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Digital Mapping Techniques '00 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 00-325

Using GIS for Visualizing Earthquake Epicenters, Hypocenters, Faults and Lineaments in Montana

By Patrick J. Kennelly and Michael C. Stickney

Montana Bureau of Mines and Geology
Montana Tech of the University of Montana
Butte, MT 59701-8997
Telephone: (406) 496-2986
Fax: (406) 496-4451
e-mail: pkennelly@mtech.edu

INTRODUCTION

The Montana Bureau of Mines and Geology (MBMG) uses Geographic Information Systems (GIS) technology to map and visualize elements associated with earthquake seismicity. Due to the nature of earthquakes, the mapping process is inherently three-dimensional. Epicenters are the location of a subsurface seismic event projected vertically up to the topographic surface. The true three-dimensional location of the focus of slip is referred to as the hypocenter. Mapping locations of foreshocks, the main shock, and aftershocks in three dimensions can define the subsurface fault plane on which movement has taken place.

In many cases, larger faults extend up to the ground surface. Offset along the fault can be lateral, vertical, or a combination of both. These linear offsets are often represented in topographic landforms, including fault-bounded mountain ranges and associated valleys. Linear geologic features that do not show conclusive evidence of offset or fault origin are referred to as lineaments. Analytical hillshading of Digital Elevation Models (DEMs) can accentuate traces of faults and lineaments. Visibility of faults and lineaments is a function of the three dimensional orientation of both the surface normal vector of the topographic features and the illumination vector. Traditional filtering methods used to identify linear features on remote sensing imagery can also be utilized, although the result is not as visually striking or intuitive as using analytical hillshading. The surface can also be filtered and then hillshaded to visualize lineaments. This paper presents data for two areas, the seismically active western portion of the state, and the seismically quiescent eastern portion. MBMG Special Publication 114 summarizes much of this data for western Montana (Stickney et. al., 2000). In addition to the data found in this report, this study presents planar regression analysis for hypocenter locations of the Norris earthquake swarm in western Montana and GIS visualization tools for the Brockton-Froid lineament in eastern Montana.

SEISMICITY IN MONTANA

Earthquake epicenters and fault traces have been compiled for the state of Montana, with an emphasis on the historically active western half of the state. In western Montana and throughout the Intermountain West, only the very largest historic earthquakes can with certainty be ascribed to specific faults. This is because western Montana earthquakes typically result from slip (movement) along faults at depths of 2-10 miles (3-15 km) below the ground surface. Only during the largest earthquakes (those generally larger than magnitude 6.5) does fault slip propagate up to, and offset, the Earth's surface. This offset of the Earth's surface results in a fault scarp. Young fault scarps (those less than 15,000 years old) mark steep mountain range fronts (Madison, Centennial, Absaroka, and Tendoy ranges for example). These mountain ranges are fault blocks uplifted by repeated earthquakes over millions of years and subsequently carved by ice and water into rugged mountains. Sediment eroded from the mountains filled broad valleys overlying the adjacent, downthrown fault blocks (Madison, Centennial, Emigrant, and Red Rock valleys).

The only historic surface-rupturing earthquake in Montana is the 1959 Hebgen Lake earthquake, centered just west of the northwest corner of Yellowstone National Park. The magnitude 7.5 Hebgen Lake earthquake offset the Earth's surface for a distance of 20 miles (32 km) along two principal faults and produced up to 20 feet (6 m) of vertical offset. Earthquakes as large as the 1959 earthquake occur infrequently (perhaps once in a few thousand to tens of thousands of years) on specific faults in western Montana.

It is these large but infrequent earthquakes that are preserved in the geologic record and modify the landscape, creating fault scarps along which a mountain block is uplifted or a valley floor is lowered. Many other faults have ruptured during the Quaternary (the past 1.6 million years) but the age of the last rupture is not well constrained. The long elapsed time since the last major earthquake on these faults may suggest they are no longer active, but their potential to produce an earthquake cannot be completely ignored because many faults in the Intermountain West have very long recurrence times.

