Engineering Geology and Seismology Lecture#12 CE-312 Engineering Geology and Seismology Instructor: Dr Amjad Naseer Department of Civil Engineering N-W.F.P University of Engineering and Technology, Peshawar

Outlines of the Presentation Earthquakes Causes and effects

Some facts about the Earthquake How Many Earthquakes Happen Each Year? There are over a million earthquakes annually, including those too small to be felt. How Many Earthquakes Happen Every Month? Day? Minute? Per month Approximately 80,000 Per day Approximately 2,600 Per minute Approximately 2 And one earthquake is felt approximately every 30 seconds. Of these only a relative few are capable of causing damage. Earthquakes are common natural events. How Deep Do Earthquakes Occur in the World? Earthquakes occur in the crust or upper mantle which ranges from the surface to about 800 kilometers deep (about 500 miles).

Some facts about the Earthquake How Many Earthquakes Happen Each Year? There are over a million earthquakes annually, including those too small to be felt. Description Magnitude Frequency per year Great 8.0+ 1 Major 7.0-7.9 18 Large 6.0-6.9 120 Moderate 5.0-5.9 1,000 Minor 4.0-4.9 6,000 Generally felt 3.0-3.9 49,000 Potentially perceptible 2.0-2.9 300,000

Some facts about the Earthquake Where Do Most Earthquakes Occur in the World? The surface of the earth is divided like a jigsaw puzzle into giant pieces called tectonic or crustal plates. These giant pieces move slowly over partially melted rock known as the mantle. As they move, they slide along each other; move into each other, move away from each other, or one slips under another. On these active plate boundaries about 95% of all the world's earthquakes occur. California, Alaska, Japan, South America, and the Philippines are all on plate boundaries. Only 5% are in areas of the plates far away from the boundaries. These are called mid-plate or intra-plate earthquakes and are, as yet, poorly understood.

Some facts about the Earthquake Can We Predict Earthquakes? 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. For example, if a region has experienced four magnitude 7 or larger earthquakes during 200 years of recorded history, and if these shocks occurred randomly in time, then scientists would assign a 50 percent probability (that is, just as likely to happen as not to happen) to the occurrence of another magnitude 7 or larger quake in the region during the next 50 years.

Some facts about the Earthquake Can We Predict Earthquakes? But in many places, the assumption of random occurrence with time may not be true, because when strain is released along one part of the fault system, it may actually increase on another part. Four magnitude 6.8 or larger earthquakes and many magnitude 6 - 6.5 shocks occurred in the San Francisco Bay region during the 75 years between 1836 and 1911. For the next 68 years (until 1979), no earthquakes of magnitude 6 or larger occurred in the region. Beginning with magnitude 6.0 shocks in 1979, the earthquake activity in the region increased dramatically; between 1979 and 1989, there were four magnitudes 6 or greater earthquakes, including the magnitude 7.1 Loma Prieta earthquakes. This clustering of earthquakes leads scientists to estimate that the probability of a magnitude 6.8 or larger earthquake occurring during the next 30 years in the San Francisco Bay region is about 67 percent (twice as likely as not).

Some facts about the Earthquake Can We Predict Earthquakes? Another way to estimate the likelihood of future earthquakes is to study how fast strain accumulates. When plate movements build the strain in rocks to a critical level, like pulling a rubber band too tight, the rocks will suddenly break and slip to a new position. Scientists measure how much strain accumulates along a fault segment each year, how much time has passed since the last earthquake along the segment, and how much strain was released in the last earthquake. This information is then used to calculate the time required for the accumulating strain to build to the levels that result in an earthquake. This simple model is complicated by the fact that such detailed information about faults is rare. In the United States, only the San Andreas Fault system has adequate records for using this prediction method.

Some facts about the Earthquake Can We Predict Earthquakes? Both of these methods, and a wide array of monitoring techniques, are being tested along part of the San Andreas Fault. For the past 150 years, earthquakes of about magnitude 6 have occurred an average of every 22 years on the San Andreas Fault near Park field, California. The last shock was in 1966. Because of the consistency and similarity of these earthquakes, scientists have started an experiment to "capture" the next Park field earthquake. A dense web of monitoring instruments was deployed in the region during the late 1980s. The main goals of the ongoing Park field Earthquake Prediction Experiment are to record the geophysical signals before and after the expected earthquake; to issue a short-term prediction; and to develop effective methods of communication between earthquake scientists and community officials responsible for disaster response and mitigation. This project has already made important contributions to both earth science and public policy.

