4.3.2 Earthquake
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SECTION 4.3.2: RISK ASSESSMENT – EARTHQUAKE 4.3.2 Earthquake This section provides a profile and vulnerability assessment for the earthquake hazard. According to the Pennsylvania Bureau of Topographic and Geologic Survey, the State is relatively free of earthquake activity compared to other states; however, earthquakes do occur. Pennsylvania has experienced fewer and milder earthquake events than most other eastern states (LVPC, 2006). An earthquake is the sudden movement of the Earth’s surface caused by the release of stress accumulated within or along the edge of the Earth’s tectonic plates, a volcanic eruption, or by a manmade explosion (Federal Emergency Management Agency [FEMA], 2010). Most earthquakes occur at the boundaries where the Earth’s tectonic plates meet (faults); however, less than 10 percent of earthquakes occur within plate interiors. The Lehigh Valley is in an area where plate interior-related earthquakes occur. The U.S. Geological Survey (USGS) Earthquake Hazards Program defines an earthquake hazard as anything associated with an earthquake that may affect resident’s normal activities. 4.3.2.1 Location and Extent The location of an earthquake is commonly described by its focal depth and the geographic position of its epicenter. The focal depth of an earthquake is the depth from the Earth’s surface to the region where an earthquake’s energy originates (the focus or hypocenter). The epicenter of an earthquake is the point on the Earth’s surface directly above the hypocenter. Earthquakes usually occur without warning and their effects can impact areas at great distance from the epicenter. According to the Pennsylvania Bureau of Topographic and Geologic Survey, when events occur in Pennsylvania, their impact area is very small (less than 100 kilometers in diameter). The most seismically active region in the state is in southeastern Pennsylvania in the area of Lancaster County (PEMA, 2010). Areas of Pennsylvania, including the Lehigh Valley, may be subject to the effects of earthquakes with epicenters outside the state. Pennsylvania has three earthquake hazard area zones (very slight, slight and moderate) as shown in Figure 4.3.2-1 (PEMA, 2010). The Lehigh Valley falls into the “moderate” zone, along with other municipalities and counties located within 17.5 miles from a historical epicenter. In this zone, minor earthquake damage is expected. DMA 2000 Hazard Mitigation Plan Update – Lehigh Valley, Pennsylvania 4.3.2-1 March 2013 SECTION 4.3.2: RISK ASSESSMENT – EARTHQUAKE Figure 4.3.2-1: Pennsylvania Earthquake Hazard Zones Source: PEMA, 2010 Note: The yellow highlight illustrates the location of the Lehigh Valley. 4.3.2.2 Range in Magnitude Earthquake magnitude is often measured using the Richter Scale, an open-ended logarithmic scale that describes the energy release of an earthquake. Table 4.3.2-1 summarizes the Richter Scale magnitudes and corresponding earthquake effects. Based on historical events, earthquakes in Pennsylvania do not exceed magnitudes greater than 6.0, however, the possibility of higher magnitude earthquakes cannot be ruled out. The worst-case earthquake in the Lehigh Valley would likely result in trees swaying, objects falling off walls, cracked walls and falling plaster. Table 4.3.2-1: Richter Scale Magnitudes Richter Magnitude Earthquake Effects 2.5 or less Usually not felt, but can be recorded by seismograph 2.5 to 5.4 Often felt, but only causes minor damage 5.5 to 6.0 Slight damage to buildings and other structures 6.1 to 6.9 May cause a lot of damage in very populated areas 7.0 to 7.9 Major earthquake; serious damage 8.0 or greater Great earthquake; can totally destroy communities near the epicenter Source: USGS, 2006 DMA 2000 Hazard Mitigation Plan – Lehigh Valley, Pennsylvania 4.3.2-2 March 2013 SECTION 4.3.2: RISK ASSESSMENT – EARTHQUAKE The impact an earthquake event has on an area is typically measured in terms of earthquake intensity. Intensity is most commonly measured using the Modified Mercalli Intensity (MMI) Scale based on direct and indirect measurements of seismic effects. A detailed description of the MMI Scale is shown in Table 4.3.2-2. The earthquakes that occur in Pennsylvania originate deep within the earth’s crust, not on an active fault. No injury or severe damage from earthquake events has been reported in the Lehigh Valley. Table 4.3.2-2: Modified Mercalli Intensity Scale with Associated Impacts Corresponding Richter Scale Scale Intensity Description Of Effects Magnitude I Instrumental Detected only on seismographs II Feeble Some people feel it <4.2 III Slight Felt by people resting; like a truck rumbling by IV Moderate Felt by people walking V Slightly Strong Sleepers awake; church bells ring <4.8 VI Strong Trees sway; suspended objects swing; objects fall off shelves <5.4 VII Very Strong Mild alarm, walls crack, plaster falls <6.1 Moving cars