Small- and moderate-magnitude earthquakes (magnitudes less than 6.5) generally do not alter the Earth's surface in Montana. However, they occur more frequently than surface-rupturing earthquakes and may be powerful enough to cause damage. Thus, much of the seismic hazard facing western Montana comes from smaller but more frequent earthquakes on faults lying hidden beneath the Earth's surface as well as major but infrequent earthquakes along mapped faults.

DISCUSSION OF THE MAP "QUATERNARY FAULTS AND SEISMICITY IN WESTERN MONTANA"

Topographic Data
Our digital topographic representation of western Montana is based on Digital Elevation Models (DEMs) created by the USGS. Western Montana DEMs were obtained from the Montana State Library National Resources Information System (NRIS). A full description of these data is available from the NRIS web site at http://nris.state.mt.us/. The topographic visualization was derived from 30-meter and 3-arc-second U.S. Geological Survey DEMs. The 3-arc-second DEMs include some vertical accuracy problems, primarily in the northeast part of the map area. The data from areas with contrasting data quality were smoothed in ArcInfo GRID using filtering techniques to minimize these artifacts.

The appearance of shaded relief topography was accomplished with the aid of ArcInfo TIN conversion routines and hill-shading techniques (Stickney et. al., 2000). The visualization of the topographic surface was created by artificially illuminating the DEM with an afternoon sun source (azimuth 315 degrees, altitude 55 degrees, and vertical exaggeration 1.5). The map was created by projecting the illuminated DEM data into a Lambert Conformal Conic Projection using the Montana State Plane Coordinate System with the following parameters: Central Meridian -109.5°, 1st standard parallel 45° north, 2nd standard parallel 49.0°, origin 44.25o and false easting 600,000 meters. Other data shown on the map, such as county boundaries, lakes, rivers, highways, and cities are derived from 1:100,000-scale U.S. Census Bureau Tiger files that also were obtained from NRIS.

Faults
The Special Publication No. 114 (Stickney et.al., 2000) map displays faults, earthquakes, and topography in western Montana. Funded through the Earthquake Hazards Reduction Program, the U.S. Geological Survey (USGS) compiled Quaternary faults in western Montana as part of a larger effort sponsored by the International Lithosphere Program. The USGS conducted a detailed review of published and unpublished maps and literature concerning Quaternary faults in western Montana. Fault data were entered into a data base and used to compile a map showing the locations, ages, and estimated slip rates of Quaternary faulting in western Montana. Fault traces were taken from original sources and compiled on 1:250,000-scale quadrangle base maps and digitized for use with the GIS. In addition to location and style of faulting, the data characterize the time of most recent movement and estimated slip rate for each fault. Also included are geographic and other paleoseismologic parameters and a bibliographic reference. Information from this data base is available on CD-ROM from the Montana Bureau of Mines and Geology (Haller et. al., 2000). Characteristics of several faults significantly change along the length of the fault (Red Rock and Madison faults for example), indicating that different parts of the fault (sections) behave independently of each other. Faults with two or three sections are indicated on the map and in the database with a lowercase letter following the fault number (i.e. 644a). If the available information does not imply a multi-sectioned fault, then the fault is described as a simple fault and designated with a three digit number (i.e., 687).

Most of the faults that have produced earthquakes in recent geologic time originated many millions of years ago. These ancient faults have moved in various ways as different tectonic events shaped Montana's geologic history. The Lewis and Clark zone is an example of a fault zone formed over a billion years ago, which may still have the potential to produce damaging earthquakes. About 12 major faults make up the Lewis and Clark zone which extends from the Helena region west-northwestward through Missoula to the Montana-Idaho state line near Lookout Pass, and beyond to the vicinity of Coeur d'Alene, Idaho. The Lewis and Clark zone is a general name describing this group of faults with horizontal offsets measured in kilometers to tens of kilometers as well as strongly deformed rock strata (Wallace and others, 1990). These faults accommodated slip during the formation of the overthrust belt in the mountainous western one-third of Montana some 50 to 80 million years ago. Younger slip of a different direction along several faults in the Lewis and Clark zone has helped to shape the modern landscape through formation of valleys. However, most Lewis and Clark zone faults do not have documented Quaternary movement.

