120°55'

35°10'

35°15'

0

helicopter

marine

xed-wing

120°45'

10 km

Figure 1. Index map of three magnetic

surveys.

Magnetic Map of the Irish Hills and Surrounding Areas, San Luis Obispo County, Central California

By

V.E. Langenheim, J.T. Watt, and K.M. Denton

2012

52'30" 50'

Open-File Report 2012-1080

120°55' 47'30" 120°45'

120°55' 52'30" 50' 47'30" 120°45'

15'

12'30"

35° 7'30"

10'

15'

12'30"

35° 7'30"

10'

SCALE 1:24 000

1 0 1 2 3 4 KILOMETERS

1 0 1 2 3 4 MILES

U.S. Department of the Interior

U.S. Geological Survey

Base from U.S. Geological Survey 7.5-minute quadrangles, Morro 

Bay South, 1965, Pismo Beach, 1965, Port San Luis, 1963, San 

Luis Obispo, 1965, revised 1994.

Universal Transverse Mercator projection, zone 10, datum NAD27

Central meridian -123.0 ̊

Base latitude 0.0 ̊

Fault—solid where location is accurate; dashed where approximate; dotted where concealed from 

Pacific Gas and Electric Company (2010; offshore) and Pacific Gas and Electric Company (1988; onshore)

EDNA FAULT

HOSGRI FAULT ZONE

Earthquake epicenter (Hardebeck, 2010)

MAP EXPLANATION

Contours of total magnetic field intensity relative to the 

International Geomagnetic Reference Field. Contour interval 

is 20 nanoteslas. Hachures indicate closed magnetic lows

Magnetic survey boundary

Location of maximum horizontal gradient, with amplitudes greater than the mean for the study area

Location of maximum horizontal gradient, with amplitudes less than the mean for the study area

-320

-280

-240

-200

-160

-120

 -80

 -40

 0

 40

 80

 120

 160

 200

 240

 280

 320

 360

 400

 440

Magnetic Field, 

in nanoteslas

Manuscript approved for publication April 25, 2012

Any use of trade, product, or firm names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government.

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center,

Denver, CO 80225, 1-888-ASK-USGS

Digital files available at http://pubs.usgs.gov/of/2012/1080/

Suggested citation: Langenheim, V.E., Watt, J.T., and Denton, K.M., 2012, Magnetic map of the 

Irish Hills and surrounding areas, San Luis Obispo County, central California: U.S. Geological 

Survey Open-File Report 2012-1080, scale 1:24,000. (Available at http://pubs.usgs.gov/of/2012/1080/.)

Table 1. Magnetic properties of rock samples within map area.

UTM grid convergence (GN)

and 1978 magnetic declination

at center of Morro Bay South 

1:24,000-scale map. Diagram 

is approximate

276 MILS 23 MILS

151⁄2 ̊ 1 ̊16’

SHORELINE FAULT

Rock property measurement (Sedimentary rock)

Rock property measurement (Tertiary volcanic rock)

Rock property measurement (Mesozoic igneous and metamorphic rock)

A magnetic map of the Irish Hills and surrounding areas was created as part of a cooperative research 

and development agreement with the Pacific Gas and Electric Company and is intended to promote 

further understanding of the areal geology and structure by serving as a basis for geophysical interpretations and by supporting geological mapping, mineral and water resource investigations, and 

other topical studies. Local spatial variations in the Earth's magnetic field (evident as anomalies on 

magnetic maps) reflect the distribution of magnetic minerals, primarily magnetite, in the underlying 

rocks. In many cases the volume content of magnetic minerals can be related to rock type, and abrupt 

spatial changes in the amount of magnetic minerals can be related to either lithologic or structural 

boundaries. Magnetic susceptibility measurements (table 1) from the area indicate that bodies of 

serpentinite and other mafic and ultramafic rocks tend to produce the most intense magnetic anomalies, but such generalizations must be applied with caution because some sedimentary units also can 

produce measurable magnetic anomalies. Remanent magnetization does not appear to be a significant source for magnetic anomalies because it is an order of magnitude less than the induced magnetization (table 1).

The map is a mosaic of three separate surveys collected by (1) fixed-wing aircraft at a nominal height 

of 305 m (Langenheim and others, 2009), (2) by boat with the sensor at sea level (Sliter and others, 

2009, revised 2010), and (3) by helicopter (fig. 1). The helicopter survey was flown by New-Sense 

Geophysics in October 2009 along flight lines spaced 150-m apart and at a nominal terrain clearance 

of 50 to 100 m. Tie lines were flown 1,500-m apart. Data were adjusted for lag error and diurnal field 

variations. Further processing included microleveling using the tie lines and subtraction of the reference field defined by International Geomagnetic Reference Field (IGRF) 2005 extrapolated to August 

1, 2008.

Data were interpolated to a square grid with a grid interval of 50 m using the principle of minimum 

curvature (Briggs, 1974). The magnetic base levels of three surveys were then adjusted to bring them 

onto a common magnetic datum.

Abrupt lateral changes in magnetization that may represent lithologic or structural boundaries were 

determined as follows: 

 (1) The total-field magnetic anomaly data were mathematically transformed into 

 pseudogravity anomalies (Baranov, 1957); this procedure effectively converts 

 the magnetic field to the "gravity" field that would be produced if all the 

 magnetic material were replaced by proportionately dense material.

