USGS Digital Data Series DDS-9

U.S. GEOLOGICAL SURVEY DIGITAL DATA SERIES DDS-9
National Geophysical Data Grids:
Gamma-Ray, Gravity, Magnetic, and Topographic
Data for the Conterminous United States

FILE DESCRIPTIONS
GRID FILE INFORMATION

The data published in the \ASCII directory on this CD-ROM consist of regular grids of ASCII values.Each grid has a different origin and sample spacing as defined in its first record, the header.The grid format is explained in detail below.Each grid represents data that have been projected from latitude and longitude coordinates into map coordinates of kilometers.Thus, the x-origin and y-origin values given in the header represent the distance in kilometers from the central meridian and base latitude of the geographic projection (discussed below) to the lower left corner of the grid.
Binary versions of these grid files, suitable for processing on IBM or compatible personal computers using the potential-field software contained on this CD-ROM, are located in the \DOSBIN directory on this CD-ROM.Use the EXTRACT BINARY GRID FILES submenu to access these grid files.For more information on the binary grid file format and the potential-field software, access the POTENTIAL-FIELD SOFTWARE submenu from the MAIN menu.
Projection information

All grids on this CD-ROM have been projected by using an Albers equal-area conic projection with standard parallels of 29.5 degrees and 45.5 degrees north.A central meridian of 96 degrees west and a base latitude of 0 degrees were used.The projection is referenced to the Clarke 1866 ellipsoid, which has an equatorial radius of 6378.2064 km and a polar radius of 6356.5838 km.
Grid format

Each ASCII grid consists of two header records followed by a series of data records.The first header record contains 56 alphanumeric characters of data identification, 8 alphanumeric characters containing the name of the computer program that created the grid, the integer number of columns in the grid, and the integer number of rows in the grid.The second header record contains a dummy integer value of 1, the x-coordinate in kilometers of the first (leftmost) column, the distance in kilometers between columns of the grid, the y-coordinate in kilometers of the first (bottommost) row of the grid, and the distance in kilometers between rows of the grid.

Each data record contains five values in scientific notation.The bottommost row is presented first, starting from the leftmost column.The first value in each row is a dummy value, usually zero (0.000000000E+00). If a row ends in the middle of a record, the record is padded with zeros. Areas of the grid containing no data are represented by a special value, 0.999999968E+38.

The gamma-ray data grids

Aerial gamma-ray surveys measure the gamma-ray flux produced by the radioactive decay of the naturally occurring elements K-40, U-238, and Th-232 in the top few centimeters of rock or soil (Duval, Cook, and Adams, 1971).If the gamma-ray system is properly calibrated (for example, see Grasty and Darnley, 1971), the data can be expressed in terms of the estimated concentrations of the radioactive elements.The potassium concentration data are usually expressed in units of percent potassium (percent K), uranium as parts per million equivalent uranium (ppm eU), and the thorium as parts per million equivalent thorium(ppm eTh).The term equivalent is used because the technique actually measures the gamma-ray flux from the decay of bismuth (Bi-214), which is a decay product of U-238, and from the decay of thallium (Tl-208), which is a decay product of Th-232.Radioactive disequilibrium in the thorium decay series may cause the measured equivalent uranium and equivalent thorium to differ from the actual uranium andthorium present in the surface rocks and soils. Because Rn-222 is a daughter product of the U-238 decay series, the U-238 concentrations can also be used to estimate the amounts of Rn-222 in the near-surface soil gas.

During the period 1975-83, the U.S. Department of Energy carried out the National Uranium Resource Evaluation (NURE) Program, which included aerial gamma-ray surveys of most of the conterminous United States.Although many of the airborne gamma-ray systems used to make these surveys were calibrated, many of the earlier surveys were done without calibration and conversion to the concentrations of the radioactive elements.Detailed examinations of the digital data available on magnetic tape also showed that many of the "calibrated" surveys do not match the data from other "calibrated" surveys of adjacent areas.For these reasons, the data must be corrected to obtain a consistent data base for the conterminous United States.
Because uranium, thorium, and potassium concentration data are useful in geologic studies and because the NURE data are the only nationwide data base on the natural radiation environment, the U.S. Geological Survey (USGS) reprocessed the aerial gamma-ray data to produce maps showing surface concentrations of potassium, uranium, and thorium for the conterminous United States.These maps have been released as USGS Open- File Reports (Duval and others, 1989, 1990).Some of the reprocessed data have also been released in profile form (Duval, 1995).

