PROF. BARONGO JUSTUS O

The East African Rift System (EARS) has natural hazards – earthquakes, volcanic eruptions, and landslides along the faulted margins, and in response to ground shaking. Strong damaging earthquakes have been occurring in the region along the EARS throughout historical time, example being the 7.4 (Ms) of December 1910. The most recent damaging earthquake is the Karonga earthquake in Malawi, which occurred on 19th December, 2009 with a magnitude of 6.2 (Ms). The earthquake claimed four lives and destroyed over 5000 houses. In its effort to improve seismic hazard assessment in the region, Eastern and Southern Africa Seismological Working Group (ESARSWG) under the sponsorship of the International Program on Physical Sciences (IPPS) carried out a study on active fault mapping in the region.

The fieldwork employed geological and geophysical techniques. The geophysical techniques employed are ground magnetic, seismic refraction and resistivity surveys but are reported elsewhere. This article gives findings from geological techniques. The geological techniques aimed primarily at mapping of active faults in the area in order to delineate presence or absence of fault segments. Results show that the Karonga fault (the Karonga fault here referred to as the fault that ruptured to the surface following the 6th–19th December 2009 earthquake events in the Karonga area) is about 9 km long and dominated by dip slip faulting with dextral and insignificant sinistral components and it is made up of 3–4 segments of length 2–3 km. The segments are characterized by both left and right steps.

Although field mapping show only 9 km of surface rupture, maximum vertical offset of about 43 cm imply that the surface rupture was in little excess of 14 km that corresponds with Mw = 6.4. We recommend the use or integration of multidisciplinary techniques in order to better understand the fault history, mechanism and other behavior of the fault/s for better urban planning in the area.

A detailed analysis of the time-domain INPUT airborne electromagnetic response to a horizontal layer of variable conductivity and thickness reveals that there are combinations of conductivity and thickness of the layer for which the response behaves like that of a thin sheet (thickness << diffusion depth), those for which it behaves like that of a finite layer (thickness between those of thin sheet and half-space) and those for which it behaves like that of a half-space (thickness >> diffusion depth). Plots of thickness versus conductivity at which the response changes from one category of behaviour to another produces three distinct zones we have referred to as 'thin sheet response zone', 'finite layer response zone' and 'half-space response zone', respectively. The boundaries between these three zones move to higher conductances with increasing sample times. A damped least-squares inversion of the synthetic time-domain airborne electromagnetic response involving all the six channels of the INPUT system and based on singular value decomposition produces a distinct 'boundary' separating pairs of layer conductivity and thickness which can be uniquely resolved from those which cannot. The results further show that conductivity and thickness pairs within the thin-sheet response zone, as expected, cannot be uniquely resolved but those within the finite-layer response zone can be resolved. Using carefully interpreted conductivities and thicknesses of the conductive weathered layer from reconnaissance ground resistivity sounding data from an area flown earlier with an INPUT system, I demonstrate how to apply the general 'response diagram' arising from the above results to select between a thin sheet, finite layer and half-space model for the interpretation of time-domain airborne electromagnetic data for geological mapping.

The concept of point‐pole and point‐dipole in interpretation of magnetic data is often employed in the analysis of magnetic anomalies (or their derivatives) caused by geologic bodies whose geometric shapes approach those of (1) narrow prisms of infinite depth extent aligned, more or less, in the direction of the inducing earth’s magnetic field, and (2) spheres, respectively. The two geologic bodies are assumed to be magnetically polarized in the direction of the Earth’s total magnetic field vector (Figure 1). One problem that perhaps is not realized when interpretations are carried out on such anomalies, especially in regions of high magnetic latitudes (45–90 degrees), is that of being unable to differentiate an anomaly due to a point‐pole from that due to a point‐dipole source. The two anomalies look more or less alike at those latitudes (Figure 2). Hood (1971) presented a graphical procedure of determining depth to the top/center of the point pole/dipole in which he assumed prior knowledge of the anomaly type. While it is essential and mandatory to make an assumption such as this, it is very important to go a step further and carry out a test on the anomaly to check whether the assumption made is correct. The procedure to do this is the main subject of this note. I start off by first using some method that does not involve Euler’s differential equation to determine depth to the top/center of the suspected causative body. Then I employ the determined depth to identify the causative body from the graphical diagram of Hood (1971, Figure 26)