Why do faults reopen

In addition, hundreds of moderate earthquakes magnitude 5. These earthquakes were felt and recorded in personal journals as far away as Louisville, Kentucky and Cincinnati, Ohio. Current knowledge about the faults system within the NMSZ is obtained by studying seismograph recordings of the seismic waves produced by the frequent small earthquakes.

Several organizations such as St. Scientists use this network to record and study small microseismic earthquakes magnitude less than 2. The measured locations of microseismic earthquakes indicate trends that scientist have used to identify active faults. The trends indicate a four-segment, zig-zag fault system with a total length of about miles stretching from east central Arkansas northeastward through Missouri, Tennessee, Kentucky and into southern Illinois. Location of earthquake epicenters in and near the New Madrid Seismic Zone circles scaled according to magnitude.

Some scientists believe the probability of a large earthquake magnitude 7. Due to their infrequency, the recurrence interval of moderate to large earthquakes in the NMSZ is very hard to predict. In addition to seismic waves, scientists are using several other methods of research to better understand the fault system. While scientists cannot predict an earthquake, they can provide a Probabilistic Seismic Hazard Analysis for an area.

By studying the geologic materials of the seismic zone, the stress buildup of an area and the historical record, a probability of exceedance can be calculated. By using paleoseismology techniques scientist are able to recognize buried evidence of large prehistoric earthquakes preserved in geologic materials soil and rock.

The Walnut Creek tracing is located under basin sediment that has for years been deposited from the San Gabriel and surrounding mountains. Damage associated with Earthquakes are dependent on a number of variable factors, the strength or magnitude of the earthquake, the length time of the shaking, the distance from the earthquake, the substrate that is being affected by the quake, and the construction of the structure.

As was demonstrated in the Northridge earthquake, not only was there significant damage in the immediate area, but there was also severe damage in the City of Santa Monica, an area quite removed from the epicenter. This as been attributed to the nature of the ground built on. Depending on which segments, or combination of segments rupture, the damage to the southern California could be moderate to severe. The Long Beach resulted in the Field Act, affecting school construction.

The Sylmar earthquake brought another set of increased structural standards. Similar re-evaluations occurred after the Loma Prieta and Northridge earthquakes. These code changes have resulted in stronger and more earthquake resistant structures. The Alquist-Priolo Earthquake Fault Zoning Act was passed in to mitigate the hazard of surface faulting to structures for human occupancy.

This state law was a direct result of the San Fernando Earthquake, which was associated with extensive surface fault ruptures that damaged numerous homes, commercial buildings, and other structures. Surface rupture is the most easily avoided seismic hazard. The Seismic Hazards Mapping Act, passed in , addresses non-surface fault rupture earthquake hazards, including liquefaction and seismically induced landslides.

The effects of earthquakes span a large area, and large earthquakes occurring in many parts of the Southern California region would probably be felt throughout the region. However, the degree to which the earthquakes are felt, and the damages associated with them may vary.

At risk from earthquake damage are large stocks of old buildings and bridges: many high tech and hazardous materials facilities: extensive sewer, water, and natural gas pipelines; earth dams; petroleum pipelines; and other critical facilities and private property located in the county.

The relative or secondary earthquake hazards, which are liquefaction, ground shaking, amplification, and earthquake-induced landslides, can be just as devastating as the earthquake. The California Geological Survey has identified areas most vulnerable to liquefaction. Liquefaction occurs when ground shaking causes wet granular soils to change from a solid state to a liquid state. Map 5 identifies the areas in the City of West Covina that have soils vulnerable to liquefaction.

Southern California has many active landslide areas, and a large earthquake could trigger accelerated movement in these slide areas, in addition to jarring loose other unknown areas of landslide risk.

Map 5 identifies the areas in the City of West Covina that slopes, if undeveloped, may be susceptible to earthquake induced landslides. Risk analysis is the third phase of a hazard assessment.