EARTHQUAKE EPICENTERS

Also depicted on the map are selected earthquake epicenters determined by the MBMG, which operates a network of seismograph stations in western Montana. Network data have been used to determine epicenters and magnitudes for over 14,000 earthquakes occurring from 1982 to 1998. Information about recent earthquakes is available through a link on the MBMG web site at http://mbmgsun.mtech.edu/. The number and proximity of seismometers that record an earthquake are the most important factors influencing the accuracy of an epicenter determination. Before 1995, seismograph network stations were generally limited to southwest Montana. Thus, the quality for epicentral locations of pre-1995 earthquakes in northwest Montana is generally below that for southwest Montana. For the same reason, many small northwest Montana earthquakes went undetected prior to 1995.

The quality of seismic monitoring in northwest Montana improved dramatically in 1995 when the MBMG entered into a cooperative agreement with the Confederated Kootenai and Salish Tribes (CSKT) in order to establish six seismographs on the Flathead Reservation, north of Missoula. Also in 1995, the MBMG received funding through a National Earthquake Hazards Reduction Program grant to install nine stations in west-central Montana between Helena and St. Regis. By 1998, the Montana seismograph network consisted of 31 seismographs distributed between Flathead Lake in northwest Montana and the north and west borders of Yellowstone National Park. Seismic data are recorded in Butte at the MBMG's Earthquake Studies Office (ESO), in Ronan at the CSKT Safety of Dams Office, and in Missoula at The University of Montana Geology Department. All seismic data are analyzed and archived in Butte. Additional data from seismographs operated by other agencies in surrounding states and Canada are routinely incorporated into Montana earthquake locations. Stickney (1995) described seismic instrumentation and data-analysis procedures employed in preparation of the Montana earthquake catalog.

A subset of 5,148 earthquake epicenters from western Montana was selected from the MBMG earthquake catalog and shown on this map. These selected earthquakes include all earthquakes with Richter magnitudes over 2.5 and those earthquakes of magnitude 1.5 or larger with better quality epicentral locations. Earthquake epicenters that lie more than 6 miles (10 km) outside the Montana border are not shown. The distribution of earthquake epicenters generally reflects the northern Intermountain Seismic Belt and eastern Centennial Tectonic Belt (Stickney and Bartholomew, 1987).

Star symbols show earthquakes of magnitude 5.5 or greater since 1900. The epicenter locations for historic Montana earthquakes are not as accurately determined as those after 1965 because prior to1965, few if any seismograph stations operated in Montana. Pre-1982 epicenters were taken from the National Oceanic and Atmospheric Administration hypocenter files, or later studies of these earthquakes if available.

EARTHQUAKE HYPOCENTERS

An earthquake hypocenter is the actual location of initial slip on a fault plane. This differs from the epicenter, which is the hypocenter projected vertically to the ground surface. Large earthquakes may have foreshocks, and many aftershocks follow, providing numerous hypocenter locations for imaging an active fault plane or fault zone. Mapping hypocenters in three-dimensions can provide information on orientation of the fault plane, relationship of the main shock to foreshocks and aftershocks, timing of movement on the three-dimensional fault plane, and discrimination between seismic events occurring on one well-defined fault plane versus multiple planes within a fault zone.

Hypocenter locations in three dimensions allow visualization of seismic events. The Norris earthquake swarm of 1987 offered an opportunity for MBMG to deploy field seismographs in the epicentral area to accurately measure aftershocks. Hypocenters were determined for nearly 600 events ranging from Richter magnitude -2.1 to 3.3. ArcView's 3D Analyst extension was used to visualize these well-constrained hypocenters. Initial observations indicate that the aftershocks originated in a complex fault zone. The "cloud" of hypocenters could be interpreted as deep movement on the Bradley Creek fault, which has a scarp in this area, as well as other antithetic or synthetic faults at depth. As a first estimate, a planar regression was performed on all hypocenter locations. The resulting plane is a poor fit, with a Root Mean Squared (RMS) error of 618. The resulting plane, however, has a strike of N17W, a dip of 42° E, and would create a surficial trace on the DEM very close to the Bradley Creek fault scarp.