The magnetic anomaly data grid

Magnetic anomalies are produced by variations in the distribution of iron minerals, usually magnetite, in the rocks of the Earth's crust.Igneous and metamorphic rocks can be very magnetic.By comparison, sedimentary rocks are usually nonmagnetic.Magnetic anomalies therefore provide a way of mapping exposed and buried crystalline rocks.
The grid of magnetic anomaly data for the conterminous United States and adjacent marine areas (Godson, 1986) was created from digitized contours of the east half of the Composite Magnetic Anomaly Map of the United States, Part A (U.S. Geological Survey, 1982), and the Composite Magnetic Anomaly Map of the Conterminous United States West of 96 Degrees Longitude (Bond and Zietz, 1987),with additional data used in the compilation of the Magnetic Anomaly Map of North America (Geological Society of America, Committee for the Magnetic Anomaly Map of North America, 1987).A regional gradient present in the 1982 map was removed by using a corrected geomagnetic reference field (Godson, 1986).The data, originally gridded on a 2-km interval using the spherical Transverse Mercator projection of the Magnetic Anomaly Map of North America, were reprojected to the Albers projection used on this CD-ROM and regridded on a 2-km interval using a minimum curvature gridding program (Webring, 1981).An interpretation of the 1982 anomaly map was presented by Hinze and Zietz (1985).

The Bouguer gravity anomaly data grid

Gravity anomalies are produced by density variations within the rocks of the Earth's crust and upper mantle.Mapping of these density variations is the primary use of gravity anomalies.
Gravity measurements made on the surface of the Earth must be corrected in various ways before they can be made into an anomaly map.The free-air correction reduces the measurement to sea level by assuming that there is no intervening mass.The simple Bouguer correction accounts for the intervening mass as a uniform slab of constant density, and the complete Bouguer correction includes the effects of constant density topography within 166.7 km of the measurement location.A gravity reference field is subtracted from the corrected measurements to produce the free-air, simple Bouguer, or complete Bouguer anomaly.
The grid of gravity data for the conterminous United States and adjacent marine areas (Godson and Scheibe, 1982; Godson, 1985) was constructed from Defense Mapping Agency gravity data files.The onshore data consisted of nearly one million Bouguer gravity anomaly values computed by using a reduction density of 2.67 grams per cubic centimeter.The offshore data consisted of approximately 800,000 free-air gravity anomaly values. Because the Bouguer anomaly equals the free-air anomaly at sea level, there is no discontinuity in the gridded data at the shoreline.All computations were performed by using the International Gravity Standardization Net of 1971 (International Association of Geodesy, 1974) and the 1967 Geodetic Reference System formula for theoretical gravity (International Association of Geodesy, 1971).
In areas of substantial relief, terrain corrections were computed about each station location at radial distances of 0.895 km to 166.7 km by using a density of 2.67 grams per cubic centimeter.The data were projected and gridded on a 4-km interval using minimum curvature (Webring, 1981).The gridding procedure resulted in the extrapolation of grid values up to 40 km beyond the limits of the data; therefore, values around the edges should be viewed with caution.These gridded data were published in map form as the Gravity Anomaly Map of the United States (Society of Exploration Geophysicists, 1982).This map was further discussed by O'Hara and Lyons (1985) and Kane and Godson (1985, 1989).

The isostatic residual gravity anomaly data grid

Isostatic residual gravity anomaly maps are produced by subtracting long- wavelength anomalies produced by masses deep within the crust or mantle from the Bouguer anomalymap.The long-wavelength anomalies are assumed to result from isostatic compensation of topographic loads.Isostatic residual gravity anomaly maps therefore reveal more clearly than Bouguer anomaly maps the density distributions within the upper crust that are of interest in many geologic and tectonic studies.
The grid of isostatic residual gravity anomaly data (Simpson and others, 1986) was produced from the grid of Bouguer gravity anomaly data (Godson and Scheibe, 1982) by using an Airy-Heiskanen compensation model (Heiskanen and Moritz, 1967) with three parameters.The depth to the compensating root at sea level was chosen to be 30 km.The density contrast across the root was chosen to be 0.35 grams per cubic centimeter, and the density of the topography was chosen to be 2.67 grams per cubic centimeter.Other reasonable choices of these parameters would produce similar-looking residual maps.
The computer program and topographic data sets used to produce the data
grid were described by Simpson and others (1983a,b). The data were
published in map form by Jachens and others (1985).Interpretations of the
isostatic residual gravity anomaly map were presented by Simpson and others
(1986) and by Jachens and others (1989).

