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City of Pleasanton—Hidden Canyon Residences and Preserve Draft EIR Geology, Soils, and Seismicity FirstCarbon Solutions 3.6-5 https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx Surface Rupture Surface rupture occurs when strong shaking generated from an earthquake causes movement along an active fault (one that has ruptured in the past 11,000 years) to break through, or cause offset directly on the earth’s surface. Fault lines capable of producing surface rupture can either trigger vertical offsets which are typically associated with normal and reverse faulting, or lateral offsets usually generated from strike-slip faulting. In addition, surface rupture can happen either rapidly during strong seismic shaking, or gradually from aseismic creep which can offset structures and foundations over long periods of time. Because of surface rupture posing greater threats to structures located directly, or in proximity to active fault lines susceptible to producing surface rupture, the State of California established regulatory zones surrounding active fault lines under the Alquist-Priolo Earthquake Fault Zoning Act of 1972 to prevent the construction of structures within approximately 50 feet of active fault lines. The San Francisco Bay Area has approximately 13 fault zones that are classified as Special Study Zones under the Alquist-Priolo Earthquake Fault Zoning Act that pose the risk of rupturing during strong seismic shaking. Rock units on the project site are transected by northwest trending faults with one subsidiary fault within the Devany Creek channel south of the project site. However, the fault lines that transect the project site cut through Tertiary geologic units and do not offset Quaternary age geologic units. As such, these faults are considered inactive and are not delineated within an Alquist-Priolo Earthquake Fault Zone.10 The closest active fault line capable of producing surface rupture is the Calaveras Fault which strikes in a northwest direction approximately 0.7 mile east of the project site. Seismicity and Faulting Seismic ground shaking is caused by the sudden movement along active fault lines resulting in the release of stored energy in the form of seismic waves. The intensity of ground shaking is influenced by many factors such as the type of local geology, distance from the epicenter, direction of rupture, and depth of the hypocenter. Seismic ground shaking is calculated either using the Richter Scale which calculates the amplitude of the largest wave and the epicenter distance using a logarithmic scale, and the Modified Mercalli Intensity Scale (MMI) which is arbitrary based on observations from earthquake intensity on a scale from 1-12 designated by roman numerals. The project site is located in the San Francisco Bay Area which is a seismically active region with major fault lines and numerous lesser-known faults. Many of the fault lines are right lateral strike- slip faults that strike in a northwest direction and are associated with the San Andreas Fault Zone which separates the Pacific Plate and the North American Plate. The project site is situated on inactive fault lines that have folded the local geology which are incapable of generating strong seismic shaking. However, the ABAG Resilience Program rates shaking severity according to the MMI. The ABAG places the site of the proposed project within the Very Strong–MMI VIII shaking severity zone for the Calaveras Fault.11 An MMI between six and 10 is considered high intensity and capable of damaging buildings. The closest major active fault lines to the project site are the Calaveras Fault 10 Ninyo & Moore. 2025. Update Geotechnical Review Hidden Canyon Residences and Preserve, Dublin Canyon Road, Pleasanton, California. February 12, 2025. 11 Association of Bay Area Governments (ABAG). MTC/ABAG Hazard Viewer Map. Website: https://mtc.maps.arcgis.com/apps/webappviewer/index.html?id=4a6f3f1259df42eab29b35dfcd086fc8. Accessed May 7, 2025.