Risk analysis involves estimating the damage and costs likely to be experienced in a geographic area over a period of time. Factors included in assessing earthquake risk include population and property distribution in the hazard area, the frequency of earthquake events, landslide susceptibility, buildings, infrastructure, and disaster preparedness of the region. This type of analysis can generate estimates of the damages to the region due to an earthquake event in a specific location.

FEMA's software program, HAZUS, uses mathematical formulas and information about building stock, local geology and the location and size of potential earthquakes, economic data, and other information to estimate losses from a potential earthquake. For greater Southern California there are multiple worst case scenarios, depending on which fault might rupture, and which communities are in proximity to the fault.

But damage will not necessarily be limited to immediately adjoining communities. Depending on the hypocenter of the earthquake, seismic waves may be transmitted through the ground to unsuspecting communities. In the Northridge earthquake, Santa Monica suffered extensive damage, even though there was a range of mountains between it and the origin of the earthquake. Damages for a large earthquake almost anywhere in Southern California are likely to run into the billions of dollars.

Although building codes are some of the most stringent in the world, ten's of thousands of older existing buildings were built under much less rigid codes. California has laws affecting unreinforced masonry buildings URM's and although many building owners have retrofitted their buildings, hundreds of pre buildings still have not been brought up to current standards.

The City of West Covina has one unreinforced masonry buildings. Non-structural bracing of equipment and contents is often the most cost-effective type of seismic mitigation. Inexpensive bracing and anchoring may be the most cost effective way to protect expensive equipment. Non-structural bracing of equipment and furnishings will also reduce the chance of injury for the occupants of a building.

Earthquake damage occurs because humans have built structures that cannot withstand severe shaking. Buildings, airports, schools, and lifelines highways and utility lines suffer damage in earthquakes and can cause death or injury to humans. The welfare of homes, major businesses, and public infrastructure is very important. Addressing the reliability of buildings, critical facilities, and infrastructure, and understanding the potential costs to government, businesses, and individuals as a result of an earthquake, are challenges faced by the city.

Dams There are a total of dams in Los Angeles County, owned by 23 agencies or organizations, ranging from the Federal government to Home Owner Associations. These dams hold billions of gallons of water in reservoirs. Releases of water from the major reservoirs are designed to protect Southern California from floodwaters and to store domestic water.

Seismic activity can compromise the dam structures, and the resultant flooding could cause catastrophic flooding. Following the Sylmar earthquake the Lower Van Norman Dam showed signs of structural compromise, and tens of thousands of persons had to be evacuated until the dam could be drained.

The dam has never been refilled. Buildings The built environment is susceptible to damage from earthquakes. Buildings that collapse can trap and bury people. Lives are at risk and the cost to clean up the damages is great. In most California communities, including the city of West Covina, many buildings were built before when building codes were not as strict.

In addition, retrofitting is not required except under certain conditions and can be expensive. Therefore, the number of buildings at risk remains high. The California Seismic Safety Commission makes annual reports on the progress of the retrofitting of unreinforced masonry buildings. Infrastructure and Communication Residents in the City of West Covina commute frequently by automobiles and public transportation such as buses and rail.

An earthquake can greatly damage bridges and roads, hampering emergency response efforts and the normal movement of people and goods. While this may seem small, earthquakes can happen at any time, and a southern San Andreas Fault earthquake could cause serious damage to Los Angeles and the surrounding region. Therefore, advisories provide a reminder that we live in earthquake country and that being knowledgeable about the hazards that surround us, and preparing ourselves are some of the most important things we can do.

For up-to-date earthquake news, follow Temblor on Facebook , Twitter , and LinkedIn , or sign up to receive email updates. Skip to content. This suggests that the major faults are not optimally aligned, and need additional stress to fail. Additionally, the high seismicity rates along the San Jacinto and Elsinore faults might be due to their more abundant splay faults. Latest Nigeria. Facebook Twitter WhatsApp Telegram.