The topographic data grid

The topographic data grid for the conterminous United States and adjacent areas was constructed from 30x30 second digital terrain files used by the U.S. Geological Survey for the reduction of gravity data.Elevations are in meters; sea level elevations are listed as 1 meter.

The topographic-bathymetric data grid

The topographic and bathymetric data grid for the conterminous United States and adjacent areas was constructed from 5x5 minute North American topographic data and 5x5 minute Synthetic Bathymetric Profiling System data available from the National Oceanic and Atmospheric Administration.The data were interpolated onto an 8x8 km grid in the Albers projection (Simpson and others, 1983b).Elevation units are in meters relative to sea level.

Mount Vesuvius - Italy Article by Jessica Ball Mount Vesuvius Introduction Vesuvius is the only active volcano in mainland Europe, and has produced some of the continent’s largest volcanic eruptions. Located on Italy’s west coast, it overlooks the Bay and City of Naples and sits in the crater of the ancient Somma volcano. Vesuvius is most famous for the 79 AD eruption which destroyed the Roman cities of Pompeii and Herculaneum. Though the volcano’s last eruption was in 1944, it still represents a great danger to the cities that surround it, especially the busy metropolis of Naples. Map: Where is Vesuvius? Map showing the location of Mount Vesuvius on the west coast of Italy. Map by Geology.com and MapResources. Nearby Volcanoes: EtnaStromboli Mount Vesuvius: Plate Tectonic Setting Vesuvius is part of the Campanian volcanic arc, a line of volcanoes that formed over a subduction zone created by the convergence of the African and Eurasian plates. This subduction zone stretches the length of the Italian peninsula, and is also the source of other volcanoes like Mount Etna, the Phlegraean Fields (Campi Flegrei), Vulcano, and Stromboli. Under Vesuvius, the lower part of the subducting slab has torn and detached from the upper part to form what is called a "slab window". This makes Vesuvius’ rocks slightly different chemically from the rocks erupted from the other Campanian volcanoes. Mount Vesuvius: Eruption History Mount Vesuvius has experienced eight major eruptions in the last 17,000 years. The 79 AD eruption is oneof the most well known ancient eruptions in the world, and may have killed more than 16,000 people. Ash, mud and rocks from this eruption buried the cities of Pompeii and Herculaneum. Pompeii is famous for the casts the hot ash formed around victims of the eruptions. The unfortunate people suffocated on ash in the air, which then covered them and preserved amazing details of their clothing and faces. Starting in 1631, Vesuvius entered a period of steady volcanic activity, including lava flows and eruptions of ash and mud. Violent eruptions in the late 1700s, 1800s and early 1900s created more fissures, lava flows, and ash-and-gas explosions. These damaged or destroyed many towns around the volcano, and sometimes killed people; the eruption of 1906 had more than 100 casualties. The most recent eruption was in 1944 during World War II. It caused major problems for the newly-arrived Allied forces in Italy when ash and rocks from the eruption destroyed planes and forced evacuations at a nearby airbase.

San Andreas Fault

The San Andreas Fault is the sliding boundary between the Pacific Plate and the North American Plate. It slices California in two from Cape Mendocino to the Mexican border. San Diego, Los Angeles and Big Sur are on the Pacific Plate. San Francisco, Sacramento and the Sierra Nevada are on the North American Plate. And despite San Francisco’s legendary 1906 earthquake, the San Andreas Fault does not go through the city. But communities like Desert Hot Springs, San Bernardino, Wrightwood, Palmdale, Gorman, Frazier Park, Daly City. Point Reyes Station and Bodega Bay lie squarely on the fault and are sitting ducks.

The San Andreas Fault is a transform fault. Imagine placing two slices of pizza on the table and sliding them past one another where they touch along a common straight edge. Bits of pepperoni from one side crumble across the boundary onto the anchovy side. The same thing happens with the fault, and the geology and landforms along the mighty rift are extremely complicated.

