Estimation of causative fault parameters of the Rudbar earthquake of June 20, 1990 from near field SH-wave data

We analyze the strong motion accelerograms recorded for the large (M_S=7.7, M_W=7.3, m_b=6.4) Rudbar earthquake of June 20, 1990. The earthquake had a complex source process. We have identified the imprints of rupture of three localized asperities on the major causative fault on the accelerograms. These asperities are interpreted to correspond to (i) the main shock that initiated the rupture process and was located in the domino block between the Kabateh and Zard Goli faults, (ii) a foreshock that occurred about 10 s earlier in the Kabateh fault and (iii) a later shock, on the western end of the Baklor fault, which terminated the bilateral rupture process at the western end. We estimate the strike, dip and slip of these causative sub-event rupture planes using the SH spectral amplitudes, based on a point source representation of sub-events and a non-linear least square formulation for inversion of the amplitude data. The results of our inversion of the near field data are comparable to other studies based on teleseismic data.     

On the estimation of volume of strain in earthquakes

Summary A relation between the volume of strain and the area of faulting in earthquakes has been derived. An empirical linear relationship between the magnitudeM of an earthquake and the logarithm of the volume of strainV has been obtained using data from 20 earthquakes.     

Focal mechanism study of South American earthquakes

Summary This study presents the focal mechanism solutions of four South American earth-quakes. These solutions have been determined using the sign of the initial motion ofP wave, and the polarization angle ofS wave. A regional study of the strike of the fault plane, the direction of motion, and the regional stress pattern has been made in which the focal mechanism solutions determined byStauder andBollinger were also used.     

Probabilities of occurrence of great earthquakes in the Himalaya

Long-term conditional probabilities of occurrence of great earthquakes along the Himalaya plate boundary seismic zone have been estimated. The chance of occurrence of at least one great earthquake along this seismic zone over a period of 100 years (beginning the year 1999) is estimated to be about 0.89. The 100-year probability of such an earthquake occurring in the Kashmir seismic gap is about 0.27, in the central seismic gap about 0.52 and in the Assam gap about 0.21. The 25-year probabilities of their occurrence in these gaps are 0.07, 0.17, and 0.05 respectively. These probability estimates may be used profitably to assess the seismic hazard in the Himalaya and the adjoining Ganga plains.     

Fine structure of seismotectonics in western Shillong massif,north east India

The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction of the Indian-EuroAsian plate convergence in the eastern Himalaya.     

Estimation of hypocentral parameters of local earthquakes when crustal layers have constantP-velocities and dipping interfaces

The paper describes an algorithm for estimating the hypocentral coordinates and origin time of local earthquakes when the wave speed model to be employed is a layered one with dipping interfaces. A constrained least-squared error problem has been solved using the penalty function approach, in conjunction with the sequential unconstrained optimization technique of Fiacco and McCormick. Joint confidence intervals for the computed parameters are estimated using the approach of Bard for nonlinear problems. These results show that when a hypocentre lies outside the array of recording stations and head waves from a dipping interface are involved, then its inclination must be taken into account for dip angles exceeding 5°.     

Simultaneous inversion of the aftershock data of the 1993 Killari earthquake in Peninsular India and its seismotectonic implications

The aftershock sequence of the September 30th, 1993 Killari earthquake in the Latur district of Maharashtra state, India, recorded by 41 temporary seismograph stations are used for estimating 3-D velocity structure in the epicentral area. The local earthquake tomography (LET) method of Thurber (1983) is used. About 1500P and 1200S wave travel-times are inverted. TheP andS wave velocities as well asV _P/V_Sratio vary more rapidly in the vertical as well as in the horizontal directions in the source region compared to the adjacent areas. The main shock hypocentre is located at the junction of a high velocity and a low velocity zone, representing a fault zone at 6–7 km depth. The estimated average errors ofP velocity andV _P/V_Sratio are ±0.07 km/s and ±0.016, respectively. The best resolution ofP and S-wave velocities is obtained in the aftershock zone. The 3-D velocity structure and precise locations of the aftershocks suggest a ‘stationary concept’ of the Killari earthquake sequence.     

Seismicity and the state of stress from investigations of local earthquakes in the Kumaon Himalaya

Results of investigations of local earthquakes in the region of the Main Central Thrust (MCT) in the Kumaon Himalaya, between and adjacent to the valleys of the Bhagirathi and Yamuna rivers, are presented. Records of over 250 earthquakes were analysed and the following facts emerged:

1. (1) Earthquakes in the Himalayas occur in specific areas and belts. One such belt has been identified in the region under investigation, hypocentral estimates being more reliable for earthquakes occurring in the middle segment approximately 70 km long of this belt, crossing the Yamuna river between the villages of Barkot and Syanachatti.

2. (2) All but a few epicentres in this middle segment, lie to the southwest of the surface trace of the MCT in a zone with a width of 10–30 km.

3. (3) Most of the earthquakes in this segment occur at depths of less than 10 km below the ground surface, the maximum estimated depth being 32 km.

Using observations of first motion for a composite focal mechanism solution, the nodal planes were observed to be near vertical and the compression axis near horizontal and normal to the local strike of the seismic belt and of the MCT.We conclude that although the Main Central Thrust itself is not seismically active in this region, there is considerable activity immediately to the southwest of it. Furthermore, the mode of faulting as inferred from the records of these earthquakes, is strike slip.     

A Study of Source Parameters, Site Amplification Functions and Average Effective Shear Wave Quality Factor Qseff from Analysis of Accelerograms of the 1999 Chamoli Earthquake, Himalaya

The accelerograms of the 1999 Chamoli earthquake and nine of its aftershocks, which occurred in Uttaranchal Himalaya, have been analyzed to investigate their source parameters, the site amplification functions and the average effective shear-wave quality factor Q_seff in the region. The fault plane solution of the main shock is obtained using the spectral amplitudes of SH waves (approximated by transverse components of accelerograms) of the high-energy packets observed in the accelerograms of the main shock. It is found to be comparable with the reported solutions in other studies. Similarly the other source parameters (viz., seismic moment = (5.03±1.7) × 10²⁵ dyne-cm, stress drop = 65 bars, source duration = 5.2 s and moment magnitude = 6.4) estimated for the main shock are consistent with the values obtained in other studies. The stress drops estimated for the aftershocks vary from 23 bars to 153 bars and the seismic moment from 1.4 × 10²³ dyne-cm to 2.9 × 10²³ dyne-cm. The average estimated values of the effective shear-wave quality factor Q_seff vary from 655±359 in the Uttaranchal sector of Himalaya and 1475±130 in the Delhi region. In general, the Q_seff value increases with an increase in the epicentral distance reflecting the penetration of the waves into deeper layers of the crust as the epicentral distance of the observation point increases. These values of Q_seff indicate that in general the curst is at low temperatures that will promote brittle behavior and conditions for episodic failure as compared to creep, under the accumulated strains from plate collision at the Himalaya plate boundary. The site amplification characteristics at sites have been identified from the frequency bands of significant amplification observed in the spectral ratios of the horizontal to the vertical component records. The decay of peak ground acceleration (PGA) values with distance has been investigated using the empirical regression curves vis-à-vis the site amplification factors.