HAZDist

HAZDist is an R wrapper around the fault-distance routines from Norman Abrahamson's HAZ Fortran program. It computes the source-to-site distance metrics required by Ground Motion Prediction Equations (GMPEs) for finite-fault seismic hazard analysis.

繁體中文說明請見 README.zh-TW.md

Features

Function	Description
getfaultcoord()	Extract fault geometry (top/bottom coordinates) from an sf object
getfaultdist()	High-level wrapper — given fault and site coordinates, returns all distance metrics
SetFltBottom()	Compute fault bottom coordinates from dip angle and down-dip width
ConvertCoordinates2()	Convert geographic coordinates (lon/lat) to a fault-strike local Cartesian system
calcfltgrid()	Build a finite-element fault grid for distance calculations
CalcDist()	Call the HAZ Fortran core to compute Rjb, Rrup, ZTOR, Rx, Ry, Ry0, etc.

System Requirements

• R ≥ 4.1.0
• Fortran compiler — gfortran must be available when installing from source
  • macOS: brew install gcc via Homebrew
  • Windows: install Rtools
  • Linux: sudo apt install gfortran (Debian/Ubuntu)
• GNU make
• R package dependency: sf

Installation

# Install the sf dependency first
install.packages("sf")

# Install HAZDist from GitHub
remotes::install_github("personlin/HAZDist")
# or using pak
pak::pak("personlin/HAZDist")

Quick Start

The package ships with FT, an example sf dataset of the Shanchiao Fault (山腳斷層) in Taiwan.

library(HAZDist)

# Load the built-in fault dataset (sf object)
data(FT)

# Step 1: Extract geometry for fault segment 1
fault <- getfaultcoord(
  faultlines    = FT,
  id            = 1,
  c.id          = "ID",
  c.fault.top   = "DEPTH1_KM",   # depth to fault top (km)
  c.fault.thick = "WIDTH_KM",    # down-dip width (km)
  c.dip         = "FAULT_DIP1"   # dip angle (degrees)
)

# Step 2: Define the site location (WGS84 lon/lat)
site <- data.frame(x = 121.001, y = 24.7794)

# Step 3: Compute all distance metrics
result <- getfaultdist(fault, site, seismoDepth = 0)
print(result)

Output fields from getfaultdist()

Field	Description
distJB	Joyner–Boore distance Rjb (km)
distRup	Closest distance to the rupture surface Rrup (km)
Rx	Horizontal distance perpendicular to strike (km); positive on hanging-wall side
Ry / Ry0	Along-strike distance (km)
ZTOR	Depth to top of rupture (km)
HWFlag	Hanging-wall flag (1 = hanging-wall side)
hypoDepth	Assumed hypocentral depth (km)
RupWidth / RupLen	Rupture width / length (km)
distSeism	Seismogenic depth distance (km)
distepi / disthypo	Epicentral / hypocentral distance (km)

Dataset

FT is an sf LINESTRING object representing the Shanchiao Fault system. Key attribute columns:

Column	Description
ID	Fault segment ID
FAULT_NAME	Fault name
FAULT_TYPE	Fault mechanism (reverse, normal, etc.)
LENGTH_KM	Segment length (km)
FAULT_DIP1	Dip angle (degrees)
DEPTH1_KM	Depth to fault top (km)
WIDTH_KM	Down-dip width (km)

Reference

Abrahamson, N. A. (2000). HAZ: A computer program for the probabilistic seismic hazard analysis. Pacific Gas and Electric Company.

License

HAZDist is released under the GPL (≥ 3) license.