HAZARD IDENTIFICATION

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AL A M O AR E A CO U N C I L O F GO V E R N M E N T S

RE G I O N A L MI T I G A T I O N AC T I O N PL A N UP D A T E

6.5.10 Geologic Hazards: Earthquakes and Sinkholes

6.5.10.1 Hazard Identifications

Descriptions of the Earthquake and Sinkhole Hazards Earthquakes

An earthquake is the motion or trembling of the ground produced by sudden displacement of rock in the Earth's crust. Earthquakes result from crustal strain, volcanism, landslides, or the collapse of caverns. Earthquakes can affect hundreds of thousands of square miles; cause damage to property measured in the tens of billions of dollars; result in loss of life and injury to hundreds of thousands of persons; and disrupt the social and economic functioning of the affected area.

Most earthquakes are caused by the release of stresses accumulated as a result of the rupture of rocks along opposing fault planes in the Earth’s outer crust. These fault planes are typically found along borders of the Earth's ten tectonic plates. These plate borders generally follow the outlines of the continents, with the North American plate following the continental border with the Pacific Ocean in the west, but following the mid-Atlantic trench in the east. As earthquakes occurring in the mid-Atlantic trench usually pose little danger to humans, the greatest earthquake threat in North America is along the Pacific Coast.

Sinkholes

Sinkholes are a natural and common geologic feature in areas with underlying limestone and other rock types that are soluble in natural water. Most limestone is porous, allowing the acidic water of rain to percolate through their strata, dissolving some limestone and carrying it away in solution. Over time, this persistent erosional process can create extensive underground voids and drainage systems in much of the carbonate rocks. Collapse of overlying sediments into the underground cavities produces sinkholes.

The three general types of sinkholes are: subsidence, solution, and collapse. Collapse sinkholes are most common in areas where the overburden (the sediments and water contained in the unsaturated zone, surficial aquifer system, and the confining layer above an aquifer) is thick, but the confining layer is breached or absent. Collapse sinkholes can form with little warning and leave behind a deep, steep sided hole. Subsidence sinkholes form gradually where the overburden is thin and only a veneer of sediments is overlying the limestone. Solution sinkholes form where no overburden is present and the limestone is exposed at land surface.

Severity of the Earthquake and Sinkhole Hazards Earthquakes

Earthquakes are measured in terms of their magnitude and intensity. Magnitude is measured using the Richter Scale, an open-ended logarithmic scale that describes the energy release of an earthquake through a measure of

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the Modified Mercalli Intensity Scale of earthquake intensity and its correspondence to the Richter Scale is given in Table 6.5.10.1-2.

Table 6.5.10.1-1 Richter Scale

Richter Magnitude Earthquake Effects

Less than 3.5 Generally not felt, but recorded. 3.5-5.4 Often felt, but rarely causes damage.

Under 6.0 At most slight damage to well-designed buildings. Can cause major _{damage to poorly constructed buildings over small regions.} 6.1-6.9 Can be destructive in areas up to about 100 kilometers across where _{people live.} 7.0-7.9 Major earthquake. Can cause serious damage over larger areas. 8 or greater Great earthquake. Can cause serious damage in areas several hundred _{kilometers across.}

Table 6.5.10.1-2

Modified Mercalli Intensity Scale for Earthquakes

Scale Intensity Description of Effects Corresponding Richter Scale 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 VIII Destructive Moving cars uncontrollable; masonry fractures, poorly _{constructed buildings damaged}

IX Ruinous Some houses collapse; ground cracks; pipes break _open <6.9 X Disastrous Ground cracks profusely; many buildings destroyed; _{liquefaction and landslides widespread} <7.3 XI Very Disastrous Most buildings and bridges collapse; roads, railways, pipes and cables destroyed; general triggering of

other hazards <8.1

XII Catastrophic Total destruction; trees fall; ground rises and falls in _waves >8.1

Source: North Carolina Division of Emergency Management.

As is discussed further in this section, the majority of occurrences of the earthquake hazard in the planning area were limited to a III or IV on the Mercalli Scale, indicating that they were slight or moderate, and were generally felt by

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people either at rest or engaged in walking. One incident was recorded as a V, which is strong enough to ring church bells and wake sleepers.

Based on the historical occurrences and the distance from identified fault lines, the planning area should expect to continue to experience the same range on the Mercalli Scale – III or IV, with the occasional V event.

Sinkholes

Unlike other geologic hazards, no scale currently exists for measuring or determining the severity of sinkholes. However, it is possible to determine where sinkholes are possible to occur, based on the structure and condition of the earth in the area. Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by ground water circulating through them. As the rock dissolves, spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces get too big. If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur. These collapses can be small or they can be large, and they can occur under a house or road.