The plates are slowly moving past one another at a couple of inches a year - about the same rate that your fingernails grow. But this is not a steady motion, it is the average motion. For years the plates will be locked with no movement at all as they push against one another. Suddenly the built-up strain breaks the rock along the fault and the plates slip a few feet all at once. The breaking rock sends out waves in all directions and it is the waves that we feel as earthquakes.


Google Map of the San Andreas
Zoom in on the Fault!
In many places like the Carrizo Plain (San Luis Obispo County) and the Olema Trough (Marin County), the fault is easy to see as a series of scarps and pressure ridges. In other places, it is more subtle because the fault hasn’t moved in many years and is covered with alluvium, or overgrown with brush. In San Bernardino and Los Angeles Counties, many of the roads along the fault cut through great mountains of gouge, the powdery, crumbled rock that has been pulverized by the moving plates.

The hallmark of the San Andreas Fault is the different rocks on either side of it. Being about 28 million years old, rock from great distances have been juxtaposed against rocks from very different locations and origins. The Salinian block of granite in central and northern California originated in Southern California, and some even say northern Mexico. Pinnacles National Monument in Monterey County is only half of a volcanic complex, the other part being 200 miles southeast in Los Angeles County and is known as the Neenach Volcanics.

There are many myths and legends about the San Andreas Fault, the biggest being that it will one day crack and California will slide into the sea. WRONG! It won’t happen and it can’t happen. Nor is there any thing such as “earthquake weather” or preferred times of day when earthquakes hit.

The San Andreas Fault is more accessible than any other fault in the world. With California’s large population and temperate climate, there are many roads that snake along the fault. They are uncrowded and peaceful, perfect for family outings. There is abundant camping, bird watching, wild flowers and wildlife, rock collecting and natural beauty along the way. State and National parks are strung along the fault like beads on a string. All it takes is a good map, a comfortable car and a desire to see the world’s most famous fault.

