Houston

===Geology===
[[File:Houston Texas 14Mar2018 SkySat.jpg|thumb|Aerial view of central Houston, showing Downtown and surrounding neighborhoods, March 2018]]
Houston is a flat, [[marshy]] area where an extensive drainage system has been built. The adjoining prairie land drains into the city, which is prone to flooding.<ref>{{cite web |url=http://www.houstonchronicle.com/local/explainer/article/The-trouble-with-living-in-a-swamp-Houston-7954514.php |title=The trouble with living in a swamp: Houston floods explained |newspaper=[[Houston Chronicle]] |first=Dylan |last=Baddour |date=May 31, 2016 |access-date=August 31, 2017 |archive-url=https://web.archive.org/web/20170829232536/http://www.houstonchronicle.com/local/explainer/article/The-trouble-with-living-in-a-swamp-Houston-7954514.php |archive-date=August 29, 2017 |url-status=live }}</ref> Underpinning Houston's land surface are [[Consolidation (geology)|unconsolidated]] clays, clay shales, and poorly cemented sands up to several miles deep. The region's geology developed from river deposits formed from the erosion of the [[Rocky Mountains]]. These [[sediment]]s consist of a series of sands and clays deposited on decaying organic marine matter, that over time, transformed into oil and natural gas. Beneath the layers of sediment is a water-deposited layer of [[halite]], a rock salt. The porous layers were compressed over time and forced upward. As it pushed upward, the salt dragged surrounding sediments into [[salt dome]] formations, often trapping oil and gas that seeped from the surrounding porous sands. The thick, rich, sometimes black, surface soil is suitable for rice farming in suburban outskirts where the city continues to grow.<ref>[https://tshaonline.org/handbook/online/articles/hch07 Harris County] {{Webarchive|url=https://web.archive.org/web/20171203083934/https://tshaonline.org/handbook/online/articles/hch07 |date=December 3, 2017 }}. ''[[Handbook of Texas]] Online.'' Retrieved on January 10, 2007.</ref><ref>[https://tshaonline.org/handbook/online/articles/afr01 Rice Culture] {{Webarchive|url=https://web.archive.org/web/20171203083049/https://tshaonline.org/handbook/online/articles/afr01 |date=December 3, 2017 }}. ''[[Handbook of Texas]] Online.'' Retrieved on January 10, 2007.</ref>

The Houston area has over 150 active [[Fault (geology)|faults]] (estimated to be 300 [[active fault]]s) with an aggregate length of up to {{convert|310|mi|km}},<ref>{{cite web |title=Mapping Active Faults in the Houston Area using LIDAR Data, #50034 (2006) |access-date=July 10, 2010 |first=R. |last=Engelkemeir |work=Online Journal for E&P Geoscientists |url=http://www.searchanddiscovery.net/documents/2006/06078engelkemeir/index.htm?q=%2Btext%3A%22active+faults+houston+area%22+-isMeetingAbstract%3Amtgabsyes}}</ref><ref>Earl R. Verbeek, Karl W. Ratzlaff, Uel S. Clanton. "[http://pubs.usgs.gov/mf-maps/mf1136/mf1136/ Faults in Parts of North-Central and Western Houston Metropolitan Area, Texas] {{Webarchive|url=https://web.archive.org/web/20060909202115/http://pubs.usgs.gov/mf-maps/mf1136/mf1136/ |date=September 9, 2006 }}", [[United States Geological Survey]], September 16, 2005. Retrieved on December 14, 2006.</ref><ref>Sachin D. Shah and Jennifer Lanning-Rush. [http://pubs.usgs.gov/sim/2005/2874/ Principal Faults in the Houston, Texas, Metropolitan Area] {{Webarchive|url=https://web.archive.org/web/20111025111836/http://pubs.usgs.gov/sim/2005/2874/ |date=October 25, 2011 }}, ''U.S. Geological Survey''. Retrieved on February 23, 2012.</ref> including the [[Long Point–Eureka Heights fault system]] which runs through the center of the city. Land in some areas southeast of Houston is sinking because water has been pumped out of the ground for many years. It may be associated with slip along the faults; however, the slippage is slow and not considered an earthquake, where stationary faults must slip suddenly enough to create seismic waves.<ref>[https://web.archive.org/web/20070715065340/http://www.ig.utexas.edu/research/projects/eq/faq/tx.htm Texas Earthquakes], ''University of Texas Institute for Geophysics'', July 2001. Retrieved on August 29, 2007.</ref> These faults also tend to move at a smooth rate in what is termed "[[Aseismic creep|fault creep]]",<ref name="USGS_Subsidence_Fault_Creep" /> which further reduces the risk of an earthquake.