|Typhoon Mekkhala at 2 p.m. Eastern Time Friday|
3:00 a.m. Saturday local time in Manila
|Projected rainfall over the next 48 hours in millimeters.|
From the CNN article cited.
Although this is significant and disruptive rainfall, it is an order of magnitude less than the heaviest rainfalls recorded in typhoons. In August 2009, Typhoon Morakot dropped 2900 mm (114") in Taiwan three days. This included the highest, single-day regional record at 1403 mm (55 inches). The damage to infrastructure from flooding and slides was enormous.
Such heavy rainfalls have not gone unnoticed by people who study earthquakes. Seven months after Typhoon Morakot dropped this rain, a M6.4 earthquake rattled Taiwan. According to Shimon Wdowinski and I. Tsukanov, as reported by Richard Lovett of National Geographic Magazine, heavy rainfall may be triggering these earthquakes. (Note: I could not find a peer-reviewed paper from them, only an AGU 2011 Fall meeting abstract#U53E-06). The M7.0 earthquake in 2010 in Haiti came 18 months after it had been deluged by two hurricanes and two tropical storms. There are difficulties in making correlations between events so far apart in time, and for which there are only a few data points:
- 2009 Typhoon Morakot, M6.2 in 2009 and M6.4 in 2010
- 1996 Typhoon Herb, M6.2 in 1998 and M7.6 in 1999
- 1969 Typhoon Flossie, M6.2 in 1972
But, Wdowinski's analysis suggested that Taiwan's M6+ earthquakes were five times more likely to occur within four years after such storms than if the storms had no effect. Wdowinski suggests that is not the weight of the water that triggers the earthquakes, but the unloading of the crust by landslides and sediment redistribution from the land into the sea. This lightens the stress on the crustal rocks and, if a fault is near failure, makes it easier to slip, that is a typhoon alters the timing of an imminent earthquake. The AGU abstract says that mesh free finite element modeling and Coulomb failure stress analysis were used to calculate the increase in failure stresses at the hypo centers by 300-1500 Pa, ultimately triggering the earthquakes. The say that the statistical analysis indicated "a very low probability (1-5%) for a random earthquake occurrence process to give the observed typhoon-earthquake correlation.
A slightly different model was proposed by Thomas Adar for the Himalayas. In the monsoon seasons, water flows from the Himalayas into the lowlands where its weight causes a slight bending of the Indian tectonic plate causing the edge of the plate to deform slightly. During the wet season, the bending offsets the tectonic strain on the fault caused by the plate motions and reduces the short-term risk of earthquakes. But, in winter, when the lowlands dry out, the plate unbends and the earthquake rate increases.