By David K. Lynch,

earthquakes

Naturally occurring earthquakesFault typesTectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behaviour. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.[1]Earthquake fault typesMain article: Fault (geology)There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction ofdip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as adivergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other ; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.Earthquakes away from plate boundariesWhere plate boundaries occur within continental lithosphere, deformation is spread out over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g. the “Big bend” region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms.[2]All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.[3]Shallow-focus and deep-focus earthquakesThe majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'mid-focus' or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).[4] These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth at which the subducted lithosphereshould no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure.[5]Earthquakes and volcanic activityEarthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, as during the Mount St. Helens eruption of 1980.[6] Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers andtiltmeters (a device which measures the ground slope) and used as sensors to predict imminent or upcoming eruptions.[7]Earthquake clustersMost earthquakes form part of a sequence, related to each other in terms of location and time.[8] Most earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.[9]AftershocksMain article: AftershockAn aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.[8]Earthquake swarmsMain article: Earthquake swarmEarthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.[10]Earthquake stormsMain article: Earthquake stormSometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.[11][12]Size and frequency of occurrenceThere are around 500,000 earthquakes each year. 100,000 of these can actually be felt.[13][14] Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azoresin Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.[15] Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years.[16] This is an example of the Gutenberg-Richter law.The Messina earthquake and tsunami took as many as 200,000 lives on December 28, 1908 in Sicily and Calabria.[17]The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The USGSestimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.[18] In recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS.[19]Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate.[20][21] Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.With the rapid growth of mega-cities such as Mexico City, Tokyo and Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.[22]Induced seismicityMain article: Induced seismicityWhile most earthquakes are caused by movement of the Earth's tectonic plates, human activity can also produce earthquakes. Four main activities contribute to this phenomenon: constructing large dams and buildings, drilling and injecting liquid into wells, and by coal mining andoil drilling.[23] Perhaps the best known example is the 2008 Sichuan earthquake in China's Sichuan Province in May; this tremor resulted in 69,227 fatalities and is the 19th deadliest earthquake of all time. The Zipingpu Dam is believed to have fluctuated the pressure of the fault 1,650 feet (503 m) away; this pressure probably increased the power of the earthquake and accelerated the rate of movement for the fault.[24]The greatest earthquake in Australia's history was also induced by humanity, through coal mining. The city of Newcastle was built over a large sector of coal mining areas. The earthquake was spawned from a fault which reactivated due to the millions of tonnes of rock removed in the mining process.[25]Measuring and locating earthquakesMain article: SeismologyEarthquakes can be recorded by seismometers up to great distances, because seismic waves travel through the whole Earth's interior. The absolute magnitude of a quake is conventionally reported by numbers on the Moment magnitude scale (formerly Richter scale, magnitude 7 causing serious damage over large areas), whereas the felt magnitude is reported using the modified Mercalli scale (intensity II-XII).Every tremor produces different types of seismic waves which travel through rock with different velocities: the longitudinal P-waves (shock- or pressure waves), the transverse S-waves (both body waves) and several surface waves (Rayleigh and Love waves). The propagation velocity of the seismic waves ranges from approx. 3 km/s up to 13 km/s, depending on the density and elasticity of the medium. In the Earth's interior the shock- or P waves travel much faster than the S waves (approx. relation 1.7 : 1). The differences in travel time from the epicentre to the observatory are a measure of the distance and can be used to image both sources of quakes and structures within the Earth. Also the depth of the hypocenter can be computed roughly.In solid rock P-waves travel at about 6 to 7 km per second; the velocity increases within the deep mantle to ~13 km/s. The velocity of S-waves ranges from 2–3 km/s in light sediments and 4–5 km/s in the Earth's crust up to 7 km/s in the deep mantle. As a consequence, the first waves of a distant earth quake arrive at an observatory via the Earth's mantle.Rule of thumb: On the average, the kilometer distance to the earthquake is the number of seconds between the P and S wave times 8[1]. Slight deviations are caused by inhomogenities of subsurface structure. By such analyses of seismograms the Earth's core was located in 1913 by Beno Gutenberg.Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world is divided into 754 Flinn-Engdahl regions (F-E regions), which are based on political and geographical boundaries as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones belong to larger F-E regions.Effects/impacts of earthquakes1755 copper engraving depicting Lisbon in ruins and in flames after the 1755 Lisbon earthquake, which killed an estimated 60,000 people. A tsunami overwhelms the ships in the harbor.The effects of earthquakes include, but are not limited to, the following:Shaking and ground ruptureShaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquakemagnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation.[26] The ground-shaking is measured by ground acceleration.Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.Ground rupture is a visible breaking and displacement of the Earth's surface along the trace of the fault, which may be of the order of several metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.[27]Landslides and avalanchesMain article: LandslideEarthquakes, along with severe storms, volcanic activity, coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue.[28]FiresFires of the 1906 San Francisco earthquakeEarthquakes can cause fires by damaging electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, more deaths in the 1906 San Francisco earthquake were caused by fire than by the earthquake itself.[29]Soil liquefactionMain article: Soil liquefactionSoil liquefaction occurs when, because of the shaking, water-saturated granularmaterial (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits. This can be a devastating effect of earthquakes. For example, in the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.[30]TsunamiThe tsunami of the 2004 Indian Ocean earthquakeMain article: TsunamiTsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers (62 miles), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373–497 miles per hour), depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.[31]Ordinarily, subduction earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.[31]FloodsMain article: FloodA flood is an overflow of any amount of water that reaches land.[32] Floods occur usually when the volume of water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and as a result some of the water flows or sits outside of the normal perimeter of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which then collapse and cause floods.[33]The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslide dam formed by the earthquake, known as theUsoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly 5 million people.[34]Tidal forcesResearch work has shown a robust correlation between small tidally induced forces and non-volcanic tremor activity.[35][36][37][38]Human impactsDamaged infrastructure, one week after the 2007 Peru earthquakeEarthquakes may lead to disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse or destabilization (potentially leading to future collapse) of buildings. Earthquakes can also precede volcanic eruptions, which cause further problems; for example, substantial crop damage, as in the "Year Without a Summer" (1816).[39]Major earthquakesMain article: List of earthquakesThe largest earthquake that has been measured was the 9.5 magnitude one in Chile in 1960.[13][14]PreparationIn order to determine the likelihood of future seismic activity, geologists and other scientists examine the rock of an area to determine if the rock appears to be "strained". Studying the faults of an area to study the buildup time it takes for the fault to build up stress sufficient for an earthquake also serves as an effective prediction technique.[40] Measurements of the amount of accumulated strain energy on the fault each year, time passed since the last major temblor, and the energy and power of the last earthquake are made.[40] Together the facts allow scientists to determine how much pressure it takes for the fault to generate an earthquake. Though this method is useful, it has only been implemented on California's San Andreas Fault.[40]Today, there are ways to protect and prepare possible sites of earthquakes from severe damage, through the following processes: earthquake engineering, earthquake preparedness, household seismic safety, seismic retrofit (including special fasteners, materials, and techniques),seismic hazard, mitigation of seismic motion, and earthquake prediction. Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries (US, Japan etc.) and late 1970s for many other parts of the world (Turkey, China etc.),[41], many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. Furthermore, state-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world - such as the ASCE-SEI 41 [42] and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines [43].