uncontrollable, masonry fractures, poorly constructed VIII Destructive buildings damaged <6.9 IX Ruinous Some houses collapse, ground cracks, pipes break open Ground cracks profusely, many buildings destroyed, liquefaction and X Disastrous <7.3 landslides widespread Most buildings and bridges collapse; roads, railways, pipes, and XI Very Disastrous <8.1 cables destroyed; general triggering of other hazards XII Catastrophic Total destruction, trees fall, ground rises and falls in waves >8.1 Source: PEMA, 2012 Environmental impacts of earthquakes can be numerous, widespread, and devastating, particularly if indirect impacts are taken into account. Some examples are shown below but are unlikely to occur in the Lehigh Valley: Induced tsunamis and flooding or landslides and avalanches Poor water quality Damage to vegetation Breakage in sewage or toxic material containments Secondary impacts including: train derailments and spillage of hazardous materials and utility interruption Seismic hazards are often expressed in terms of Peak Ground Acceleration (PGA) and Spectral Acceleration (SA). USGS defines PGA and SA as the following: ‘PGA is what is experienced by a particle on the ground. Spectral Acceleration (SA) is approximately what is experienced by a building, as modeled by a particle mass on a massless vertical rod having the same natural period of vibration as the building’ (USGS, Date Unknown). Both PGA and SA can be measured in g (the acceleration due to gravity) or expressed as a percent acceleration force of gravity (%g). PGA and SA hazard maps provide insight into location specific vulnerabilities (NYSDPC, 2011). DMA 2000 Hazard Mitigation Plan – Lehigh Valley, Pennsylvania 4.3.2-3 March 2013 SECTION 4.3.2: RISK ASSESSMENT – EARTHQUAKE PGA is a common earthquake measurement that shows three things: the geographic area affected, the probability of an earthquake of each given level of severity, and the strength of ground movement (severity) expressed in terms of percent of acceleration force of gravity (%g). In other words, PGA expresses the severity of an earthquake and is a measure of how hard the earth shakes (or accelerates) in a given geographic area (NYSDPC, 2011). National maps of earthquake shaking hazards have been produced since 1948. They provide information essential to creating and updating the seismic design requirements for building codes, insurance rate structures, earthquake loss studies, retrofit priorities and land use planning used in the U.S. Scientists frequently revise these maps to reflect new information and knowledge. Buildings, bridges, highways and utilities built to meet modern seismic design requirements are typically able to withstand earthquakes better, with less damages and disruption. After thorough review of the studies, professional organizations of engineers update the seismic-risk maps and seismic design requirements contained in building codes (Brown et al., 2001). A probabilistic assessment was conducted for the 100-, 500- and 2,500-year mean return periods (MRP) through a Level 1 analysis in HAZUS-MH version 2.1 to analyze the earthquake hazard for the Lehigh Valley. The HAZUS analysis evaluates the statistical likelihood that a specific event will occur and what consequences will occur. A 100-year MRP event is an earthquake with a 1% chance that the mapped ground motion levels (PGA) will be exceeded in any given year. For a 500-year MRP, there is a 0.2% chance the mapped PGA will be exceeded in any given year. For a 2,500-year MRP, there is a 0.04% chance the mapped PGA will be exceeded in any given year. Figures 4.3.2-2 through 4.3.2-4 illustrate the geographic distribution of PGA (%g) across the Lehigh Valley for the 100-, 500- and 2,500-year MRP events. A discussion of the estimated potential losses estimated by HAZUS-MH for each MRP and the associated PGA is discussed in the ‘Vulnerability Assessment’ subsection below. DMA 2000 Hazard Mitigation Plan – Lehigh Valley, Pennsylvania 4.3.2-4 March 2013 SECTION 4.3.2: RISK ASSESSMENT – EARTHQUAKE Figure 4.3.2-2. Peak Ground Acceleration Modified Mercalli Scale in the Lehigh Valley for a 100-Year MRP Earthquake Event Source: HAZUS-MH 2.1 Note: The peak ground acceleration for the 100-year MRP is 1.12 – 1.18 %g. DMA 2000 Hazard Mitigation Plan – Lehigh Valley, Pennsylvania 4.3.2-5 March 2013 SECTION 4.3.2: RISK ASSESSMENT – EARTHQUAKE Figure 4.3.2-3. Peak Ground Acceleration Modified Mercalli Scale in the Lehigh Valley for a 500-Year MRP Earthquake Event Source: HAZUS-MH 2.1 Note: The peak ground acceleration for the 500-year MRP is 4.64 to 5.44%g. DMA 2000 Hazard Mitigation Plan – Lehigh Valley, Pennsylvania 4.3.2-6 March 2013 SECTION 4.3.2: RISK ASSESSMENT – EARTHQUAKE Figure 4.3.2-4. Peak Ground Acceleration Modified Mercalli Scale in the Lehigh Valley for a 2,500-Year MRP Earthquake Event Source: HAZUS 2.1 Note: The peak ground acceleration for the 2,500-year MRP is15.68 to 18.06 %g.