The day after Halloween 2015 Phoenix Arizona got shaken up a bit when three earthquakes rattled some bones Sunday just before midnight. Black Canyon City was the closest population center (pop. ~2000) to the epicenter. The largest was a magnitude 4.1 and no damage was yet identified.
Another earthquake hit the country of Afghanistan on 2015-10-26 09:09:32 (UTC) according to the United State Geological Survey (USGS) (tinyurl.com/od3gobe). It registered M 7.5 on the Richter scale in the Hindu Kush region 270 km northeast of Kabul. The reverse fault movement was some 210 km underground, an intermediate depth. The destruction included some 4000 dwellings and killed at least 80 people.
This is a fairly active area. According to the USGS report “at the latitude of the earthquake, the India subcontinent moves northward and collides with Eurasia at a velocity of about 37 mm/yr.”
San Ramon has experienced what is being called a record breaking 408 earthquake swarm (tinyurl.com/qxw2yko). Located to the east of San Francisco bay, San Ramon is a community of approximately 76, 000 people. The swarm began on Tuesday, October 13, 2015 and continued through today (October 27) with a couple of minor quakes registering 2.0 or larger per day. The largest registered at 3.9 on the Richter scale at 4:21 PM local time on October 19. The earthquakes in this swarm occurred 5 or so miles down along the “Calaveras Fault zone near the north end of the Pleasanton Fault.” They were strike-slip quake events. This general area has a history of earthquake swarms with 120 over 31 days in 2013 and 351 over 42 days in 1990 near Alamo, CA. Minor shaking will likely continue over the next several weeks. A larger event is “unlikely,” however, a 6.7 or greater earthquake is 8% likely to occur sometime in the next 30 years.
Well, that is not exactly what they say – but it is close. Using data from before and after the 2014 M 5.1 La Habra earthquake, NASA developed a model of the main and surrounding shocks. GPS and Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) measurements showed “a broad pattern of deformation that would be expected of a M5.1 earthquake.” (tinyurl.com/putd88o) The six modeled “structures and slips” account for the estimated seismic moment, that is a measure of the amount of earth movement due to the quake. The authors stated that their modeled “results are consistent with north-south shortening and westward escape of the crust near Los Angeles.” Guess we continue the “go west, young man” (and woman) adventure.
NASA continued the analysis by looking at the Gutenburg-Richter relation for earthquakes in the region from 1994 to 2015 and estimated that there is a 99.9% probability that another greater than 5 magnitude earthquake will hit before April 1, 2018. The probability that it will be greater than 6 is much less at 35%. But, still. All of this for an area within 100 km (60 miles) of the La Habra epicenter (Figure 1), which covers most of the greater Los Angeles area. The rational behind the estimate is that more pent up energy is buried deep and the 2014 shake left it unreleased.
Figure 1. 100 km radius around La Habra, CA.
This is not the first, and certainly won’t be the last, prediction of a big earthquake in Southern California. The USGS gives a 85% probability for the >5 M earthquake, for instance. More on that later.