Geology Map Symbol KML Generator

Geologic maps use a combination of colors, lines, and symbols to give a a third dimension to the rock layers on a flat map surface.

With the Geology Map Symbol Generator you can add geologic map symbols to google earth using a web-based symbol generator.

This is still a Beta version which simply posts html form data to php script.

Geology Map of the San Diego 30' x 60' Quadrangle

Geology map of the San Diego 30' x 60' Quadrangle
Preliminary Geologic Map at 1:100,000 Scale to provide the public timely access to digital geology, prepared by the California Geological Survey Regional Geologic Mapping Project.


GEOLOGIC SUMMARY
The San Diego 30' X 60' quadrangle was prepared by the Department of Conservation, California Geological Survey pursuant to a U.S. Geological Survey STATEMAP cooperative mapping award (# 1434-94-A-1224). It is a product of the Southern California Areal Mapping Project (SCAMP), a cooperative U.S. Geological Survey-California Geological Survey mapping project, http://scamp.wr.usgs.gov/. This map is a compilation of published geological mapping (Fig. 1). The published mapping has been modified only to the extent necessary to integrate variables in nomenclature and scale. The onshore part of the map was digitized at a scale of 1:24,000 and the offshore part has been enlarged from 1:250,000. The quadrangle is between 32.5° and 33.0° N. latitude and 117.0° and 118.0° W. longitude. It encompasses the greater San Diego area, the second largest metropolitan area of California.
The area is tectonically active and is dissected by four major northwest-trending, oblique right slip faults that lie within the western part of the Pacific/North American Plate boundary. They include the Rose Canyon-Newport-Inglewood Fault Zone along the coastal margin, the Palos Verdes-Coronado Bank Fault Zone on the inner shelf, the San Diego Trough Fault Zone (origin of the 1986, ML=5.3, Oceanside earthquake) in the central offshore and the San Clemente Fault Zone on the outer offshore margin. Within the greater San Diego metropolitan area, the Rose Canyon Fault Zone as depicted by Kennedy and others (1975), Moore and Kennedy (1975), Kennedy and Welday (1980), Clarke and others (1987), Treiman (1993) and Kennedy and Clarke (2001) includes the Mount Soledad, Old Town, Point Loma, Silver Strand, Coronado and Spanish Bight faults. The Rose Canyon Fault Zone displaces Holocene sediment in Rose Canyon 7 km north of San Diego Bay where a late Pleistocene slip rate of 1-2 mm/yr has been estimated (Lindvall and Rockwell, 1995). A study of the recency and character of faulting in the greater San Diego metropolitan area suggests a long-term Tertiary slip rate for the Rose Canyon Fault Zone of about 1-2 mm/yr (Kennedy and others, 1975). Although there is significant late Quaternary deformation in the San Diego region the seismicity is relatively low (Simons, 1977).
The San Diego quadrangle is underlain by a thick sequence (>5 km) of Mesozoic fore-arc and fore-arc basin andesitic flows and coarse-grained volcaniclastic breccias that have been in large part metamorphosed to low-grade greenshist facies and are pervasively penetratively deformed. However, in the upper part of the section these rocks are not metamorphosed and are only moderately deformed. Marine sedimentary interbeds in Penasquitos Canyon, near Del Mar, contain the fossil Buchia piochii, which indicates a Late Jurassic (Tithonian) age for these strata (Fife and others, 1967; Jones and Miller, 1982). Zircon U/Pb ages from the metavolcanic rocks are reported to range from 137 Ma to 119 Ma (Anderson, 1991) indicating that they are coeval with the surrounding plutonic rocks of the western Peninsular Ranges batholith. The batholithic rocks are mostly granodiorite and tonalite and based on U-Pb isotopic ages range from 140 Ma to 105 Ma (Silver and Chappell, 1988). Much of the basement rock has been deeply weathered and altered. The weathered bedrock and Quaternary alluvial deposits derived from them contain expansible clays, mostly smectite.
