If you want to determine the “Origin Time” (actual time the earthquake occurred at the source), take the s-p times above and multiply by 1.37. Take this time and subtract that from the P arrival time to get the approximate origin time.

Scientists estimate earthquake probabilities in two ways: by studying the history of large earthquakes in a specific area and the rate at which strain accumulates in the rock. Scientists study the past frequency of large earthquakes in order to determine the future likelihood of similar large shocks.

Seismic monitoring utilizes sensitive seismographs to record the ground motion from seismic waves created by earthquakes or other sources. Seismograms from seismic monitoring stations can be used to determine the location, origin time, and magnitude (as well as other characteristics) of earthquakes.

normal fault – a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.

Where do earthquakes occur?

• The world’s greatest earthquake belt, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81 percent of our planet’s largest earthquakes occur.
• The Alpide earthquake belt extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic.

Earthquakes occur on faults – strike-slip earthquakes occur on strike-slip faults, normal earthquakes occur on normal faults, and thrust earthquakes occur on thrust or reverse faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other.

Normal Faults: This is the most common type of fault. It forms when rock above an inclined fracture plane moves downward, sliding along the rock on the other side of the fracture. Normal faults are often found along divergent plate boundaries, such as under the ocean where new crust is forming.

How do we know a fault exists? Earthquakes on the fault have left surface evidence, such as surface ruptures or fault scarps (cliffs made by earthquakes); Earthquakes recorded by seismographic networks are mapped and indicate the location of a fault.

The danger of living near fault lines Living near fault lines is inherently dangerous but difficult to avoid. The dangers people face include not only tremors but also other threats: Widespread smoke and ash can pollute the air and block out the sun for miles in every direction.

Reverse faults are exactly the opposite of normal faults. If the hanging wall rises relative to the footwall, you have a reverse fault. Reverse faults occur in areas undergoing compression (squishing). The fault planes are nearly vertical, but they do tilt to the left.

Reverse faults, also called thrust faults, slide one block of crust on top of another. These faults are commonly found in collisions zones, where tectonic plates push up mountain ranges such as the Himalayas and the Rocky Mountains.

Reverse faults have a characteristic topographic signature caused by uplift of the hanging wall and associated folding above the fault, producing lobate ridges (Schultz et al. 2010).

Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. When the rocks break, the earthquake occurs.

A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If the angle of the fault plane is lower (often less than 15 degrees from the horizontal) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an overthrust or overthrust fault.

This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small.

There are three main types of fault which can cause earthquakes: normal, reverse (thrust) and strike-slip. Figure 1 shows the types of faults that can cause earthquakes. Figures 2 and 3 show the location of large earthquakes over the past few decades.

Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse. A thrust fault is a reverse fault with a dip of 45 degrees or less.

Dip-slip faults In a normal fault, the hanging wall moves downward, relative to the footwall. Low-angle normal faults with regional tectonic significance may be designated detachment faults. A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall.

There are two types of strike-slip and two types of dip-slip fault. The two types of strike- slip fault are right-lateral (or dextral) and left-lateral (or sinistral) while the two types of dip- slip fault are normal and reverse (or thrust) (Figure 7).

The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter. Sometimes an earthquake has foreshocks. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens.

The epicenter is the point on the earth’s surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.

Richter magnitudes. . Events with magnitudes greater than 4.5 are strong enough to be recorded by a seismograph anywhere in the world, so long as its sensors are not located in the earthquake’s shadow. The following describes the typical effects of earthquakes of various magnitudes near the epicenter.