San Diego Geologic History and Natural Hazards Summary - Research Paper Example

San Diego geologic History and Natural Hazards Summary Introduction San Diego County can be put into 3 distinct geomorphic regions that include Peninsular Range region, the Coastal Plain region to the west of the Peninsular Ranges and the Salton Trough region to the east of the Peninsular Ranges. There is a reflection of basic geologic difference between the 3 regions of this geomorphic division with the plutonic rocks metasedimentary and Mesozoic metavolcanic predominating in the Peninsular Ranges, and Cenozoic sedimentary rocks primarily predominating to the east and west of the central mountain range. The contact experienced between these geologic regions is irregular and this reflects the ancient area topography before being buried by the thick sequence of Tertiary and Cretaceous sedimentary rocks that was deposited by ancient seas and ancient rivers over the last 75 million years (Abbott, P, 23) Both these rocks with their existing biological record such as the fossils, document the biological and geological evolution of this western North America part. It is recorded that at a certain point in time, there was an ancient oceanic crustal plate consumption that created volcanic islands archipelago and also this same plate subduction generated massive magma volumes which in the later stage congealed in the crust that resulted to plutonic rock. It is also observed that local rocks record an early period when erosion unroofed and tectonic forces uplifted the plutonic rocks that were deeply buried forming a rugged and steep mountainous coastline. This is similar to the one present today along the west coast of South America. Also recorded are periods of subtropical climates and higher rainfall that supported coastal rain forests with exotic floras and faunas; relentless erosion, drowned coastlines periods and great rivers; periods of extreme renewed and aridity volcanism; widespread crustal shear and faulting periods and the new seaways formation (Bergen et al, 34). The above are just some of the stories of geology preserved in the San Diego County. Deciphering the biological and geological record is a process that is ongoing and every year brings new insights and new discoveries. Geographical regions of San Diego affected by Natural Hazards The processes of natural geology which represent a hazard to health, property or life are taken to be geologic hazards. Natural geologic hazards which affect property and people in San Diego County include earthquakes that can result to ground shaking, liquefaction and surface fault rupture, expansive weathering soils, and phenomena mass wasting such as rockfalls and landslides (Bray and Kelson,5).The hazard of surface rupture has been identified as Alquist-Priolo Earthquake Fault Zones (AP Zones). Two major AP Zones extend to southeast from northwest across the northeast half of the County, the San Jacinto and Elsinore fault zones; present the highest fault-rupture threat in the unincorporated San Diego County part (Anderson et al,13).Liquefaction primarily occurs in loose, saturated and fine to medium-grained soils in places where the groundwater table is basically at most 50-feet below the surface. There may be a potential for liquefaction within the County in areas with loose sandy soils together with a shallow groundwater table, that are typically located in floodplains and alluvial river basins/ valleys. Previous landslides together with landslide-prone sedimentary formations are located in western unincorporated County portions( Bowman,21). However, it is known that landslides can as well take place in the granitic terrain in the eastern County area, despite the prevalence is so minimal (URS, 2004). Most of the landslides that are significant have happened along coastal bluffs and other areas within incorporated County parts (URS, 2004). Existing landslides reactivations can be triggered by situations that include irrigation or heavy rainfall, seismic shaking or grading. Particular clay soils types normally shrink when dried and expand when they are saturated. These are referred to as expansive soils, and can stage a threat to the improvement integrity that is built on them without proper engineering ( Bowman,21). Particularly, if the appropriate design measures are not included and the activities related to human that result from the project leads to the soil’s moisture content to change. Primarily, these soils are normally derived from weathering of volcanic ash and feldspar minerals. Areas that have the potential of experiencing expansive soils within the County happen predominately in the coastal plains. This is an area of uplands and dissected marine terraces. They can as well be found in slopes and valleys in the mountains and foothills of the Peninsular Ranges Region, and in the desert to a lesser extent (Bowman, 21). A tsunami can be defined as a series of large waves which results from a sudden disturbance that displaces water. Triggers for a tsunami include submarine landslides, earthquakes, meteor impacts or volcanic eruptions. The coastline of the County is on Camp Pendleton and largely within incorporated cities and in this unincorporated County is not expected to have tsunamis that would affect lands (Bryant, 6). Human’s role in turning this natural hazards into Disasters Landslides occur when masses of earth, debris or rock move down a slope, that include rock falls, shallow debris flows, deep slopes failure. Human activities influence Landslides by grading and other activities such as construction, slopes irrigation, mining activity among others. Natural factors influence these natural hazards by soil types/ geology, topography, precipitation and water subsurface/ surface flow. Mitigation efforts for Future Events Avoidance is the major goal for preventing or mitigating hazards associated to fault rupture zones. It is required by the County to have no structure for human occupancy across the trace of an active fault and that there should be at least a 50-foot setback from the active fault trace for such structures (American Society of Civil Engineers, 18). Hazards related to ground shaking are or can be mitigated by following the UBC Seismic Hazards Standards for construction in Seismic Zone 4 or the ‘‘County Near-Source Seismic Shaking Zone’’. Most construction projects in the County inevitably might be impacted by seismic shaking; In relation to this, construction standards have been put in place to ensure that structures have the capacity to withstand seismic events (Ainza, 34). Mitigation of Land slide hazards where there is a potential for failure is beyond the level that is acceptable and not preventable through practical means, the hazard needs to be avoided. Developments are required to be built effectively far away from the threat so that they will or can’t be affected if the slope fails. In conclusion, despite it is not possible to mitigate or prevent all geologic hazards, their effects of destructive can be reduced to levels that are acceptable or avoided through appropriate site design and location (California Code of Regulations, 34) Works Cited Abbott, P. L. The rise and fall of San Diego [subtitled] 150 million years of history recorded in sedimentary rocks. Sunbelt Publications, San Diego. pp. i-xxiv + 1-231, illustrated 1999.Print Ainza, Manuel. Associate Structural Engineer, County of San Diego, Department of Planning and Land Use, Departmental communication, 2002.Print American Society of Civil Engineers (ASCE). Civil Engineering Magazine, August 1997.Print Anderson, John G., Thomas K. Rockwell, and Duncan C. Agnew. Past and Possible Future Earthquakes of Significance to the San Diego Region, Earthquake Spectra, Volume V, Number 2, May 1989.Print Bergen, F.W., H.J. Clifford, and S.G. Spear.. Geology of San Diego County, Legacy of the Land. Sunbelt Publications, San Diego. Pp. 1-175, illustrated, 1996.Print Bowman, R.H. Soil survey of San Diego Area, California. USDA. Soil Conserv. Serv., Washington, DC. 1973.Print Bray, J.D. and K.I. Kelson.Observations of surface fault rupture from the 1906 earthquake in the context of current practice. Earthquake Spectra 22(S2):S69-S89, 2006.Print Bryant, W.A. (compiler). Digital Database of Quaternary and Younger Faults from the Fault Activity Map of California, version 2.0: California Geological Survey Web Page. 2005.Print California Code of Regulations Title 14, Division 2, Chapter 8, Article 10. California Department of Conservation California Geological Survey (CGS). Guidelines for Evaluating the Hazard of Surface Fault Rupture – Note 49, 2002.Print Read More