations are within 15–70km of epicenters, and the resulting displacement waveforms are generally simple, displaying prominent P, Sp, and S wave pulses. Particle motion of P and S waves suggest near-vertical raypaths consistent with preliminary depth estimates. The direct S wave on the horizontal component is characterized by lower frequency content than the converted Sp phase on the vertical component. This difference in frequency content between S and Sp phases can be explained in terms of different attenuation effects for P and S waves in the unconsolidated sediments. The Sp phase is generated by S-to-P phase conversion at the base of Mesozoic sediments of the Kachchh basin. Travel-time inversion (VELEST) of 2565 P and 2380 S arrivals from 658 well located aftershocks recorded at 8–14 three-component local seismic stations led to 1 D velocity models indicated very slow sediments in the upper 0–2 km depth range (Vp: 2.92 km/s and Vs: 0.90 km/s) and an increasing trend of velocities with depth at 2–40 km depth. The estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel-time difference between S and converted Sp phases reaches a maximum of (1.534 ± 0.117) km beneath Bandri (near the location of 2001 Bhuj mainshock) and attains a minimum sediment thickness of (0.858 ± 0.104) km beneath Ramvav and Burudia. The spectral ratios between Sp and S from 159 three-component accelerograms have been used to study seismic wave attenuation in the Kachchh rift basin. The estimated Qs vs. Qp relations for 12 accelerograph sites vary from Qs = 0.184 Qp (at Chobari) to Qs = 0.505 Qp (at Dudhai). For stations Chobari, Chopdwa, Jahawarnagar, Vondh and Tapar, the spectral ratio slopes and hence the calculated Qs vs. Qp relations are effectively the same, and the correlation coefficients are quite high (0.91–0.93). Stations Adhoi, Manfara, New Dudhai, Dudhai and Sikara have similar Qs vs. Qp relationships to each other and also have high correlation coefficients (0.78–0.87). The spectral ratios for stations Anjar and Ramvav are small and poorly constrained, resulting in less reliable Qs vs. Qp relations. This could be due to noisy data, fewer available waveforms, or scattering due to velocity heterogeneities and/or interface irregularities.

Seismicity of the Northwestern Region of Libya: An Example of Continental Seismicity

Libya is situated on the Mediterranean foreland of the African Shield; its seismicity and tectonics reflect the geological history of the north African craton. Libya is not considered a highly active seismic region (Gutenberg and Richter 1965). However, several earthquakes of magnitude > 6.0 have occurred in Libya during the past century. A number of authors have investigated the seismicity and seismotectonics of Libya (Seiberg 1932; Minami 1963; Gordan and Engdahl 1963; Campbell 1968; Goodchild 1968; Kebeasy 1980; Westway 1990; Suleiman and Doser 1994; Al-Heety and Eshwehdi 2004). These investigations show that seismic activity in Libya is documented back to A.D. 262 and A.D. 365, when two large events destroyed the ancient city of Cyrene. Most of the seismic activity is concentrated on the coast of the northwestern part of Libya in the Al Jabal Al Akhdar region. The seismicity of this coastal region extends to the north as far as Greece. Another active seismic zone is concentrated along the eastern margin of the NW-SE trending Hun Graben. The region offshore of Tripoli is active as well. The southern part of Libya appears to be largely aseismic. In previous investigations of seismicity in Libya, all data were collected from international sources such as the National Earthquake Information Center (NEIC), the International Seismological Centre (ISC), the European-Mediterranean Seismological Centre (EMSC), and the bulletins of the national networks ( e.g. , those in southern Europe, Egypt, Tunisia, and others). Until very recently, there has been no national seismic network; the Libyan Seismological Network began operation in early 2006. This study represents the first attempt to study the seismicity of Libya using records of a local seismic station, Gharyan station. (Gharyan station data are available from the office of the Libyan Seismological Observatory and are available …