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Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.
Slikensides of fault crosses the Taylor Highway 123 miles north of Tetlin Junction, Alaska
SLICKENSIDES are smoothly polished surfaces caused by frictional movement between rocks along the two sides of a fault
PANDEMIC EDITING
The shelter in place gave me the opportunity to look at images that I haven’t seen in a LONG TIME.
A Look Back 30 Years
We were on a 9000 mile round trip from Palo Alto, California to Inuvik, NWT and back home via Alaska. About 3000 consecutive miles were on dirt/gravel roads which took a toll on our internal organs from the constant vibrations from corrugated roads. We thought about leaving the Land Cruiser at Inuvik and flying home returning the next year to retrieve it.....But staying in one spot and not driving for a few days allowed us to heal and helped us get over the ill conceived idea.
This seismogram is from the Chernabura Island seismic station in Alaska. The noise was caused by a magnitude 7.3 offshore earthquake south of the Alaskan Peninsula. The quake occurred at 9:37 AM, local time, on 16 July 2025. The epicenter was 70 to 71 kilometers ~south of the abandoned town of Unga, Alaska. The hypocenter was about 20 kilometers deep. Shaking resulted from right-lateral slip along a north-northwest to south-southeast striking, east-dipping fault fault zone, or left-lateral slip along an east-west striking, very steeply dipping fault zone. This is the sixth magnitude 7+ earthquake of 2025.
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Info. from the United States Geological Survey:
The July 16, 2025, magnitude 7.3 earthquake southeast of Sand Point, Alaska (south of the Alaska Peninsula), occurred as the result of strike-slip faulting near the subduction zone interface between the Pacific and North America plates, either within the upper (North America) Plate or within the downgoing (Pacific) slab. The preliminary focal mechanism solution indicates rupture occurred on either a moderately dipping right-lateral strike-slip fault striking towards the north-northwest or on a steeply dipping left-lateral strike-slip fault striking towards the east, and therefore that this earthquake was not a thrust event on the plate interface itself. At the location of this event, the Pacific plate converges with North America to the northwest at a rate of about 65 millimeters per year, subducting at the Alaska-Aleutian Trench 100 km to the southeast of the earthquake. The majority of large earthquakes in this area are thrust events on the plate interface, unlike the strike-slip event of July 16, 2025.
While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike slip faulting events of the size of the July 16, 2025, earthquake are typically about 90 x 15 kilometers (length x width).
Large earthquakes are common in the Alaska-Aleutian subduction zone. Since 1900, 9 other earthquakes M7 and larger have occurred within 250 kilometers of the July 16, 2025, event, including the October 19, 2020, magnitude 7.6, which had a similar mechanism. The largest of these was a magnitude 8.6 earthquake on April 1, 1946, which generated a large tsunami that caused destruction and loss of life both locally on Unimak Island and more distantly at Hilo, Hawaii. The Alaska-Aleutian Trench also hosted the second largest earthquake recorded by modern seismic instrumentation, the magnitude 9.2 March 27, 1964, earthquake, which ruptured to within about 350 kilometers of this event.
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Info. at:
earthquake.usgs.gov/earthquakes/eventpage/us7000qd1y/exec...
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An earthquake is a natural shaking or vibrating of the Earth caused by sudden fault movement and a rapid release of energy. Earthquake activity is called "seismicity". The study of earthquakes is called "seismology". The actual underground location of an earthquake is the hypocenter, or focus. The site at the Earth's surface, directly above the hypocenter, is the epicenter. Minor earthquakes may occur before a major event - such small quakes are called foreshocks. Minor to major quakes after a major event are aftershocks.
Most earthquakes occur at or near tectonic plate boundaries, such as subduction zones, mid-ocean ridges, collision zones, and transform plate boundaries. They also occur at hotspots - large subsurface mantle plumes (Examples: Hawaii, Yellowstone, Iceland, Afar).
Earthquakes generate four types of shock waves: P-waves, S-waves, Love waves, and Rayleigh waves. P-waves and S-waves are body waves - they travel through solid rocks. Love waves and Rayleigh waves travel only at the surface - they are surface waves. P-waves are push-pull waves that travel quickly and cause little damage. S-waves are up-and-down waves (like flicking a rope) that travel slowly and cause significant damage. Love waves are side-to-side surface waves, like a slithering snake. Rayleigh waves are rotational surface waves, somewhat like ripples from tossing a pebble into a pond.
Earthquakes are associated with many specific hazards, such as ground shaking, ground rupturing, subsidence (sinking), uplift (rising), tsunamis, landslides, fires, and liquefaction.
Some famous major earthquakes in history include: Shensi, China in 1556; Lisbon, Portugal in 1755; New Madrid, Missouri in 1811-1812; San Francisco, California in 1906; Anchorage, Alaska in 1964; and Loma Prieta, California in 1989.
On my second visit to Lake Cowal back in early December, I was told about a place that was of great interest to geologists visiting the nearby gold mine. This trip I headed out to have a look.
Following is the information I was given to explain what I was going to see.
"The outcrop is part of the Booberoi Fault and features sheared Late Silurian-Early Devonian Edols Conglomerate (Sherwin, 1996) showing stretched quartzite and vein-quartz pebbles in a quartz white mica-chlorite matrix, which was probably a muddy sandstone. Mesoscopic kinematic indicators show at least two movement episodes: a horizontal sinstral movement and a near vertical reverse (west side up) movement. The movement direction can be discerned from the asymmetry of the matrix surrounding the pebbles and the mineral elongation within the matrix."
Video - youtu.be/gFIb1JXFTb8
Grey limestone clasts in a red mudstone matrix from the Triassic basin of Maryland. From Frederick, Maryland.
On my second visit to Lake Cowal back in early December, I was told about a place that was of great interest to geologists visiting the nearby gold mine. This trip I headed out to have a look.
Following is the information I was given to explain what I was going to see.
"The outcrop is part of the Booberoi Fault and features sheared Late Silurian-Early Devonian Edols Conglomerate (Sherwin, 1996) showing stretched quartzite and vein-quartz pebbles in a quartz white mica-chlorite matrix, which was probably a muddy sandstone. Mesoscopic kinematic indicators show at least two movement episodes: a horizontal sinstral movement and a near vertical reverse (west side up) movement. The movement direction can be discerned from the asymmetry of the matrix surrounding the pebbles and the mineral elongation within the matrix."
Video - youtu.be/gFIb1JXFTb8
Help the flying geologist locate this hanging valley/fault system. Looking south... somewhere in the rockies.
Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.
A small reverse fault in a sample from a banded iron formation (BIF). This fault would have been caused by compressional forces on the rock.
Locality: near Jerome, Arizona
Dimensions: 4 x 8 x 7 cm
Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.