 (2) The horizontal gradient of the pseudogravity field was calculated everywhere 

 by numerical differentiation.

 (3) Locations of locally steepest horizontal gradient (shown on map as red-filled circles) were 

 determined by numerically searching for maxima in the horizontal gradient grid 

 (Blakely and Simpson, 1986).

Boundaries between bodies having different densities are characterized by steep gradients in the 

gravity field they produce and, if the boundaries have moderate-to-steep dips (greater than 45°), 

locally the maximum horizontal gradients will be located over the surface traces of the boundaries 

(Blakely and Simpson, 1986), especially for shallow sources. Similarly, boundaries between bodies 

having different magnetizations are characterized by steep gradients in the pseudogravity field and, 

therefore, the procedure described above can be used to locate these boundaries. For example, the 

steepest horizontal gradients shown on map by red-filled circles coincide locally with strands of the 

Edna, Hosgri, and Shoreline Faults.

REFERENCES CITED

Baranov, V.I., 1957, A new method for interpretation of aeromagnetic maps—Pseudo-gravimetric 

 anomalies: Geophysics, v. 22, p. 359-383.

Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or grav-

 ity anomalies: Geophysics, v. 51, p. 1494-1498.

Briggs, I.C., 1974, Machine contouring using minimum curvature: Geophysics, v. 39, p. 39-48.

Hall, C.A., Jr., 1973a, Geologic map of the Morro Bay South and Port San Luis quadrangles, San Luis 

 Obispo County, California: U.S. Geological Survey Miscellaneous FIeld Studies Map MF-511, 

 scale 1:24,000.

Hall, C.A., Jr., 1973b, Geology of the Arroyo Grande 15-minute quadrangle, San Luis Obispo county, 

 California: California Division of Mines and Geology Map Sheet 24, scale 1:62,500, 8 p.

Hardebeck, J.L., 2010, Seismotectonics and fault structure of the California Central Coast: Bulletin of 

 the Seismological Society of America, v. 100, no. 3, p. 1031-1050.

Langenheim, V.E., Jachens, R.C., and Moussaoui, K., 2009, Aeromagnetic Survey Map of the Central 

 California Coast Ranges: U.S. Geological Survey Open-File Report 2009-1044,

 http://pubs.usgs.gov/of/2009/1044/.

Pacific Gas and Electric Company, 1988, Final Report of the Diablo Canyon Long Term Seismic

 Program. Docket Nos. 50-275, and 50-323, 700 p, 21 plates.

Pacific Gas and Electric Company, 2010, Appendix B: Geology Plates in Report on the Analysis of the

 Shoreline Fault Zone, Central Coastal California: Report to the U.S. Nuclear Regulatory

 Commission, 4 plates, scale 1:12,000. [http://www.pge.com/myhome/edusafety/

 systemworks/dcpp/shorelinereport/].

Sliter, R.W., Triezenberg, P.J., Hart, P.E., Watt, J.T., Johnson, S.Y., and Scheirer, D.S., 2009, revised 2010, 

 High-resolution seismic reflection and marine magnetic data along the Hosgri Fault Zone, 

 Central California: U.S. Geological Survey Open-File Report 2009-1100, version 1.1, 

 http://pubs.usgs.gov/of/2009/1100/.

HOSGRI FAULT ZONE

flight-line artifact

Rock Type1

Number 

of 

samples 

(k/NRM)

Average 

Magnetic 

Susceptibility 

(x 10-3 SI)

Magnetic 

Susceptibility 

Range

Average NRM 

Magnetization 

(A/m)

NRM 

Magnetization 

Range (A/m)

Average 

Volume 

Susceptibility 

(x 10-3 SI)

Range Volume 

Susceptibility 

(x 10-3 SI)

Average 

Koenigsberger 

Ratio 

Koenigsberger 

Ratio Range

Sedimentary rocks

Pismo Formation 8/0 0.06 0.00 0.14 * * * * * *

Monterey Formation chert 3/0 0.04 0.02 0.05 * * * * * *

Sandstone 2/0 0.17 0.11 0.22 * * * * * *

Graywacke 1/0 0.41 0.41 * * * * * *

Chert 1/0 0.11 0.11 * * * * * *

Tertiary volcanic rocks

Obispo Formation tuff 4/0 0.07 0.05 0.09 * * * * * *

Obispo Formation diabase, 

weathered 4/4 0.33 0.25 0.41 3.67E-04

2.56E-04 

5.19E-04 0.37 0.39-0.42 0.03 0.016 0.052

Obispo Formation diabase 13/8 11.97 0.13 32.64 4.33E-02 7.91E-06 

7.02E-02 13.07 0.34-31.60 0.06 0.009 0.147

Mesozoic igneous and 

metamorphic rocks

Basalt 8/2 26.25 15.65 38.10 1.36E-01 0.125 0.146 31.67 29.77 - 33.56 0.10 0.098 0.102

Meta-volcanic rocks 7/0 1.46 0.00 6.33 * * * * * *

Serpentinite 3/0 26.27 6.90 39.60 * * * * * *

[k/NRM, magnetic susceptibility/natural remanent magnetization; SI, standard international unit; A/m, ampere per meter; *, no data]

1From Hall (1973a, b)