 Norris earthquake best fit plane from regression analysis

Figure 1. Norris earthquake best fit plane from regression analysis for nearly 600 hypocenter locations and 283 points located on the fault scarp.

Assuming a planar fault surface, the location of the scarp itself also can be included to better constrain this potential fault plane. The arc associated with this fault trace was densified at 30 meters, resulting in 283 points. These points were assigned z values from the DEM, combined with the hypocenter points, and fitted to a second plane (figure 1). This plane was once again a poor fit, with an RMS value of 521. The new plane has a strike of N24W and a dip of 45° E. Results could be refined in several ways. The most obvious would be to divide the hypocenter data into different groupings, either by identifying multiple planes from visual inspection, or based on additional data such as first motion studies. In this manner, multiple fault planes could provide an overall better fit to all hypocenter and fault scarp data.

LINEAMENTS AND SEISMICITY IN EASTERN MONTANA

Lineaments
All magnitude 5.5 or greater earthquakes in Montana this century have occurred in the Intermountain Seismic Belt, except one - the May 16, 1909 earthquake in northeast Montana. Because of its early date, no local seismographs existed to record it; however, its widespread area of perceptibility and strong shaking near the epicenter suggest a magnitude of at least 5.5.

Identifying fault scarps in the less seismically active eastern half of Montana is challenging. One fault candidate is the Brockton-Froid lineament in northeastern Montana. The lineament has been interpreted by field mapping efforts of Colton (1963a) of the U.S. Geological Survey as a northeast-southwest (N55E) trending fault zone more than 50 km (30 mi) in length. The entire zone is straight, with the northeastern-most portion consisting of a single lineament. In the central portion, the zone consists of two parallel traces defining a small graben-like structure. At the southwestern end, the zone splays into several less well defined lineaments.

Vertical relief along the trend is evident in the Quaternary glacial till covering the area, implying relatively recent movement. Surficial deposits also vary across the inferred graben, changing from Quaternary alluvium outside the zone to Quaternary gravel within the inferred graben. Two traverses done with auger holes show thicker Quaternary sequences in the central graben as compared to outside of this structure (Colton, 1963b). No trenching has yet been done across the lineament.

The Brockton-Froid lineament is clearly identified from 30 m. Digital Elevation Models (DEMs) using analytical hillshading and remote sensing filtering techniques. Analytical hillshading applies a gray color to each pixel in the DEM based on the angle between the selected illumination vector and the surface normal vector. The illumination vector is defined by two angles, an attitude (compass direction between 0° and 360°) and an altitude (horizontal angle between 0° and 90°). The surface normal is a vector normal to a surface defined by a grid cell and its closest 8 neighboring grid cells. The grayness of grid cells will vary as a function of the cosine of the angle made in three dimensions between the surface normal and the illumination vector.

Figure 2. The Brockton-Froid lineament using analytical hillshading for visualization. The lineament is illuminated by a light source located at an azimuth of N35W and an angle from horizontal of 45°.

The Brockton-Froid lineament remains visible with large changes both in the attitude and altitude of the illumination vector. The ideal illumination direction would be at a right angle to the N55E trending zone. A hillshading of the DEM with an attitude of N35W and an altitude of 45° illuminates the lineament well (figure 2). Other examples of hillshading with large variations in attitude (+-60°) and altitude (+- 42°) produce an easily identified lineament. Only when the illumination direction is nearly parallel to the lineament (attitude = N55E) or when the illumination is vertical or horizontal (altitude = 0° or 90°) does the lineament disappear. The robustness of the lineament results from a laterally continuous trend which can readily be distinguished from the background. Efforts to find other lineaments in eastern Montana would require only a limited number of illumination vectors to cover all potential lineament orientations.