The western part of the quadrangle is underlain by a relatively thick (>1,000 m) succession of Upper Cretaceous, Tertiary and Quaternary sedimentary rocks that unconformably overlie basement rocks. They consist of marine, paralic, and continental claystone, siltstone, sandstone and conglomerate. The Upper Cretaceous rocks are composed of marine turbidites and continental fan deposits assigned to the Rosario Group (Kennedy and Moore, 1971). The Lusardi Formation, the basal formation of the Rosario Group is a nonmarine boulder fanglomerate deposited along the western margin of a tectonic highland upon a deeply weathered surface of the older Cretaceous and Jurassic plutonic and metamorphic basement rocks. Clasts within the Lusardi Formation are composed exclusively of these weathered basement rocks. The Lusardi Formation is overlain by the Point Loma Formation, the middle part of the Rosario Group. It is composed mostly of marine sandstone, siltstone and conglomerate sequences that together form massive turbidite deposits. The Point Loma Formation is Campanian and Maestrichtian in age (Sliter, 1968; Bukry and Kennedy, 1969) and underlies most of the Point Loma Peninsula and the hills southeast of La Jolla. It is conformably overlain by the uppermost part of the Rosario Group, marine sandstone and conglomerate of the Maestrichtian (Sliter, 1968; Bukry and Kennedy, 1969) Cabrillo Formation. Following the deposition of the Rosario Group, the San Diego coastal margin underwent uplift and erosion until the middle Eocene when nine partially intertonguing middle and upper Eocene sequences composed of siltstone, sandstone, and conglomerate were deposited during several major transgressive-regressive cycles. The succession is over 700 meters thick and grades from nonmarine fan and dune deposits on the east through lagoonal and nearshore beach and beach-bar deposits to marine continental shelf deposits on the west near the present-day coastline. The age and environmental interpretation of the Eocene sequence is based on the mapped distribution of lithofacies coupled with the presence of a pelagic fossil calcareous nannoplankton flora in the continental shelf facies (e.g., Bukry and Kennedy, 1969), a shallow water molluscan fauna in the nearshore facies (e.g., Givens and Kennedy, 1979), and a fossil terrestrial vertebrate mammal fauna in the paralic facies (e.g., Golz, 1973). Cross bedding, cobble imbrications, paleo-stream gradients and clast petrology indicate a local eastern source for these rocks. The nonmarine facies of the Eocene formations are typically well indurated and cemented whereas the lagoonal facies are soft and friable. The nearshore facies are well indurated, well sorted, and locally concretionary. The marine deposits are typically fine-grained, indurated, and cemented. Following the deposition of Eocene rocks the San Diego margin was again elevated and eroded. During the Oligocene, continental and shallow water lagoonal deposits of the Otay Formation, were deposited. The Otay Formation is light-gray and light- brown, medium- and coarse-grained, arkosic sandstone intertongued with light-brown siltstone and light-gray claystone. Much of the claystone is composed of lightgray bentonite in beds up to 1 m in thickness. Following Oligocene time the San Diego coastal margin underwent uplift and extensive erosion. The next major marine transgression did not occur until Pliocene time when the strata of the San Diego Formation were deposited. The San Diego Formation rests unconformably upon Oligocene, Eocene and Upper Cretaceous beds across its outcrop from Pacific Beach to the International border with Mexico. The San Diego Formation is late Pliocene in age and contains a rich molluscan fauna (e.g., Arnold, 1903; Demere, 1983). It consists mostly of yellowish-brown and gray, fine- to mediumgrained, marine sandstone and reddish-brown, transitional marine and nonmarine pebble and cobble conglomerate. Following the deposition of the San Diego Formation and continuing to the present time, the San Diego coastal margin has undergone relatively steady uplift (Fig. 2). A series of continually evolving marine abrasion platforms have been carved and uplifted during this time and are manifest in the marine terraces and their deposits that are ubiquitous to the San Diego coastal region. The deposits consist of nearshore marine, beach, estuarine, lagoonal and continental dune facies that were deposited across a marine/nonmarine transition zone and along a coastal strandline. Changes in sea level coupled with regional uplift give rise to the preservation and/or obliteration of both the abrasion platforms and their overlying deposits (e.g. Lajoie, and others, 1991; Kern and Rockwell, 1992; Kern, 1996a, 1996b).
The authors appreciate very helpful reviews by Victoria R.Todd and J. Philip Kern