An earthquake (also known as a quake, tremor or temblor) is the perceptible shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth’s crust that creates seismic waves. Earthquakes can be violent enough to toss people around and destroy whole cities. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.
Earthquakes are caused mostly by rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake’s point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter.
Tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the fault surface that increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behavior. Once the fault 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.
地震（英語：Earthquake）是指在地殼內釋放能量所造成的震動，可由自然現象如地殼突然運動、火山活動及隕石撞擊引起，亦可由人為活動如地下核試造成。歷史曾記載的災害性地震主要由地殼突然運動所造成，地殼在板塊運動的過程中累積應力，當地殼無法繼續累積應力時破裂釋放出地震波，使地面發生震動，震動可能引發山泥傾瀉甚或火山活動。如果地震在海底發生，海床的移動甚至會引發海嘯。
震動的發源處稱為震源。大多數震源都在地殼和上地幔頂部，即岩石圈內。根據震源的深度，地震可分為三類：淺源地震（深度在70公里內）、中源地震（深度在70-300公里）和深源地震（深度在300公里以上）。由震源豎一垂直線至地面上的位置稱為震中。震中是地表距離震源最近的地方，因此地震波最早到達這處，震動也最為強烈，破壞程度也最大，測量震央的方式一般是透過由三點不同地點的P波與S波走時差與到達時間，而又以P波（纵波）傳遞速度較快，S波（横波）較具傷害力。
There are three main types of fault, all of which may cause an interplate earthquake:
o normal,
o reverse (thrust) and
o strike-slip.
Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent 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.
The Richter magnitude scale (also Richter scale) assigns a magnitude number to quantify the size of an earthquake. As measured with a seismometer, an earthquake that registers 5.0 on the Richter scale has a shaking amplitude 10 times greater than an earthquake that registered 4.0 at the same distance. As energy release is generally proportional to the shaking amplitude raised to the 3/2 power, an increase of 1 magnitude corresponds to a release of energy 31.6 times that released by the lesser earthquake.
Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, megathrust earthquakes, including almost all of those of magnitude 8 or more. Strike-slip faults, particularly continental transforms, can produce major earthquakes up to about magnitude 8. Earthquakes associated with normal faults are generally less than magnitude 7.
断层（英语：fault）是指岩石破裂后，兩側岩石发生显著的相对位移。断层大小不等，大的断层可纵贯整个岩石圈，水平则可绵延几千公里。
断层有断层面、断层线和断层盘三要素。
断层面是指岩层受力后发生相对位移的破裂面。呈面状展布，具有一定的走向、倾向、倾角；断层面有的光滑，有的是擦痕，有的呈波状起伏较粗糙。
断层线是指断层面与地面的交线。可直可弯，甚至形成一条宽窄不等、成带状分布的破碎地带，称为断层破碎带。
断层盘是指断层面两侧的断块称为断层盘。位于断层面上方的称为上盘，下方的称为下盘；相对上升的一盘称为上升盘，相对下降的一盘称为下降盘。 依照斷層兩側岩體相對位移的方式，可以將斷層分類。
傾向断层
斷層滑動方向和傾向平行的斷層稱為走向滑移斷層。傾向是指斷層面和水平面的夾角。「正断层」是上盘相对下降，下盘相对上升。由于受水平张力作用，断层面较粗糙，擦痕一般不太发育，产状较陡；其破碎带中常形成棱角明显的断层角砾岩。
走滑断层
英语：Strike-slip fault，断层两侧岩块，沿着断层面走向的水平方向相对移动的断层。
斜向断层
斷層的滑移包含走向和傾向分量。
里氏地震规模（英语：Richter magnitude scale），亦稱近震震級（local magnitude, ML），是表示地震规模大小的標度。它是由观测点处地震儀所記錄到的地震波最大振幅的常用對數演算而來。所以，震級相差1代表振幅相差10倍，而所釋出的能量則相差約32倍。
TAIWAN´S TECTONIC ENVIRONMENT
Taiwan is located in a complex, tectonically active region. The Philippine Sea plate is subducting northwestward along the Ryukyu Trench in the north and the Eurasian plate underthrusts the Philippine Sea plate along the Manila Trench in the south. The Philippine Sea plate is moving northwestward at the rate of 7-8 cm/yr relative to the Eurasia plate, creating the Taiwan collision zone. The major seismic faults in Taiwan correspond to the various suture zones between the various terranes. As a result, Taiwan has numerous mud volcanoes and hot springs. These faults have also produced several major quakes throughout the history of the island.
