January 31, 2017
It has been over a year since I was in Bangladesh after coming here twice a year for the previous five years. This will be a packed trip doing many different things, collecting samples, fixing equipment, visiting rivers and hopefully meeting with the public and government officials about the earthquake hazard. My paper last year showed that there is the potential for an earthquake of at least Mw8.2 here, an area with ~140,000,000 people. However, with no knowledge of when the last megaquake was or how often that comes, we don’t know when it might occur, in years or centuries. The article received widespread coverage in the press and caused a panic in the region. Now I feel an obligation to help steer things towards better preparation and building construction.
Our first task is related to a past earthquake. The last time we were here, Céline and I collected samples from an abandoned river that shifted ~20 km to the west. We think the shift was caused by an earthquake, but we don’t know if it was a moderate M7 or a large M8.5. Either one could be pretty destructive to people living on the soft delta
sediments. We now have dates for the samples. The three ages we got for the last sediments deposited before the shift, or avulsion, were 3,800, 3,800 and 3,600 years ago. The method we used was OSL, optically stimulated luminescence, dating. It measures electrons trapped in quartz crystals. The electrons are so weakly trapped that sunlight can set them free. Thus we date how long it has been since the sediment has seen the sun. The one hitch is the possibility that when the sediment was transported down the river, not all the electrons were freed. This is known as incomplete bleaching and would result in too old an age. Our solution is to collect samples from the modern river to see if there is any residual age that would shift our estimate for the earthquake.
Four of us arrived together, Chris Small and Dan Sousa, who use remote sensing to study the changes in the delta: rivers, coastline, vegetation, Liz Chamberlain, a graduate student specializing in OSL and myself. On arrival, we were met by Saddam Hossain and headed NE towards the Kushiara and Meghna Rivers. We stayed the first night in a “resort” in the folded hills of Sylhet where tea is grown. As a result of the attack on the Holey Artisan Bakery last year, the country is taking precautions. When we entered the Sylhet Division, we were met with a police escort. They stayed with us all through the night and their relief through the next day until we left the division on our way to Dhaka. This is a new experience for me. It did have the advantage of being able to drive without stopping for tolls and the police used
their siren to help with passing cars. Still we only got to our room around midnight, a long drive after two long flights.
Our first stop was the Kushiara River, a bit upstream of the river avulsion, but above the lake that forms every summer during the monsoon. My concern is that the lake decants the sands so the sediments upstream and downstream of the lake are different. Thus we will sample both. We drove to the small town of Sherpur and the police facilitated renting a small boat. Sailing along the river, we spotted a good spot where the bank was eroding and we could easily collect samples. This was as simple as cleaning a spot and then hammering an iron tube into the deposits. The main precaution is that the sample must not be
exposed to light or it will lose all its electrons. We then decided to collect a sample from the river bottom from the boat. This proved more challenging and it took several tries to get the boat into the correct depth of water and collect a sample with the sampler at the end of a several meter long augur. We found we had to work fast as the boat would drift in the strong current. By noon, we had our two samples and headed to the Meghna River on the way back towards Dhaka.
The much larger and more industrialized Meghna River was a bit more challenging. We need an area undisturbed by people. We rented a country boat at the ghat (dock) near the Bhairab Bazaar Bridge over the river and sailed off. After almost an hour on the river, we found it.
The nose of a large island and an eroding cut bank nearby. The point of the island was protected by sand bars, so Liz and I got out and sampled the bottom with the augur. No movement to worry about when the boat is aground. Then we headed to the cut bank exposure and took our fourth and final sample. The set of samples is different then I envisioned, but with Liz’s guidance, they will fit the bill well. By the time we got back to the ghat it was dusk. Time for a slow and traffic filled drive to Dhaka. We got in just in time to rush off to our favorite restaurant for a celebratory dinner at our favorite restaurant before it closed. This trip is off to a great start.