The figure below illustrates the formation of a typical sinkhole.

Figure 6.5.10.1-1 Typical Sinkhole Formation

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A significant number of sinkholes tend to occur in the years that follow a drought. When an area has a long-term lack of rain and water levels decrease, there’s usually a correlated link to an increase in incidences of sinkholes being reported. Historically, years where dry weather has been followed by wet weather have resulted in some of the greatest increases in sinkhole occurrences. Given that Texas is prone to periodic droughts – some of which can be exception - sinkholes are a geologic feature that can be expected to occur in the planning area. The planning area can expect to experience sinkholes from the very small to the very large, with the potential for significant property damage.

Impacts to People and Property from the Earthquake and Sinkhole Hazards Earthquakes

Most property damage and earthquake-related deaths are caused by the failure and collapse of structures due to ground shaking. The level of damage depends upon the amplitude and duration of the shaking, which are directly related to the earthquake size, distance from the fault, site and regional geology. Other damaging earthquake effects include landslides, the down-slope movement of soil and rock (mountain regions and along hillsides), and liquefaction, in which ground soil loses the ability to resist shear and flows much like quick sand. In the case of liquefaction, anything relying on the substrata for support can shift, tilt, rupture, or collapse.

Specific, local data is required to fully assess the potential impacts of earthquakes to the planning area. See Section 8 (Mitigation Action Plan) for action to address data deficiency.

Sinkholes

Sinkholes occur in many shapes, from steep-walled holes to bowl or cone shaped depressions. Sinkholes are dramatic because the land generally stays intact for a while until the underground spaces get too big. If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur. Under natural conditions, sinkholes form slowly and expand gradually. However, human activities such as dredging, constructing reservoirs, diverting surface water, and pumping groundwater can accelerate the rate of sinkhole expansions, resulting in the abrupt formation of collapse sinkholes.

Although a sinkhole can form without warning, specific signs can signal potential development:  Slumping or falling fence posts, trees, or foundations;

 Sudden formation of small ponds;  Wilting vegetation;

 Discolored well water; and/or  Structural cracks in walls, floors.

Sinkhole formation is aggravated and accelerated by urbanization. Development increases water usage, alters drainage pathways, overloads the ground surface, and redistributes soil. According to the Federal Emergency Management Agency (FEMA), the number of human-induced sinkholes has doubled since 1930, insurance claims for damages as a result of sinkholes has increased 1,200 percent from 1987 to 1991, costing nearly $100 million. Specific, local data is required to fully assess the potential impacts of sinkholes to the planning area. See Section 8 (Mitigation Action Plan) for action to address data deficiency.

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Occurrences of the Earthquake and Sinkhole Hazards Earthquakes

Though earthquakes are not likely to affect Texas, their effects have been felt throughout history. Table 6.5.10.1-3 lists recorded earthquakes in the planning area.

Because of the limited impact of sinkholes on the region, past occurrences and their probability have not been mapped.

Table 6.5.10.1-3

Recorded Earthquakes in the Planning Area Date of

Occurrence

Modified Mercalli Intensity

(if known) Location County

Miles from Epicenter

08/16/1931 III Fredericksburg Gillespie 512

08/16/1931 IV Hondo Medina 506

08/16/1931 III Jourdanton Atascosa 576

08/16/1931 V Karnes City Karnes 637

08/16/1931 IV New Braunfels Comal 593

08/16/1931 III Pearsall Frio 528

08/16/1931 IV San Antonio Bexar 564

06/20/1936 unknown San Antonio Bexar 718

07/23/1983 III Schertz Guadalupe 91

07/23/1983 III Campbellton Atascosa 16

03/03/1984 IV Christine Atascosa 7

03/03/1984 V Jourdanton Atascosa 11

03/03/1984 IV Pleasanton Atascosa 11

03/03/1984 IV Poteet Atascosa 24

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Figure 6.5.10.1-3

Areas of the US Prone to Sinkhole Development

Source: US Geological Survey

Based on historic occurrences, the above image, and the ground conditions and periodic drought cycles that occur in the planning area, and using the established criteria described in the introduction to this section, the probability of a future occurrence of the sinkhole hazard is moderate.

Location and Extent of the Earthquake and Sinkhole Hazards Sinkholes

Sinkholes are of interest to Central and Western Texas because they are one of the predominant landform features of those areas of the state. Their development may be sudden and may result in property damage or loss of life. Sinkholes can vary greatly in both diameter and depth. Recorded occurrences have ranged from a few feet to more than 2,000 feet, both in depth and diameter. Portions of Gillespie, Kerr, Bandera, Medina, Frio, Comal, Bexar and Atascosa counties are located in karst regions, which are susceptible to cave and sinkhole development due to the geology and geomorphology of the area. However, limited property damage or loss of life due to sinkholes in these areas has been recorded. Given the geologic composition of the area, the planning area can expect to experience a variety of sinkholes, in varying depths and diameters.

