[1] The temporal and spatial pattern of seismicity associated with reservoir water level fluctuations, injection of high-pressure fluids in deep boreholes, and seasonal groundwater recharge provide a unique setting to study the hydrological properties of the seismogenic fractures. Pore pressure diffusion is primarily responsible for the build up of fluid pressures and the onset of seismicity. The hydrologic property controlling pore pressure diffusion is hydraulic diffusivity c, which is directly related to intrinsic permeability k. By analyzing more than 90 case histories of induced seismicity, we determined the hydraulic diffusivity value of fractures associated with seismicity to lie between 0.1 and 10 m . We call this range the seismogenic permeability k s . Fractures with k s were found to be associated with Darcian flow. Fractures with permeability less than k s were aseismic, as the pore pressure increase was negligible. In fractures with permeability larger than k s , aseismic non-Darcian flow was observed. Seismicity was uniquely associated with fractures with seismogenic permeability. Thus seismogenic permeability is an intrinsic property of fractures where pore pressure diffusion is associated with seismicity.

S U M M A R YThe occurrence of the 2008 May 12 Wenchuan earthquake (M 7.9) near the eastern edge of the Tibetan Plateau triggered a debate whether it was influenced by the newly impounded Zipingpu reservoir, located only about 21 km east of the earthquake epicentre. We calculated change in stresses due to the 3-D reservoir water load using Boussinesq theory and pore pressure diffusion due to the actual variation of water level in the reservoir. Using the stresses and pore pressure due to the reservoir impoundment, we calculated the Coulomb stress on the NW dipping fault plane of the 2008 Wenchuan earthquake. We find that the reservoir operation did not cause any increase in the Coulomb stress at the earthquake hypocentre and the region around it, rather it decreased the stress by a nominal amount of about 1 KPa. Thus, we suggest that the reservoir probably did not play a role in the occurrence of this earthquake. However, our analysis suggests that the slightly enhanced low-magnitude shallow seismicity after the reservoir impoundment, but prior to the 2008 Wenchuan earthquake, was positively influenced by the reservoir impoundment.

Plate motion in the Indo‐Burmese Arc‐Andaman‐Sumatra region of Burma plate is poorly resolved. This is mainly due to lack of relevant data and complex tectonics of the region. We analyze (1) azimuths of coseismic displacements due to the 2004 Sumatra‐Andaman and 2005 Nias earthquakes; (2) estimates of interseismic deformation in the Indo‐Burmese Arc, Andaman, Sumatra, and Sagaing Fault regions (all based on GPS measurements); (3) long‐term plate motion rates across Sumatra Fault System, Sagaing Fault, and Andaman Sea from geomorphological and other geophysical studies, and (4) the earthquake focal mechanisms in the region. We suggest that the SSW motion of Sunda plate with respect to Indian plate may be partitioned into the dextral strike‐slip motion across the Sagaing Fault in the north and Sumatra Fault System in south in the back‐arc region, and the arc‐normal motion across the Sumatra subduction zone, which becomes oblique in Andaman and southern Indo‐Burmese Arc region and dextral in the northern Indo‐Burmese Arc region of the fore arc. Under the rigid plate approximation, we estimate a pole for India‐Burma plate pair at 27 ± 1°N, 82 ± 1.1°E with an angular velocity of 0.845 ± 0.12°/Ma and for Burma‐Sunda at 22.3 ± 1.1°N, 109.3 ± 2.5°E with an angular velocity of 0.67 ± 0.12°/Ma. Thus the plate motion in the northern and southern regions of Burma plate, namely, the Indo‐Burmese Arc and Andaman‐Sumatra Arc, may be explained by a single pole and does not require a boundary between the two.

.[1] Continuing seismicity for about 30 years near a large western embayment of the Lake Nasser, about 50 km from the Aswan High Dam in Egypt, has led to a debate about the possibility of its relation with the reservoir impoundment. The largest event in the region occurred on 14 November 1981 (M 5.3), 20 km beneath the Wadi Kalabsha embayment, a westward extension of the Lake Aswan. Since then, continuous monitoring of seismic activity has given an excellent opportunity to study the spatiotemporal distribution of seismicity in the area. Most of the immediate aftershocks of the 1981 main shock were located in the Gebel Marawa area at depths between 15 and 30 km. Depths of almost all earthquakes away from this zone were shallower than 12 km. To quantify the effect of the reservoir impoundment on the seismicity of the Aswan area, we calculated changes in stress and pore pressure due to the reservoir impoundment using Green's function approach. The change in Coulomb stress (DS) is calculated on the fault planes responsible for majority of the seismicity of the region. We found that for all the seismogenic faults, DS is negative, i.e., stabilizing, when we consider the effect of the reservoir load only, whereas it is positive, i.e., destabilizing, when we include pore pressure. For example, at the hypocenter of the main earthquake, shear stress, normal stress, and pore pressure due to reservoir operation are estimated as 5.5, 13.2, and 13.5 kPa, respectively, which suggest that DS is À3.1 kPa when we do not consider the effect of pore pressure and 5.7 kPa when contribution from pore pressure is considered. Hence, the seismicity in the Aswan lake region is driven by the pore pressure due to reservoir impoundment.