Thursday 6 February 2014

Earthquakes.... and a Geography lesson !!



Lorca....May 2011.....and I wonder if you were as surprised as I was to hear about a 5.1 level earthquake that took place in the close proximity of Condado....

The images of destruction and the tragic loss of life were things I had never imagined happening in Europe, but on further investigation I discovered that the Alhama region sits on a prominent and well known earthquake fault line, indeed its even known as the  "Alhama de Murcia fault" and its a prominent oblique fault that runs from south-west to north-east for over 100 km through the Murcia province.
Concerned by this I did a little research on Wiki ref earthquakes and our locality, apologies if this takes to you back to "Geography 101" at school....but its interesting stuff..... easy concepts 1st

Tectonic plates are large plates of rock that make up the foundation of the Earth's crust and the shape of the continents. The tectonic plates comprise the bottom of the crust and the top of the Earths Mantle.  There are ten major plates on Earth and many more minor ones. They float on a plastic-like part of the Earth's mantle called the asthenosphere. The plates are most famously known for being the source of earthquakes.
The tectonic plates are about 100 km (60 miles) in thickness, with continental plates tending to be thicker than oceanic ones. The composure of the two types of plate is also quite different. Oceanic plates consist of thicker basaltic rocks, compressed by the pressure of kilometers of water. Continental plates have a lower average density, containing granitic rocks with a heavy composition of aluminum and silica.. The mantle underneath the tectonic plates is constantly recirculating, causing the plates to float around slowly in a process called tectonic drift. This process was described well by the theory  of plate Tectonics, which solved several scientific dilemmas about the distribution of species when it was introduced. When plates push up against each other, they create mountain ranges and volcanoes. Mt. Everest was created in this way !!

The shortening between the African and the Iberian plates is absorbed by a number of faults distributed over a very wide zone with very low slip rates and long periods of seismic loading. Thus a seismotectonic map based only on faults associated with seismicity or with expressive geomorphic features is incomplete. It is possible to characterize seismogenic faults using paleoseismology.

First, paleoseismological results based on trenching analysis in the eastern Betics (Lorca-Totana segment of the Alhama de Murcia fault) are presented. The main paleoseismic parameters of this fault segment are (1) a minimum of two to three M w 6.5–7 earthquakes in the last 27 kyr (shortly before 1650 A.D., between 830 and 2130 B.C. and shortly before 16.7 ka, respectively), with a mean recurrence period of 14 kyr, and a very short elapsed time, and (2) a net slip rate of 0.07–0.6 mm/yr during the last 30 kyr. These results were extrapolated to the rest of the known active faults in the eastern Betics and were added to the slip rates of the active faults at the African margin. The total slip rate of the transect, which crosses de Alhama de Murcia fault in Spain and reaches the Cheliff basin (Algeria), would represent 21–82% of the total shortening between Africa and Eurasia estimated from plate motion models and seismic moment summation. A number of factors could account for this discrepancy: (1) hidden seismogenic faults in the emerged areas, (2) absence of correlation between current and late Pleistocene slip rates, (3) extensive small faults that are undetected and that absorb a significant amount of the deformation, and (4) possible overestimation of the convergence rates.
The earthquake in Spain that occurred on 11 May 2011 appears to be the latest in a series of earthquakes occurring along the Alhama de Murcia fault; a prominent oblique fault that runs from south-west to north-east for over 100 km through Murcia province.  Geological studies have confirmed the importance of this fault as one of the structures controlling crustal shortening as Europe collides with Africa. The fault can be partitioned into a number of segments, and of these, only the segment close to Lorca appears to be seismically active in historical times. The shallow depth of the 11 May earthquake, which contributed to the amount of damage caused, and which has been estimated to have been as little as 3.5 km, appears to be typical of the behaviour of the Alhama de Murcia fault . Earthquakes in the range of 5 to 6 in magnitude have occurred at regular intervals, notably in 1579, 1674, 1783, 1818, 1855 and 1907.

The most recent activity on this fault segment was in 1977 and 1978; however, these earthquakes were only around magnitude 4...... long may this dormant period continue...