At the time this plan update was developed there was no known source that could be used to map past occurrences, or to determine extent or probability of sinkholes. More information may be included in the five-year update of this plan. See the MAP in Section 9 for mitigation actions included to address this data deficiency.

Earthquakes

The areas of greatest tectonic instability occur at the perimeters of the slowly moving plates, as these locations are subjected to the greatest strains from plates traveling in opposite directions and at different speeds. Deformation

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6.5.10.2 Risk Assessment

This subsection of the Plan Updates provides estimates of future losses from these geologic hazards, i.e. risk. Each of the loss calculations is based on best available data, but they must be considered estimates because highly detailed engineering was not performed as part of this planning process.

Methodology and Limitations

After discussion and review of the best available data regarding this hazard, and in consideration of the potential impacts of this hazard to the planning area, the EMC determined that the risk assessment should include both qualitative and quantitative analyses, to the extent possible, but that the quantitative analysis should be limited to earthquakes, which have a more definable hazard area.

Discussion indicated that while sinkholes are a concern and are a threat to the planning area, their area of impact relative to the planning are as a whole made them less of a concern, as did the lack of options to effectively mitigate the hazard. It was determined that, given the lack of ability to define specific areas of occurrence, no quantitative assessment would be performed.

Qualitative

Each participating jurisdiction was asked to provide a qualitative risk assessment ranking regarding the earthquake and sinkhole hazards. (For definitions of these rankings, please see Table 6.4-1, earlier in this section.) The results of this assessment for earthquake are presented in the table below.

Table 6.5.10.2-1

Qualitative Risk Assessment Results – Geologic Hazards: Earthquakes

Jurisdictions Ranking Hazards as Low _{Hazards as Moderate}Jurisdictions Ranking Jurisdictions Ranking _{Hazards as High} Bandera County

City of Bandera Bexar County

City of Alamo Heights City of Balcones Heights City of Converse

City of Helotes City of Kirby City of Leon Valley City of Live Oak City of St. Hedwig City of Somerset City of Terrell Hills

Gillespie County City of Fredericksburg Guadalupe County City of New Berlin City of Schertz City of Seguin Karnes County City of Karnes City of Kenedy City of Runge City of Falls City Kerr County City of Ingram

City of San Antonio Atascosa County City of Charlotte City of Christine City of Jourdanton City of Lytle City of Pleasanton City of Poteet City of Pearsall

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Jurisdictions Ranking Hazards as Low _{Hazards as Moderate}Jurisdictions Ranking Jurisdictions Ranking _{Hazards as High} Frio County

City of Dilley City of La Vernia City of Stockdale

San Antonio River Authority

The results of this assessment for sinkholes are presented in the table below.

Table 6.5.10.2-2

Qualitative Risk Assessment Results – Geologic Hazards: Sinkholes

Jurisdictions Ranking Hazards as Low _{Hazards as Moderate}Jurisdictions Ranking Jurisdictions Ranking _{Hazards as High} Bandera County

City of Bandera Bexar County

City of Alamo Heights City of Balcones Heights City of Converse

City of Helotes City of Kirby City of Leon Valley City of Live Oak City of St. Hedwig City of Somerset City of Terrell Hills City of Universal City City of Von Ormy City of Windcrest Comal County City of Garden Ridge City of Bulverde City of New Braunfels Frio County

City of Dilley

Gillespie County City of Fredericksburg Guadalupe County City of New Berlin City of Schertz City of Seguin Karnes County City of Karnes City of Kenedy City of Runge City of Falls City Kerr County City of Ingram City of Kerrville Medina County City of Castroville City of Devine City of Hondo City of Natalia Wilson County City of La Vernia City of Stockdale

San Antonio River Authority

City of San Antonio Atascosa County City of Charlotte City of Christine City of Jourdanton City of Lytle City of Pleasanton City of Poteet City of Pearsall Quantitative

Each participating jurisdiction provided information regarding the location and type of critical facilities within their planning area. These facilities were mapped in relation to the Texas Earthquake Risk Zones provided by the Texas Hazard Mitigation Package (THMP) as shown in Figure 6.5.10.2-1. Relative risk areas defined by ground

acceleration associated with earthquakes are categorized into 12 classes using the “natural breaks” method. This method defines classes based on natural groupings of data values; break points are identified by looking for patterns inherent in the data; then ranges are set where there are relatively big jumps in the data values. Given the lack of building specific data (only location and use type were available at this time) this analysis assumes that all facilities located within the Earthquake Risk Zones are potentially at risk from seismic activity. Table 6.5.10.2-3 lists the facilities at risk from earthquakes by jurisdiction.

