Historic Seismic Zoning Map for New York State

This seismic zoning map for the State of New York was completed in 1993 as part of the technically approved but not yet legally adopted draft for the seismic provisions of the New York State Building Code (NYSBC). It is stepped along county lines to facilitate code administration. It displays four seismic zones (A, B, C, and D) with associated seismic zone factors Z = 0.09, 0.12, 0.15, and 0.18 respectively. The Z numerically corresponds to effective peak acceleration in g on rock/stiff soil S1 conditions (shear wave velocities of about 2,500 ft/sec).

— from NEHRP Map Collection

Zone A:  Z = 0.09
Zone B:  Z = 0.12
Zone C:  Z = 0.15
Zone D:  Z = 0.18

The NYSBC map is not a probabilistic map but rather represents a consensus of the group responsible for its development. Comparison of this map with recently computed probabilistic ground motion maps indicates that in zones of lowest seismic hazard, it corresponds to 10 percent exceedance probability in 100 years exposure time; in regions of intermediate seismic hazard (Zone C, Z = 0.15), it coincides closely with the 10 percent in 50-year maps; and in Zone D (Z = 0.18), it falls below what is expected from the 50-year maps. This indicates that, in general, this map smoothes out the extreme highs and lows that exist in computed ground motion maps. In short, the NYSBC map does not represent a single uniform hazard level.

NEHRP Recommended Provisions for Seismic Regulations for New Buildings (Reports: FEMA 222A & 223A). Washington D.C.: Building Seismic Safety Council, 1994.

Incorporation of Seismic Considerations in the New York State Building Code

Gergely, Peter. MCEER Bulletin, Volume 7, Number 2, April 1993, page 5.

A draft of seismic design provisions for the New York State Uniform Fire Prevention and Building Code was completed in January 1993. They are based, to a large extent, on seismic provisions for New York City which have been subsequently signed into law. Since this is the first time earthquake-resistant design has been considered in the State, many technical and practical issues had to be resolved. MCEER has been represented by Klaus Jacob and Peter Gergely on the Earthquake Code Advisory Committee.

The most important question was the seismic risk (zone) map for the State, (see next article by Klaus Jacob) together with a major change in the handling of the soil effect. Establishing ground motion and design force levels is complicated because it is difficult to design for a low probability-high consequence event.

In the Eastern United States, the difference between the typical 250-year event and a much longer return period earthquake (say one with a 2,500-year return period) is large. How much of the design should be dictated by the much greater but low probability earthquake? One could guard against collapse due to the large earthquake either by designing for larger forces (greater strength) or by relying on ductility (more complex structural detailing). Only Zone A on the map is classified as a zone of low risk, the rest of the State has moderate risk. Ordinary moment resisting frames are allowed only in Zone A.

One and two-family detached dwellings, which are not more than two stories high above the basement, need not be designed for earthquakes. However, it is likely that the proposed federal insurance bill will require minor "soft" mitigation, such as fastening of water heaters and tieing walls to roofs and foundations.

The draft proposal is based on the Uniform Building Code, but there are numerous exceptions. Simple rules were introduced for required building separation. For irregular or tall structures, dynamic force analysis should be considered, but it is not required.

NYS Seismic Vulnerability: Code Implications for Buildings, Bridges, and Municipal Facilities

Jacob, Klaus. MCEER Bulletin, Volume 7, Number 2, April 1993, pages 4-5.

Most regions of New York State are characterized by a moderate level of seismicity and seismic hazard. The highest levels of seismicity are concentrated in the northern Adirondacks, the New York City Metropolitan area, and Western New York, including Attica and Buffalo. Since 1886, at least four earthquakes with Richter Magnitude M > 5 have occurred in these areas, and numerous smaller, but widely felt earthquakes have occurred throughout the State.

Earthquakes with magnitudes M > 6 are possible, although none are documented in the short historic record. These more significant earthquakes are expected to be rare (about once every few hundred years). On the other hand, in highly populated areas like New York City or Buffalo, multi- billion dollar losses can be expected from single earthquakes with magnitudes of about M >5.5 to 6.

To reduce the earthquake risk to New York State, a variety of seismic hazard reduction measures are about to become effective in the near future. They concern seismic codes and regulations in the following areas of public interest:

  • Seismic Building Codes
  • Seismic Design Guidelines for New highway bridges
  • Seismic Retrofit Guidelines for Existing Bridges
  • EPA Guidelines for for Municipal Solid Waste Landfills (MSWLF's)

Seismic Building Codes have been drafted for both New York State and New York City. Both are quite similar and represent essentially partly modified versions of the Uniform Building Code (UBC). New York State is divided into four seismic zones: A, B, C, D, with seismic zone factors of Z = 0.09, 0.12, 0.15 and 0.18, respectively (measuring effective peak acceleration in fractions of g, where g equals the earth's gravity acceleration).

The building code seismic hazard map for New York State uses these four zone factors, which are based on an exceedance probability of 10% in about 100 years (other codes usually use 50 years). Five seismic soil-type factors, S0 = 2/3, S1 = 1.0, S2 = 1.2, S3 = 1.5, and S4 = 2.5, respectively, modify the reference design spectrum (defined for S1 conditions) according to local geological site conditions ranging from very hard rocks (S0) to very soft soils (S4). A soil liquefaction screening procedure is also included in the code.

Regional Seismic Breakdown
Seismic Zone Factor Z Z as % of g Seismic Soil Type Factor
A 0.09 S0 — 2/3
B 0.12 S1 — 1.0
C 0.15 S2 — 1.2
D 0.18 S3 — 1.5
S4 — 2.5

The New York State Department of Transportation (NYSDOT) has issued an internal Engineering Instruction (dated 3128/90) requiring all new highway bridges in New York State to be designed for seismic forces assuming a statewide seismic zone factor of Z = 0.19; otherwise, the instruction essentially follows the AASHTO Standard Specifications. An Engineering Instruction for seismic rehabilitation design for existing bridge structures has been issued more recently (10/14/92). Priority and level of seismic retrofit measures envisioned under this Instruction depend on functional importance of the bridge and other factors.

On October 9, 1991, the Federal EPA issued a Revised Minimum Federal Criteria for Municipal Solid Waste Landfills (MSWLF's). Notably, among other site limitations, there are at least three geologic/earthquake-related guidelines included. These are:

  • Sites for new MSWLF's should not be located closer than 200 feet from faults that have undergone Holocene movement.
  • For the seismic design, new MSWLF's seismic zone factors for peak effective horizontal acceleration shall be used that are based on maps with an exceedance probability of 10% in 250 years. This may imply a zone factor of about Z = 0.38 at a few of the most seismically exposed sites in New York State.
  • Site stability with respect to foundations and resistance to differential settling and mass movement must be demonstrated for new MSWLF's effective 10/3/93, and for existing MSWLF's effective 10/9/96.

While not specifically mentioned, this should include the demonstration of the stability of MSWLF sites with respect to soil liquefaction effects under seismic conditions by engineering studies.

The introduction of these seismic provisions into land use planning, engineering, design and construction practice will need the joint attention of the practicing engineering, geotechnical and seismological community, and of regulatory bodies and decision makers, especially where site-specific studies are needed or may lead to more economic solutions.