Earthquake Geology

Abstract:

The history of earthquakes in Bangladesh and surrounding areas indicates that many severe earthquakes have occurred in this landmass. The past regional earthquakes caused considerable damage in different parts of the country, including the Dhaka City. The rapid growth of population and urbanization, together with frequent occurrences of earthquakes in the eastern part of Bangladesh has sharpened the need of understanding the affect of earthquake in the city.

A study has been made to infer the pattern of possible ground motion to be occurred during a moderate earthquake in and around Dhaka City. The inferences are made on the basis of tectonic influence, geomorphology and lithological boundary condition of subsurface materials, their generalized geotechnical properties, state of infrastructure design and construction practice in the Dhaka City. The study reveals that the city has been developed on an advantageous geological location consisting of raised Madhupur Clay Formation or older alluvium in respect to the surrounding floodplains of young alluvial deposits. Considering the variations in geomorphology, ground condition, their constituent geological materials and geotechnical properties the Dhaka City and surrounding area is divided into three broad earthquake risk zones. The zones are - Zone 1: The Central high area having Ground Condition Class 1; Zone 2: Complex of high and low areas having Ground Condition Class 2 and Zone 3: Complex of low area having Ground Condition Class 3.


The Central high area forms the axial zone of the terrace and extends northward. The Madhupur Clay Formation is well exposed throughout this zone. This zone is designated as earthquake risk zone 1. The ground condition of this zone is rated as of Class 1 type which is composed of very stiff to hard reddish brown Clay-Silt having good engineering properties of materials and considered to produce less ground motion.

The Complex of high and low areas is considered as earthquake risk zone 2. This zone is formed of small domes or nodes of Madhupur Clay Formation that is either exposed at lower elevation or overlain by young alluvium or fill materials. The ground condition of this zone is rated as of Class 2 type. The elevation of this zone is below the central zone; the materials are moister and have lower shear strength than those of zone 1. The inter-depressions of this zone are some times filled up with very soft clay, organic clay or peat deposits. The materials are often compressible and may suffer strong ground motion to cause severe destruction during earth-movement. 

The Complex of low area is located in the eastern and western periphery of Dhaka City and considered as earthquake risk zone 3. This zone is formed of very soft clay silt in the east and flood plain silt-sand in the west. The ground condition of this zone is rated as of Class 3 type. The general elevation of this zone is below the adjacent complex of high and low areas; the materials are very soft and susceptible to compression and liquefaction.

Present study on effect on existing infrastructures indicates that there are three types of existing infrastructures in terms of design and construction such as well engineered infrastructures, poorly engineered infrastructures and non-engineered infrastructures. It has been observed that the structures designed by qualified engineers and architects and constructed under close supervision by maintaining proper quality of materials are expected to survive during a moderate earthquake irrespective of earthquake risk zones of Dhaka City.

Northern Attica in Greece is characterized by a set of north dipping, subparallel normal faults. These faults were considered to have low tectonic activity, based on historical earthquake reports, instrumental seismicity and slip rate estimates. This study presents new data for one of these faults, the Milesi Fault. We run GIS based geomorphological analyses on fault offset distribution, field mapping of postglacial fault scarps and ground penetrating radar profiling to image hangingwall deformation. The first palaeoseismological trenching in this part of Greece allowed obtaining direct data on slip rates and palaeoearthquakes. The trenching revealed downthrown and buried palaeosols, which were dated by radiocarbon. The results of our investigations show that the slip rates are higher than previously thought and that at least four palaeoearthquakes with magnitudes of aroundM6.2 occurred during the last 4000–6000 yr.We calculate an average recurrence interval of 1000–1500 yr and a maximum throw rate of ∼0.4– 0.45 mm a−1. Based on the new geological earthquake data we developed a seismic hazard scenario, which also incorporates geological site effects. Intensities up to IX must be expected for Northern Attica and the southeastern part of Evia. Earthquake environmental effects like liquefaction and mass movements are also likely to occur. This scenario is in contrast to the official Greek seismic hazard zonation that is based on historical records and assigns different hazard zones for municipalities that will experience the same intensity by earthquakes on the Milesi Fault. We show that the seismic hazard is likely underestimated in our study area and emphasize the need to incorporate geological information in such assessments

Most catastrophic earthquakes occur along fast moving faults although some of them 27 are triggered by slow moving ones. Long paleoseismic histories are infrequent in the 28 latter faults. An exceptionally long paleoseismic record (more than 300 ka) of a slow 29 moving structure is presented: the southern tip of the Alhama de Murcia fault (AMF, 30 Eastern Betic Shear Zone), characterized by morphological expression of current 31 tectonic activity and by a lack of historical seismicity. At its tip, the fault divides into a 32 splay with two main faults bounding the Góñar fault system. At this area, the 33 condensed sedimentation and the distribution of the deformation in several structures 34 provided us with more opportunities to obtain a complete paleoseismic record than at 35 other segments of the fault. The tectonic deformation of the system was studied by an 36 integrated structural, geomorphological and paleoseismological approach. 37 Stratigraphic and tectonic features at six paleoseismic trenches indicate that old 38 alluvial units have been repeatedly folded and thrusted over younger ones along the 39 different traces of the structure. The correlation of the event timing inferred for each of 40 these trenches and the application of an improved protocol for the Infrared Stimulated 41 Luminescence (IRSL) dating of K-feldspar allowed us to constrain a paleoseismic 42 record as old as 325 ka. A minimum of 6 possible paleoearthquakes of M w = 6 -7 and 43 a maximum mean recurrence interval of 29 ka were identified. This provided 44 compelling evidence of the underestimation of the seismic hazard in the region. 45 46 47

Remotely triggered earthquakes and aftershocks constitute a great challenge in assessing seismic risk. A growing body of observations indicates that significant earthquakes can be triggered by moderate to great earthquakes occurring at distances of up to thousands of kilometres. Currently we lack the knowledge to predict the location of triggered events. We present numerical simulations showing that dynamic
interactions between material heterogeneities (e.g. compliant fault zones, sedimentary basins) and seismic waves focus and enhance stresses sufficiently to remotely trigger earthquakes. Numerical simulations indicate that even at great distances (>100 km), the amplified transient dynamic stress near heterogeneities is equivalent to stress levels near the source rupture tip (<5 km). Such stress levels are widely considered capable of nucleating an earthquake rupture on a pre-stressed fault. Analysis of stress patterns in dynamic rupture simulations which include a heterogeneous zone with a range of material and geometrical properties reveals various mechanisms of stress enhancement. We conclude that both
stiff and weak heterogeneities may focus stress waves to form zones of enhanced stress, and that bimaterial interfaces distort under static and dynamic loading in a way that induces local stress concentrations.
Our work provides insights for understanding non-uniform distribution of remotely triggered seismicity and recurrence of such events along complex fault-systems and near magmatic intrusions and geothermal zones.