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Figure 6.5.10.2-1

Texas Earthquake Risk Zones for the AACOG Planning Area

Source: Texas Hazard Mitigation Package Website

Table 6.5.10.2-3

Quantitative Risk Assessment Results – Earthquake

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Jurisdiction Facilities in Zone ₂ Facilities in Zone ₄ Facilities in Zone ₆ Facilities in Zone ₈ 15 – Water

Supply 20 – Schools 11 – Water Supply

Given the lack of building specific data (only location and use type were available at this time) this analysis assumes that all facilities located within the planning area are potentially at risk from seismic activity. Table 6.5.10.2-4 lists the facilities at risk by jurisdiction.

Table 6.5.10.2-4

Quantitative Risk Assessment Results – Sinkhole

Jurisdiction Emergency _Services Residential / _{Heavily Occupied Utilities} Transportation / _Other Atascosa 10 – Fire Station _{2 – Police Station} 9 – Nursing Home _{21 – Schools}

1 – Hospital 1 – Courthouse 1 – Power Plant 14 – Water Supply 6 – Airport Bandera 3 – Emergency Response 9 – Fire Departments 2 – Police Departments 3 – Hospitals 1 – Nursing Home 8 – Schools 4 – Daycares 7 – Communications 2 – Power Supply 1 – Water Supply 1 – City Hall 2 – Courthouses 5 – Courts 26 – Other 1 – Transportation Bexar 2 – Emergency Response 70 – Fire Department 19 – Fire Stations 21 – Police Department 27 – Police Stations 68 – Hospital 168 – Nursing Homes 521 – Schools 5 – Stadiums 41 – Day Cares 46 – Communications 20 – Power Supply 1 – Power Plant 17 – Public Works 74 – Water Supply 23 – Airport 11 – City Halls 7 – Corrections 3 – Courthouse 5 – Military 27 – Other 5 –Town Halls 2 – Transportation Comal 5 – Fire Stations

1 – Hospital 11 – Nursing Homes 38 – Schools 1 – Communications 1 – Power Plant 12 – Water Supply 5 – Airport 1 – Courthouse Frio 3 – Fire Stations 3 – Police Stations 3 – Hospitals 3 – Nursing Homes 7 – Schools 2 – Communications 1 – Power Plant 4 – Water Supply 5 – Airport 1 – Courthouse

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Jurisdiction Emergency _Services Residential / _{Heavily Occupied Utilities} Transportation / _Other

Guadalupe 13 – Fire Stations 2 – Police Stations 1 – Hospital 11 – Nursing Homes 36 – Schools 1 – Airport 1 – Water Supply 3 – Communications 4 – Power Plants 11 – Water Supply 13 – Airport 1 – Courthouse Karnes 2 – Emergency Response 1 – Fire Department 5 – Fire Stations 3 – Police Departments 1 – Police Station 3 – Hospitals 12 - Nursing Homes 15 – Schools 1 – Courthouse

5 – Water Supply 6 – Airport 3 – City Halls 5 – Daycares 11 – Other 6 – Unclassified Kerr 5 – Fire Departments 8 – Fire Stations Homes 3 – Police Departments 1 – Police Station 9 – Daycares 5 – Hospitals 11 – Nursing Homes 79 – Schools 6 – Communications 2 – Fuel Supply 3 – Power Supply 3 – Water Supply 10 – Airport 1 – City Hall 1 – Courthouse 4 – Other 4 – Transportation Medina 6 – Fire Stations 3 – Police Stations 1 – Hospital 6 – Nursing Homes 22 – Schools 1 – Communication 5 – Water Supply 7 – Airport 1 – Courthouse Wilson 3 – Fire Departments 8 – Fire Stations 3 – Police Departments 20 – Day Cares 2 – Hospitals 5 – Nursing Homes 32 – Schools 10 – Communications 5 – Fuel Distributors 2 – Power Supply 26 – Water Supply 8 – Airports 4 – City Halls 1 – Courthouse 7 – Other

All critical facilities in the planning area are at risk from geologic hazards; however, it is not anticipated that many would actually be affected by geologic hazards, under the present conditions.

Data to collect in order to improve this methodology prior to the Plan Update includes: Data regarding building construction (materials, protection systems, etc.)

Risk Assessment Conclusions

The geology of the planning area makes it vulnerable to sinkhole formation. However, when sinkholes do occur they tend to be relatively contained and small in size, in relation to the planning area, which limits the potential for damage from this hazard.

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A significant number of critical facilities within each jurisdiction fall within the various Earthquake Zones. Jurisdictions should consider an evaluation of these facilities to determine if mitigation actions are needed.