ALBERT

Description: Jan Mayen is an active volcanic island situated along the mid-Atlantic Ridge north of Iceland. It is closely connected with the geodynamic processes associated with the interaction between the Jan Mayen Fracture Zone (JMFZ) and the slowly spreading Kolbeinsey and Mohns Ridges. Despite the significant tectonic activity expressed by the frequent occurrence of medium to large earthquakes, detailed correlation between individual events and the causative faults along the JMFZ has been lacking. Recently acquired detailed bathymetric data in the vicinity of Jan Mayen has allowed us to document such correlation for the first time. The earthquake of 14 April 2004 (M (sub w) 6), which occurred along the JMFZ, was studied in detail and correlated with the bathymetry. Locations of aftershocks within the first 12 hours after the mainshock outline a 10-km-long fault plane. Interactions between various fault systems are demonstrated through locations of later aftershocks, which indicate that supposedly normal fault structures to the north of the ruptured fault, in the Jan Mayen Platform, have been reactivated. Correlation of the waveforms shows that events located on these structures are significantly different from activity at neighboring structures. Coulomb stress modeling gives an explanation to the locations of the aftershocks but cannot reveal any information about their mechanisms.

Description: Following the disastrous earthquakes in Izmit and Duzce along the North Anatolian fault in 1999, the earthquake hazard in the Istanbul area became a great concern. In this study we simulate strong ground motions caused by a scenario earthquake (M 7.5) in the Marmara Sea, and investigate the effect of varying the input parameters on the broadband frequency ground motion. Simulations are based on a multiasperity source model that involves the combined rupture of the North Anatolian fault segments beneath the Marmara Sea. We use a hybrid model combining a deterministic simulation of the low frequencies (0.1-1.0 Hz) with a semistochastic simulation of the high frequencies (1.0-10.0 Hz). Computation at each frequency range is performed separately and the total ground motion is combined in the time domain. Computations are linear and are performed at bedrock level, thereby not taking any effect of local geological conditions into account. We calculate a total of 17 earthquake scenarios corresponding to different source and attenuation parameters to study their effect on the ground motion. The most significant parameters in terms of ground-shaking level are the rise time, rupture velocity, rupture initiation point, and stress drop. The largest variability of strong ground motions is observed in regions adjacent to asperities and is associated with frequencies higher than 5 Hz. For lower frequencies our simulated velocity spectra within the Istanbul area are fairly stable among scenarios. The average standard deviations of all ground-motion measures are less than 35% of the mean.

Description: On 26 December 2004, a devastating earthquake of M 9.3 occurred offshore northern Sumatra. Due to the size of this earthquake and the accompanying tsunami wave, disastrous consequences have been observed in several countries around the Indian Ocean. The tectonics in the region are characterized by the oblique, north-northeast-oriented subduction of the Indian-Australian plate under the Sunda microplate with a rate of 6-6.5 cm/yr. This oblique convergence results in strain partitioning, where the trench-perpendicular thrust faulting along the subducting slab accommodates the east-west component of the motion, whereas the north-south component of the motion is probably accommodated by the right-lateral strike-slip faulting along the Great Sumatran fault and the Mentawi fault. Source parameters of the 26 December 2004 event have been used for modeling the resulting ground motions in the nearby affected regions. Results give an insight on the importance of ground shaking in the total destruction of places like Banda Aceh, northern Sumatra, Indonesia. The modeling is performed for a multiasperity finite fault using a hybrid procedure combining deterministic modeling at low frequencies and semistochastic modeling at high frequencies. Results show that strong shaking was distributed over a large area including northwestern Sumatra and its offshore islands. In Banda Aceh, which experienced significant damage, bedrock velocities reached 60 cm/sec with duration of the shaking of ca. 150 sec. The largest ground motions occurred near the strongest asperities of the fault plane, where velocities of 200 cm/sec are modeled for bedrock conditions.

Description: Prediction equations for macroseismic intensity are the backbone of seismic hazard assessment, of source parameter estimation, and of shake map generation in cases where an output in terms of intensity is desired. This is especially required when a direct relation to the damage associated with ground shaking is of interest or if ground shaking estimates will be used for informing nonseismologists such as emergency response teams or the general public. In the current study we derive ground-motion prediction equations for macroseismic intensity valid for the Marmara Sea region, northwest Turkey. The relations have a physical basis and are easy to implement for the user. In one relation, the finite extent of the fault rupture is accounted for by defining distance as the Joyner-Boore distance leading to the relation I (sub S) =0.376 M (sub w) +5.913-2.656 log R (super 2) (sub JB) +h (super 2) /h (super 2) -0.0020(R (super 2) (sub JB) +h (super 2) -h), where M (sub w) is the moment magnitude, R (sub JB) is the Joyner-Boore distance, and h is the hypocentral depth. Furthermore, a relation based on the epicentral distance (R (sub epi) ) is derived for application in cases where the extent of the fault plane is unknown: I (sub S) =0.793 M (sub w) +3.417-2.157 log R (super 2) (sub epi) +h (super 2) /h (super 2) -0.0065(R (super 2) (sub epi) +h (super 2) -h). The relations are valid for the ranges 5〈 or =I〈 or =10, 5.9〈 or =M (sub w) 〈 or =7.4, and R〈 or =350 km. It is shown that inclusion of the rupture dimensions leads to an improvement in the ability of the relation to fit observations in the near field for large earthquakes. Comparison to already existing intensity prediction equations for the region shows that the new relations provide better estimates of the macroseismic intensity distribution, especially in the region near the rupturing fault plane.

Description: On 12 May 2008, a devastating earthquake of M (sub w) 7.9 occurred in the Sichuan Province of China. The earthquake had disastrous consequences and cost more than 69,000 lives. The rupture occurred along a 300-km-long fault dipping northwest along the Longmen Shan fold-thrust belt, which separates the Longmen Shan of the Tibetan Plateau in the northwest from the Sichuan Basin in the southeast and is associated with a significant change in the crustal thickness. We simulate the ground shaking due to the Wenchuan earthquake by applying a hybrid broadband frequency strong ground-motion simulation technique that combines deterministic simulation of low frequencies (0.1-1.0 Hz) with semistochastic modeling of high frequencies (1.0-10 Hz). We use three available finite fault-slip models obtained from waveform inversion as input models for the earthquake scenarios. The resulting simulations reveal large variations in ground shaking due to the rupture complexity in terms of a variety of factors, including the location of asperities and the width of the fault plane. The applied methodology successfully reproduces the strong ground-motion distribution and frequency content of the seismic waves. The simulated ground motions, when calibrated with the recorded data, can also be used to verify the finite fault-slip models based on different teleseismic data and inversion schemes. Comparison with the damage distribution from reconnaissance field observations confirms the fault rupture complexity. The applied simulation methodology provides a promising platform for predictive studies.