Effects of Earthquake Strong Ground Motion or Ground Shaking The most destructive of all earthquake hazards is caused by seismic waves reaching the ground surface at places where human-built structures, such as buildings and bridges, are located. When seismic waves reach the surface of the earth at such places, they give rise to what is known as strong ground motion. Strong ground motion causes buildings and other structures to move and shake in a variety of complex ways. Many buildings cannot withstand this movement and suffer damages of various kinds and degrees. Most deaths, injuries, damages and economic losses caused by earthquakes result from strong ground motion acting upon buildings and other man-made structures not capable of withstanding such motion. It is for this reason that it is often said, "Earthquakes don't kill people, buildings do”.

Fig: 2.4.2 Hector Mines E/q, USA Effects of Earthquake Surface Rupture Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture. Ground failure, rather than ground shaking, is the principal cause of damage to water and sewer lines. The brittle sewer pipes tended to fail under much lower strains than water lines, so damage to sewer lines is considerably more extensive. Identifying where and to what degree subgrade utilities are at risk from earthquakes can be accomplished by accurately delineating regions at risk of ground failure during earthquake shaking. Fig: 2.4.2 Hector Mines E/q, USA

Effects of Earthquake Landslides Buildings aren't the only thing to fail under the stresses of seismic waves. Often unstable regions of hillsides or mountains fail. In addition to the obvious hazard posed by large landslides, even non lethal slides can cause problems when they block highways they can be inconvenient or cause problems for emergency and rescue operations.

Effects of Earthquake Liquefaction One of the most important types of ground failure which can occur during an earthquake is known as liquefaction. What is Liquefaction? Liquefaction refers to a process resulting in a soil’s loss of shear strength, due to a transient excess of pore water pressure. Soil with a high water table being strongly shaken during an earthquake; that is, cyclically sheared. The soil particles initially have large voids between them. Due to shaking, the particles are displaced.

Effects of Earthquake Liquefaction

Effects of Earthquake Tsunamis Another important class of earthquake effects are tsunamis, which are generated by earthquakes which have occurred beneath the ocean floor. Tsunamis are immense sea waves. "Tsunami" is actually a Japanese word meaning "huge wave". Japan is one of the most seismically active countries in the world and has experienced many earthquakes and tsunamis. These waves travel across the ocean at speeds as great as 597 miles per hour and may be 15 meters (49 feet) high or higher by the time they reach the shore.

Effects of Earthquake Effects of Earthquake on Buildings Fig: 2.5 How the building damages during an Earthquake (Courtesy of National Disaster management Division India)

Effects of Earthquake Effects of Earthquake on Buildings

Effects of Earthquake Effects of Earthquake on Buildings

Effects of Earthquake Effects of Earthquake on Buildings

Locating an Earthquake’s Epicenter The source of an earthquake within the earth is the actual place of rock slippage along a fault. The hypocenter or Focus, the point where the fault starts to move, can be located by using P and S waves. The point at the earth's surface directly above the hypocenter is called the epicenter.

Locating an Earthquake’s Epicenter Around the world, abrupt motions of the earth are continuously monitored by Seismographs.

Locating an Earthquake’s Epicenter ). Seismogram Seismograph

Locating an Earthquake’s Epicenter Seismic wave behavior P waves arrive first, then S waves, then L and R After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter (D).

Locating an Earthquake’s Epicenter

Locating an Earthquake’s Epicenter S-P (S minus P) time formula: The correlation between distance and the difference between arrival times is given by Where D is the distance to the source, vs is the velocity of the secondary wave and vp is the velocity of the primary wave. These velocities range from 3 to 8 km/sec (Primary) and 2 to 5 km/sec (Secondary).

Seismic Travel-time Curve If the speeds of the seismic waves are not known, use Travel-Time curve for that region to get the distance 1. Measure time between P and S wave on seismogram 2. Use travel-time graph to get distance to epicenter Ideally

3-circle steps: 3-circle method: 1) Read S-P time from 3 seismograms. 2) Compute distance for each event/recording station pair (D1, D2, D3) using S-P time formula. 3) Draw each circle of radius Di on map. 4) Overlapping point is the event location. Assumption: Source is relatively shallow; epicenter is relatively close to hypocenter. north D1 D2 D3