菲律賓海板塊的隱沒方向在台灣改變，在台灣南方，歐亞大陸板塊朝東隱沒到菲律賓海板塊之下，形成馬尼拉海溝及其東側的呂宋火山島弧，到了台灣北方，菲律賓海板塊則是朝北隱沒到歐亞大陸板塊之下，造成琉球海溝及其北側的琉球火山島弧和沖繩海槽。台灣位於菲律賓海板塊和歐亞大陸板塊的聚合邊界，為一個造山作用的活躍的活動構造區，根據GPS觀測資料，菲律賓海板塊以每年8.2公分的移動速度向西北移動.
1999 JIJI EARTHQUAKE
In the early morning (01:47 local time) of September 21, 1999, the largest earthquake of the century in Taiwan (Mw=7.6, ML=7.3) struck the central island near the small town of Ji-Ji.The hypocenter was located by the Central Weather Bureau Seismological Center at 23.87°N, 120.75°E, with a depth of about 7 km. The earthquake was in an unusual location for Taiwan, which experiences the majority of its earthquakes off the eastern coast, such quakes normally causing little damage. The earthquake with vertical thrust and left lateral strike-slip offsets occurred along the Chelongpu Fault (車籠埔斷層) with extensive surface ruptures for about 85 km. This area was previously identified as an active fault. The maximum displacement of about 9.8 meters is among the largest fault movements ever measured for modern earthquakes. Seismic reflection data indicate that the Chelungpu fault dips shallowly (25°-30°) toward the east,so the observed surface displacement may only be about half of the total slip on the fault. There was severe destruction in the towns of Chungliao, Nantou,Taichung, FengYuan,and Tungshi, with over 2300 fatalities and 8700 injuries.
921大地震，正式名稱為集集大地震，是1999年9月21日上午1時47分15.9秒（臺灣當地時間）發生於臺灣中部山區的逆斷層型地震，造成臺灣全島均感受到嚴重搖晃，共持續102秒，乃臺灣自二戰後傷亡損失最大的自然災害。震央在北緯23.85度、東經120.82度，約於南投縣集集鎮境內，震源深度8.0公里[1]，芮氏規模7.3，美國地質調查局測得矩震級7.6~7.7。該地震肇因於車籠埔斷層的錯動，並在地表造成長達85公里的破裂帶[5]，另外也有學者認為是由車籠埔斷層及大茅埔－雙冬斷層兩條活動斷層同時再次活動所引起。
此地震造成2,415人死亡，29人失蹤，11,305人受傷，51,711間房屋全倒，53,768間房屋半倒。 斷層走向為北偏東18度，傾角19度。此次地震屬內陸淺層地震，斷層地表破裂面全長約105公里，造成地表斷層最大垂直錯動量達到11公尺、最大水平錯動量達10公尺以上，平均錯動量約4公尺，破壞力相當強大。
921大地震，南投縣受創最鉅，10多個鄉鎮市重創，縣內的各項設施、經濟產業、觀光旅遊、民眾生命財產等都受到重大損失。災情主要集中在震央及斷層所經附近的埔里、竹山、名間、中寮、國姓、草屯等地，尤其是中寮鄉在這次的大震之下受創最為慘重。
Secondary effects of a strong earthquake, such as landslides can cause much trouble for people living in the area. A landslide is a form of mass wasting that includes a wide range of ground movements, such as rockfalls, deep failure of slopes, and shallow debris flows.
The Jiji earthquake caused about ten thousand landslides, each of which was larger than 625 square meters and that the total area of the landslides exceeded hundred square kilometers. Many highway slopes have been weakened, or even fractured by the shaking of the Jiji earthquake. The Central Cross-Island Highway (中部橫貫公路) or Provincial Highway No.8 is one of few highway systems that connected the west coast with the east of Taiwan. The Jiji earthquake caused massive damage to the highway and cut the highway in multiple places between Dongshih and Lishan (34 Km to 62 Km). The highway is still closed between Lishan and Guguan and is not expected to be repaired.
