Civilizations have existed in the proximity of the Indus River Valley regions of modern Pakistan and India from at least 3000 B.C. onward. Geographically, the region encompasses a swath of the Makran coast, the alluvial plain and delta of the Indus River, and the Runn of Kachchh. The regional tectonic setting is controlled by the collision of the Indian and Eurasian plates and the subduction of the Arabian plate beneath the Eurasian plate. Earthquakes have undoubtedly struck many ancient sites, but finding their footprint in a riparian environment represents a challenge for archaeoseismology. However, some insight into seismoarchaeological indicators can be gleaned from examining the earthquake effects produced by historical infrequent large-magnitude events that have occurred in the region. Studies of these earthquakes emphasize the importance of repeated reconstructions, direct faulting, river damming from seismic uplift, and coastal elevation change as indicators of past earthquakes. Examples of past earthquake effects are presented for Banbhore in the Indus Delta, Brahmanabad, and the Harappan sites of Kalibangan and Dholavira. Future hermeneutic investigations in the region need to incorporate a seismological/tectonic perspective and not rely solely on serendipity.

Abstract Anomalous velocity and porosity are common indicators of abnormal pore pressure. Therefore, it is important to be able to link velocity to porosity and rock texture in a rational, first-principle-based manner. The critical-porosity concept allows for building such rock physics models. Critical porosity is the porosity above which the rock can exist only as a suspension. In sandstones the critical porosity is 36-40%, that is, the porosity of a random close pack of well-sorted rounded quartz grains. This pack is commonly the starting point for the formation of consolidated sandstones. Using this starting point for effective medium modeling, rational models can be built that relate velocity to porosity depending on rock texture and lithology.

Abstract Stepovers are fundamental features along strike-slip faults of various lengths. Two types of stepover between strike-slip faults are considered in this paper: (1) along-strike stepovers that are due to en echelon arrangement of faults in map view, and (2) down-dip stepovers that are due to en echelon arrangement of faults in cross section. Along-strike stepovers produce pull-apart basins und push-up ranges depending on the sense of stepover. Down-dip stepovers of both senses may produce strike-slip faults in orientations different from the initial major strike-slip faults that are arranged en echelon. Some possible mechanisms that produce stepovers and control the sense of stepover are (1) bending of initially straight faults. (2) faulting within a weak zone oriented slightly off a local failure plane. (3) segmentation of faults to accommodate curved fault traces. (4) horizontal slip across pre-existing extensional fractures or dip-slip faults that have steps. (5) a change of physical parameters such as elastic moduli and pore pressure, and (6) stress field resulting from fault interaction.

The gaps of volcanic activity and the associated shallow-dipping seismicity in South America can be explained by the consumption of the thick-rooted, buoyant, aseismic Nazca and Juan Fernandez Ridges and perhaps also the Cocos Ridge. The ridges erase the trench where they collide with the overriding continent. The point of collision migrates north or south along the plate boundary, depending on the orientation of the ridge relative to the direction of plate motion. This migration leaves behind a zone in which subduction is temporarily stopped; lack of subduction leads to the cessation of volcanism, perhaps owing to lack of water needed for partial melting. Although the present aseismic ridges probably consist of basaltic cumulates, there is some indication that earlier-consumed parts of these ridges (or different, previously consumed ridges) contained continental fragments that are now embedded in the western coast of South America.