A Source Study of the Bhuj, India, Earthquake of 26 January 2001 (M_w 7.6)

We study the source time function (STF) and radiated seismic energy (E_R) of the M_w 7.6 Bhuj earthquake using the empirical Green's function (EGF) technique. Our estimations of the STF and E_R are based on teleseismic P waves and regional seismograms, respectively. We find that the STFs as a function of azimuth have a similar shape and nearly constant duration of 18 sec. This suggests that the rupture propagation was essentially radial. The STFs show a sharp rise in the first 6 sec. The E_R estimated from the EGF technique is 2.1 × 10^(23) erg. We find that E_R's computed from integration of corrected velocity-squared spectra of teleseismic P waves and regional seismograms are in excellent agreement with the ER obtained from the EGF technique. Since the seismic moment, M_0, is 3.4 × 10^(27) dyne cm, we obtain E_R/M_0 = 6.2 × 10^(-5). The radiation efficiency, η_R, during the Bhuj earthquake was low, about 0.23. The sharp rise of the STFs and η_R = 0.23 can be explained by Sato and Hirasawa's (1973) quasi-dynamic, circular source model with an effective stress of ∼ 300 bar and the ratio of rupture to shear-wave velocity, V_R/β, of ∼ 0.5. The corresponding estimate of slip velocity at the center of the fault is 156 cm/sec. V_R/β ∼ 0.5 is in reasonable agreement with the duration of the STF and the reported dimension of the aftershocks, as well as with the results of inversion of teleseismic body waves. The observations may also be explained by a frictional sliding model, with gradual frictional stress drop and significant dissipation of energy on the fault plane. This model requires an average dynamic stress drop of about 120 bar and V_R/β ∼ 0.7 to explain both the rapid rise in the first 6 sec of the STFs and, along with a static stress drop of 200 bar, the observed E_R/M_0. High static stress drop is a common feature of most crustal earthquakes in stable continental regions. An examination of the available data, however, does not suggest that most of them also have relatively low radiation efficiency

Estimating tsunami potential of earthquakes in the Sumatra-Andaman region based on broadband seismograms in India

Estimating tsunami potential of earthquakes in the Sumatra–Andaman region based on broadband seismograms in IndiaIn this paper, we report that the ratio of broadband energy (0.01–2 Hz) to highfrequency energy (0.3–2 Hz), Er, estimated from regional seismograms of India, might be a useful parameter in estimating tsunami potential of earthquakes in the Sumatra–Andaman region. Er is expected to be sensitive to the depth as well as to the source characteristics of an earthquake. Since a shallow and slow earthquake has a greater tsunamigenic potential, Er may be a useful diagnostic parameter. We base our analysis on broadband seismograms of the great earthquakes of Sumatra–Andaman (2004, Mw * 9.2) and Nias (2005, Mw 8.6), 41 of their aftershocks, and the earthquakes of north Sumatra (2010, Mw 7.8) and Nicobar (2010, Mw 7.4) recorded at VISK, a station located on the east coast of India. In the analysis, we also included the two recent, great strike-slip earthquakes of north Sumatra (2012, Mw 8.6, 8.2) recorded at VISK and three south Sumatra earthquakes (2007, Mw 8.5; 2007, Mw 7.9; 2010, Mw 7.8) recorded at PALK, a station in Sri Lanka. We find that Er is a function of depth; shallower earthquakes have higher Er values than the deeper ones. Thus, Er may be indicative of tsunamigenic potential of an earthquake. As Mw and Er increase so does the tsunami potential. In addition to the parameter Er, the radiated seismic energy, Es, may be estimated from the regional seismograms in India using empirical Green’s function technique. The technique yields reliable Es for the great Sumatra and Nias earthquakes. Er and Es computed from VISK data, along with Mw and focal mechanism, may be useful in estimating tsunami potential along the east coast of India from earthquakes in the Sumatra–Andaman region in less than 20 min.En este trabajo, informamos de que la relación entre la energía de banda ancha (0,01-2 Hz) y la energía de alta frecuencia (0,3-2 Hz), Er, estimada a partir de sismogramas regionales de la India, podría ser un parámetro útil para estimar el potencial de tsunami de los terremotos en la región de Sumatra-Andamán. región de Sumatra-Andamán. Se espera que Er sea sensible a la profundidad así como a las características de la fuente de de un terremoto. Dado que un terremoto poco profundo y lento tiene un mayor potencial tsunamigénico, Er puede ser un parámetro de diagnóstico útil. Basamos nuestro análisis en sismogramas de banda ancha de los grandes terremotos de Sumatra-Andaman (2004, Mw 9,2) y Nias (2005, Mw 8,6), 41 de sus réplicas, y los terremotos del norte de Sumatra (2010, Mw 7,8) y Nicobar (2010, Mw 7,4) registrados en VISK, una estación situada en la costa oriental de la India. En el análisis, también se incluyeron los dos grandes terremotos recientes de deslizamiento del norte de Sumatra (2012, Mw 8,6, 8,2) registrados en VISK y tres terremotos del sur de Sumatra (2007, Mw 8,5; 2007, Mw 7,9; 2010, Mw 7,8) registrados en PALK, una estación de Sri Lanka. Encontramos que Er es una función función de la profundidad; los terremotos menos profundos tienen valores de Er más altos que los más profundos. Por lo tanto, Er puede ser un indicador del potencial tsunamigénico de un terremoto. A medida que la Mw y la Er aumentan también lo hace el potencial de tsunami. Además del parámetro Er, la energía sísmica radiada, Es, puede estimarse a partir de los sismogramas regionales de la India utilizando la técnica de la función de Green empírica empírica de Green. Esta técnica permite obtener Es fiables para los grandes terremotos de Sumatra y Nias. Er y Es calculados a partir de los datos de VISK, junto con Mw y el mecanismo focal, pueden ser útiles para estimar el potencial de tsunami a lo largo de la costa oriental de la India a partir de los terremotos de la región de Sumatra-Andamán en menos de 20 minutos.Universidad Nacional, Costa Rica.Universidad Nacional Autónoma de México, DF, México.India Meteorological Department, India.Observatorio Vulcanológico y Sismológico de Costa Ric