山崩又稱山体滑坡、山泥傾瀉、土溜，是指在重力的影響下塊體沿着一段山坡下滑的現象，又稱作坍方。若是土體坍方時，混和雨水或河水則演變成土石流。 地滑又稱走山，是融合岩屑和土壤的岩體沿層面滑動或移動，與山崩不同。山崩最主要的原因是山坡上的岩石或土壤吸收了大量的水（比如由于暴雨或者融雪），导致岩石或土壤内部的摩擦力降低，土壤或岩石丧失其稳固性下滑。當斜坡上的引力較抗力強,斜坡便不穩固,有倒塌的危險 其它原因有.
中橫公路在1999年921大地震之後，由於位在震央九份二山附近，道路毀損相當嚴重，特別是谷關到德基水庫這一段，還有台八甲線壩新路口到德基段。據公路局資料顯示，在路工部份，邊坡崩塌里程達80%以上，高度有數百公尺，路基全部流失處多達26處，受創而造成路面寬度不足則有134處。此外亦有數座隧道坍塌或半毀，增加了搶修通車的難度。當時圍困在谷關的遊客約有上百人，在9月21日之後，一直受困到9月23日搶通台八線谷關往台中路線之後，才得以脫困。直到四個月後，也就是2000年的1月18日，公路局德基搶修小組和谷關搶修小組，才接起這一段路線。但接起路線後並未馬上開放通車，由於其中有24公里屬極危險路段，只要下雨或有餘震，就會造成二次嚴重坍方。
Chiu-fen-erh-shan landslide
This gigantic landslide was induced by the Jiji earthquake and is located only a few kilometers from the epicenter. The estimated volume moved is 50 million cubic meters. The slid materials buried 39 humans and blocked a confluence of two streams.
This area is underlain mainly by the Miocene sedimentary rocks. The strata, that can be found in this region are:
Shihmen Formation - consist mainly of light gray muddy sandstone with thin-bedded dark grey shale. The sandstone is fine grained and compact.
Changhukeng Shale - is composed of dark grey marine shale with subordinate sandstone. The shale is generally massive.
Kueichulin Formation - is represented by grey to yellowish brown massive sandstone with subordinate shale.
The structure exposed on the upper part of the slip surface indicates that the strata were subjected to creep deformation before the Chi-chi earthquake. Gravitational creep is the slow, imperceptible deformation of slope materials under stresses lower than those required to produce sudden failure.
The long-term creeping loosens rocks which becomes more permeable. The higher permeability results in higher infiltration capacity and consequently leads to a low drainage density within the creeping area. The high infiltration capacity decreases surface runoff, which erodes land surface to form a gully. Therefore, the low drainage density in the Chiu-fen-erh-shan landslide area is most likely the resultant effect of creeping, and can act as a geomorphological precursor to the slope instability.
THE CACHE:
As this is a virtual, „Earth cache“-type, don´t look for a physical container. To fulfil the log requirements, however, you have to send me via e-mail or messenger answers to following questions:
At WP1:
1, Describe the characteristics (structure, layers, colour etc…) of the soil you find in the landslide area. From the mentioned strata, which one do you think is here the most dominant ? How thick are the different layers ?
2, Look at the slope of the landslide. Estimate the angle of the slope at the upper and what at the lower end.
3, Apart from the earthquake, an other major meteorological phenomenon has contributed to the landslide. Which one ?
WP2: Slanted House:
4, Looking at the front of the house, how big is the vertical difference between the left and right end of the building (in meters) ? Which end is higher ?
5, Enter the house. Can you feel any “different” sensations ? If so, how would you explain them ?
6, As usual, a photo from the location isn´t mandatory, however if you would like to share your pictures from this amazing place, this would be very much appreciated.
Last but not least:
A) For a log permission, you HAVE TO submit your answers. If you´re visiting in a group, that´s fine, but the log will be accepted only from those cachers, who put the effort in to submit the answers. No answer results in a log deletion without further notice.
B) Feel free to log the cache straight after you hit “submit”. Should there be any problem, I´ll contact you.
Ref.: Wikipedia, Britannica, Earthquake Engineering, EOS, Elsevier