Filters are often used with remote sensing data to help define linear features present in imagery. Filters apply a small grid called a convolution kernel to each grid cell and neighboring grid cells. The values are then summed and divided by the number of grid cells in the kernel. The same techniques can be applied to gridded DEM data. Two filters were effective on this data, a filter to detect NE-SW oriented edges, and a filter to detect compass gradients. The best filter for detecting the Brockton-Froid lineament was the NW-oriented compass gradient filter. The kernel for this filter is designed to sum to zero in flat areas, sum to a positive number for NE-SW linear features increasing to the NW, and sum to a negative number for NE-SW linear features decreasing to the NW. This technique, however, produces a less visually continuous lineation than the analytical hillshading. The edge detection filter is not designed to sum to zero for flat areas; edges enhanced are overprinted on the topographic model. If, however, analytical hillshading is applied to the DEM after it undergoes NE-SW trending edge detection filtering, the result clearly shows the lineament. Although the lineament itself is no sharper than in traditional analytical hillshading, the filtering process eliminates other linear features not oriented NE-SW, allowing the lineament to stand out more clearly.

ACKNOWLEDGEMENTS

Funds to produce MBMG Special Publication 114, Quaternary Faults and Seismicity Map of Western Montana came from the Hazard Grant Mitigation Program administered by the Disaster and Emergency Services Division of the Montana Department of Military Affairs. Larry Akers and Jerry Smithers of DES were helpful in guiding us through the grant application process and program administration-their assistance is gratefully acknowledged. Richard Dart of the USGS supplied the digital fault data in ARC/INFO format. The MBMG Earthquake Studies Office, Confederated Salish and Kootenai Tribes' Safety of Dams Office, and the University of Montana Geology Department provided seismograph data for locating and cataloging western Montana earthquakes. The National Earthquake Hazards Reduction Program has provided two previous grants (awards 1434-94-G-2516 and 1424-95-G-2628) to the MBMG that expanded seismic monitoring capabilities and re-analysis of previously recorded earthquake data. Finally, thanks to GIS specialists Bill Myers and Paul Thale (MBMG) for GIS production of the map and cartographer Susan Smith (MBMG) for cartographic production.

REFERENCES

Colton, R. B., 1963a, Geologic map of the Brockton Quadrangle, Roosevelt and Richland Counties, Montana (#3653): U.S. Geological Survey Miscellaneous Geologic Investigations I-362, 1 sheet, scale 1:62,500. Colton, R. B., 1963b, Geologic map of the Poplar Quadrangle, Roosevelt, Richland, and McCone Counties, Montana (#3654): U.S. Geological Survey Miscellaneous Geologic Investigations I-367, 1 sheet, scale 1:62,500.

Haller, K.M., Dart, R.L., Machette, M.N., Stickney, M.C., 2000, Data for Quaternary faults in western Montana: Montana Bureau of Mines and Geology Open-file Report 411, 241 p., 1 compact disk. Stickney, M.C., 1995, Montana seismicity report for 1990: Montana Bureau of Mines and Geology Miscellaneous Contribution 16, 44 p.

Stickney, M.C., Haller, K.M., and Machette, M.N., 2000, Quaternary faults and seismicity in western Montana: Montana Bureau of Mines and Geology Special Publication 114, Scale 1:750,000, 1 sheet.

Stickney, M.C. and Bartholomew, M.J., 1987, Seismicity and late Quaternary faulting of the northern Basin and Range Province, Montana and Idaho: Bulletin of the Seismological Society of America, v. 77, p. 1602-1625.

Wallace, C.A., Lidke, D.J., and Schmidt, R.G., 1990, Faults of the central part of the Lewis and Clark line and fragmentation of the Late Cretaceous foreland basin in west-central Montana: Geological Society of America Bulletin, v. 102, p. 1021-1037.

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